ZXLD1356

ZXLD1356

Document number: DS33470 Rev. 3 - 2

23 of 29

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December 2010

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Diode selection

For maximum efficiency and performance, the rectifier (D1) should be a fast low capacitance Schottky diode* with low reverse

leakage at the maximum operating voltage and temperature.

They also provide better efficiency than silicon diodes, due to a combination of lower forward voltage and reduced recovery time.

It is important to select parts with a peak current rating above the peak coil current and a continuous current rating higher than

the maximum output load current. It is very important to consider the reverse leakage of the diode when operating above 85°C.

Excess leakage will increase the power dissipation in the device and if close to the load may create a thermal runaway condition.

The higher forward voltage and overshoot due to reverse recovery time in silicon diodes will increase the peak voltage on the LX

output. If a silicon diode is used, care should be taken to ensure that the total voltage appearing on the LX pin including supply

ripple, does not exceed the specified maximum value.

*A suitable Schottky diode would be B1100B (Diodes Inc).

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 (approx.). 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

excessive on-resistance of the output transistor. The output transistor is not full enhanced until the supply voltage exceeds

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.

LX

ZXLD1356

Rs

L1

Cled

LED

D1