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| A Product Line of Diodes Incorporated ZXLD1352 350mA LED driver with internal switch and enhanced PWM dimming range Description The ZXLD1352 is a continuous mode inductive step-down converter, designed for driving single or multiple series connected LEDs efficiently from a voltage source higher than the LED voltage. The device operates from an input supply between 7V and 30V and provides an externally adjustable output current of up to 350mA. Depending upon supply voltage and external components, this can provide up to 8 watts of output power. The ZXLD1352 includes the output switch and a high-side output current sensing circuit, which uses an external resistor to set the nominal average output current. Output current can be adjusted above, or below the set value, by applying an external control signal to the 'ADJ' pin. The ADJ pin will accept either a DC voltage or a PWM waveform. DC voltages between 0.3V and 2.5V allow adjustment of output current from 25% to 200% of nominal. 1000:1 adjustment of output current is possible using PWM control. Applying a voltage of 0.2V or lower to the ADJ pin turns the output off and switches the device into a low current standby state. The device is assembled in a TSOT23-5 pin package. Features * * * * * * * * * * * * * Simple low parts count Internal 30V NDMOS switch 350mA output current Single pin on/off and brightness control using DC voltage or PWM 1000:1 PWM dimming range Soft-start High efficiency (up to 95%(*)) Wide input voltage range: 7V to 30V 40V transient capability Output shutdown Up to 1MHz switching frequency Inherent open-circuit LED protection Typical 4% output current accuracy Applications * * * * * Low voltage halogen replacement LEDs Automotive lighting Low voltage industrial lighting LED back-up lighting Illuminated signs (*) Using standard external components as specified under electrical characteristics. Efficiency is dependent upon the number of LEDs driven and on external component types and values. Pin connections LX 1 GND 2 5 Typical application circuit D1 ZLLS1000 VIN VIN (12V - 30V) Rs 0.33 L1 47 H ADJ 3 TSOT23-5 Top view 4 ISENSE C1 1F PWM VIN ADJ ISENSE LX ZXLD1352 GND GND Issue 1 - August 2008 (c) Diodes Incorporated 2008 1 www.zetex.com www.diodes.com ZXLD1352 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 18) Operating temperature (TOP) Storage temperature (TST) Junction temperature (Tj MAX) -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) -0.3V to +6V 500mA 450mW -40 to 105C -55 to 150C 150C These are stress ratings only. Operation above the absolute maximum rating may cause device failure. Operation at the absolute maximum ratings, for extended periods, may reduce device reliability. Thermal resistance Junction to ambient (R JA) 200C/W Electrical characteristics (test conditions: VIN=12V, Tamb=25C unless otherwise stated) (*) Symbol VIN VSU IINQoff IINQon VSENSE Parameter Input voltage Conditions Min. 7 Typ. 4.8 20 Max. 30 Unit V V Internal regulator start-up threshold VIN rising Quiescent supply current with output off Quiescent supply current with output switching Mean current sense threshold voltage (defines LED current setting accuracy) ADJ pin grounded 30 500 105 A A mV ADJ pin floating f=250kHz Measured on ISENSE pin with respect to VIN VADJ =1.25V 95 250 100 VSENSEHYS Sense threshold hysteresis ISENSE VREF VREF / T VADJ VADJoff ISENSE pin input current Internal reference voltage Temperature coefficient of VREF External control voltage range on ADJ pin for dc brightness control () 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 Resistance between ADJ pin and VREF Continuous LX switch current LX Switch `On' resistance LX switch leakage current VADJ falling 0.3 0.15 VSENSE =VIN -0.1 Measured on ADJ pin with pin floating 1.21 15 1.25 1.25 50 2.5 0.2 0.25 10 1.29 % A V ppm/C V V VADJon VADJ rising 0.2 0.25 0.3 V RADJ ILXmean RLX ILX(leak) 35 65 0.37 1.5 2 1 k A A Issue 1 - August 2008 (c) Diodes Incorporated 2008 2 www.zetex.com www.diodes.com ZXLD1352 Electrical characteristics (test conditions: VIN=12V, Tamb=25C unless otherwise stated) (*) (continued) Symbol DPWM(LF) Parameter Duty cycle range of PWM signal applied to ADJ pin during PWM dimming mode Brightness control range Conditions