ZXLD1350 HIGH POWER LED DRIVER WITH INTERNAL SWITCH DESCRIPTION The ZXLD1350 is a continuous mode PWM inductive converter, designed for driving 1 watt white LEDs efficiently from a voltage source higher than the LED voltage. The device operates from an input supply of between 9V and 30V and provides an externally adjustable output current of up to 350mA. The ADJ pin will accept either a dc voltage or a PWM waveform. Depending upon the control frequency, this will provide either a continuous or a gated output current. The PWM filter components are contained within the chip. A capacitor from the ADJ pin to ground provides an adjustable soft-start function. The ZXLD1350 includes the output switch and an output current sensing circuit, which uses an external resistor to set the nominal output current. A continuous logic low signal on the ADJ pin will turn the device off. (shutdown current <1μA) Once set, the nominal output current can be adjusted to a lower value, by applying an external control signal to the ‘ADJ’ pin. The device is assembled in a SOT23 5 pin package. FEATURES Internal 30V NDMOS switch Up to 350mA output current Can drive up to 3 series connected 1Watt LEDs Brightness control using dc voltage or PWM Internal PWM filter Optional soft-start High efficiency (80% typ) Wide input voltage range: 9V to 30V Low quiescent current: (100A typ) 1A max shutdown current Up to 1MHz switching frequency PIN CONNECTIONS APPLICATIONS Low voltage halogen replacement LEDs Automotive lighting Low voltage industrial lighting LED flashlights Mining LED backup light TYPICAL APPLICATION CIRCUIT SOT23-5 pack TARGET SPEC ISSUE F September 2004 ZXLD1350 1 ZXLD1350 ABSOLUTE MAXIMUM RATINGS (Voltages to GND unless otherwise stated) Input Voltage (VIN) 30V (40V for 20 sec) -0.3V to 30V 500mA 320mW -40 to 105C -55 to 150C 150C LX Output Voltage (VLX) Switch output current (ILX) Power Dissipation (Ptot) Operating Temperature (TOP) Storage Temperature (TST) Junction Temperature (Tj MAX) ELECTRICAL CHARACTERISTICS: Test conditions: VIN=VEN=12V, TAMB=25C unless otherwise stated1 Symbol Parameter VIN IIN Input voltage Supply current ISD VSENSE VHYS ISENSE VADJ(nom) VADJ Shutdown current VSENSE pin threshold voltage VSENSE pin hysteresis Isense pin input current Internal voltage reference External dc control voltage applied to ADJ pin Minimum dc voltage on ADJ pin to hold device out of shutdown mode Minimum duty cycle of PWM signal on ADJ pin to hold device out of shutdown mode Operating frequency VADJoff D fLX fLXmax DLX ILXpk RLX ILX(leak) Conditions Min Typ 9 ILX = 0, Output not switching ADJ pin held at 0V Measured with respect to VIN Measured as proportion of VSENSE VIN-100mV <VSENSE < VIN ADJ pin floating 12 100 Max Unit s 30 V µA 1 µA mV % µA V V 100 30 10 1.23 0 VADJ(nom) 0.5 PWM frequency >10kHz 0.4 ADJ pin floating L=47µH, IOUT=350mA @ 3.4V Maximum operating frequency Duty cycle of output switch at fLXmax LX Switch max current LX Switch ‘On’ resistance LX Switch leakage current V 400 0.2 KHz 1 0.8 MHz 0.5 A µA 1 1 Note 1: Production testing of the device is performed at 25 C. Functional operation of the device over a –40C to +105C temperature range is guaranteed by design, characterisation and process control. TARGET SPEC ISSUE F September 2004 ZXLD1350 2 ZXLD1350 PIN DESCRIPTION Name LX GND ADJ ISENSE VIN Pin # Description Drain of NDMOS switch Ground (0V) Internal voltage reference pin (1.23V) Connect to GND to shut-down the device Drive with dc voltage ((0.5<V<1.23) or PWM signal to adjust output current. Connect a capacitor from this pin to ground to provide a soft-start function. Connect resistor Rs from this pin to VIN to sense nominal output current IOUT=0.1/ Rs Input voltage (9V to 30V). Decouple to ground with capacitor close to device. BLOCK DIAGRAM TARGET SPEC ISSUE F September 2004 ZXLD1350 3 ZXLD1350 Device Description The device, in conjunction with the coil (L1) and current sense resistor (Rs), forms a self-oscillating continuous mode inductive converter. With reference to the typical application and block