A Product Line of Diodes Incorporated ZXLD1362 60V 1A LED DRIVER WITH AEC-Q100 Description Pin Assignments The ZXLD1362 is a continuous mode inductive stepdown converter with integrated switch and high side current sense. VIN LX It operates from an input supply from 6V to 60V driving single or multiple series connected LEDs efficiently externally adjustable output current up to 1mA. GND ADJ The ZXLD1362 has been qualified to AEC-Q100 Grade 1 enabling operation in ambient temperatures from -40 to 125°C. The output current can be adjusted by applying a DC voltage or a PWM waveform. 100.1 adjustment of output current is possible using PWM control. ISENSE TSOT23-5 Top View Applying 0.2V or lower to the ADJ pin turns the output off and switches the device into a low current standby state. Features Typical Application Circuit • Simple low parts count • Single pin on/off and brightness control using DC voltage or PWM • High efficiency (up to 95%) • Wide input voltage range: 6V to 60V • Up to 1MHz switching frequency • Qualified to AEC-Q100 Grade 1 • Thermally enhanced TSOT23-5: θJA 82°C/W • Typical 2% output current accuracy Rs V IN (24V) 0.1V L1 68m C1 D1 4.7µF 100nF VIN I SE N S E ADJ ZXLD1362 LX GND GND ZXLD1362 Document number: DS33472 Rev. 2 - 2 1 of 28 www.diodes.com December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 Block Diagram D1 VIN L1 RS 5 5V C1 4.7mF 4 VIN ISENSE 1 LX R1 Voltage regulator + 0.2V + Low voltage detector MN + Adj 3 R4 50K D1 1.25V Gnd R5 20K 600KHz R2 + R3 1.35V 2 Figure 1. Block Diagram Pin Description LX Pin No. 1 GND 2 Ground (0V) ADJ 3 Multi-function On/Off and brightness control pin: • Leave floating for normal operation.(VADJ = VREF = 1.25V giving nominal average output current o 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 • Connect a capacitor from this pin to ground to increase soft-start time. • Soft-start time increases approximately 0.2ms/nF) ISENSE 4 Connect resistor RS from this pin to VIN to define nominal average output current IOUTnom = 0.1/RS (Note: RSMIN=0.1Ω with ADJ pin open circuit) VIN 5 Input voltage (6V to 60V). Decouple to ground with 4.7µF of higher X7R ceramic capacitor close to device Name Description Drain of NDMOS switch ZXLD1362 Document number: DS33472 Rev. 2 - 2 2 of 28 www.diodes.com December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 Absolute Maximum Ratings (Voltages to GND Unless Otherwise Stated) Symbol Parameter VIN Input Voltage VSENSE ISENSE Voltage V +0.3 to -5 V (measured with respect to VIN) -0.3 to +60 LX Output Voltage VADJ ILX Adjust Pin Input Voltage Switch Output Current Power Dissipation TST TJ MAX Unit (65V for 0.5 sec) VLX PTOT Rating -0.3 to +60 V (65V for 0.5 sec) (Refer to Package thermal de-rating curve on page 16) Storage Temperature Junction Temperature -0.3 to +6 1.25 V A 1 W -55 to 150 150 °C °C 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. ESD Susceptibility Human Body Model Machine Model Rating 500 75 Unit V V Semiconductor devices are ESD sensitive and may be damaged by exposure to ESD events. Suitable ESD precautions should be taken when handling and transporting these devices. The human body model is a 100pF capacitor discharge through a 1.5kΩ resistor pin. The machine model is a 200pF capacitor discharged directly into each pin Thermal Resistance Symbol Parameter Junction to Ambient Junction to Board θJA ΨJB Rating 82 33 Unit °C/W °C/W Recommended Operating Conditions Symbol VIN tOFFMIN tONMIN fLXmax TOP Notes: Parameter Input Voltage Range (Note 1) Minimum switch off-time Minimum switch on-time Recommended maximum operating frequency (Note 2) Operating Temperature range Min 6 -40 Max 60 800 800 625 125 Units V ns ns kHz °C 1. VIN > 16V to fully enhance output transistor. Otherwise out current must be derated - see graphs. Operation at low supply may cause excessive heating due to increased on-resistance. Tested at 7V guaranteed for 6V by design. 