PWM frequency 100Hz - 1KHz PWM amplitude= VREF Measured on ADJ pin Min. 0.01 Typ. Max. 1 Unit 1000:1 ADJ pin floating L=100H (0.82 ) IOUT=350mA @ VLED=3.4V Driving 1 LED LX switch `ON' LX switch `OFF' 200 200 1 0.3 50 0.7 ns fLX Operating frequency (See graphs for more detail) 250 KHz ns ns MHz TONmin TOFFmin fLXmax DLX TPD Minimum switch `ON' time Minimum switch `OFF' time Recommended maximum operating frequency Recommended duty cycle range of output switch at fLXmax Internal comparator propagation delay NOTES: (*) Production testing of the device is performed at 25C. Functional operation of the device and parameters specified over a -40C to +105C temperature range, are guaranteed by design, characterization and process control. () 100% brightness corresponds to VADJ = VADJ(nom) = VREF. Driving the ADJ pin above VREF will increase the VSENSE threshold and output current proportionally. Pin description Name LX GND ADJ Pin No. 1 2 3 Description Drain of NDMOS switch Ground (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 4 5 Ordering information Device ZXLD1352ET5TA Reel size (mm) 180 Reel width (mm) 8 Quantity per reel 3,000 Device mark 1352 Issue 1 - August 2008 (c) Diodes Incorporated 2008 3 www.zetex.com www.diodes.com ZXLD1352 Block diagram RS VIN L1 D1 VIN VIN R1 Current sense circuit Voltage regulator 5V + C1 Shutdown circuit ISENSE LX Comparator - ADJ 50k 600KHz R2 + MN Vref 1.25V R3 GND Issue 1 - August 2008 (c) Diodes Incorporated 2008 4 www.zetex.com www.diodes.com ZXLD1352 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 and the 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 no output from the current sense circuit. Under this condition, the (-) input to the comparator is at ground and its output is high. This turns MN on and switches the LX pin low, causing current to flow from VIN to ground, via RS, L1 and the LED(s). The current rises at a rate determined by VIN and L1 to produce a voltage ramp (VSENSE) across RS. The supply referred voltage VSENSE is forced across internal resistor R1 by the current sense circuit and produces a proportional current in internal resistors R2 and R3. This produces a ground referred rising voltage at the (-) input of the comparator. When this reaches the threshold voltage (VADJ), the comparator output switches low and MN turns off. The comparator output also drives another NMOS switch, which bypasses internal resistor R3 to provide a controlled amount of hysteresis. The hysteresis is set by R3 to be nominally 15% of VADJ. When MN is off, the current in L1 continues to flow via D1 and the LED(s) back to VIN. The current decays at a rate determined by the LED and diode forward voltages to produce a falling voltage at the input of the comparator. When this voltage returns to VADJ, the comparator output switches high again. This cycle of events repeats, with the comparator input ramping between limits of VADJ 15%. Switching thresholds With VADJ =VREF, the ratios of R1, R2 and R3, define an average VSENSE switching threshold of 100mV (measured on the ISENSE pin with respect to VIN). The average output current IOUTnom is then 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 for noise suppression between the ADJ pin and the threshold comparator and an internal current limiting resistor (50k nom) between ADJ and the internal reference voltage. This allows the ADJ pin to be overdriven with either DC or PWM signals to adjust the output current. The filter is first order, comprising one section with a cut-off frequency of nominally 600kHz. Details of the different modes of adjusting output current are given in the applications section. Output shutdown The ADJ pin drives the shutdown circuit. When the input voltage to this circuit falls 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 the shutdown circuit. Quiescent supply current during shutdown is nominally 20 A and switch leakage is below 1 A. Issue 1 - August 2008 (c) Diodes Incorporated 2008 5 www.zetex.com www.diodes.com ZXLD1352 VIN LX voltage 0V Toff VIN 115mV SENSE voltage 85mV 100mV VSENSEVSENSE+ Ton IOUTnom +15% Coil current IOUTnom IOUTnom -15% 0V Comparator input voltage 0.15VADJ VADJ 0.15VADJ Comparator output 5V 0V Figure 1 Operating waveforms Issue 1 - August 2008 (c) Diodes Incorporated 2008 6 www.zetex.com