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ZXLD1356Q

AUTOMOTIVE GRADE 60V 550mA LED DRIVER

Diodes Incorporated. 

ZXLD1356Q

Q

Application Information (Cont.)

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:

A value of 1µF will reduce the supply ripple current by a factor three (approximately). 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

Below the undervoltage lockout threshold (VSD) the drive to the output transistor is turned off to prevent device operation with excessive on-

resistance of the output transistor. The output transistor is not full enhanced until the supply voltage exceeds approximately 17V. At supply

voltages between VSD and 17V care must be taken to avoid excessive power dissipation due to the on-resistance.

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 (25mm)2 PCB

with 1oz copper standing in still air.

Note that the device power dissipation will most often be at 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.

ZXLD1356Q

Document number: DS37077 Rev. 2 - 2

22 of 28

www.diodes.com

October 2015

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