ZXLD1371Q

ZXLD1371Q

Document number: DS37116 Rev. 1 - 2

30 of 42

www.diodes.com

September 2015

© Diodes Incorporated

ZXLD1371Q

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 for a switching current of up to about 1.5A would be PDS3100Q (Diodes Inc).

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:

Boost and Buck-Boost

PP

LED

LED

SW

PP

L

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

Boost and Buck-Boost

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.

PP

LED

LED

SW

PP

L

OUTPUT

I

x

r

x

f

x

8

I

C











12

I

I

PP

LED

RMS

COUTPUT





MAX

MAX

LED

COUTPUTRMS

D

1

D

I

I



