NB679

— 28V VIN, FIXED 5V-8A BUCK CONVERTER WITH LDO

NB679 Rev. 1.03

www.MonolithicPower.com

6

7/27/2017

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PIN FUNCTIONS

NB679

PIN #

Name

Description

1

VIN

Supply voltage. VIN supplies power for the internal MOSFET and regulator. The NB679

operates from a 5.5 V to 24 V input rail. An input capacitor is needed to decouple the input

rail. Use wide PCB traces and multiple vias to make the connection. Apply at least two

layers for this input trace.

2

PGND

Power ground. Use wide PCB traces and enough vias to handle the load current to make

the connection. Make the PGND trace to the Vin decoupling capacitor as wide as

possible.

3

PG

Power good output. The output of PG is an open-drain signal. It is high if the output

voltage is higher than 95 percent of the nominal voltage or lower than 105 percent of the

nominal voltage.

4

NC

No connection.

5

VOUT

VOUT is used to sense the output voltage of the buck regulator. Connect VOUT to

the output capacitor of the regulator directly. Also, VOUT acts as the input of the internal

LDO switch over power input. Keep the VOUT sensing trace far away from the SW node.

Avoid vias on the VOUT sensing trace. A >25 mil trace is required.

6

LDO

Internal LDO output. Decouple with a minimum 4.7 µF ceramic capacitor as close to

LDO as possible. X7R or X5R grade dielectric ceramic capacitors are recommended for

their stable temperature characteristics. If ENLDO is high, it switches over to the LDO to

buck after PG is ok.

7

SW

Switch

output. Connect SW to the inductor and bootstrap capacitor. SW is driven up to

the VIN voltage by the high-side switch during the on-time of the PWM duty cycle. The

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

switch and the internal diode fixes the negative voltage. Use wide and short PCB traces to

make the connection. Try to minimize the area of the SW pattern.

8

BST

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

supply across the high-side switch driver.

9

VCC

Internal VCC LDO output. The driver and control circuits are powered from this voltage.

Decouple with a minimum 1 µF ceramic capacitor as close to VCC as possible. X7R or

X5R grade dielectric ceramic capacitors are recommended for their stable temperature

characteristics.

10

AGND

Signal logic ground. Kelvin connection to PGND.

11

EN

Buck enable. EN is a digital input that turns the buck regulator on or off. When the power

supply of the control circuit is ready, drive EN high to turn on the buck regulator, and drive

it low to turn off the buck regulator. Connect EN to 3V3 through a pull-up resistor or a

resistive voltage divider for automatic start-up. Do NOT float this pin. EN can be used to

set USM. When EN is in the range of 1.4 V to 1.8 V, it enters USM. If EN is in the range of

2.6V to 3.6V, it operates in normal mode.

12

ENLDO

LDO enable pin. ENLDO is pulled up internally to high. Leave ENLDO open to enable the

LDO. Drive it low to turn off the LDO.