MPQ4458 –INDUSTRIAL GRADE,1A, 4MHz, 36V STEP-DOWN CONVERTER

MPQ4458 Rev. 1.0

www.MonolithicPower.com

14

12/5/2012

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Try to run the feedback trace as far from the

inductor and noisy power traces as possible. It

is 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 MPQ4458. 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

MPQ4458

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.