MP4458 – 1A, 4MHz, 36V STEP-DOWN CONVERTER

MP4458 Rev. 1.02

www.MonolithicPower.com

14

3/4/2013

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2013 MPS. All Rights Reserved.

Since the internal bootstrap circuitry has higher

impedance, which may not be adequate to

charge the bootstrap capacitor during each

charging period, an external bootstrap charging

diode is strongly recommended if the switching

frequency is above 2MHz (see External

Bootstrap

Diode

section

for

detailed

implementation information).

With higher switching frequencies, the inductive

reactance (XL) of a capacitor dominates, such

that the ESL of the input/output capacitor

determines the input/output ripple voltage at

higher switching frequencies. As a result, high

frequency ceramic capacitors are strongly

recommended as input decoupling capacitors

and output filtering capacitors.

Layout becomes more important when the

device switches at higher frequency. It is

essential

to

place

the

input

decoupling

capacitor, catch diode and the MP4458 as

close together as possible, with traces that are

very short and fairly wide. This can help to

greatly reduce the voltage spikes on SW and

also lower the EMI noise level.

Try to run the feedback trace as far from the

inductor and noisy power traces as possible. It

is 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

close to the MP4458. Do not place the

compensation components close to or under

the 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 these

nodes. Switching losses are expected to

increase at high switching frequencies. To help

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.

PC Board Layout

The high current paths (GND, IN and SW)

should be placed very close to the device with

short, direct and wide traces. The input

capacitor needs to be as close as possible to

the IN and GND pins. The external feedback

resistors should be placed next to the FB pin.

Keep the switch node traces short and away

from

the

feedback

resistor

divider

and

compensation network.

External Bootstrap Diode

An external bootstrap diode may enhance the

efficiency of the regulator, the applicable

conditions of external BST diode are:

Duty cycle is high: D=

IN

OUT

V

V

>65%

In these cases, an external BST diode is

recommended from the output of the voltage

regulator to BST pin, as shown in Figure 3

MP4458

SW

BST

C

L

BST

C

5V or 3.3V

OUT

External BST Diode

IN4148

+

Figure 3—Add Optional External Bootstrap

Diode to Enhance Efficiency

The recommended external BST diode is

IN4148, and the BST cap is 0.1~1µF.