www.diodes.com ZXLD1352 Typical operating waveforms [VIN=12V, RS=0.3 , L=100H] Normal operation. Output current (Ch3) and LX voltage (Ch1) 2 3 Ch3 100mA Ch 2 20.0V M 400s 5.0 S/s A Ch2 \ 12.0 V 200 ns/pt Start-up waveforms. Output current (Ch3), LX voltage (Ch2) Issue 1 - August 2008 (c) Diodes Incorporated 2008 7 www.zetex.com www.diodes.com ZXLD1352 Typical operating conditions For typical application circuit driving 1W Luxeon(R) white LED(s) at VIN =12V and Tamb=25C unless otherwise stated. Efficiency vs No. of LEDs L=100uH, Rs=0.33 Ohms 100 1.2 Duty Cycle vs Input Voltage L=100uH, Rs=0.33 Ohms 95 1 LED 2 LED 3 LED 4 LED 5 LED 6 LED 7 LED 8 LED 1 1 LED 2 LED 3 LED 4 LED 5 LED 6 LED 7 LED 8 LED 90 Efficiency (%) 0.8 Duty Cycle 85 0.6 80 0.4 75 0.2 70 5 10 15 VIN (V) 20 25 30 0 5 10 15 VIN (V) 20 25 30 Operating Frequency vs Input Voltage L=100uH, Rs=0.33 Ohms 600 8 6 Deviation from nominal set current (%) Output current variation with Supply Voltage L=100uH, Rs=0.33 Ohms 500 1 LED 2 LED 3 LED 4 LED 5 LED 6 LED 7 LED 8 LED 4 2 0 5 -2 -4 -6 10 15 20 25 30 1 LED 2 LED 3 LED 4 LED 5 LED 6 LED 7 LED Frequency (kHz) 400 300 200 100 0 5 10 15 VIN (V) 20 25 30 -8 VIN (V) Issue 1 - August 2008 (c) Diodes Incorporated 2008 8 www.zetex.com www.diodes.com ZXLD1352 Typical operating conditions (continued) Efficiency vs No. of LEDs L=47uH, Rs=0.33 Ohms 100 Efficiency (%) 1 Duty Cycle vs Input Voltage L=47uH, Rs=0.33 Ohms 1 LED 2 LED 3 LED 4 LED 5 LED 6 LED 7 LED 1 LED 2 LED 3 LED 4 LED 5 LED 6 LED 7 LED 5 10 15 VIN (V) 20 25 30 95 90 85 80 75 70 5 10 15 VIN (V) 20 25 30 Duty Cycle 0.8 0.6 0.4 0.2 0 Operating Frequency vs Input Voltage L=47uH, Rs=0.33 Ohms 800 700 600 500 400 300 200 100 0 5 10 15 VIN (V) 20 25 30 20 Deviation from nominal set current (%) Output Current Variation vs Supply Voltage L=47uH, Rs=0.33 Ohms 1 LED 2 LED 3 LED 4 LED 5 LED 6 LED 7 LED 15 10 5 0 -5 5 10 15 20 25 30 1 LED 2 LED 3 LED 4 LED 5 LED 6 LED 7 LED Frequency (kHz) -10 VIN (V) -15 Issue 1 - August 2008 (c) Diodes Incorporated 2008 9 www.zetex.com www.diodes.com ZXLD1352 Typical operating conditions (continued) Efficiency vs No. of LEDs L=220uH, Rs=0.33 Ohms 1 100 0.8 1 LED 2 LED 3 LED 4 LED 5 LED 6 LED 7 LED 8 LED 1 LED 2 LED 3 LED 4 LED 5 LED 6 LED 7 LED 8 LED Duty Cycle vs Input Voltage L=220uH, Rs=0.33 Ohms 95 Efficiency (%) 90 Duty Cycle 0.6 85 0.4 80 0.2 75 5 10 15 VIN (V) 20 25 30 0 5 10 15 VIN (V) 20 25 30 Operating Frequency vs Input Voltage L=220uH, Rs=0.33 Ohms 350 300 250 2 1 Output Current Variation vs Input Voltage L=220uH, Rs=0.33 Ohms Deviation from nominal set current (%) 200 150 100 50 0 5 10 15 VIN (V) 20 25 30 1 LED 2 LED 3 LED 4 LED 5 LED 6 LED 7 LED 8 LED 0 5 -1 -2 -3 -4 -5 -6 VIN (V) 10 15 20 25 30 1 LED 2 LED 3 LED 4 LED 5 LED 6 LED 7 LED 8 LED Issue 1 - August 2008 (c) Diodes Incorporated 2008 Frequency (kHz) 10 www.zetex.com www.diodes.com ZXLD1352 Typical operating conditions (continued) Vref vs Vin over nominal supply voltage range 1.2425 1.4 1.2 1 Vref vs Vin at low supply voltage Vref (V) Vref (V) 1.242 0.8 0.6 0.4 0.2 0 1.2415 5 10 15 Vin (V) 20 25 30 0 1 2 3 4 5 Vin (V) 6 7 8 9 10 Supply Current vs Vin (Operating) 500 4 00 Iin (u A ) Supply Current vs Vin (Quiescent) 20 15 I in (u A) 10 5 0 0 5 10 15 Vin (V) 20 25 30 300 200 100 0 0 5 10 15 Vin (V) 20 25 30 Output Current vs VADJ 35 0 30 0 2 50 Iout mean (mA) 20 0 Rs=0.3 Ohm Rs=0.56 Ohm Rs=1 Ohm 150 100 50 0 0 0.5 1 1.5 VADJ (V) 2 2.5 3 Issue 1 - August 2008 (c) Diodes Incorporated 2008 11 www.zetex.com www.diodes.com ZXLD1352 Typical operating conditions (continued) VADJ vs Temperature L=100uH, Rs=0.33 Ohms 1.255 Output Current Change vs Temperature VIN=7V, L=100uH, Rs=0.33 Ohms 2 Deviation from nominal set value (%) 1.25 Vin = 7V Vin = 9V Vin = 12V Vin = 30V Vadj (V) 1 1.245 0 -60 -40 -20 -1 0 20 40 60 80 100 120 140 1.24 1.235 -50 0 50 Temperature (Deg C) 100 150 -2 Temperature (Deg C) LX Switch 'On' Resistance vs Temperature 2.6 2.4 2.2 2 Ohms Output Current Change vs Temperature VIN=12V, L=100uH, Rs=0.33 Ohms 0. 