MP4561 – 1.5A, 2MHz, 55V STEP-DOWN CONVERTER

MP4561 Rev. 1.0

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14

11/5/2012

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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.

The compensation components should be placed

closed to the MP4561. Do not place the

compensation components close to or under high

dv/dt SW node, or inside the high di/dt power

loop. If you have to do so, the proper ground

plane must be in place to isolate them.

Switching loss is expected to be increased at

high switching frequency. 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.

External Bootstrap Diode

An external bootstrap diode may enhance the

efficiency of the regulator. 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

cycle operation (when VOUT / VIN > 65%)

applications.

The bootstrap diode can be a low cost one such

as IN4148 or BAT54.

MP4561

SW

BST

5V

Figure 3—External Bootstrap Diode

At no load or light load, the converter may

operate in pulse skipping mode in order to

maintain the output voltage in regulation. Thus

there is less time to refresh the BST voltage. In

order to have enough gate voltage under such

operating conditions, the difference of (VIN –

VOUT) should be greater than 3V. For example,

if the VOUT is set to 3.3V, the VIN needs to be

higher than 3.3V+3V=6.3V to maintain enough

BS voltage at no load or light load. To meet this

requirement, EN pin can be used to program the

input UVLO voltage to Vout+3V.