ZXLD1360资料

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ZXLD1360

1A LED driver with internal switch

Description

The ZXLD1360 is a continuous modeinductive step-down converter, designed fordriving single or multiple series connectedLEDs efficiently from a voltage source higherthan the LED voltage. The device operatesfrom an input supply between 7V and 30V andprovides an externally adjustable outputcurrent of up to 1A. Depending upon supplyvoltage and external components, this canprovide up to 24 watts of output power.

The ZXLD1360 includes the output switch anda high-side output current sensing circuit,which uses an external resistor to set thenominal average output current.

Output current can be adjusted above, orbelow the set value, by applying an externalcontrol signal to the 'ADJ' pin.

The ADJ pin will accept either a DC voltage or aPWM waveform. Depending upon the controlfrequency, this will provide either a continuousor a gated output current. The PWM filtercomponents are contained within the chip.

The PWM filter provides a soft-start feature bycontrolling the rise of input/output current. Thesoft-start time can be increased using anexternal capacitor from the ADJ pin to ground.Applying a voltage of 0.2V or lower to the ADJpin turns the output off and switches the deviceinto a low current standby state.

Features

•••••••••••••

Simple low parts count

Internal 30V NDMOS switch1A output current

Single pin on/off and brightness controlusing DC voltage or PWMInternal PWM filterSoft-start

High efficiency (up to 95%)

Wide input voltage range: 7V to 30V40V transient capabilityOutput shutdown

Up to 1MHz switching frequency Inherent open-circuit LED protectionTypical 4% output current accuracy

Applications

•••••

Low voltage halogen replacement LEDsAutomotive lighting

Low voltage industrial lightingLED back-up lightingIlluminated signs

Pin connections

LX 1GND 2ADJ 3TSOT23-5 Top view 4ISENSE 5Typical application circuit

VIN (7V - 30V)VIN Rs 0.1⍀ C1 4.7␮FL1 47␮H D1VIN ISENSE LX N/C ADJ ZXLD1360GND GNDIssue 1 - March 2007

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ZXLD1360

Absolute maximum ratings (voltages to GND unless otherwise stated)Input voltage (VIN)

ISENSE voltage (VSENSE)LX output voltage (VLX)

Adjust pin input voltage (VADJ)Switch output current (ILX)Power dissipation (Ptot)

(Refer to package thermal de-rating curve on page 16)

-0.3V to +30V (40V for 0.5 sec)

+0.3V to -5V (measured with respect to VIN)-0.3V to +30V (40V for 0.5 sec)-0.3Vto+6V1.25A1W

-40 to 125°C-55 to 150°C150°C

Operating temperature (TOP)Storage temperature (TST)Junction temperature (Tj MAX)

These are stress ratings only. Operation outside the absolute maximum ratings may cause device failure. Operation at the absolute maximum ratings, for extended periods, may reduce device reliability.

Thermal resistance

Junction to ambient (R⍜JA)SymbolVINVSUVSDIINQoffIINQonVSENSE

ParameterInput voltage

125°C/W

Electrical characteristics (test conditions: VIN=12V, Tamb=25°C unless otherwise stated)(a)

Conditions

Min.7

Typ.5.655.55

Max.30

UnitVVV

Internal regulator start-up thresholdVIN risingInternal regulator shutdown VIN fallingthreshold

Quiescent supply current ADJ pin groundedwith output off Quiescent supply current ADJ pin floatingwith output switchingf = 250kHzMean current sense threshold Measured on ISENSE voltagepin with respect to VIN(Defines LED current setting accuracy)V

ADJ = 1.25VISENSE pin input currentInternal reference voltage

VSENSE = VIN -0.1Measured on ADJ pin with pin floating

201.8100

405.0105

␮AmAmV

VSENSEHYSSense threshold hysteresisISENSEVREF

±151.251.2550

0.3

VADJ falling

0.15

0.2

2.50.2510

%␮AVppm/KVV

⌬VREF /⌬TTemperature coefficient of VREFVADJVADJoff

External control voltage range on ADJ pin for DC brightness control(b)DC voltage on ADJ pin to switch device from active (on) state to quiescent (off) state

DC voltage on ADJ pin to switch device from quiescent (off) state to active (on) state

VADJonVADJrising

0.20.250.3V

NOTES:

(a)Production testing of the device is performed at 25°C. Functional operation of the device and parameters specified overa -40°C to +105°C temperature range, are guaranteed by design, characterization and process control.

(b)100% brightness corresponds to VADJ = VADJ(nom) = VREF. Driving the ADJ pin above VREF will increase the VSENSE.threshold and output current proportionally.

