MP1584 – 3A, 1.5MHz, 28V STEP-DOWN CONVERTER

MP1584 Rev. 1.0

www.MonolithicPower.com

14

8/8/2011

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High Frequency Operation

The switching frequency of MP1584 can be

programmed up to 1.5MHz with an external

resistor.

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 MP1584 (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 MP1584. 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.

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.

MP1584

SW

BS

5V

Figure 2—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.