MP1567 – 1.2A SYNCHRONOUS RECTIFIED STEP-DOWN CONVERTER

MP1567 Rev. 2.3

www.MonolithicPower.com

4

1/3/2006

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2006 MPS. All Rights Reserved.

TM

PIN FUNCTIONS

Pin#

Name

Function

1

BS

Power Switch Boost. BS powers the gate of the high-side N-Channel power MOSFET switch.

Connect a 10nF or greater capacitor between BS and SW.

2

IN

Internal Power Input. IN supplies the power to the MP1567 through the internal LDO

regulator. Bypass IN to PGND with a 10µF or greater capacitor. Connect IN to the input

source voltage.

3

SW

Output Switching Node. SW is the source of the high-side N-Channel switch and the drain of

the low-side N-Channel switch. Connect the output LC filter between SW and the output.

4

PGND

Power Ground. PGND is the source of the N-Channel MOSFET synchronous rectifier.

Connect PGND to SGND as close to the MP1567 as possible.

5

SGND

Signal Ground.

6

SS

Soft-Start Input. Place a capacitor from SS to SGND to set the soft-start period. The MP1567

sources 2µA from SS to the soft-start capacitor at start up. As the voltage at SS rises, the

feedback threshold voltage increases to limit inrush current at start up.

7

FB

Feedback Input. FB is the inverting input of the internal error amplifier. Connect a resistive

voltage divider from the output voltage to FB to set the output voltage.

8

COMP

Compensation Node. COMP is the output of the error amplifier. Connect a series RC network

to compensate the regulation control loop.

9

BP

Internal 2.4V Regulator Bypass. Connect a 10nF capacitor between BP and SGND to bypass

the internal regulator. Do not apply any load to BP.

10

EN

On/Off Control Input. Drive EN high to turn on the MP1567; low to turn it off. For automatic

startup, connect EN to IN.

OPERATION

The MP1567 measures the output voltage

through an external resistive voltage divider and

compares that to the internal 0.9V reference to

generate the error voltage at COMP. The

current-mode regulator uses the voltage at

COMP and compares it to the inductor current

to regulate the output voltage. The use of

current-mode regulation improves transient

response and improves control loop stability.

At the beginning of each cycle, the high-side

N-Channel MOSFET is turned on, forcing the

inductor current to rise. The current at the drain

of

the

high-side

MOSFET

is

internally

measured and converted to a voltage by the

current sense amplifier. That voltage is

compared to the error voltage at COMP. When

the inductor current raises sufficiently, the PWM

comparator turns off the high-side switch and

turns on the low-side switch, forcing the

inductor current to decrease. The average

inductor current is controlled by the voltage at

COMP, which in turn, is controlled by the output

voltage. Thus the output voltage controls the

inductor current to satisfy the load.

Since the high-side N-Channel MOSFET

bootstrap capacitor from SW to BS is required

to drive the high-side MOSFET gate. When SW

is driven low (through the low-side MOSFET),

the BS capacitor is internally charged. The

voltage at BS is applied to the high-side

MOSFET gate to turn it on, and maintains that

voltage until the high-side MOSFET is turned

off and the low-side MOSFET is turned on, and

the cycle repeats. Connect a 10nF or greater

capacitor from BS to SW to drive the high-side

MOSFET gate. Using a larger capacitor does

little to improve performance.