ZXLD1370

Document number: DS32165 Rev. 6 - 2

25 of 40

www.diodes.com

December 2016

© Diodes Incorporated

ZXLD1370

Diodes 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. The Schottky diode also provides better efficiency than silicon PN 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. In particular, it is recommended to have a voltage rating at least 15% higher than the maximum transistor voltage to ensure

safe operation during the ringing of the switch node and a current rating at least 10% higher than the average diode current. The power rating is

verified by calculating the power loss through the diode.

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

external MOSFET. If a silicon diode is used, care should be taken to ensure that the total voltage appearing on the Drain of the external

MOSFET, including supply ripple, does not exceed the specified maximum value.

*A suitable Schottky diode would be PDS3100 (Diodes Incorporated).

Output Capacitor

An output capacitor may be required to limit interference or for specific EMC purposes. For boost and buck-boost regulators, the output capacitor

provides energy to the load when the freewheeling diode is reverse biased during the first switching subinterval. An output capacitor in a buck

topology will simply reduce the LED current ripple below the inductor current ripple. In other words, this capacitor changes the current waveform

through the LED(s) from a triangular ramp to a more sinusoidal version without altering the mean current value.

In all cases, the output capacitor is chosen to provide a desired current ripple of the LED current (usually recommended to be less than 40% of

the average LED current).

Buck

PP

LED

LED

SW

PP

L

OUTPUT

I

x

r

x

f

x

8

I

C











Boost and Buck-Boost

PP

LED

LED

SW

PP

LED

OUTPUT

I

x

r

x

f

I

x

D

C









Where:









datasheet of the LED manufacturer).

The output capacitor should be chosen to account for derating due to temperature and operating voltage. It must also have the necessary RMS

current rating. The minimum RMS current for the output capacitor is calculated as follows:

Buck

12

I

I

PP

LED

RMS

COUTPUT







Boost and Buck-Boost

MAX

MAX

LED

RMS

COUTPUT

D

1

D

I

I







Ceramic capacitors with X7R dielectric are the best choice due to their high ripple current rating, long lifetime, and performance over the voltage

and temperature ranges.