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ZXLD1350

30V 350mA LED DRIVER with AEC-Q100

Diodes Incorporated. 

Device Description

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Diodes Incorporated

ZXLD1350

30V 350mA LED DRIVER with AEC-Q100

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 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

15mA and switch leakage is below 1mA.

ZXLD1350

Document number: DS33468 Rev. 8 - 2

5 of 22

www.diodes.com

March 2011

© Diodes Incorporated

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