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ZXLD1352

30V 350mA LED DRIVER WITH 1000:1 PWM DIMMING AND AEC-Q100

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Diodes Incorporated

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 20mA and switch leakage is below 1mA.

ZXLD1352

Document number: DS33469 Rev. 3 - 2

5 of 21

www.diodes.com

December 2010

© Diodes Incorporated

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