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ZXLD1360

Electrical characteristics (test conditions: VIN=12V, Tamb=25°C unless otherwise stated) (cont.)SymbolRADJILXmeanRLXILX(leak)

ParameterConditionsResistance between ADJ pin and 0< VADJ < VREFVREFVADJ > VREF +100mVContinuous LX switch currentLX Switch ‘On’ resistanceLX switch leakage current

PWM frequency

<500Hz

PWM amplitude = VREFMeasured on ADJ pinPWM frequency >10kHzPWM amplitude = VREFMeasured on ADJ pinTime taken for output current to reach 90% of final value after voltage on ADJ pin has risen above 0.3V

ADJ pin floatingL = 33␮H (0.093⍀)

IOUT = 1A @ VLED = 3.6VDriving 1 LEDLX switch ‘ON’LX switch ‘OFF’

0.01

@ ILX = 1 A

0.5

Min.13513.5

Typ.

Max.2502511.051

Unitk⍀A⍀␮A

DPWM(LF)Duty cycle range of PWM signal

applied to ADJ pin during low frequency PWM dimming modeBrightness control range

DPWM(HF)Duty cycle range of PWM signal

applied to ADJ pin during high frequency PWM dimming modeBrightness control range

TSSSoft start time

100:10.16

5:1

500

␮s

fLX

Operating frequency

(See graphs for more detail)

280240 (*)200 (*)800

0.3

0.7

KHzns nsnsMHz

TONminTOFFmin

Minimum switch ‘ON’ timeMinimum switch ‘OFF’ time

TONmin_RECRecommended minimum switch LX switch 'ON'

'ON' time

fLXmaxRecommended maximum

operating frequency

Recommended duty cycle range DLX

of output switch at fLXmaxTPD

Internal comparator propagation delay

NOTES:

(*)Parameters are not tested at production. Parameters are guaranteed by design, characterization and process control.

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ZXLD1360

Pin description

LX 1GND 2ADJ 3TSOT23-5 Top view NameLXGNDADJ

Pin no.123

5VIN 4ISENSE Description

Drain of NDMOS switchGround (0V)

Multi-function On/Off and brightness control pin:

•Leave floating for normal operation.(VADJ = VREF = 1.25V giving nominal average output current IOUTnom = 0.1/RS)

•Drive to voltage below 0.2V to turn off output current

•Drive with DC voltage (0.3V < VADJ < 2.5V) to adjust output current from 25% to 200% of IOUTnom

•Drive with PWM signal from open-collector or open-drain transistor, to adjust output current. Adjustment range 25% to 100% of IOUTnom for f>10kHz and 1% to 100% of IOUTnom for f < 500Hz

•Connect a capacitor from this pin to ground to increase soft-start time. (Default soft-start time = 0.5ms. Additional soft-start time is approx.0.5ms/nF)Connect resistor RS from this pin to VIN to define nominal average output current IOUTnom = 0.1/RS

(Note: RSMIN = 0.1⍀ with ADJ pin open-circuit)

Input voltage (7V to 30V). Decouple to ground with 4.7␮F or higher X7R ceramic capacitor close to device

ISENSE

VIN

Ordering information

Device

ZXLD1360ET5TA

Reel size(mm)180

Reel width(inches)

Quantityper reel3,000

Device mark1360

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ZXLD1360

Block diagram

VIND1RSL15VIN4ISENSE1LX5VC14.7␮FVoltageregulatorR1-+0.2V-+Low voltagedetectorAdj3R4200KR520KR2-+MND11.25V-+1.35VR3Gnd2Issue 1 - March 2007

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ZXLD1360

Device description

The device, in conjunction with the coil (L1) and current sense resistor (RS), forms a self-oscillating continuous-mode buck converter.

Device operation (Refer to block diagram and Figure 1 - Operating waveforms)

Operation can be best understood by assuming that the ADJ pin of the device is unconnected andthe voltage on this pin (VADJ) appears directly at the (+) input of the comparator.

When input voltage VIN is first applied, the initial current in L1 and RS is zero and there is nooutput from the current sense circuit. Under this condition, the (-) input to the comparator is atground and its output is high. This turns MN on and switches the LX pin low, causing current toflow from VIN to ground, via RS, L1 and the LED(s). The current rises at a rate determined by VINand L1 to produce a voltage ramp (VSENSE) across RS. The supply referred voltage VSENSE isforced across internal resistor R1 by the current sense circuit and produces a proportional currentin internal resistors R2 and R3. This produces a ground referred rising voltage at the (-) input ofthe comparator. When this reaches the threshold voltage (VADJ), the comparator output switcheslow and MN turns off. The comparator output also drives another NMOS switch, which bypassesinternal resistor R3 to provide a controlled amount of hysteresis. The hysteresis is set by R3 to benominally 15% of VADJ.

When MN is off, the current in L1 continues to flow via D1 and the LED(s) back to VIN. The currentdecays at a rate determined by the LED(s) and diode forward voltages to produce a falling voltageat the input of the comparator. When this voltage returns to VADJ, the comparator output switcheshigh again. This cycle of events repeats, with the comparator input ramping between limits ofVADJ ± 15%.

