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| ZXLD1360 1A LED driver with internal switch Description The ZXLD1360 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 1A. Depending upon supply voltage and external components, this can provide up to 24 watts of output power. The ZXLD1360 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. 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. The PWM filter provides a soft-start feature by controlling the rise of input/output current. The soft-start time can be increased using an external capacitor from the ADJ pin to ground. 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. Features * * * * * * * * * * * * * Simple low parts count Internal 30V NDMOS switch 1A output current Single pin on/off and brightness control using DC voltage or PWM Internal PWM filter 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 Pin connections LX 1 GND 2 ADJ 3 TSOT23-5 Top view 4 ISENSE 5 Typical application circuit VIN (7V - 30V) Rs 0.1 VIN L1 47 H C1 4.7 F D1 VIN ISENSE LX N/C ADJ ZXLD1360 GND GND Issue 1 - March 2007 (c) Zetex Semiconductors plc 2007 1 www.zetex.com 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) -0.3V to +6V 1.25A 1W -40 to 125C -55 to 150C 150C 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) 125C/W Electrical characteristics (test conditions: VIN=12V, Tamb=25C unless otherwise stated)(a) Symbol VIN VSU VSD IINQoff IINQon VSENSE Parameter Input voltage Conditions Min. 7 Typ. 5.65 5.55 Max. 30 Unit V V V Internal regulator start-up threshold VIN rising Internal regulator shutdown VIN falling threshold Quiescent supply current ADJ pin grounded with output off Quiescent supply current ADJ pin floating with output switching f = 250kHz Mean current sense threshold Measured on ISENSE voltage pin with respect to VIN (Defines LED current setting accuracy) V ADJ = 1.25V ISENSE pin input current Internal reference voltage VSENSE = VIN -0.1 Measured on ADJ pin with pin floating 20 1.8 100 40 5.0 105 A mA mV 95 VSENSEHYS Sense threshold hysteresis ISENSE VREF 15 1.25 1.25 50 0.3 0.15 0.2 2.5 0.25 10 % A V ppm/K V V VREF / T Temperature coefficient of VREF VADJ VADJoff External control voltage range on ADJ pin for DC brightness control(b) DC voltage on ADJ pin to switch VADJ falling device from active (on) state to quiescent (off) state DC voltage on ADJ pin to switch VADJ rising device from quiescent (off) state to active (on) state VADJon 0.2 0.25 0.3 V NOTES: (a) 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. (b) 100% brightness corresponds to VADJ = VADJ(nom) = VREF. Driving the ADJ pin above VREF will increase the VSENSE. threshold and output current proportionally. Issue 1 - March 2007 (c) Zetex Semiconductors plc 2007 2 www.zetex.com ZXLD1360 Electrical characteristics (test conditions: VIN=12V, Tamb=25C unless otherwise stated) (cont.) Symbol RADJ ILXmean RLX ILX(leak) Parameter Conditions Resistance between ADJ pin and 0< VADJ < VREF VREF VADJ > VREF +100mV Continuous LX switch current LX Switch `On' resistance LX switch leakage current PWM frequency <500Hz PWM amplitude = VREF Measured on ADJ pin PWM frequency >10kHz PWM amplitude = VREF Measured on ADJ pin Time taken for output current to reach 90% of final value after voltage on ADJ pin has risen above 0.3V ADJ pin floating L = 33 H (0.093 ) IOUT = 1A @ VLED = 3.6V Driving 1 LED LX switch `ON' LX switch `OFF' LX switch 'ON' 0.01 @ ILX = 1 A 0.5 Min. 135 13.5 Typ. Max. 250 25 1 1.0 5 1 A Unit k A DPWM(LF) Duty cycle range of PWM signal applied to ADJ pin during low frequency PWM dimming mode Brightness control range DPWM(HF) Duty cycle range of PWM signal applied to ADJ pin during high frequency PWM dimming mode Brightness control range TSS Soft start time 100:1 0.16 1 5:1 500 s fLX Operating frequency (See graphs for more detail) 280 240 (*) 200 (*) 800 1 0.3 50 0.7 KHz ns ns ns MHz TONmin TOFFmin Minimum switch `ON' time Minimum switch `OFF' time TONmin_REC Recommended minimum switch 'ON' time fLXmax Recommended maximum operating frequency Recommended duty cycle