MP28256 – 21V, 3A, 500kHz SYNCHRONOUS STEP-DOWN CONVERTER

MP28256 Rev. 1.0

www.MonolithicPower.com

7

11/3/2011

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PIN FUNCTIONS

QFN14

(3x4mm)

Pin #

Name

Description

1

IN

Supply Voltage. The MP28256 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 BST pins is required to form a

floating supply across the high-side switch driver.

7

SYNC

This pin serves as frequency synchronous clock input.

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 100mV.

9

EN

EN=1 to enable the MP28256. For automatic start-up, connect EN pin to VIN with

100kHz resistor. There is an internal 1mΩ pull-down resistor at EN pin.

10

SS

Soft Start. Connect an external capacitor to program the soft start time for the switch

mode regulator.

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.