NB669, 24V, HIGH CURRENT SYNCHRONOUS BUCK CONVERTER WITH LDO

NB669 Rev. 1.01

www.MonolithicPower.com

5

7/23/2013

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PIN FUNCTIONS

PIN #

Name

Description

1

VIN

Supply Voltage. The VIN pin supplies power for internal MOSFET and regulator. The

NB669 operates from a +6.5V to +24V input rail. An input capacitor is needed to

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

connection.

2

PGND

Power Ground. Use wide PCB traces and multiple vias to make the connection.

3

NC

Not connected.

4

PG

Power good output. The output of this pin is an open drain signal and is high if the

output voltage is higher than 95% of the nominal voltage. There is a delay from Vout

≥ 95% to PGOOD goes high.

5

CLK

300kHZ CLK output to drive the external charge pump

6

LDO

Internal 5V LDO output. Decouple with a minimum 4.7µF ceramic capacitor as close

to the pin as possible. X7R or X5R grade dielectric ceramic capacitors are

recommended for their stable temperature characteristics.

Once the output voltage of the Buck regulator is ready, it will switch over the LDO

output to save the power loss.

7

VOUT

Output voltage sense. For the NB669, the output of the Buck regulator is fixed to 5V.

VOUT pin is used to sense the output voltage of the Buck regulator, connect this pin

to the output capacitor of the regulator directly. This pin also acts as the input of the

5V LDO switch over power input.

Keep the VOUT sensing trace far away from the SW node. Vias should also be

avoided on the VOUT sensing trace.

8, 9

Exposed Pad

15, 16

SW

Switch Output. Connect this pin to the inductor and bootstrap capacitor. This pin 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 the SW pin 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.

10

BST

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

floating supply across the high-side switch driver.

11

VCC

Internal 5V LDO output. The driver and control circuits are powered from this

voltage. Decouple with a minimum 1µF ceramic capacitor as close to the pin as

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

stable temperature characteristics.

12

ENLDO

100mA LDO and VCC enable pin. ENLDO is internally pulled up to high. Leave this

pin open to enable the LDO. Drive it low to turn off all the regulators.

13

EN

Buck regulator and charge pump clock enable pin. EN is a digital input that turns the

Buck regulator and CLK on or off. When the power supply of the control circuit is

ready, drive EN high to turn on the Buck regulator and charge pump clock, drive it

low to turn them off.

14

AGND

Analog ground. The internal reference is referred to AGND.