range DLX of output switch at fLXmax TPD Internal comparator propagation delay ns NOTES: (*) Parameters are not tested at production. Parameters are guaranteed by design, characterization and process control. Issue 1 - March 2007 (c) Zetex Semiconductors plc 2007 3 www.zetex.com ZXLD1360 Pin description LX 1 GND 2 ADJ 3 TSOT23-5 Top view Name LX GND ADJ Pin no. 1 2 3 5 VIN 4 ISENSE 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 < 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 4 VIN 5 Ordering information Device ZXLD1360ET5TA Reel size (mm) 180 Reel width (inches) 8 Quantity per reel 3,000 Device mark 1360 Issue 1 - March 2007 (c) Zetex Semiconductors plc 2007 4 www.zetex.com ZXLD1360 Block diagram VIN D1 RS L1 5 VIN 4 ISENSE 1 LX 5V C1 4.7 F Voltage regulator R1 + 0.2V + Low voltage detector MN Adj 3 R4 200K R5 20K R2 + D1 1.25V + 1.35V R3 Gnd 2 Issue 1 - March 2007 (c) Zetex Semiconductors plc 2007 5 www.zetex.com ZXLD1360 Device description The device, in conjunction with the coil (L1) and current sense resistor (RS), forms a selfoscillating 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(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 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 between the ADJ pin and the threshold comparator and an internal 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 VSENSE switching threshold and adjust the output current. The filter is third order, comprising three sections, 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 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 5 A. Issue 1 - March 2007 (c) Zetex Semiconductors plc 2007 6 www.zetex.com ZXLD1360 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 - March 2007 (c) Zetex Semiconductors plc 2007 7 www.zetex.com ZXLD1360 Actual operating waveforms [VIN=15V, RS=0.1 , L=33H] Normal operation. Output current (Ch1) and LX voltage (Ch2) Actual operating waveforms [VIN=30V, RS=0.1 , L=33H] Normal operation. Output current (Ch1) and LX voltage (Ch2) Issue 1 - March 2007 (c) Zetex Semiconductors plc 2007 8 www.zetex.com ZXLD1360 Typical operating conditions Efficiency 1,3 and 7 LEDs L = 33 H 100 Deviation from nominal set current (%) 10 8 6 4 2 0 -2 -4 -6 -8 -10 5 10 15 20 25 30 Supply Voltage VIN (V) 1 Led 3 Led 7 Led Output current variation with Supply Voltage L = 33 H 95 90 Efficiency (%) 85 80 75 70 65 60 0 10 20 Supply Voltage VIN (V) 1 Led 3 Led 7 Led 30 40 Operating Frequency vs Input Voltage L = 33 H 600 500 400 100 90 80 70 1 Led 3 Led 7 Led Duty Cycle % vs Input Voltage L = 33 H Freq (kHz) Duty (%) 60 50 40 30 20 10 1 Led 3 Led 7 Led 300 200 100 0 5 10 15 20 25 30 0 0 5 10 15 20 25 30 35 Supply Voltage V (V) IN Supply Voltage V (V) IN Issue 1 - March 2007 (c) Zetex Semiconductors plc 2007 9 www.zetex.com ZXLD1360 Typical operating conditions ZXLD1360 Output Current L=33H 1060 10.0% 8.0% ZXLD1360 Output Current L=33H 1040 Output Current Deviation (%) 0 5 10 15 20 25 30 35 6.0% 4.0% 2.0% 0.0% -2.0% -4.0% -6.0% 1020 Output Current (mA) 1000 980 960 940 920 -8.0% 900 -10.0% 0 5 10 15 20 25 30 35 Supply Voltage VIN (V) 1LED 2LED 3LED 4LED 5LED 6LED 7LED 8LED 1LED 2LED Supply Voltage VIN (V) 3LED 4LED 5LED 6LED 7LED 8LED ZXLD1360 Switching Frequency L=33H 600 100 90 500 80 70 ZXLD1360 Duty Cycle L=33H Switching Frequency (kHz) 400 Duty Cycle (%) 0 5 10 15 20 25 30 35 60 50 40 30 20 10 300 200 100 0 0 0 5 10 15 20 25 30 35 Supply Voltage VIN(V) 1 LED 2 LED 3 LED 4 LED 5 LED 6 LED 7 LED 8 LED 1 LED 2 LED Supply Voltage VIN(V) 3 LED 4 LED 5 LED 6 LED 7 LED 8 LED Issue 1 - March 2007 (c) Zetex Semiconductors plc 2007 10 www.zetex.com ZXLD1360 Typical operating conditions ZXLD1360 Output Current L=47H 1060 10.0% 8.0% ZXLD1360 Output Current L=47H 1040 Output Current Deviation (%) 6.0% 4.0% 2.0% 0.0% -2.0% -4.0% -6.0% 1020 Output Current (mA) 1000 980 960 940 920 -8.0% -10.0% 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 900 Supply Voltage VIN (V) 1LED 2LED 3LED 4LED 5LED 6LED 7LED 8LED 1LED 2LED Supply VoltageVIN (V) 3LED 4LED 5LED 6LED 7LED 8LED ZXLD1360 Switching Frequency