MPQ28261 – 21V, 3A, 500kHz SYNCHRONOUS STEP-DOWN CONVERTER

MPQ28261 Rev. 1.12

www.MonolithicPower.com

5

10/10/2013

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PIN FUNCTIONS

QFN14

(3x4mm)

Pin #

Name

Description

1

IN

Supply Voltage. Supplies power for the internal MOSFET and regulator. The MPQ28261

operates from a 4.5V-to-21V input rail. Requires a low-ESR and low-inductance

capacitor (C1) to decouple the input rail. Place the input capacitor close to this pin and

connect it with wide PCB traces and multiple vias.

2,3,4,5

SW

Switch Output. Connect to the inductor and bootstrap capacitor. The internal high-side

inductor current drives the SW pins negative during the off-time. The on-resistance of the

low-side switch and the internal Schottky diode fixes the negative voltage. Connect using

wide PCB traces and multiple vias.

6

BST

Bootstrap. Use a capacitor connected between SW and BST pins to form a floating

supply across the high-side switch driver.

7

EN

Enable. Connected internally to a 1MΩ pull-down resistor. EN=1 to enable the

MPQ28261. Pull EN low to disable the part.

8

FB

Feedback. Connect to the tap of an external resistor divider from the output to GND to

set the output voltage. The frequency fold-back comparator lowers the oscillator

frequency when the FB voltage is below 100mV to prevent current-limit runaway during a

short circuit fault condition. Place the resistor divider as close to FB pin as possible.

Avoid vias on the FB traces.

9

PG

Power-Good Output. Active high. Pin output goes to open drain of an internal switch.

Power-good low-to-high threshold is 90% of regulation value. There is delay from FB ≥

90% to PG high during start up, which is about 1/4 of SS time. Power-good high-to-low

threshold is 80% of regulation value.

10

SS

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

mode regulator. The soft-start pin connects to an internal reference source and to the

non-inverting input of an error amplifier. When the soft-start period begins, an internal

10µA current source begins charging the capacitor. The soft-start voltage continues to

rise until the voltage exceeds the reference voltage of 0.6V.

11

VCC

Bias Supply. Typical level of around 5.1V. Decouple with a 0.1μF-up-to-0.22μF capacitor.

12,13,

Exposed

Pad

GND

System Ground. Reference ground for the regulated output voltage. Requires special

care for PCB layout. Connect exposed pad to GND plane for optimal thermal

performance.

14

AGND

Signal Ground. AGND is not internally connected to System Ground—connect to System

Ground on PCB.