MP28128 – 2.5A, 1.5MHz SYNCHRONOUS STEP-DOWN CONVERTER

MP28128 Rev. 0.92

www.MonolithicPower.com

7

2/13/2009

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© 2009 MPS. All Rights Reserved.

FUNCTIONAL DESCRIPTION

PWM Control

The MP28128 is a constant frequency peak-

current-mode control PWM switching regulator.

Refer to the functional block diagram. The high side

N-Channel DMOS power switch turns on at the

beginning of each clock cycle. The current in the

inductor

increases

until

the

PWM

current

comparator trips to turn off the high side DMOS

switch. The peak inductor current at which the

current comparator shuts off the high side power

switch is controlled by the COMP voltage at the

output

of

feedback

error

amplifier.

The

transconductance from the COMP voltage to the

output current is set at 11.25A/V.

This current-mode control greatly simplifies the

feedback compensation design by approximating

the switching converter as a single-pole system.

Only Type II compensation network is needed,

which is integrated into the MP28128. The loop

bandwidth is adjusted by changing the upper

resistor value of the resistor divider at the FB pin.

The internal compensation in the MP28128

simplifies the compensation design, minimizes

external component counts, and keeps the flexibility

of external compensation for optimal stability and

transient response.

Enable and Frequency Synchronization

(EN/SYNC PIN)

This is a dual function input pin. Forcing this pin

below 0.4V for larger than 4us shuts down the part;

forcing this pin above 1.6V for larger than 4µs turns

on the part. Applying a 1MHz to 2MHz clock signal

to this pin also synchronizes the internal oscillator

frequency to the external clock. When the external

clock is used, the part turns on after detecting the

first few clocks regardless of duty cycles. If any ON

or OFF period of the clock is longer than 4µs, the

signal will be intercepted as an enable input and

disables the synchronization.

Soft-Start and Output Pre-Bias Startup

When the soft-start period starts, an internal current

source begins charging an internal soft-start

capacitor. During soft-start, the voltage on the soft-

start capacitor is connected to the non-inverting

input of the error amplifier. The soft-start period

lasts until the voltage on the soft-start capacitor

exceeds the reference voltage of 0.8V. At this point

the reference voltage takes over at the non-

inverting error amplifier input. The soft-start time is

internally set at 120µs. If the output of the MP28128

is pre-biased to a certain voltage during startup, the

IC will disable the switching of both high-side and

low-side switches. until the voltage on the internal

soft-start capacitor exceeds the sensed output

voltage at the FB pin.

Over current Protection

The MP28128 offers cycle-to-cycle current limiting

for both high-side and low-side switches. The high-

side current limit is relatively constant regardless of

duty cycles. When the output is shorted to ground,

causing the output voltage to drop below 70% of its

nominal output, the IC is shut down momentarily

and begins discharging the soft start capacitor. It

will restart with a full soft-start when the soft- start

capacitor is fully discharged. This hiccup process is

repeated until the fault is removed.

Power Good Output (POK PIN)

The MP28128 includes an open-drain Power Good

output that indicates whether the regulator output is

within ±10% of its nominal output. When the output

voltage moves outside this range, the POK output

is pulled to ground. There is a 30µs deglitch time

when the POK output change its state.

Bootstrap (BST PIN)

The gate driver for the high-side N-channel DMOS

power switch is supplied by a bootstrap capacitor

connected between the BS and SW pins. When the

low-side switch is on, the capacitor is charged

through an internal boost diode. When the high-side

switch is off and the low-side switch turns on, the

voltage on the bootstrap capacitor is boosted above

the input voltage and the internal boost-strap diode

prevents the capacitor from discharging.