Switching thresholds

With VADJ = VREF, the ratios of R1, R2 and R3 define an average VSENSE switching threshold of100mV (measured on the ISENSE pin with respect to VIN). The average output current IOUTnom isthen defined by this voltage and RS according to:IOUTnom = 100mV/RS

Nominal ripple current is ±15mV/RS

Adjusting output current

The device contains a low pass filter between the ADJ pin and the threshold comparator and aninternal current limiting resistor (200k⍀ nom) between ADJ and the internal reference voltage.This allows the ADJ pin to be overdriven with either DC or pulse signals to change the VSENSEswitching threshold and adjust the output current. The filter is third order, comprising threesections, each with a cut-off frequency of nominally 4kHz.

Details of the different modes of adjusting output current are given in the applications section.

Output shutdown

The output of the low pass filter drives the shutdown circuit. When the input voltage to this circuitfalls below the threshold (0.2V nom.), the internal regulator and the output switch are turned off.The voltage reference remains powered during shutdown to provide the bias current for theshutdown circuit. Quiescent supply current during shutdown is nominally 20␮A and switchleakage is below 5␮A.

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ZXLD1360

VINLX voltage0V VIN 115mVSENSE voltageToff Ton 85mV100mVVSENSE-VSENSE+IOUTnom +15%Coil current0V 0.15VADJVADJ0.15VADJIOUTnomIOUTnom -15%Comparatorinput voltageComparatoroutput5V 0V Figure 1 - Operating waveforms

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ZXLD1360

Actual operating waveforms [VIN=15V, RS=0.1⍀, L=33µH]

Normal operation. Output current (Ch1) and LX voltage (Ch2)

Actual operating waveforms [VIN=30V, RS=0.1⍀, L=33µH]

Normal operation. Output current (Ch1) and LX voltage (Ch2)

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ZXLD1360

Typical operating conditions

Efficiency 1,3 and 7 LEDsL = 33␮H101009590Efficiency (%)Output current variation with Supply VoltageL = 33␮HDeviation from nominal set current (%)820-2-4-6-8-1051015202530Supply Voltage VIN (V)1 Led3 Led7 Led85807570656001020Supply Voltage VIN (V)1 Led3 Led7 Led3040600500400Operating Frequency vs Input VoltageL = 33␮H1009080701 Led3 Led7 LedDuty Cycle % vs Input VoltageL = 33␮HFreq (kHz)Duty (%)6050403020101 Led3 Led7 Led300200100051015202530005101520253035Supply Voltage VIN (V)Supply Voltage VIN (V)Issue 1 - March 2007

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ZXLD1360

Typical operating conditions

ZXLD1360 Output CurrentL=33μH106010.0%8.0%ZXLD1360 Output CurrentL=33μH1040Output Current Deviation (%)051015202530356.0%4.0%2.0%0.0%-2.0%-4.0%-6.0%1020Output Current (mA)1000980960940920-8.0%900-10.0%05101520253035Supply Voltage V (V)IN1LED2LED3LED4LED5LED6LED7LED8LEDSupply Voltage VIN (V)1LED2LED3LED4LED5LED6LED7LED8LEDZXLD1360 Switching FrequencyL=33μH600100905008070ZXLD1360 Duty CycleL=33μHSwitching Frequency (kHz)400Duty Cycle (%)051015202530356050403020103002001000005101520253035Supply Voltage VIN(V)1 LED2 LED3 LED4 LED5 LED6 LED7 LED8 LEDSupply Voltage VIN (V)1 LED2 LED3 LED4 LED5 LED6 LED7 LED8 LEDIssue 1 - March 2007

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ZXLD1360

Typical operating conditions

ZXLD1360 Output CurrentL=47μH106010.0%8.0%ZXLD1360 Output CurrentL=47μH1040Output Current Deviation (%)6.0%4.0%2.0%0.0%-2.0%-4.0%-6.0%1020Output Current (mA)1000980960940920-8.0%-10.0%0510152025303505101520253035900Supply Voltage VIN (V)1LED2LED3LED4LED5LED6LED7LED8LED1LED2LEDSupply VoltageVIN (V)3LED4LED5LED6LED7LED8LEDZXLD1360 Switching FrequencyL=47μH600100905008070ZXLD1360 Duty CycleL=47μHSwitching Frequency (kHz)400Duty Cycle (%)051015202530356050403020103002001000005101520253035Supply Voltage VIN (V)1 LED2 LED3 LED4 LED5 LED6 LED7 LED8 LED1 LED2 LEDSupply Voltage VIN (V)3 LED4 LED5 LED6 LED7 LED8 LEDIssue 1 - March 2007

