MP28255 – 4A, 21V, SYNCHRONOUS STEP-DOWN CONVERTER WITH INTERNAL MOSFETS

MP28255 Rev. 1.01

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4

3/11/2011

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PIN FUNCTIONS

Pin #

Name

Description

1

IN

Supply Voltage. The MP28255 operates from a +4.5V to +21V input rail. C1 is

needed to decouple the input rail. Use wide PCB traces and multiple vias to make

the connection.

2,3,4,5

SW

Switch Output. Use wide PCB traces and multiple vias to make the connection.

6

BST

Bootstrap. A capacitor connected between SW and BS pins is required to form a

floating supply across the high-side switch driver.

7

EN/SYNC

EN=1 to enable the chip. External clock can be applied to EN pin for changing

switching frequency. For automatic start-up, connect EN pin to VIN by proper EN

resistor divider as Figure 2 shows.

8

FB

Feedback. An external resistor divider from the output to GND, tapped to the FB

pin, sets the output voltage. To prevent current limit run away during a short circuit

fault condition the frequency fold-back comparator lowers the oscillator frequency

when the FB voltage is below 500mV.

9

PG

Power Good Output, the output of this pin is open drain. Power good threshold is

90% low to high and 70% high to low of regulation value.

10,

SS

Soft-Start control input. SS controls the soft-start period. Connect a capacitor from

SS to Gnd to set the soft-start period.

11

VCC

Bias Supply. Decouple with 0.1µF~0.22µF cap. And the capacitance should be no

more than 0.22µF.

12,13

GND

System Ground. This pin is the reference ground of the regulated output voltage.

For this reason care must be taken in PCB layout.

14

AGND

Signal Ground. AGND is not internally connected to System Ground, make sure

AGND connected to system Ground in PCB layout.

Exposed

Pad

No Internal Connection. It is recommended to connect exposed pad to GND plane

for optimal thermal performance.