NB621 – 8A, 25V, 600kHz SYNCHRONOUS STEP-DOWN CONVERTER WITH INTEGRATED LDO

NB621 Rev. 0.9

www.MonolithicPower.com

9

12/10/2013

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PIN FUNCTIONS

Pin #

Name

Description

1

BST

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

floating supply across the high-side switch driver.

2, 3, 4, 5

SW

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

6, 9, 24, 27, 28

N/C

No Connection.

7

GND

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

For this reason care must be taken in PCB layout.

8

SDRV

Low Side Gate Driver Output.

10

AGND

Analog Ground.

11

VCC

Bias Supply. Decouple with 1µF capacitor.

12

AAM

Connect to GND forces NB621 in synchronous mode. Connects to a voltage set by

2 resistor dividers forces NB621 into non-synchronous mode when load is small.

13

SS

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

switch mode regulator.

14

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 400mV.

15

PG

Power good output, the output of this pin is an open drain and is high if the output

voltage is higher than 90% of the nominal voltage. There is a 20µs delay to pull PG

if the output voltage is lower than 10% of regulation value.

16

EN/SYNC

EN=1 to enable the NB621. External clock can be applied to EN pin for changing

switching frequency. For automatic start-up, connect EN pin to VIN with 100KΩ

resistor.

17,18,19,20,21

IN

Supply Voltage. The NB621 operates from a +4.5V to +25V input rail. C1 is

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

the connection.

22

SELH

Internal LDO output voltage selection. When grounded, the LDO output will be

regulated to 5V. When pulled high, the LDO output will be regulated to 3.3V.

23

ENLDO

Internal LDO regulator enable pin

25

LDO

Internal LDO regulator Output. Connect a 10uF decoupling capacitor to GND.

26

REGO

Switch Mode Regulator Output connection pin. This pin is used to switch the

internal LDO output to the switch mode regulator output when it is available.