MPM3632C – SYNCHRONOUS, STEP-DOWN MODULE W/ INTEGRATED INDUCTOR

MPM3632C Rev. 1.03

www.MonolithicPower.com

13

9/10/2019

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OPERATION

Pulse-Width Modulation (PWM) Operation

The

MPM3632C

is

a

fully

integrated,

synchronous, rectified, step-down, switch-mode

converter. Constant-on-time (COT) control is

employed to provide fast transient response

and ease loop stabilization. At the beginning of

each cycle, the high-side MOSFET (HS-FET) is

turned on if the feedback voltage (VFB) is below

the reference voltage (VREF), which indicates an

insufficient output voltage. The on period is

determined by both the output voltage and input

voltage to make the switching frequency fairly

constant over the input voltage range.

After the on period elapses, the HS-FET turns

off and is turned on again when VFB drops

below VREF. By repeating operation in this way,

the converter regulates the output voltage. The

integrated low-side MOSFET (LS-FET) is

turned on when the HS-FET is off to minimize

conduction loss. There is a dead short between

the input and GND if both the HS-FET and LS-

FET are turned on at the same time. This is

called a shoot-through. To avoid shoot-through,

a dead time (DT) is generated internally

between the HS-FET off and LS-FET on period

or the LS-FET off and HS-FET on period.

Internal compensation is applied for COT

control to provide a more stable operation, even

when ceramic capacitors are used as output

capacitors.

This

internal

compensation

improves jitter performance without affecting

line or load regulation.

Regular-Load Operation

Continuous conduction mode (CCM) is when

the output current is high and the inductor

current is always above zero amps (see Figure

2). When VFB is below VREF - VDC_ERROR, the HS-

FET is turned on for a fixed interval determined

by the internal one-shot on-timer. When the HS-

FET is turned off, the LS-FET is turned on until

the next period. In CCM, the switching

frequency is fairly constant. This is called pulse-

width modulation (PWM) operation.

V

REF-DC_ERROR

V

REF

Figure 2: Heavy-Load Operation

DC Auto-Tune Loop

The MPM3632C applies a DC auto-tune loop to

balance the DC error between VFB and VREF by

adjusting the comparator input reference to

make VFB always follow VREF. This is a slow

loop and improves the load and line regulation

without affecting the transient performance. The

relationship between VFB, VREF, and REF is

shown in Figure 3.

Figure 3: DC Auto-Tune Loop Operation

Internal Regulator

A 3.3V internal regulator powers most of the

internal circuitries. When EN is high, this

regulator takes VIN and operates in the full VIN

range. When VIN is higher than 3.3V, the

output of the regulator is in full regulation. When

VIN is lower than 3.3V, the output voltage

decreases and follows the input voltage.

Enable Control (EN)

EN is a digital control pin that turns the

regulator on and off. Drive EN high to turn on

the regulator. Drive EN low to turn off the

regulator. EN is a high-voltage input node, so

connecting EN to the input can enable the part

to start-up automatically. EN can support an

18V input voltage.