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ZXLD1360

Typical operating conditions

ZXLD1360 Output CurrentL=100μH106010.0%8.0%ZXLD1360 Output CurrentL=100μH10401020Output Current Deviation (%)6.0%4.0%2.0%0.0%-2.0%-4.0%-6.0%Output Current (mA)1000980960940920-8.0%-10.0%0510152025303505101520253035900Supply Voltage VIN (V)1LED2LED3LED4LED5LED6LED7LED8LED1LED2LEDSupply Voltage VIN (V)3LED4LED5LED6LED7LED8LEDZXLD1360 Switching FrequencyL=100μH600100905008070400ZXLD1360 Duty CycleL=100μHSwitching Frequency (kHz)Duty Cycle (%)605040302030020010010005101520253035005101520253035Supply Voltage VIN (V)1 LED2 LED3 LED4 LED5 LED6 LED7 LED8 LED1 LED2 LEDSupply Voltage VIN (V)3 LED4 LED5 LED6 LED7 LED8 LEDIssue 1 - March 2007

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ZXLD1360

Typical operating conditions

Vref vs Supply Voltage1.41.23721.23711.2371.2369Vref (V)Vref (V)0.81.23681.23670.41.23661.23651.2301234Supply Voltage VIN (V)5678051015202530 Vref (V)Vref vs Supply Voltage1.210.60.2035Supply Voltage VIN (V)Supply Current vs Supply Voltage600181650014400Iin (µA)Iin (µA)121081002005101520253035Supply Voltage VIN (V)00510Shutdown Current vs Supply Voltage3002001520253035Supply Voltage VIN (V)LED Current vs Vadj12001000LED Current (mA)800600400200001ADJ Pin Voltage (V)R=100mΩR=150mΩ2R=330mΩ3Issue 1 - March 2007

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ZXLD1360

Typical operating conditions

ZXLD1360 Response Time vs TemperatureTypical minimum LX 'on' and 'off' time350300250\"On\" Resistance (Ω)Lx Switch \"On\" Resistance vs Temperature0.800.700.600.500.400.300.20Response Time (ns)200150100500-55-35-15525Min LX on456585105125Ambient Temperature (°C)-50050100150200oAmbient Temperature ( C )Min LX offVadj vs Temperature1.24L = 470uH, Rs = 0.33 Ohms100.4100.2Voltage across Rsense (0.333 Ohm) vs Temperature 1.2351001.2399.81.22512V, single LED1.2212V, three LED24V, single LED24V, three LED1.215Vsense (V)Vadj (V)99.699.424V, single LED99.29998.8.612V, three LED12V, single LED24V, three LED1.21-55-35-15525658510512598.4-55-35-155256585105125Ambient Temperature ( oC )Ambient Temperature ( oC )Output current change vs TemperatureVIN = 12V, L= 470uH, Rs = 0.33 Ohms0.50.4Output current change vs TemperatureVIN = 24V, L= 470uH, Rs = 0.33 OhmsDeviation from nominal set value (%)0.4Deviation from nominal set value (%)0.30.20.10-0.1-0.2-0.3-0.4-0.5-55-35-15525658510512512V, single LED12V, three LED0.20-0.2-0.424V, single LED24V, three LED-0.6-0.8-1-55-35-1552565 o85105125Ambient Temperature ( oC )Ambient Temperature (C )Issue 1 - March 2007

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ZXLD1360

Application notes

Setting nominal average output current with external resistor RS

The nominal average output current in the LED(s) is determined by the value of the externalcurrent sense resistor (RS) connected between VIN and ISENSE and is given by:IOUTnom = 0.1/RS [for RS Ն 0.1⍀]

The table below gives values of nominal average output current for several preferred values ofcurrent setting resistor (RS) in the typical application circuit shown on page 1:

RS (⍀)0.10.130.15

Nominal average output current (mA)1000760667

The above values assume that the ADJ pin is floating and at a nominal voltage of VREF (=1.25V).Note that RS = 0.1⍀ is the minimum allowed value of sense resistor under these conditions tomaintain switch current below the specified maximum value.

It is possible to use different values of RS if the ADJ pin is driven from an external voltage. (Seenext section).

Output current adjustment by external DC control voltage

The ADJ pin can be driven by an external dc voltage (VADJ), as shown, to adjust the output currentto a value above or below the nominal average value defined by RS.

+ ADJ ZXLD1360GND DC GND The nominal average output current in this case is given by:IOUTdc = (VADJ /1.25) x 100mV x RS [for 0.3< VADJ <2.5V]

Note that 100% brightness setting corresponds to VADJ = VREF. When driving the ADJ pin above1.25V, RS must be increased in proportion to prevent IOUTdc exceeding 1A maximum.

The input impedance of the ADJ pin is 200k⍀ ±25% for voltages below VREF and 20k⍀ ±25% forvoltages above VREF +100mV.