5 Deviation from nominal set value (%) 0.25 1.8 1.6 1.4 1.2 1 -60 -40 -20 0 20 40 60 80 100 120 140 160 Temperature (Deg C) 0 -60 -40 -20 -0.25 0 20 40 60 80 100 120 140 -0.5 Temperature (Deg C) Output Current Change vs Temperature VIN=30V, L=100uH, Rs=0.33 Ohms 4 Deviation from nominal set value (%) 3 2 1 0 -60 -40 -20 0 20 40 60 80 100 120 140 Temperature (Deg C) Issue 1 - August 2008 (c) Diodes Incorporated 2008 12 www.zetex.com www.diodes.com ZXLD1352 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 external current sense resistor (RS) connected between VIN and ISENSE and is given by: IOUTnom = 0.1/RS [for RS>0.27 ] The table below gives values of nominal average output current for several preferred values of current setting resistor (RS) in the typical application circuit shown on page 1: RS ( ) 0.27 0.3 0.33 0.39 Nominal average output current (mA) 370 333 300 256 The above values assume that the ADJ pin is floating and at a nominal voltage of VREF (=1.25V). Note that RS=0.27 is the minimum allowed value of sense resistor under these conditions to maintain 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. (See next 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 current to a value above or below the nominal average value defined by RS. + ADJ ZXLD1352 GND DC GND The nominal average output current in this case is given by: IOUTdc = 0.08*VADJ/RS [for 0.3< VADJ <2.5V] Note that 100% brightness setting corresponds to VADJ = VREF. When driving the ADJ pin above 1.25V, RS must be increased in proportion to prevent IOUTdc exceeding 370mA maximum. The input impedance of the ADJ pin is 50k 25%. Issue 1 - August 2008 (c) Diodes Incorporated 2008 13 www.zetex.com www.diodes.com ZXLD1352 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, as shown below, to adjust the output current to a value above or below the nominal average value set by resistor RS: PWM VADJ ADJ 0V ZXLD1352 GND GND Driving the ADJ input via open collector transistor The recommended method of driving the ADJ pin and controlling the amplitude of the PWM waveform is to use a small NPN switching transistor as shown below: ADJ PWM ZXLD1352 GND GND This scheme uses the 50k resistor between the ADJ pin and the internal voltage reference as a pull-up resistor for the external transistor eg MMBT3904. Driving the ADJ input from a microcontroller Another possibility is to drive the device from the open drain output of a microcontroller. The diagram below shows one method of doing this: MCU 10k ADJ ZXLD1352 GND If the NMOS transistor within the microcontroller has high Gate / Drain capacitance, this arrangement can inject a negative spike into ADJ input of the ZXLD1352 and cause erratic operation but the addition of a Schottky clamp diode (eg Diodes Inc. SD103CWS) to ground and inclusion of a series resistor (3.3k) will prevent this. See the section on PWM dimming for more details of the various modes of control using high frequency and low frequency PWM signals Issue 1 - August 2008 (c) Diodes Incorporated 2008 14 www.zetex.com www.diodes.com ZXLD1352 Shutdown mode Taking the ADJ pin to a voltage below 0.2V will turn off the output and supply current will fall to a low standby level of 20A nominal. Note that the ADJ pin is not a logic input. Taking the ADJ pin to a voltage above VREF will increase output current above the 100% nominal average value. (See graphs for details). Soft-start An external capacitor from the ADJ pin to ground will provide soft-start delay, by increasing the time taken for the voltage on this pin to rise to the turn-on threshold and by slowing down the rate of rise of the control voltage at the input of the comparator. The graph below shows the variation of soft-start time for different values of capacitor. Soft Start Time vs Capacitance from ADJ pin to Ground 10 8 Soft Start time (ms) 6 4 2 0 0 5 10 Capacitance (nF) 15 20 25 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, so the device will not be damaged, unlike in many boost converters, where the back EMF may damage 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 in series with the supply source impedance and lowers overall efficiency. This capacitor has to supply the relatively high peak current to the coil and smooth the current ripple on the input supply. A minimum value of 1 F is acceptable if the input source is close to the device, but higher values will improve performance at lower input voltages, especially when the source impedance 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 better dielectric are recommended. Capacitors