MP8712 – 18V, 12A, SYNCHRONOUS STEP-DOWN CONVERTER

MP8712 Rev. 1.2

MonolithicPower.com

15

3/19/2021

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OPERATION

PWM Operation

The MP8712 is a fully integrated, synchronous,

rectified, step-down, switch-mode converter.

The MP8712 uses constant-on-time (COT)

control to provide fast transient response and

ease loop stabilization. Figure 2 shows the

simplified ramp compensation block. At the

beginning of each cycle, the high-side MOSFET

(HS-FET) turns on whenever the ramp voltage

(VRAMP) is lower than the error amplifier output

voltage (VEAO), 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 enters

the off state. By cycling HS-FET between the

on and off states, the converter regulates the

output

voltage.

The

integrated

low-side

MOSFET (LS-FET) turns on when the HS-FET

is in its off state to minimize conduction loss.

Shoot-through occurs when both the HS-FET

and LS-FET are turned on at the same time,

causing a dead short between the input and

GND.

Shoot-through

reduces

efficiency

dramatically, so the MP8712 prevents this by

generating a dead time (DT) internally between

the HS-FET off and LS-FET on time and the

LS-FET off and HS-FET on time. The MP8712

enters either heavy-load operation or light-load

operation depending on the amplitude of the

output current.

Figure 2: Simplified Compensation Block

Switching Frequency

The MP8712 uses COT control. There is no

dedicated oscillator in the IC. The input voltage

is forward fed to the one-shot on-timer through

the internal frequency resistor. The duty ratio is

kept as VOUT/VIN, and the switching frequency

is a fairly constant 500kHz over the input

voltage range.

Light-Load Operation

When the MP8712 works in light-load operation,

the MP8712 reduces the switching frequency

automatically to maintain high efficiency, and

the inductor current drops almost to zero. When

the inductor current reaches zero, the LS-FET

driver goes into tri-state (Hi-Z) (see Figure 3).

The output capacitors discharge slowly to GND

through R1 and R2. This operation improves

device efficiency greatly when the output

current is low.

Figure 3: Light-Load Operation

Light-load operation is also called skip mode

since the HS-FET does not turn on as

frequently as it does during heavy-load

condition. The HS-FET turn-on frequency is a

function of the output current. As the output

current

increases,

the

current

modulator

regulation time period becomes shorter, and the

HS-FET turns on more frequently. The switching

frequency increases as well. The output current

reaches critical levels when the current

modulator time is zero and can be determined

with Equation (1):

IN

OUT

OUT

OUT

SW

IN

(V

V

) V

I

2L F

V





 

(1)

The MP8712 enters pulse-width modulation

(PWM) mode once the output current exceeds

the critical level. Afterward, the switching

frequency remains fairly constant over the

output current range.

Operating without an External Ramp

The

traditional

COT

control

scheme

is

intrinsically unstable if the output capacitor’s