diagram, operation is as follows: Control loop When an input voltage is first applied to the device, the current in the coil will be zero and there will be no output from the current sense circuit. Under this condition, the output of the comparator will be high. This will turn the NDMOS transistor on and the LX pin will be switched low, causing current to flow from VIN to ground, via the external sense resistor Rs and LED. This current will rise at a rate determined by the coil and VIN to produce a voltage drop across Rs. This voltage drop is sensed at the ISENSE pin and forced across internal resistor RA. The current in RA is mirrored into internal resistors RB and RC and produces a rising voltage at the input to the comparator. As the current rises, the voltage at the inverting input of the comparator increases <<TBC>> the internal current sense circuit will produce a voltage dropcompared against an upper threshold voltage VTH+. When this threshold is reached, the comparator will switch and reset the latch, causing the LX output switch to turn off. At this point, the current in the coil will continue to flow via the LED and schottky diode and will ramp down linearly at a rate determined by the coil and the forward drop of the LED. The falling current produces a decreasing voltage between VIN and ISENSE, which is compared against a second threshold voltage VTH-. When this threshold is reached, the lower comparator will switch and set the latch This turns the switch back on again and the cycle repeats. The loop is therefore self-oscillating and the average current in the LED will be determined by the threshold voltages. The thresholds VTH+ and VTH- are derived from an internal reference voltage Vref. This reference is brought out to the ADJ pin and may be overdriven to adjust the output current. For stable oscillation, within the specified frequency range, the thresholds VTH+ and VTH- are set nominally 15% above and below the reference voltage. This defines the output ripple current to be nominally 30% of the nominal output current. The device contains a low pass filter between the ADJ pin and the threshold comparators. This allows the ADJ pin to be overdriven with either dc or ac signals. The filter has a cut-off frequency of nominally 4kHz TARGET SPEC ISSUE F September 2004 ZXLD1350 4 ZXLD1350 TYPICAL OPERATING CHARACTERISTICS (For typical application circuit at Vin=12V and TAMB=25 C unless otherwise stated) [TBD] TARGET SPEC ISSUE F September 2004 ZXLD1350 5 ZXLD1350 APPLICATION NOTES Adjusting output current When connected as shown in the typical application circuit, with Rs=0.28, the ZXLD1350 will produce a nominal output current of 350mA. This can be adjusted by one of the two methods described below. 1) Output current adjustment by external resistor Rs The nominal output current is determined by the internal reference voltage and the value of the external current sense resistor (RS) from VIN to the current sense pin ISENSE and is given by: IOUTdc = 0.1/Rs 2) Output current adjustment by external dc control voltage The ADJ pin of the device is connected to the internal voltage reference (1.23V nom). This pin can be overdriven by an external dc voltage (VADJ) in order to override this reference and adjust the output current to a value below the nominal value in 1) above. The nominal output current is then given by: IOUTdc = 0.0813*VADJ/Rs 3) Output current adjustment by PWM control A Pulse Width Modulated (PWM) signal with duty cycle D can be applied to the ADJ pin in order to adjust the output current to a value below the nominal value in 1) above. If the PWM frequency is less than approximately 10kHz, the device will be gated ‘on’ and ‘off’ and the output will be discontinuous with an average value given by: IOUTavg 0.1D/Rs This mode may be preferred when optimum output ‘whiteness’ is required when adjusting LED brightness. It will also provide higher efficiency at the expense of greater output ripple. If the PWM frequency is higher than approximately 10kHz and the duty cycle above the specified minimum