MP4433 - 36V, 3A, LOW IQ, SYNCHRONOUS STEP-DOWN CONVERTER

MP4433 Rev. 1.01

www.MonolithicPower.com

18

1/12/2018

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OPERATION

The MP4433 is a high-frequency, synchronous,

rectified,

step-down,

switch-mode

converter

with integrated, internal, high-side and low-side

power MOSFETs. The MP4433 offers a very

compact

solution

that

achieves

3A

of

continuous output current with excellent load

and line regulation over a wide 3.3V to 36V

input supply range. The MP4433 features

switching

frequency

programmable

from

350kHz to 2.5MHz, external soft start, power

good indication, and precision current limit. Its

very low operational quiescent current makes it

suitable for battery-powered applications.

Pulse Width Modulation (PWM) Control

At moderate-to-high output current, the MP4433

operates in a fixed-frequency, peak-current-

control mode to regulate the output voltage. An

internal clock initiates a PWM cycle. At the

rising edge of the clock, the high-side power

MOSFET (HS-FET) is turned on, and the

inductor current rises linearly to provide energy

to the load. The HS-FET remains on until its

current reaches the value set by the COMP

internal error amplifier. If the current in the HS-

the HS-FET remains on, saving a turn-off

operation. When the HS-FET is off, it remains

off until the next clock cycle begins. The low-

side MOSFET (LS-FET) turns on immediately

while the inductor current flows through it. To

avoid a shoot-through, dead time is inserted to

prevent the HS-FET and LS-FET from turning

on at the same time. For each turn on and off in

a switching cycle, the HS-FET remains on and

off with a minimum on and off time limit.

Forced CCM Mode and AAM Mode

The MP4433 has selectable forced continuous

conduction

mode

(CCM)

and

advanced

asynchronous mode (AAM) (see Figure 3).

Drive SYNC above its specified threshold

before the chip starts up to force the device into

CCM with a fixed frequency, regardless of the

output load current. Once the device is in CCM,

SYNC can be pulled low again or driven with an

external clock if needed. The advantage of

CCM is a controllable frequency and smaller

output ripple, but it also has low efficiency at

light load.

Drive SYNC below its specified threshold or

leave SYNC floating before the chip starts up to

enable AAM power-save mode. The MP4433

first enters non-synchronous operation for as

long as the inductor current approaches zero at

light load. If the load is further decreased or is

mode, consuming very low quiescent current to

further improve light-load efficiency.

In sleep mode, the internal clock is blocked first,

and the MP4433 skips some pulses. Since the

reset, and the crossover time is taken as the

benchmark of the next clock. This control

scheme helps achieve high efficiency by scaling

down the frequency to reduce switching and

gate driver losses during light-load or no-load

conditions.

When the output current increases from light

switching frequency increases. If the DC value

resumes discontinuous conduction mode (DCM)

or CCM, which have a constant switching

frequency.

Forced CCM

Inductor

Current

t

t

t

Load

Decreased

AAM

Inductor

Current

t

t

t

Load

Decreased

Figure 3: Forced CCM and AAM

Error Amplifier (EA)

outputs a current proportional to the difference

between the two. This output current then

charges

or

discharges

the

internal

controls the power MOSFET current. The

optimized

internal

compensation

network

minimizes the external component counts and

simplifies the control loop design.