2. ZXLD1362 will operate at higher frequencies but accuracy will be affected due to propagation delays. ZXLD1362 Document number: DS33472 Rev. 2 - 2 3 of 28 www.diodes.com December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 Electrical Characteristics Symbol VSU VSD IINQoff IINQon VSENSE VSENSEHYS ISENSE VREF ΔVREF/ΔT VADJ VADJoff VADJon RADJ ILXmean RLX ILX(leak) DPWM(LF) DCADJ tSS fLX tONmin tOFFmin tPWmin_REC Notes: (a) (Test conditions: VIN = 24V, TA = 25°C, unless otherwise specified.) Parameter Internal regulator start-up threshold Internal regulator shutdown threshold Quiescent supply current with output off Quiescent supply current with output switching (Note 3) Mean current sense threshold voltage (Defines LED current setting accuracy) Sense threshold hysteresis ISENSE pin input current Internal reference voltage Condition ADJ pin grounded ADJ pin floating, L=68mH, 3 LEDs, f=260kHz Measured on ISENSE pin with respect to VIN VADJ=1.25V Min. Typ. 4.85 4.75 65 Max. 90 1.8 95 VSENSE = VIN -0.1 Measured on ADJ pin with pin floating Temperature coefficient of VREF External control voltage range on ADJ pin for 0.3 DC brightness control Note 4 DC voltage on ADJ pin to switch device from VADJ falling 0.15 active (on) state to quiescent (off) state DC voltage on ADJ pin to switch device from 0.2 VADJ rising quiescent (off) state to active (on) state 30 0< VADJ< VREF Resistance between ADJ pin and VREF 10.4 VADJ>VREF +100mV Continuous LX switch current LX switch ‘On’ resistance @ ILX = 1A LX switch leakage current Duty cycle range of PWM signal applied to PWM frequency <300Hz ADJ pin during low frequency PWM dimming 0.001 PWM amplitude = VREF mode Measured on ADJ pin Brightness control range DC Brightness control range Note 5 Time taken for output current to reach 90% of final value after voltage on Soft start time ADJ pin has risen above 0.3V. Requires external capacitor 22nF. See graphs for more details ADJ pin floating Operating frequency L=68mH (0.1V) (See graphs for more details) IOUT=1A @ VLED=3.6V Driving 3 LEDs Minimum switch ‘ON’ time LX switch ‘ON’ Minimum switch ‘OFF’ time LX switch ‘OFF’ Recommended minimum switch pulse width LX switch ‘ON’ or ‘OFF’ Unit V V µA mA 100 105 mV ±10 4 10 % µA 1.25 V 50 ppm/°C 2.5 V 0.2 0.27 V 0.25 0.3 V 50 14.2 65 18 1 1.0 5 0.5 kΩ A Ω µA 1 1000:1 5:1 2 ms 300 kHz 130 70 800 ns ns ns 3. Static current of device is approximately 700µA, see Graph, Page 16. 4. 100% brightness corresponds to VADJ = VADJ(nom) = VREF. Driving the ADJ pin above VREF will increase the VSENSE threshold and output current proportionally. 5. Ratio of maximum brightness to minimum brightness before shutdown VREF = 1.25/0.3. VREF externally driven to 2.5V, ratio 10:1. ZXLD1362 Document number: DS33472 Rev. 2 - 2 4 of 28 www.diodes.com December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 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 Figure 1 - Block diagram and Figure 2 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 SENSE ) across RS. The supply referred voltage VSENSE is forced across internal by VIN and L1 to produce a voltage ramp (V 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 10% 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(s) 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 ± 10%. 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 ±10mV/RS Adjusting output current The device contains a low pass filter between the ADJ pin and the threshold comparator and an internal current limiting resistor (50kΩ 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 VSENSE switching threshold and adjust the output current. 