value, the device will remain active and the output will be continuous, with a nominal value given by: IOUTnom 0.1D/Rs This mode will give minimum output ripple and reduced radiated emission. Shutdown mode Taking the ADJ to a voltage below 0.5V for more than approximately 100µs, will turn off the device and supply current will fall to a low standby level. Soft-start A capacitor from the ADJ pin to ground will provide a soft-start function, by delaying the time taken for the voltage to rise to the turn-on threshold and by slowing down the rate of rise of the internal reference voltage. TARGET SPEC ISSUE F September 2004 ZXLD1350 6 ZXLD1350 Capacitor selection A good quality, low ESR capacitor should also be used for input decoupling, as the ESR of this capacitor is effectively in series with the 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 1F is acceptable if the input source is close to the device, but higher values will improve performance at lower input voltages, when the source impedance is high. The input capacitor should be mounted as close as possible to the IC For maximum stability over temperature, capacitors with X7R dielectric are recommended, as these have a much smaller temperature coefficient than other types. A table of recommended manufacturers is provided below Manufacturer Website Murata Taiyo Yuden Kemet AVX www.murata.com www.t-yuden.com www.kement.com www.avxcorp.com Inductor selection The choice of inductor will depend on available board space as well as required performance. Small value inductors have the advantage of smaller physical size and may offer lower series resistance and higher saturation current compared to larger values. A disadvantage of smaller inductors is that they can result in reduced efficiency due to switch losses. Higher inductor values can provide better performance at lower supply voltages. <insert equations for duty cycle and ripple here> Recommended inductor values for the ZXLD1350 are in the range 10H to 100H. The inductor should be mounted as close to the device as possible with low resistance connections to the LX and VIN pins. Suitable coils for use with the ZXLD1350 are given in the table below: Part No. CMD4D11-100MC DO1608C L (H) 10 DCR () 0.457 ISAT (A) 0.5 10 22 33 47 0.16 0.37 0.51 0.64 1.1 0.7 0.58 0.5 Manufacturer Sumida www.sumida.com Coilcraft www.coilcraft.com TARGET SPEC ISSUE F September 2004 ZXLD1350 7 ZXLD1350 Diode selection The rectifier diode (D1) should be a fast low capacitance schottky diode with low reverse leakage at the working voltage. It should also have a peak current rating above the peak coil current and a continuous current rating higher than the maximum output load current. The table below gives some typical characteristics for diodes that can be used with the ZXLD1350 Diode ZHCS1000 ZHCS2000 Forward voltage at 100mA (mV) 300 300 Peak current (mA) 1000 1000 Continuous current (mA) 1000 2000 Reverse leakage at 30V (A) 15 15 Package TSOT23 TSOT23 Layout considerations PCB tracks should be kept as short as possible to minimise ground bounce and the ground pin of the device should be soldered directly to the ground plane. It is particularly important to mount the coil and the input/output capacitors close to the device to minimise parasitic resistance and inductance, which will degrade efficiency. The FB pins are high impedance inputs, so PCB track lengths to these should also be kept as short as possible to reduce noise pickup. Excess capacitance from the FB pins to ground should be avoided and the FB2 pin should be grounded if not required. TARGET SPEC ISSUE F September 2004 ZXLD1350 8 ZXLD1350 PACKAGE OUTLINE SOT235 TARGET SPEC ISSUE F September 2004 ZXLD1350 9 ZXLD1350 ORDERING INFORMATION DEVICE REEL SIZE ZXLD1350E5TA 180mm REEL WIDTH 8mm QUANTITY PER REEL PART MARK 3000 1350 Zetex plc. 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The company reserves the right to alter without notice the specification, design, price or conditions of supply of any product or service. TARGET SPEC ISSUE F September 2004 ZXLD1350 10