MP46885 – 80V INPUT, 500mA HIGH POWER LED DRIVER

MP46885 Rev. 1.0

www.MonolithicPower.com

9

7/7/2011

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External Bootstrap Diode

An external bootstrap diode may enhance the

efficiency of the LED driver. In below cases, an

external BST diode is recommended from the

5V to BST pin:



There is a 5V rail available in the system;



VIN is no greater than 5V;



VOUT is between 3.3V and 5V;

This diode is also recommended for high duty

high frequency (over 1.5MHz) applications.

The bootstrap diode can be a low cost one

such as IN4148 or BAT54.

MP46885

BS

5V

SW

Figure 2—External Bootstrap Diode

Layout Consideration

It is essential to place the input decoupling

capacitor, catch diode and the MP46885 (VIN

pin, SW pin and PGND) as close as possible,

with traces that are very short and fairly wide.

This can help to greatly reduce the voltage

spike on SW node, and lower the EMI noise

level as well.

Try to run the feedback trace as far from the

inductor and noisy power traces as possible. It

is often a good idea to run the feedback trace

on the side of the PCB opposite of the inductor

with a ground plane separating the two.

To help to improve the thermal conduction, a

grid of thermal vias can be created right under

the exposed pad. It is recommended that they

be small (15mil barrel diameter) so that the hole

is essentially filled up during the plating

process, thus aiding conduction to the other

side. Too large a hole can cause ‘solder

wicking’ problems during the reflow soldering

process. The pitch (distance between the

centers) of several such thermal vias in an area

is typically 40mil.