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 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 60μA and switch leakage is below 5μA. ZXLD1362 Document number: DS33472 Rev. 2 - 2 5 of 28 www.diodes.com December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 Device Description (continued) Figure 2. Theoretical Operating Waveforms ZXLD1362 Document number: DS33472 Rev. 2 - 2 6 of 28 www.diodes.com December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 Device Description (continued) Actual operating waveforms [VIN=15V, RS=0.1V, L=100µH] Normal operation. Output current (Ch1) and LX voltage (Ch2) Actual operating waveforms [VIN=30V, RS=0.1V, L=100µH] Normal operation. Output current (Ch1) and LX voltage (Ch2) Actual operating waveforms [VIN=60V, RS=0.1V, L=100µH] Normal operation. Output current (Ch1) and LX voltage (Ch2) ZXLD1362 Document number: DS33472 Rev. 2 - 2 7 of 28 www.diodes.com December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 Typical Characteristics ZXLD1362 Output Current L = 68µH 1100 1090 1080 Output Current (mA) 1070 1060 1050 1040 1030 1020 1010 1000 0 10 20 30 40 50 60 70 Supply Voltage (V) 1 LED 3 LED 5 LED 7 LED 9 LED 11 LED 13 LED 15 LED ZXLD1362 Output Current L = 68µH 10% 8% Output Current Deviation 6% 4% 2% 0% -2% -4% -6% -8% -10% 0 10 20 30 40 50 60 70 Supply Voltage (V) 1 LED 3 LED 5 LED 7 LED 9 LED 11 LED 13 LED 15 LED ZXLD1362 Efficiency L = 68µH 100% Efficiency (%) 90% 80% 70% 60% 50% 0 10 1 LED ZXLD1362 Document number: DS33472 Rev. 2 - 2 20 3 LED 30 40 Supply Voltage (V) 5 LED 7 LED 9 LED 8 of 28 www.diodes.com 50 11 LED 60 13 LED 70 15 LED December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 Typical Characteristics(Cont.) ZXLD1362 Switching Frequency L = 68µH 500 Switching Frequency (kHz) 400 300 200 100 0 0 10 1 LED 20 3 LED 5 LED 30 40 Supply Voltage (V) 7 LED 9 LED 50 11 LED 60 13 LED 70 15 LED ZXLD1362 Duty Cycle L = 68µH 100 90 80 Duty Cycle (%) 70 60 50 40 30 20 10 0 0 10 1 LED ZXLD1362 Document number: DS33472 Rev. 2 - 2 20 3 LED 5 LED 30 40 Supply Voltage (V) 7 LED 9 LED 9 of 28 www.diodes.com 50 11 LED 60 13 LED 70 15 LED December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 Typical Characteristics (Cont.) ZXLD1362 Output Current L = 100µH 1100 1090 1080 Output Current (mA) 1070 1060 1050 1040 1030 1020 1010 1000 0 10 20 30 40 60 50 70 Supply Voltage (V) 1 LED 3 LED 5 LED 7 LED 9 LED 11 LED 13 LED 50 60 11 LED 13 LED 15 LED ZXLD1362 Output Current L = 100µH 10% 8% Output Current Deviation 6% 4% 2% 0% -2% -4% -6% -8% -10% 0 10 20 30 40 70 Supply Voltage (V) 1 LED 3 LED 5 LED 7 LED 9 LED 15 LED ZXLD1362 Efficiency L = 100µH 100% Efficiency (%) 90% 80% 70% 60% 50% 0 10 1 LED ZXLD1362 Document number: DS33472 Rev. 2 - 2 20 3 LED 5 LED 30 40 Supply Voltage (V) 7 LED 9 LED 10 of 28 www.diodes.com 50 11 LED 60 13 LED 70 15 LED December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 Typical Characteristics (Cont.) ZXLD1362 Switching Frequency L = 100µH 500 Switching Frequency (kHz) 400 300 200 100 0 0 10 1 LED 20 3 LED 30 40 Supply Voltage (V) 5 LED 7 LED 9 LED 50 11 LED 60 13 LED 70 15 LED ZXLD1362 Switching Frequency L = 100µH 100 90 80 Duty Cycle (%) 70 60 50 40 30 20 10 0 0 10 1 LED ZXLD1362 Document number: DS33472 Rev. 2 - 2 20 3 LED 5 LED 30 40 Supply Voltage (V) 7 LED 9 LED 11 of 28 www.diodes.com 50 11 LED 60 13 LED 70 15 LED December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 Typical Characteristics (Cont.) ZXLD1362 Output Current L = 150µH 1100 1090 1080 Output Current (mA) 1070 1060 1050 1040 1030 1020 1010 1000 0 10 20 30 40 50 60 70 Supply Voltage (V) 1 LED 3 LED 5 LED 7 LED 9 LED 11 LED 13 LED 15 LED ZXLD1362 Output Current L = 150µH 10% 8% Output Current Deviation 6% 4% 2% 0% -2% -4% -6% -8% -10% 0 10 20 30 40 50 60 70 Supply Voltage (V) 1 LED 3 LED 5 LED 7 LED 9 LED 11 LED 13 LED 50 60 15 LED ZXLD1362 Efficiency L = 150µH 100% Efficiency (%) 90% 80% 70% 60% 50% 0 10 20 30 40 70 Supply Voltage (V) 1 LED ZXLD1362 Document number: DS33472 Rev. 2 - 2 3 LED 5 LED 7 LED 9 LED 12 of 28 www.diodes.com 11 LED 13 LED 15 LED December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 Typical Characteristics (Cont.) ZXLD1362 Switching Frequency L = 150µH 500 Switching Frequency (kHz) 400 300 200 100 0 0 10 1 LED 20 3 LED 5 LED 30 40 Supply Voltage (V) 7 LED 9 LED 50 60 11 LED 13 LED 50 60 70 15 LED ZXLD1362 Duty Cycle L = 150µH 100 90 80 Duty Cycle (%) 70 60 50 40 30 20 10 0 0 10 1 LED ZXLD1362 Document number: DS33472 Rev. 2 - 2 20 3 LED 5 LED 30 40 Supply Voltage (V) 7 LED 9 LED 13 of 28 www.diodes.com 11 LED 13 LED 70 15 LED December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 Typical Characteristics (Cont.) ZXLD1362 Output Current L = 220µH 1100 1090 Output Current (mA) 1080 1070 1060 1050 1040 1030 1020 1010 1000 0 10 1 LED 20 3 LED 5 LED 30 40 Supply Voltage (V) 7 LED 9 LED 50 11 LED 60 13 LED 70 15 LED ZXLD1362 Output Current L = 220µH 10% 8% Output Current Deviation 6% 4% 2% 0% -2% -4% -6% -8% -10% 0 10 1 LED 20 3 LED 30 40 Supply Voltage (V) 5 LED 7 LED 9 LED 50 11 LED 60 13 LED 70 15 LED ZXLD1362 Efficiency L = 220µH 100% Efficiency (%) 90% 80% 70% 60% 50% 0 10 1 LED ZXLD1362 Document number: DS33472 Rev. 2 - 2 20 3 LED 5 LED 30 40 Supply Voltage (V) 7 LED 9 LED 14 of 28 www.diodes.com 50 11 LED 60 13 LED 70 15 LED December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 Typical Characteristics (Cont.) ZXLD1362 Switching Frequency L = 2200µH 500 Switching Frequency (kHz) 400 300 200 100 0 0 10 1 LED 20 3 LED 5 LED 30 40 Supply Voltage (V) 50 7 LED 9 LED ZXLD1362 Duty Cycle L = 2200µH 11 LED 30 40 Supply Voltage (V) 50 60 13 LED 70 15 LED 100 90 80 Duty Cycle (%) 70 60 50 40 30 20 10 0 0 10 1 LED ZXLD1362 Document number: DS33472 Rev. 2 - 2 20 3 LED 5 LED 7 LED 9 LED 15 of 28 www.diodes.com 11 LED 60 13 LED 70 15 LED December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 Typical Characteristics (Cont.) LED Current vs Vadj 1200 1000 LED Current (mA) 800 600 400 200 0 0 1 2 3 ADJ Pin Voltage (V) R=100mΩ R=150mΩ R=330mΩ Supply current 800 Supply current (mA) 700 600 500 400 300 200 100 0 0 10 20 30 40 50 60 70 Supply voltage (V) Vref ADJ pin voltage (V) 1.243 1.2425 1.242 1.2415 1.241 1.2405 1.24 1.2395 1.239 1.2385 1.238 0 10 20 30 40 50 60 70 Supply voltage (V) Shutdow n current Shutdown current (mA) 90 80 70 60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 Supply voltage (V) ZXLD1362 Document number: DS33472 Rev. 2 - 2 16 of 28 www.diodes.com December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 Typical Characteristics (Cont.) Lx on-resistance vs supply voltage 1.6 On-resistance (Ohms) 1.4 1.2 1 -40C 20C 150C 0.8 0.6 0.4 0.2 0 0 5 10 15 20 25 30 35 Supply Voltage (V) Vadj vs Temperature 1.262 1.26 1.258 Vadj (V) 1.256 7V 9V 12V 20V 30V 1.254 1.252 1.25 1.248 1.246 1.244 -50 0 50 100 Temperature (C) 150 200 Lx on-resistance vs die temperature 1.6 On-resistance (Ohms) 1.4 1.2 1 7V 9V 12V 20V 30V 0.8 0.6 0.4 0.2 0 -50 ZXLD1362 Document number: DS33472 Rev. 2 - 2 0 50 100 Die Temperature (C) 17 of 28 www.diodes.com 150 200 December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 Application Information 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 > 0.1Ω] 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 (Ω) Nominal average output current (mA) 0.1 1000 0.13 760 0.15 667 The above values assume that the ADJ pin is floating and at a nominal voltage of VREF (=1.25V). Note that RS = 0.1Ω 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 ZXLD1362 GND DC GND The nominal average output current in this case is given by: IOUTdc = (VADJ /1.25) x (100mV/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 1A maximum. The input impedance of the ADJ pin is 50kΩ ±25% for voltages below VREF and 14.2kΩ ±25% for voltages above VREF +100mV. ZXLD1362 Document number: DS33472 Rev. 2 - 2 18 of 28 www.diodes.com December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 Application Information (Cont.) 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 ZXLD1362 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 ZXLD1362 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. 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 3.3k ADJ ZXLD1362 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 1362 and cause erratic operation but the addition of a Schottky clamp diode (cathode to ADJ) 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. ZXLD1362 Document number: DS33472 Rev. 2 - 2 19 of 28 www.diodes.com December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 Application Information (Cont.) Shutdown Mode Taking the ADJ pin to a voltage below 0.2V for more than approximately 100µs will turn off the output and supply current 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 increase output current above the 100% nominal average value. (See page 18 graphs for details). Soft-start An external capacitor from the ADJ pin to ground will provide a 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. Adding capacitance increases this delay by approximately 0.2ms/nF. 