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ZXLD1360

Output current adjustment by PWM control

Directly driving ADJ input

A Pulse Width Modulated (PWM) signal with duty cycle DPWM can be applied to the ADJ pin, asshown below, to adjust the output current to a value above or below the nominal average valueset by resistor RS:

PWM VADJ ADJ 0V ZXLD1360GND GND Driving the ADJ input via open collector transistor

The recommended method of driving the ADJ pin and controlling the amplitude of the PWMwaveform is to use a small NPN switching transistor as shown below:

ADJ PWM ZXLD1360GND GND This scheme uses the 200k resistor between the ADJ pin and the internal voltage reference as apull-up resistor for the external transistor.Driving the ADJ input from a microcontroller

Another possibility is to drive the device from the open drain output of a microcontroller. Thediagram below shows one method of doing this:

MCU 10k ADJ ZXLD1360GND If the NMOS transistor within the microcontroller has high Drain / Source capacitance , thisarrangement can inject a negative spike into ADJ input of the 1360 and cause erratic operationbut the addition of a Schottky clamp diode (cathode to ADJ) to ground and inclusion of a seriesresistor (10K) will prevent this. See the section on PWM dimming for more details of the variousmodes of control using high frequency and low frequency PWM signals.

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ZXLD1360

Shutdown mode

Taking the ADJ pin to a voltage below 0.2V for more than approximately 100µs, will turn off theoutput and supply current will fall to a low standby level of 20µA nominal.

Note that the ADJ pin is not a logic input. Taking the ADJ pin to a voltage above VREF will increaseoutput current above the 100% nominal average value. (See graphs for details).Soft-start

The device has inbuilt soft-start action due to the delay through the PWM filter. An externalcapacitor from the ADJ pin to ground will provide additional soft-start delay, by increasing thetime taken for the voltage on this pin to rise to the turn-on threshold and by slowing down therate of rise of the control voltage at the input of the comparator. With no external capacitor, thetime taken for the output to reach 90% of its final value is approximately 500µs. Addingcapacitance increases this delay by approximately 0.5ms/nF. The graph below shows thevariation of soft-start time for different values of capacitor.

Soft Start Time vs Capacitance from ADJ pin to Ground108Soft Start time (ms)200510Capacitance (nF)152025Actual operating waveforms [VIN=15V, RS=0.1⍀, L=33µH, 0nF on ADJ]Soft-start operation. Output current (Ch2) and LX voltage (Ch1)

The trace above shows the typical soft startup time (Tss) of 500␮Sec with no additionalcapacitance added to the ADJ pin.

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ZXLD1360

This time has been extended on the trace below by adding a 100nF ceramic capacitor which givesa soft start time of 40 milliseconds approximately.

Actual operating waveforms [VIN=15V, RS=0.1⍀, L=33µH ,100nF on ADJ]Soft-start operation. Output current (Ch2) and LX voltage (Ch1)

Inherent open-circuit LED protection

If the connection to the LED(s) is open-circuited, the coil is isolated from the LX pin of the chip, sothe device will not be damaged, unlike in many boost converters, where the back EMF maydamage the internal switch by forcing the drain above its breakdown voltage.Capacitor selection

A low ESR capacitor should be used for input decoupling, as the ESR of this capacitor appears inseries with the supply source impedance and lowers overall efficiency. This capacitor has tosupply the relatively high peak current to the coil and smooth the current ripple on the inputsupply. A minimum value of 4.7␮F is acceptable if the input source is close to the device, buthigher values will improve performance at lower input voltages, especially when the sourceimpedance is high. The input capacitor should be placed as close as possible to the IC.

For maximum stability over temperature and voltage, capacitors with X7R, X5R, or betterdielectric are recommended. Capacitors with Y5V dielectric are not suitable for decoupling in thisapplication and should NOT be used.

A suitable Murata capacitor would be GRM42-2X7R475K-50.The following web sites are useful when finding alternatives:www.murata.com www.t-yuden.com www.kemet.com www.avxcorp.com

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ZXLD1360

Inductor selection

Recommended inductor values for the ZXLD1360 are in the range 33␮H to 100␮H.

Higher values of inductance are recommended at higher supply voltages in order to minimizeerrors due to switching delays, which result in increased ripple and lower efficiency. Highervalues of inductance also result in a smaller change in output current over the supply voltagerange. (See graphs). The inductor should be mounted as close to the device as possible with lowresistance connections to the LX and VIN pins.

The chosen coil should have a saturation current higher than the peak output current and acontinuous current rating above the required mean output current.Suitable coils for use with the ZXLD1360 are listed in the table below:Part no.MSS1038-333MSS1038-683NPISD330MTRF

L (␮H)336833

DCR(⍀)0.0930.2130.124

ISAT(A)2.31.51.1

Manufacturer

CoilCraft www.coilcraft.comNIC www.niccomp.com

The inductor value should be chosen to maintain operating duty cycle and switch 'on'/'off' timeswithin the specified limits over the supply voltage and load current range.

