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ZXLD1356ET5TA Datasheet(PDF) 6 Page - Diodes Incorporated |
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ZXLD1356ET5TA Datasheet(HTML) 6 Page - Diodes Incorporated |
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6 / 29 page  ZXLD1356/ ZXLD1356Q Document number: DS33470 Rev. 4 - 2 6 of 29 www.diodes.com October 2012 © Diodes Incorporated ZXLD1356/ ZXLD1356Q A Product Line of Diodes Incorporated 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) 0V VIN 200mV 230mV 0V SENSE voltage VSENSE+ VSENSE- Toff Ton 170mV 0V 5V VIN 0.15VADJ 0.15VADJ IOUTnom IOUTnom +15% IOUTnom -15% VADJ LX voltage Coil current Comparator input voltage Comparator output Figure 2. Theoretical 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 200mV (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 = 200mV/RS Nominal ripple current is ±30mV/RS |
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