MP4459 – 1.5A, 4MHz, 36V STEP-DOWN CONVERTER

MP4459 Rev. 1.02

www.MonolithicPower.com

15

12/24/2013

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 MP4459 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 MP4459. 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.

External Bootstrap Diode

It is recommended that an external bootstrap

diode be added when the input voltage is no

greater than 5V or the 5V rail is available in the

system. This helps improve the efficiency of the

regulator. The bootstrap diode can be a low

cost one such as IN4148 or BAT54.

MP4459

SW

BS

5V

Figure 3—External Bootstrap Diode

This diode is also recommended for high duty

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 BS voltage. In

order to have enough gate voltage under such

should be greater than 3V. For example, if the

output voltage is set to 3.3V, the input voltage

needs to be higher than 3.3V+3V=6.3V to

maintain enough BS voltage at no load or light

loads. To meet this requirement, the EN pin can

be used to program the input UVLO voltage to