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 16 14 Soft Start Time (ms) 12 10 8 6 4 2 0 -2 0 20 40 60 Capacitance (nf) 80 100 120 Actual operating waveforms [VIN=24V, RS=0.1Ω, L=68µH, 22nF on ADJ] Soft-start operation. Output current (Ch2) and LX voltage (Ch1) ZXLD1362 Document number: DS33472 Rev. 2 - 2 20 of 28 www.diodes.com December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 Application Information (Cont.) VIN 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. To avoid transients into the IC, the size of the input capacitor will depend on the VIN voltage: VIN = 6 to 40V CIN = 2.2μF VIN = 40 to 50V CIN = 4.7μF VIN = 50 to 60V CIN = 10μF When the input voltage is close to the output voltage the input current increases which puts more demand on the input capacitor. The minimum value of 2.2μF may need to be increased to 4.7μF; 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 is recommended. Capacitors with Y5V dielectric are not suitable for decoupling in this application and should NOT be used. If higher voltages are used and the CIN is 10μF. This can be an electrolytic capacitor provide a suitable 1mF ceramic capacitor is also used and positioned as close the VIN of the IC as possible. A suitable capacitor would be NACEW100M1006.3x8TR13F. The following web sites are useful when finding alternatives: www.murata.com www.niccomp.com www.kemet.com ZXLD1362 Document number: DS33472 Rev. 2 - 2 21 of 28 www.diodes.com December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 Application Information (Cont.) Inductor selection Recommended inductor values for the ZXLD1362 are in the range 68 μH to 220 μH. 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 pages 10- 17). 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 ZXLD1362 may be selected from the MSS range manufactured by Coilcraft, or the NPIS range manufactured by NIC components. The following websites may be useful in finding suitable components. www.coilcraft.com www.niccomp.com www.wuerth-elektronik.de 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 graph Figure 3 below can be used to select a recommended inductor based on maintaining the ZXLD1362 case temperature below 60°C. For detailed performance characteristics for the inductor values 68, 100, 150 and 220μH see graphs on pages 10-17. ZXLD1362 Minimum Recommended Inductor Aluminium board, 2%Accuracy, <60°C CaseTemperature 16 15 14 13 12 Number of LEDs 11 10 9 8 7 6 5 4 3 2 1 0 0 10 20 30 40 50 60 Supply Voltage (V) Figure 3. ZXLD1362 Minimum Recommended Inductor ZXLD1362 Document number: DS33472 Rev. 2 - 2 22 of 28 www.diodes.com December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 Application Information (Cont.) 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. They also provide better efficiency than silicon diodes, due to a combination of lower forward voltage and reduced recovery time. 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 85°C. Excess leakage will increase the power dissipation in the device and if close to the load may create a thermal runaway condition. 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. *A suitable Schottky diode would be 30BQ100PBF (IR). Reducing Output Ripple Peak to peak ripple current in the LED(s) can be reduced, if required, by shunting a capacitor Cled across the LED(s) as shown below: Rs V IN LED Cled L1 D1 VIN ISE NSE LX ZXLD1362 A value of 1μF will reduce the supply 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 startup delay, by reducing the rate of rise of LED voltage. By adding this capacitor the current waveform through the LED(s) changes from a triangular ramp to a more sinusoidal version without altering the mean current value. ZXLD1362 Document number: DS33472 Rev. 2 - 2 23 of 28 www.diodes.com