MP4350 – 2.5A, 4MHz, 20V STEP-DOWN CONVERTER

MP4350 Rev. 1.0

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12

5/26/2008

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vs Switching Frequency

30

25

20

15

10

5

1500 2000 2500 3000 3500 4000

Since the internal bootstrap circuitry has higher

impedance, which may not be adequate to

charge the bootstrap capacitor during each

(1-D)×Ts 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

so that the ESL of input/output capacitor

determines the input/output ripple voltage at

higher switching frequency. As a result of that,

high frequency ceramic capacitor is strongly

recommended as input decoupling capacitor

and output filtering capacitor for such high

frequency operation.

Layout becomes more important when the

device switches at higher frequency. It is

essential

to

place

the

input

decoupling

capacitor, catch diode and the MP4350 (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. The

compensation components should be placed

closed to the MP4350.

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 those. 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. Please refer to the layout example on

EV4460 datasheet.

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.

MP4350

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

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.