The following equations can be used as a guide, with reference to Figure 1 - Operatingwaveforms.LX Switch 'On' time

L⌬I

TON=-------------------------------------------------------------------------------------VIN–VLED–Iavg(RS+rL+RLX)

Note: TONmin>240nsLX Switch 'Off' time

L⌬I

TOFF=---------------------------------------------------------------------VLED+VD+Iavg(RS+rL)

Note: TOFFmin>200nsWhere:

L is the coil inductance (H)rL is the coil resistance (⍀)RS is the current sense resistanceIavg is the required LED current (A)

⌬I is the coil peak-peak ripple current (A) {Internally set to 0.3 x Iavg}VIN is the supply voltage (V)

VLED is the total LED forward voltage (V)RLX is the switch resistance (⍀) {=0.5⍀ nominal}

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ZXLD1360

Example:

For VIN =12V, L=33␮H, rL=0.093, RS=0.1 , RLX=0.15⍀, VLED=3.6V, Iavg =1A and VD =0.49VTON = (33e-6 x 0.3)/(12 - 3.6 - 0.693) = 1.28␮sTOFF = (33e-6 x 0.3)/(3.6 + 0.49 + 0.193)= 2.31␮s

This gives an operating frequency of 280kHz and a duty cycle of 0.35.

These and other equations are available as a spreadsheet calculator from the Zetex website atwww.zetex.com/zxld1360

Note that, in practice, the duty cycle and operating frequency will deviate from the calculatedvalues due to dynamic switching delays, switch rise/fall times and losses in the externalcomponents.

Optimum performance will be achieved by setting the duty cycle close to 0.5 at the nominalsupply voltage. This helps to equalize the undershoot and overshoot and improves temperaturestability of the output current.Diode selection

For maximum efficiency and performance, the rectifier (D1) should be a fast low capacitanceSchottky diode with low reverse leakage at the maximum operating voltage and temperature.They also provide better efficiency than silicon diodes, due to a combination of lower forwardvoltage and reduced recovery time.

It is important to select parts with a peak current rating above the peak coil current and acontinuous current rating higher than the maximum output load current. It is very important toconsider the reverse leakage of the diode when operating above 85°C. Excess leakage willincrease the power dissipation in the device and if close to the load may create a thermal runawaycondition.

The higher forward voltage and overshoot due to reverse recovery time in silicon diodes willincrease the peak voltage on the LX output. If a silicon diode is used, care should be taken toensure that the total voltage appearing on the LX pin including supply ripple, does not exceed thespecified maximum value.

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ZXLD1360

Reducing output ripple

Peak to peak ripple current in the LED(s) can be reduced, if required, by shunting a capacitor Cledacross the LED(s) as shown below:

VIN Rs LEDCledL1D1 VIN ISENSE LX ZXLD1360A value of 1␮F will reduce the supply ripple current by a factor three (approx.). Proportionallylower ripple can be achieved with higher capacitor values. Note that the capacitor will not affectoperating frequency or efficiency, but it will increase start-up delay, by reducing the rate of rise ofLED voltage.

By adding this capacitor the current waveform through the LED(s) changes from a triangular rampto a more sinusoidal version without altering the mean current value . Operation at low supply voltage

The internal regulator disables the drive to the switch until the supply has risen above the start-up threshold (VSU). Above this threshold, the device will start to operate. However, with thesupply voltage below the specified minimum value, the switch duty cycle will be high and thedevice power dissipation will be at a maximum. Care should be taken to avoid operating thedevice under such conditions in the application, in order to minimize the risk of exceeding themaximum allowed die temperature. (See next section on thermal considerations). The drive tothe switch is turned off when the supply voltage falls below the under-voltage threshold (VSD).This prevents the switch working with excessive 'on' resistance under conditions where the dutycycle is high.

Note that when driving loads of two or more LEDs, the forward drop will normally be sufficientto prevent the device from switching below approximately 6V. This will minimize the risk ofdamage to the device.Thermal considerations

When operating the device at high ambient temperatures, or when driving maximum loadcurrent, care must be taken to avoid exceeding the package power dissipation limits. The graphbelow gives details for power derating. This assumes the device to be mounted on a 25mm2 PCBwith 1oz copper standing in still air.

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Maximum Power Dissipation11001000900800700Power (mW)6005004003002001000-50-30-101030507090110130150Ambient Temperature (Deg C)Note that the device power dissipation will most often be a maximum at minimum supplyvoltage. It will also increase if the efficiency of the circuit is low. This may result from the use ofunsuitable coils, or excessive parasitic output capacitance on the switch output.Thermal compensation of output current

High luminance LEDs often need to be supplied with a temperature compensated current in orderto maintain stable and reliable operation at all drive levels. The LEDs are usually mountedremotely from the device so, for this reason, the temperature coefficients of the internal circuitsfor the ZXLD1360 have been optimized to minimize the change in output current when nocompensation is employed. If output current compensation is required, it is possible to use anexternal temperature sensing network - normally using Negative Temperature Coefficient (NTC)thermistors and/or diodes, mounted very close to the LED(s). The output of the sensing networkcan be used to drive the ADJ pin in order to reduce output current with increasing temperature.