L=47H 600 100 90 500 80 ZXLD1360 Duty Cycle L=47H Switching Frequency (kHz) 70 400 Duty Cycle (%) 0 5 10 15 20 25 30 35 60 50 40 30 20 10 300 200 100 0 0 0 5 10 15 20 25 30 35 Supply Voltage VIN (V) 1 LED 2 LED 3 LED 4 LED 5 LED 6 LED 7 LED 8 LED 1 LED 2 LED Supply Voltage VIN (V) 3 LED 4 LED 5 LED 6 LED 7 LED 8 LED Issue 1 - March 2007 (c) Zetex Semiconductors plc 2007 11 www.zetex.com ZXLD1360 Typical operating conditions ZXLD1360 Output Current L=100H 1060 10.0% 8.0% ZXLD1360 Output Current L=100H 1040 1020 Output Current Deviation (%) 6.0% 4.0% 2.0% 0.0% -2.0% -4.0% -6.0% Output Current (mA) 1000 980 960 940 920 -8.0% -10.0% 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 900 Supply Voltage VIN (V) 1LED 2LED 3LED 4LED 5LED 6LED 7LED 8LED 1LED 2LED Supply Voltage VIN (V) 3LED 4LED 5LED 6LED 7LED 8LED ZXLD1360 Switching Frequency L=100H 600 100 90 500 80 ZXLD1360 Duty Cycle L=100H Switching Frequency (kHz) 70 400 Duty Cycle (%) 60 50 40 30 20 300 200 100 10 0 0 5 10 15 20 25 30 35 0 0 5 10 15 20 25 30 35 Supply Voltage VIN (V) 1 LED 2 LED 3 LED 4 LED 5 LED 6 LED 7 LED 8 LED 1 LED 2 LED Supply Voltage VIN (V) 3 LED 4 LED 5 LED 6 LED 7 LED 8 LED Issue 1 - March 2007 (c) Zetex Semiconductors plc 2007 12 www.zetex.com ZXLD1360 Typical operating conditions Vref vs Supply Voltage 1.4 1.2372 1.2371 1.237 1 1.2369 Vref (V) Vref (V) 0.8 1.2368 1.2367 0.4 1.2366 1.2365 1.2364 0 1 2 3 4 Supply Voltage VIN (V) 5 6 7 8 0 5 10 15 20 25 30 Vref (V) Vref vs Supply Voltage 1.2 0.6 0.2 0 35 Supply Voltage VIN (V) Supply Current vs Supply Voltage 600 18 16 500 14 400 Iin (A) Iin (A) 12 10 8 6 4 100 2 0 0 5 10 15 20 25 30 35 Supply Voltage VIN (V) 0 0 5 10 Shutdown Current vs Supply Voltage 300 200 15 20 25 30 35 Supply Voltage VIN (V) LED Current vs Vadj 1200 1000 LED Current (mA) 800 600 400 200 0 0 1 ADJ Pin Voltage (V) R=100m R=150m R=330m 2 3 Issue 1 - March 2007 (c) Zetex Semiconductors plc 2007 13 www.zetex.com ZXLD1360 Typical operating conditions ZXLD1360 Response Time vs Temperature Typical minimum LX 'on' and 'off' time 350 300 250 "On" Resistance ( ) Lx Switch "On" Resistance vs Temperature 0.80 0.70 0.60 0.50 0.40 0.30 0.20 Response Time (ns) 200 150 100 50 0 -55 -35 -15 5 25 45 65 85 105 125 Ambient Temperature (C) -50 0 50 100 150 200 Ambient Temperature (oC ) Min LX on Min LX off Vadj vs Temperature L = 470uH, Rs = 0.33 Ohms 1.24 100.4 100.2 1.235 100 1.23 99.8 Voltage across Rsense (0.333 Ohm) vs Temperature 1.225 12V, single LED 1.22 12V, three LED 24V, single LED 24V, three LED 1.215 Vsense (V ) Vadj (V) 99.6 99.4 24V, single LED 99.2 99 98.8 98.6 12V, three LED 12V, single LED 24V, three LED 1.21 -55 -35 -15 5 25 45 65 85 105 125 98.4 -55 -35 -15 5 25 45 65 85 105 125 Ambient Temperature ( o C ) Ambient Temperature ( o C ) Output current change vs Temperature VIN = 12V, L= 470uH, Rs = 0.33 Ohms 0.5 0.4 Output current change vs Temperature VIN = 24V, L= 470uH, Rs = 0.33 Ohms Deviation from nominal set value (%) 0.4 Deviation from nominal set value (%) 0.3 0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 -0.5 -55 -35 -15 5 25 45 65 85 105 125 12V, single LED 12V, three LED 0.2 0 -0.2 -0.4 24V, single LED 24V, three LED -0.6 -0.8 -1 -55 -35 -15 5 25 45 65 o 85 105 125 Ambient Temperature ( o C ) Ambient Temperature ( C ) Issue 1 - March 2007 (c) Zetex Semiconductors plc 2007 14 www.zetex.com 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 external current sense resistor (RS) connected between VIN and ISENSE and is given by: IOUTnom = 0.1/RS [for RS 0.1 ] 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 ( ) 0.1 0.13 0.15 Nominal average output current (mA) 1000 760 667 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 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 ZXLD1360 GND 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 above 1.25V, RS must be increased in proportion to prevent IOUTdc exceeding 1A maximum. The input impedance of the ADJ pin is 200k voltages above VREF +100mV. 25% for voltages below VREF and 20k 25% for Issue 1 - March 2007 (c) Zetex Semiconductors