MPM3686 – 18V 20A STEP-DOWN POWER MODULE

MPM3686 Rev. 1.02

www.MonolithicPower.com

11

10/13/2016

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OPERATION

Power Module Operation

The MPM3686 is a high performance single

output synchronous switching mode DC-to-DC

power supply. It can deliver 20A continuous

output current. The MPM3686 can provide an

output voltage from 0.65V to 5V over a 4.5V to

18V (or 2.5V to 18V with 5V external VCC bias)

wide input voltage range.

The MPM3686 is a complete power solution. It

integrates a constant-on-time (COT) control DC-

to-DC regulator, power devices, an inductor,

input/output

capacitors

and

some

other

supporting resistors and small capacitors. It only

needs as few as one external resistor and one

external small capacitor to operate.

The MPM3686 is controlled by both the VCC

voltage and the EN signal. It can only be turned

on when both voltages are higher than the

thresholds.

The switching frequency is determined by a

frequency set resistor. The default switching

frequency with the integrated resistor is shown in

table

1.

The

default

switching

frequency

increases with the output voltage. The switching

frequency can also be programmed externally in

the range of 200KHz to 1000KHz. The details

can be found in the section of “SWITCHING

FREQUENCY SETTING” on Page 12.

The MPM3686 utilizes constant-on-time control.

It has sufficient stability margin with simple loop

compensation. And it provides very good

transient response with a wide range of output

capacitors,

even

with

all

ceramic

output

capacitors.

The MPM3686 has a variable soft start timer to

smooth-out the output voltage during start-up.

The default (with SS pin floating) soft start timer

is about 3ms. The soft-start time can be

extended by adding a capacitor between SS pin

and AGND pin.

PWM Operation

The MPM3686 uses Constant-on-time (COT)

control to provide a fast transient response and

ease loop stabilization.

At the beginning of each cycle, the high-side

MOSFET (HS-FET) turns ON when the feedback

voltage. The input voltage and the frequency-set

resistor determine the ON period as follows:

FREQ

ON

IN

6.1 R

(k )

T(ns)

V(V) 0.4







(1)

After the ON period elapses, the HS-FET turns

By repeating this operation, the converter

regulates the output voltage to the desired level.

The integrated low-side MOSFET (LS-FET) turns

on when the HS-FET is OFF to minimize the

conduction loss. There is a dead short (or shoot-

through) between input and GND if both HS-FET

and LS-FET turn on at the same time. A dead-

time (DT) internally generated between HS-FET

OFF and LS-FETON, or LS-FET OFF and HS-

FET ON avoids shoot-through.

Heavy-Load Operation

Figure 2—Heavy Load Operation

When the output current is high and the inductor

current is always above zero amps, it is called

continuous-conduction-mode (CCM). Figure 2

determined by the one-shot on-timer as per

equation 1. When the HS-FET turns off, the LS-

FET turns on until the next period.

In CCM operation, the switching frequency is

fairly constant and is also called PWM mode.