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ZXLD1360

Layout considerations

LX pin

The LX pin of the device is a fast switching node, so PCB tracks should be kept as short aspossible. To minimize ground 'bounce', the ground pin of the device should be soldered directlyto the ground plane.

Coil and decoupling capacitors and current sense resistor

It is particularly important to mount the coil and the input decoupling capacitor as close to thedevice pins as possible to minimize parasitic resistance and inductance, which will degradeefficiency. It is also important to minimize any track resistance in series with current sense resistorRS. Its best to connect VIN directly to one end of RS and Isense directly to the opposite end of RSwith no other currents flowing in these tracks. It is important that the cathode current of theSchottky diode does not flow in a track between RS and VIN as this may give an apparent highermeasure of current than is actual because of track resistance.ADJ pin

The ADJ pin is a high impedance input for voltages up to 1.35V so, when left floating, PCB tracksto this pin should be as short as possible to reduce noise pickup. A 100nF capacitor from the ADJpin to ground will reduce frequency modulation of the output under these conditions. Anadditional series 10k⍀ resistor can also be used when driving the ADJ pin from an external circuit(see below). This resistor will provide filtering for low frequency noise and provide protectionagainst high voltage transients.

10k 100nF GND High voltage tracks

ADJ ZXLD1360GND Avoid running any high voltage tracks close to the ADJ pin, to reduce the risk of leakage currentsdue to board contamination. The ADJ pin is soft-clamped for voltages above 1.35V to desensitizeit to leakage that might raise the ADJ pin voltage and cause excessive output current. However,a ground ring placed around the ADJ pin is recommended to minimize changes in output currentunder these conditions.Evaluation PCB

The ZXLD1360EV1, 2 or 3 evaluation boards are available on request. These boards contain aLumileds K2 or multiple Ostar LEW type LEDs to allow quick testing of the 1360 device. Additionalterminals allow for interfacing to customers own LED products.

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ZXLD1360

Dimming output current using PWM

Low frequency PWM mode

When the ADJ pin is driven with a low frequency PWM signal (eg 100Hz), with a high level voltageVADJ and a low level of zero, the output of the internal low pass filter will swing between 0V andVADJ, causing the input to the shutdown circuit to fall below its turn-off threshold (200mV nom)when the ADJ pin is low. This will cause the output current to be switched on and off at the PWMfrequency, resulting in an average output current IOUTavg proportional to the PWM duty cycle.(See Figure 2 - Low frequency PWM operating waveforms).

VADJPWM VoltageTon Toff 0V VADJFilter Output300mV0V IOUTnomOutput Current0.1/RsIOUTavg200mV0 Figure 2 Low frequency PWM operating waveforms

The average value of output current in this mode is given by:IOUTavg 0.1DPWM/RS [for DPWM >0.01]

This mode is preferable if optimum LED 'whiteness' is required. It will also provide the widestpossible dimming range (approx. 100:1) and higher efficiency at the expense of greater outputripple.

Note that the low pass filter introduces a small error in the output duty cycle due to the differencebetween the start-up and shut-down times. This time difference is a result of the 200mV shutdownthreshold and the rise and fall times at the output of the filter. To minimize this error, the PWMfrequency should be as low as possible consistent with avoiding flicker in the LED(s).

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ZXLD1360

High frequency PWM mode

At PWM frequencies above 10kHz and for duty cycles above 0.16, the output of the internal lowpass filter will contain a DC component that is always above the shutdown threshold. This willmaintain continuous device operation and the nominal average output current will beproportional to the average voltage at the output of the filter, which is directly proportional to theduty cycle. (See Figure 3 - High frequency PWM operating waveforms). For best results, the PWMfrequency should be maintained above the minimum specified value of 10kHz, in order tominimize ripple at the output of the filter. The shutdown comparator has approximately 50mV ofhysteresis, to minimize erratic switching due to this ripple. An upper PWM frequency limit ofapproximately one tenth of the operating frequency is recommended, to avoid excessive outputmodulation and to avoid injecting excessive noise into the internal reference.

VADJPWM voltageTon Toff 0V VADJFilter output200mV0V 0.1/RSOutput currentIOUTnom0 Figure 3 High frequency PWM operating waveforms

The nominal average value of output current in this mode is given by:IOUTnom ≈0.1DPWM/RS [for DPWM >0.16]

This mode will give minimum output ripple and reduced radiated emission, but with a reduceddimming range (approx.5:1). The restricted dimming range is a result of the device being turnedoff when the dc component on the filter output falls below 200mV.