plc 2007 15 www.zetex.com 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, 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 ZXLD1360 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 ZXLD1360 GND GND This scheme uses the 200k 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 10k ADJ ZXLD1360 GND If the NMOS transistor within the microcontroller has high Drain / Source capacitance , this arrangement can inject a negative spike into ADJ input of the 1360 and cause erratic operation but the addition of a Schottky clamp diode (cathode to ADJ) to ground and inclusion of a series resistor (10K) 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 - March 2007 (c) Zetex Semiconductors plc 2007 16 www.zetex.com ZXLD1360 Shutdown mode Taking the ADJ pin to a voltage below 0.2V for more than approximately 100s, 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 The device has inbuilt soft-start action due to the delay through the PWM filter. An external capacitor from the ADJ pin to ground will provide additional 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. With no external capacitor, the time taken for the output to reach 90% of its final value is approximately 500s. Adding capacitance increases this delay by approximately 0.5ms/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 10 8 Soft Start time (ms) 6 4 2 0 0 5 10 Capacitance (nF) 15 20 25 Actual operating waveforms [VIN=15V, RS=0.1 , L=33H, 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 additional capacitance added to the ADJ pin. Issue 1 - March 2007 (c) Zetex Semiconductors plc 2007 17 www.zetex.com ZXLD1360 This time has been extended on the trace below by adding a 100nF ceramic capacitor which gives a soft start time of 40 milliseconds approximately. Actual operating waveforms [VIN=15V, RS=0.1 , L=33H ,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, 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 4.7 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 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 Issue 1 - March 2007 (c) Zetex Semiconductors plc 2007 18 www.zetex.com 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 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 ZXLD1360 are listed in the table below: Part no. MSS1038-333 MSS1038-683 NPIS64D330MTRF L ( H) 33 68 33 DCR () 0.093 0.213 0.124 ISAT (A) 2.3 1.5 1.1 Manufacturer CoilCraft www.coilcraft.com NIC www.niccomp.com 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>240ns 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 ( ) RS is the current sense 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 ( ) {=0.5 nominal} VD is the diode forward voltage at the required load current (V) Issue 1 - March 2007 (c) Zetex Semiconductors plc 2007 19 www.zetex.com ZXLD1360 Example: For VIN =12V, L=33 H, rL=0.093, RS=0.1 , RLX=0.15 , VLED=3.6V, Iavg =1A and VD =0.49V TON = (33e-6 x 0.3)/(12 - 3.6 - 0.693) = 1.28 s TOFF = (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 at www.zetex.com/zxld1360 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. 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 85C. 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. Issue 1 - March 2007 (c) Zetex Semiconductors plc 2007 20 www.zetex.com ZXLD1360 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: VIN Rs LED Cled L1 D1 VIN ISENSE LX ZXLD1360 A value of 1 F 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 start-up 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 . 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). The drive to the 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 duty cycle is high. 