with Y5V dielectric are not suitable for decoupling in this application and should NOT be used. A table of recommended manufacturers is provided below: Manufacturer Murata Taiyo Yuden Kemet AVX Website www.murata.com www.t-yuden.com www.kemet.com www.avxcorp.com Issue 1 - August 2008 (c) Diodes Incorporated 2008 15 www.zetex.com www.diodes.com ZXLD1352 Inductor selection Recommended inductor values for the ZXLD1352 are in the range 47 H to 220 H. Higher values of inductance are recommended at higher supply voltages in order to minimize errors due to switching delays, which result in increased ripple and lower efficiency. Higher values of inductance also result in a smaller change in output current over the supply voltage range. (See graphs). The inductor should be mounted as close to the device as possible with low resistance connections to the LX and VIN pins. The chosen coil should have a saturation current higher than the peak output current and a continuous current rating above the required mean output current. Suitable coils for use with the ZXLD1352 are listed in the table below: Part No. DO1608C MSS6132ML CD104-MC NP04SB470M L ( H) 47 47 68 100 220 47 DCR () 0.64 0.38 0.58 0.82 0.55 0.27 ISAT (A) 0.5 0.56 0.47 0.39 0.53 0.38 Manufacturer CoilCraft Sumida Taiyo Yuden The inductor value should be chosen to maintain operating duty cycle and switch 'on'/'off' times within 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 - Operating waveforms. LX Switch 'On' time LI T ON = --------------------------------------------------------------------------------------V IN - V LED - I avg ( R S + rL + R LX ) Note: TONmin>200ns LX Switch 'Off' time LI T OFF = ---------------------------------------------------------------------V LED + VD + I avg ( R S + rL ) Note: TOFFmin>200ns Where: L is the coil inductance (H) rL is the coil resistance ( ) Iavg 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 ( ) VD is the diode forward voltage at the required load current (V) Issue 1 - August 2008 (c) Diodes Incorporated 2008 16 www.zetex.com www.diodes.com ZXLD1352 Example: For VIN =12V, L=47 H, rL=0.64 , VLED=3.4V, Iavg =350mA and VD =0.36V TON = (47e-6 x 0.105)/(12 - 3.4 - 0.672) = 0.622 s TOFF = (47e-6 x 0.105)/(3.4 + 0.36 + 0.322)= 1.21 s This gives an operating frequency of 546kHz and a duty cycle of 0.34. These and other equations are available as a spreadsheet calculator from the Zetex website. Go to www.zetex.com/ZXLD1352 Note that in practice, the duty cycle and operating frequency will deviate from the calculated values due to dynamic switching delays, switch rise/fall times and losses in the external components. Optimum performance will be achieved by setting the duty cycle close to 0.5 at the nominal supply voltage. This helps to equalize the undershoot and overshoot and improves temperature stability of the output current. Diode selection For maximum efficiency and performance, the rectifier (D1) should be a fast low capacitance Schottky diode with low reverse leakage at the maximum operating voltage and temperature. The recommended diode for use with this part is the ZLLS1000. This has approximately ten times lower leakage than standard Schottky diodes, which are unsuitable for use above 85C. It also provides better efficiency than silicon diodes, due to a combination of lower forward voltage and reduced recovery time. The table below gives the typical characteristics for the ZLLS1000: Diode Forward voltage at 100mA (mV) 310 Continuous current (mA) 1000 Reverse Leakage At 30V 85C ( A) 300 Package Manufacturer ZLLS1000 TSOT23 Zetex If alternative diodes are used, it is important to select parts with a peak current rating above the peak coil current and a continuous current rating higher than the maximum output load current. It is very important to consider the reverse leakage of the diode when operating above 85C. Excess leakage will increase the power dissipation in the device. The higher forward voltage and overshoot due to reverse recovery time in silicon diodes will increase the peak voltage on the LX output. If a silicon diode is used, care should be taken to ensure that the total voltage appearing on the LX pin including supply ripple, does not exceed the specified maximum value. Issue 1 - August 2008 (c) Diodes