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Intentionally left blank

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Package outline - TSOT23-5

DIMAA1A2bcDEE1ee1LL2a°

Min.-0.010.840.300.12

Millimeters

Max.1.000.100.900.450.20

2.90 BSC2.80 BSC1.60 BSC0.95 BSC1.90 BSC

0.30

0.25 BSC4°

12°

4°

0.50

0.0118Min.-0.00030.03300.01180.0047

Inches

Max.0.03930.00390.030.01770.0078

0.114 BSC0.110 BSC0.062 BSC0.0374 BSC0.0748 BSC

0.0196

0.010 BSC

12°

Note: Controlling dimensions are in millimeters. Approximate dimensions are provided in inches

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ZXLD1360

Definitions

Product change

Zetex Semiconductors reserves the right to alter, without notice, specifications, design, price or conditions of supply of any product orservice. Customers are solely responsible for obtaining the latest relevant information before placing orders.Applications disclaimer

The circuits in this design/application note are offered as design ideas. It is the responsibility of the user to ensure that the circuit is fit forthe user’s application and meets with the user’s requirements. No representation or warranty is given and no liability whatsoever isassumed by Zetex with respect to the accuracy or use of such information, or infringement of patents or other intellectual property rightsarising from such use or otherwise. Zetex does not assume any legal responsibility or will not be held legally liable (whether in contract,tort (including negligence), breach of statutory duty, restriction or otherwise) for any damages, loss of profit, business, contract,opportunity or consequential loss in the use of these circuit applications, under any circumstances.Life support

Zetex products are specifically not authorized for use as critical components in life support devices or systems without the express writtenapproval of the Chief Executive Officer of Zetex Semiconductors plc. As used herein:A. Life support devices or systems are devices or systems which:

1.are intended to implant into the body or

2.support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in thelabelling can be reasonably expected to result in significant injury to the user.

B. A critical component is any component in a life support device or system whose failure to perform can be reasonably expected to

cause the failure of the life support device or to affect its safety or effectiveness.Reproduction

The product specifications contained in this publication are issued to provide outline information only which (unless agreed by thecompany in writing) may not be used, applied or reproduced for any purpose or form part of any order or contract or be regarded as arepresentation relating to the products or services concerned. Terms and Conditions

All products are sold subjects to Zetex’ terms and conditions of sale, and this disclaimer (save in the event of a conflict between the twowhen the terms of the contract shall prevail) according to region, supplied at the time of order acknowledgement.For the latest information on technology, delivery terms and conditions and prices, please contact your nearest Zetex sales office.Quality of product

Zetex is an ISO 9001 and TS16949 certified semiconductor manufacturer.

To ensure quality of service and products we strongly advise the purchase of parts directly from Zetex Semiconductors or one of ourregionally authorized distributors. For a complete listing of authorized distributors please visit: www.zetex.com/salesnetwork

Zetex Semiconductors does not warrant or accept any liability whatsoever in respect of any parts purchased through unauthorized sales channels.ESD (Electrostatic discharge)

Semiconductor devices are susceptible to damage by ESD. Suitable precautions should be taken when handling and transporting devices.The possible damage to devices depends on the circumstances of the handling and transporting, and the nature of the device. The extentof damage can vary from immediate functional or parametric malfunction to degradation of function or performance in use over time.Devices suspected of being affected should be replaced.Green compliance

Zetex Semiconductors is committed to environmental excellence in all aspects of its operations which includes meeting or exceedingregulatory requirements with respect to the use of hazardous substances. Numerous successful programs have been implemented toreduce the use of hazardous substances and/or emissions.

All Zetex components are compliant with the RoHS directive, and through this it is supporting its customers in their compliance withWEEE and ELV directives.Product status key:“Preview”Future device intended for production at some point. Samples may be available“Active”Product status recommended for new designs“Last time buy (LTB)”Device will be discontinued and last time buy period and delivery is in effect“Not recommended for new designs”Device is still in production to support existing designs and production“Obsolete”Production has been discontinuedDatasheet status key:“Draft version”This term denotes a very early datasheet version and contains highly provisional information, which

may change in any manner without notice.

“Provisional version”This term denotes a pre-release datasheet. It provides a clear indication of anticipated performance.

However, changes to the test conditions and specifications may occur, at any time and without notice.

“Issue”This term denotes an issued datasheet containing finalized specifications. However, changes to

specifications may occur, at any time and without notice.Zetex sales officesEurope

Zetex GmbH

Kustermann-parkBalanstraße 59D-811 MünchenGermany

Telefon: (49) 45 49 49 0Fax: (49) 45 49 49 49europe.sales@zetex.com

Americas

Zetex Inc

700 Veterans Memorial HighwayHauppauge, NY 11788USA

Telephone: (1) 631 360 2222Fax: (1) 631 360 8222usa.sales@zetex.com

Asia Pacific

Zetex (Asia Ltd)

3701-04 Metroplaza Tower 1Hing Fong Road, Kwai Fong

Telephone: (852) 26100 611Fax: (852) 24250 494asia.sales@zetex.com

Corporate Headquarters

Zetex Semiconductors plc

Zetex Technology Park, ChaddertonOldham, OL9 9LLUnited Kingdom

Telephone: (44) 161 622 4444Fax: (44) 161 622 4446hq@zetex.com

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