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. Issue 1 - March 2007 (c) Zetex Semiconductors plc 2007 21 www.zetex.com ZXLD1360 Maximum Power Dissipation 1100 1000 900 800 700 Power (mW) 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 ZXLD1360 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. Issue 1 - March 2007 (c) Zetex Semiconductors plc 2007 22 www.zetex.com ZXLD1360 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 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 High voltage tracks ADJ ZXLD1360 GND 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 The ZXLD1360EV1, 2 or 3 evaluation boards are available on request. These boards contain a Lumileds K2 or multiple Ostar LEW type LEDs to allow quick testing of the 1360 device. Additional terminals allow for interfacing to customers own LED products. Issue 1 - March 2007 (c) Zetex Semiconductors plc 2007 23 www.zetex.com 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 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 2 - Low frequency PWM operating waveforms). VADJ PWM Voltage Ton Toff 0V VADJ Filter Output 300mV 200mV 0V IOUTnom Output Current 0.1/Rs IOUTavg 0 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 widest possible dimming range (approx. 100:1) and higher efficiency at the expense of greater output ripple. Note that the low pass filter introduces a small error in the output duty cycle due to the difference between the start-up and shut-down times. This time difference is a result of the 200mV shutdown threshold and the rise and fall times at the output of the filter. To minimize this error, the PWM frequency should be as low as possible consistent with avoiding flicker in the LED(s). Issue 1 - March 2007 (c) Zetex Semiconductors plc 2007 24 www.zetex.com ZXLD1360 High frequency PWM mode At PWM frequencies above 10kHz and for duty cycles above 0.16, the output of the internal low pass filter will contain a DC component that is always above the shutdown threshold. This will maintain continuous device operation and the nominal average output current will be proportional to the average voltage at the output of the filter, which is directly proportional to the duty cycle. (See Figure 3 - High frequency PWM operating waveforms). For best results, the PWM frequency should be maintained above the minimum specified value of 10kHz, in order to minimize ripple at the output of the filter. The shutdown comparator has approximately 50mV of hysteresis, to minimize erratic switching due to this ripple. An upper PWM frequency limit of approximately one tenth of the operating frequency is recommended, to avoid excessive output modulation and to avoid injecting excessive noise into the internal reference. VADJ PWM voltage Ton Toff 0V VADJ Filter output 200mV 0V 0.1/RS Output current IOUTnom 0 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 reduced dimming range (approx.5:1). The restricted dimming range is a result of the device being turned off when the dc component on the filter output falls below 200mV. Issue 1 - March 2007 (c) Zetex Semiconductors plc 2007 25 www.zetex.com ZXLD1360 Intentionally left blank Issue 1 - March 2007 (c) Zetex Semiconductors plc 2007 26 www.zetex.com ZXLD1360 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 - March 2007 (c) Zetex Semiconductors plc 2007 27 www.zetex.com ZXLD1360 Definitions Product change Zetex Semiconductors 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 Zetex 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. 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 written approval 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 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 Zetex' terms 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 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 our regionally 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 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 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 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. 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 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 Zetex Semiconductors plc Zetex Technology Park, Chadderton Oldham, OL9 9LL United Kingdom Telephone: (44) 161 622 4444 Fax: (44) 161 622 4446 hq@zetex.com (c) 2007 Published by Zetex Semiconductors plc Issue 1 - March 2007 (c) Zetex Semiconductors plc 2007 28 www.zetex.com |
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