ZXLD1352 Просмотр технического описания (PDF) - Zetex => Diodes

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ZXLD1352

350mA LED driver with internal switch and enhanced PWM dimming range

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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 20␮A and switch leakage is below 1␮A.

Issue 1 - August 2008

5

© Diodes Incorporated 2008

www.zetex.com

www.diodes.com

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