Incorporated 2008 17 www.zetex.com www.diodes.com ZXLD1352 Reducing output ripple Peak to peak ripple current in the LED can be reduced, if required, by shunting a capacitor Cled across the LED(s) as shown below: VIN Rs LED Cled L1 D1 VIN ISENSE LX ZXLD1352 A value of 1 F will reduce nominal ripple current by a factor three (approx.). Proportionally lower ripple can be achieved with higher capacitor values. Note that the capacitor will not affect operating frequency or efficiency, but it will increase start-up delay, by reducing the rate of rise of LED voltage. Operation at low supply voltage The internal regulator disables the drive to the switch until the supply has risen above the startup threshold (VSU). Above this threshold, the device will start to operate. However, with the supply voltage below the specified minimum value, the switch duty cycle will be high and the device power dissipation will be at a maximum. Care should be taken to avoid operating the device under such conditions in the application, in order to minimize the risk of exceeding the maximum allowed die temperature. (See next section on thermal considerations). Note that when driving loads of two or more LEDs, the forward drop will normally be sufficient to prevent the device from switching below approximately 6V. This will minimize the risk of damage to the device. Thermal considerations When operating the device at high ambient temperatures, or when driving maximum load current, care must be taken to avoid exceeding the package power dissipation limits. The graph below gives details for power derating. This assumes the device to be mounted on a 25mm2 PCB with 1oz copper standing in still air. Maximum Power Dissipation 500 400 Power (mW) 300 200 100 0 -50 -30 -10 10 30 50 70 90 110 130 Ambient Temperature (Deg C) Issue 1 - August 2008 (c) Diodes Incorporated 2008 18 www.zetex.com www.diodes.com ZXLD1352 Note that the device power dissipation will most often be a maximum at minimum supply voltage. It will also increase if the efficiency of the circuit is low. This may result from the use of unsuitable 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 order to maintain stable and reliable operation at all drive levels. The LEDs are usually mounted remotely from the device, so for this reason, the temperature coefficients of the internal circuits for the ZXLD1352 have been optimized to minimize the change in output current when no compensation is employed. If output current compensation is required, it is possible to use an external 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 network can be used to drive the ADJ pin in order to reduce output current with increasing temperature. Layout considerations LX pin The LX pin of the device is a fast switching node, so PCB tracks should be kept as short as possible. To minimize ground 'bounce', the ground pin of the device should be soldered directly to the ground plane. Coil and decoupling capacitors It is particularly important to mount the coil and the input decoupling capacitor close to the device to minimize parasitic resistance and inductance, which will degrade efficiency. It is also important to take account of any track resistance in series with current sense resistor RS. ADJ pin The ADJ pin is a high impedance input, so when left floating, PCB tracks to this pin should be as short as possible to reduce noise pickup. A 100nF capacitor from the ADJ pin to ground will reduce frequency modulation of the output under these conditions. An additional 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 protection against high voltage transients. 10k 100nF GND ADJ ZXLD1352 GND High voltage tracks Avoid running any high voltage tracks close to the ADJ pin, to reduce the risk of leakage due to board contamination. Any such leakage may raise the ADJ pin voltage and cause excessive output current. A ground ring placed around the ADJ pin will minimize changes in output current under these conditions. Issue 1 - August 2008 (c) Diodes Incorporated 2008 19 www.zetex.com www.diodes.com ZXLD1352 Dimming output current using PWM When the ADJ pin is driven with a low frequency PWM signal (eg 100Hz), with a high level voltage VADJ and a low level of zero, the output current will be switched on and off at the PWM frequency, resulting in an average output current IOUTavg proportional to the PWM duty cycle. (See Figure 2 VADJ PWM Voltage Ton Toff 0V IOUTnom Output Current 0.1/Rs IOUTavg 0 Figure 2 Low frequency PWM operating waveforms The average value of output current is given by: IOUTavg = 0.1DPWM/RS [for DPWM >0 01] PWM dimming is preferable to DC dimming if optimum LED 'whiteness' is required. It will also provide the widest possible dimming range (approx. 