MP1567 – 1.2A SYNCHRONOUS RECTIFIED STEP-DOWN CONVERTER

MP1567 Rev. 2.3

www.MonolithicPower.com

6

1/3/2006

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© 2006 MPS. All Rights Reserved.

TM

APPLICATION INFORMATION

COMPONENT SELECTION

Internal Low-Dropout Regulator

The internal power to the MP1567 is supplied

from the input voltage through an internal 2.4V

low-dropout linear regulator, whose output is

BP. Bypass BP to SGND with a 10nF or greater

capacitor to insure the MP1567 operates

properly. The internal regulator cannot supply

more current than is required to operate the

MP1567, therefore do not apply any external

load to BP.

Soft-Start

The MP1567 includes a soft-start timer that

slowly ramps the output voltage at startup to

prevent excessive current at the input. This

prevents premature termination of the battery

voltage at startup due to input current overshoot

at startup.

When power is applied to the MP1567 a 2µA

internal current source charges the external

capacitor at SS. As the capacitor charges, the

voltage at SS will rise. The MP1567 internally

limits the feedback threshold voltage at FB to

that of the voltage at SS. This forces the output

voltage to rise at the same rate as the voltage

at SS, forcing the output voltage to ramp

linearly from 0V to the desired regulation

voltage during soft-start. The soft-start period is

determined by the equation:

5

C

45

.

0

×

=

Where C5 (in nF) is the soft-start capacitor from

period. Determine the capacitor required for a

given soft-start period by the equation:

SS

t

22

.

2

5

C

×

=

Use values for C5 between 10nF and 22nF to

set the soft-start period (between 4ms and

10ms).

Setting the Output Voltage

Set the output voltage by selecting the resistive

voltage divider ratio. The voltage divider drops

the output voltage to the 0.9V feedback

threshold voltage. Use 10kΩ for the low-side

resistor of the voltage divider. Determine the

high side resistor by the equation:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

=

1

R

V

V

V

2

R

FB

FB

OUT

Where R2 is the high-side resistor, R1 is the

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous, so a capacitor is required to

supply the AC current to the step-down

converter while maintaining the DC input

voltage. A low ESR capacitor is required to

keep the noise at the IC to a minimum. Ceramic

capacitors are preferred, but tantalum or low

ESR electrolytic capacitors will also suffice.

Use an input capacitor with a value greater than

10µF. The capacitor can be electrolytic,

tantalum or ceramic. However, since it absorbs

the input switching current it requires an

adequate ripple current rating. Use a capacitor

with a RMS current rating greater than 1/2 of

the DC load current.

For insuring stable operation, place the input

capacitor as close to the IC as possible.

Alternately, a smaller high quality 0.1µF

ceramic capacitor may be placed closer to the

IC with the larger capacitor placed further away.

If using this technique, it is recommended that

the larger capacitor be a tantalum or electrolytic

type. All ceramic capacitors should be placed

close to the MP1567.

Selecting the Output Capacitor

The output capacitor is required to maintain the

DC output voltage. Low ESR capacitors are

preferred to keep the output voltage ripple to a

minimum. The characteristics of the output

capacitor also affect the stability of the

regulation control system. Ceramic, tantalum or

low

ESR

electrolytic

capacitors

are

recommended.

In the case of ceramic capacitors, the

impedance at the switching frequency is

dominated by the capacitance, and so the