December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 Application Information (Cont.) Operation at low supply voltage Below the under-voltage lockout threshold (VSD) the drive to the output transistor is turned off to prevent device operation with excessive on-resistance of the output transistor. The output transistor is not full enhanced until the supply voltage exceeds approximately 17V. At supply voltages between VSD and 17V care must be taken to avoid excessive power dissipation due to the on-resistance. If the supply voltage is always less than 30V continuous (or less than 40V for less than 0.5s) an alternative device is available, the ZXLD1360Q. 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 1100 1000 900 800 Power (mW) 700 600 500 400 300 200 100 0 -50 -30 -10 10 30 50 70 90 110 130 150 Ambient Temperature (Deg C) 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 ZXLD1362 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. ZXLD1362 Document number: DS33472 Rev. 2 - 2 24 of 28 www.diodes.com December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 Application Information (Cont.) 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 and current sense resistor It is particularly important to mount the coil and the input decoupling capacitor as close to the device pins as possible to minimize parasitic resistance and inductance, which will degrade efficiency. It is also important to minimize any track resistance in series with current sense resistor RS. Its best to connect VIN directly to one end of RS and ISENSE directly to the opposite end of RS with no other currents flowing in these tracks. It is important that the cathode current of the Schottky diode does not flow in a track between RS and VIN as this may give an apparent higher measure 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 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 3.3kΩ 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. 3.3k ADJ 100nF ZXLD1362 GND GND High voltage tracks Avoid running any high voltage tracks close to the ADJ pin, to reduce the risk of leakage currents due to board contamination. The ADJ pin is soft-clamped for voltages above 1.35V to desensitize it 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 current under these conditions. Evaluation PCB ZXLD1362 evaluation boards are available on request. These boards contain LEDs to allow quick testing of the 1362 device. Additional terminals allow for interfacing to customers own LED products. ZXLD1362 Document number: DS33472 Rev. 2 - 2 25 of 28 www.diodes.com December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 Application Information (Cont.) 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 voltage VADJ and a low level of zero, the output of the internal low pass filter will swing between 0V and VADJ, 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 PWM frequency, resulting in an average output current IOUTavg proportional to the PWM duty cycle. (See Figure 4 - Low frequency PWM operating waveforms). Figure 4. Low Frequency PWM Operating Waveforms The average value of output current in this mode is given by: IOUTavg = 0.1DPWM/RS [for DPWM >0.001] This mode is preferable 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. ZXLD1362 Document number: DS33472 Rev. 2 - 2 26 of 28 www.diodes.com December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 Ordering Information Device Part Mark Package Code Packaging (Note 4) Reel size (mm) Reel width (mm) ZXLD1362ET5TA 1362 ET5 TSOT23-5 180 8 Part AEC-Q100 Quantity Number grade per reel Suffix 3000 TA 1 Package Outline Dimensions TSOT23-5 ZXLD1362 Document number: DS33472 Rev. 2 - 2 27 of 28 www.diodes.com December 2010 © Diodes Incorporated A Product Line of Diodes Incorporated ZXLD1362 IMPORTANT NOTICE DIODES INCORPORATED MAKES NO WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, WITH REGARDS TO THIS DOCUMENT, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE (AND THEIR EQUIVALENTS UNDER THE LAWS OF ANY JURISDICTION). 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