1000:1) and higher efficiency at the expense of greater output ripple. Issue 1 - August 2008 (c) Diodes Incorporated 2008 20 www.zetex.com www.diodes.com ZXLD1352 Package outline - TSOT23-5 DIM A A1 A2 b c D E E1 e e1 L L2 a Min. 0.01 0.84 0.30 0.12 Millimeters Max. 1.00 0.10 0.90 0.45 0.20 2.90 BSC 2.80 BSC 1.60 BSC 0.95 BSC 1.90 BSC 0.30 0.25 BSC 4 12 4 0.50 0.0118 Min. 0.0003 0.0330 0.0118 0.0047 Inches Max. 0.0393 0.0039 0.0354 0.0177 0.0078 0.114 BSC 0.110 BSC 0.062 BSC 0.0374 BSC 0.0748 BSC 0.0196 0.010 BSC 12 Note: Controlling dimensions are in millimeters. Approximate dimensions are provided in inches Issue 1 - August 2008 (c) Diodes Incorporated 2008 21 www.zetex.com www.diodes.com ZXLD1352 Definitions Product change Diodes Incorporated reserves the right to alter, without notice, specifications, design, price or conditions of supply of any product or service. 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 for the user's application and meets with the user's requirements. No representation or warranty is given and no liability whatsoever is assumed by Diodes Inc. with respect to the accuracy or use of such information, or infringement of patents or other intellectual property rights arising from such use or otherwise. Diodes Inc. 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 Diodes Zetex products are specifically not authorized for use as critical components in life support devices or systems without the express written approval of the Chief Executive Officer of Diodes Incorporated. 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 the labelling 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 the company in writing) may not be used, applied or reproduced for any purpose or form part of any order or contract or be regarded as a representation relating to the products or services concerned. Terms and Conditions All products are sold subjects to Diodes Inc and conditions of sale, and this disclaimer (save in the event of a conflict between the two when 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 Diodes Zetex sales office. Quality of product Diodes Zetex Semiconductors Limited 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 Diodes Inc. or one of our regionally authorized distributors. For a complete listing of authorized distributors please visit: www.zetex.com or www.diodes.com Diodes Incorporated 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 extent of 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 Diodes Zetex Semiconductors is committed to environmental excellence in all aspects of its operations which includes meeting or exceeding regulatory requirements with respect to the use of hazardous substances. Numerous successful programs have been implemented to reduce the use of hazardous substances and/or emissions. All Diodes Zetex components are compliant with the RoHS directive, and through this it is supporting its customers in their compliance with WEEE 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 discontinued Datasheet 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. Diodes Zetex sales offices Europe Zetex GmbH Kustermann-Park Balanstrae 59 D-81541 Munchen Germany Telefon: (49) 89 45 49 49 0 Fax: (49) 89 45 49 49 49 europe.sales@zetex.com Americas Zetex Inc 700 Veterans Memorial Highway Hauppauge, NY 11788 USA Telephone: (1) 631 360 2222 Fax: (1) 631 360 8222 usa.sales@zetex.com Asia Pacific Diodes Zetex (Asia) Ltd 3701-04 Metroplaza Tower 1 Hing Fong Road, Kwai Fong Hong Kong Telephone: (852) 26100 611 Fax: (852) 24250 494 asia.sales@zetex.com Corporate Headquarters Diodes Incorporated 15660 N. Dallas Parkway Suite 850, Dallas, X57248, USA Telephone (1) 972 385 2810 www.diodes.com Issue 1 - August 2008 (c) Diodes Incorporated 2008 22 www.zetex.com www.diodes.com |
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