MP2227 – 24V, 3A, 1.3MHz SYNCHRONOUS STEP-DOWN CONVERTER

MP2227 Rev. 1.1

www.MonolithicPower.com

9

11/21/2011

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© 2011 MPS. All Rights Reserved.

FUNCTIONAL DESCRIPTION

PWM Control

The MP2227 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

MP2227. The loop bandwidth is adjusted by

changing the upper resistor value of the resistor

divider at the FB pin. The internal compensation

in the MP2227 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 longer than 4us shuts down the

part; forcing this pin above 1.6V for longer 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 MP2227 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 MP2227 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.

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 Low-side switch is off and the

high-side switch turns on, the voltage on the

bootstrap capacitor is boosted above the input

voltage and the internal bootstrap diode

prevents the capacitor from discharging.

No external bootstrap diode is required for

typical applications. For applications with low

input VCC voltage or where output voltage is

very close to input voltage, an external Schottky

diode may be connected from the VCC to BS

pins to charge the bootstrapped capacitor more

strongly for increased gate drive voltage. When

using the external bootstrap diode, a resistor at

the regulator output or a minimal load current

may be required as the bootstrapped capacitor