MP1527DM datasheet(8/14 Pages) MPS

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MP1527

2A, 1.3MHz

Step-Up Converter

MP1527 Rev 1.8_8/31/05

Monolithic Power Systems, Inc.

Monolithic Power Systems

Functional Description

The MP1527 uses a 1.3MHz fixed-frequency,

current-mode

regulation

architecture

regulate the output voltage. The MP1527

measures the output voltage through an

external resistive voltage divider and compares

that to the internal 1.22V reference to generate

the error voltage at COMP. The current-mode

regulator compares voltage at the COMP pin

to the inductor current to regulate the output

voltage. The use of current-mode regulation

improves transient response and control loop

stability.

At the beginning of each cycle, the n-channel

MOSFET switch is turned on, forcing the

inductor current to rise. The current at the

source of the switch 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 rises sufficiently, the PWM comparator

turns off the switch forcing the inductor current

to the output capacitor through the external

rectifier. This forces the inductor current to

decrease. The peak 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.

Internal Low-Dropout Regulator

The internal power to the MP1527 is supplied

from the input voltage (IN) 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 MP1527

operates properly. The internal regulator can

not supply any more current than is required to

operate the MP1527, therefore do not apply

any external load to BP.

Soft-Start

The MP1527 includes a soft-start timer that

limits the voltage at COMP during start-up to

prevent excessive current at the input. This

prevents premature termination of the source

voltage at startup due to input current

overshoot at startup. When power is applied to

the MP1527, or with power applied when

enable is asserted, a 2µA internal current

source charges the external capacitor at SS.

As the capacitor charges, the voltage at SS

rises.

The MP1527 internally clamps the

voltage at COMP to 0.7V above the voltage at

SS. This limits the inductor current at start-up,

forcing the input current to rise slowly to the

current required to regulate the output voltage

during soft-start.

The soft-start period is determined by the

equation:

tSS = 2.75 *10

5 * CSS

Where CSS (in F) is the soft-start capacitor from

SS to SGND, and tSS (in seconds) is the soft-

start period.

Determine the capacitor required for a given

soft-start period by the equation:

CSS = 3.64 *10

-6 * tSS

Use values for CSS between 10nF and 22nF to

set the soft-start period.

Fault Timer-Latch Function

The MP1527 includes an output fault detector

and timer-latch circuitry to disable the regulator

in the event of an undervoltage, overcurrent, or

thermal overload.

Once the soft-start is

complete, the fault comparator monitors the

voltage at FB. If the voltage falls below the

1.098V fault threshold, the capacitor at SS

charges through an internal 2µA current

source.

If the fault condition remains long

enough for the capacitor at SS to charge to

1.2V, the FAULT output is pulled low and the

power switch is turned off, disabling the output.

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