MP8861 – 2.85V - 18V, 6A, SYNCHRONOUS, STEP-DOWN CONVERTER

MP8861 Rev. 1.1

www.MonolithicPower.com

15

5/28/2020

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OPERATION

Pulse-Width Modulation (PWM) Operation

The MP8861 is a fully integrated, synchronous,

rectified, step-down, switch-mode converter.

The MP8861 uses constant-on-time (COT)

control to provide fast transient response and

ease loop stabilization. Figure 3 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 the HS-FET on and off,

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 and reducing efficiency dramatically. The

MP8861 prevents shoot-through by generating

a dead time (DT) internally between the HS-

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

off and HS-FET on period. The MP8861 enters

either

heavy-load

operation

or

light-load

operation depending on the amplitude of the

output current.

Figure 3: Simplified Compensation Block

Switching Frequency

The MP8861 uses constant-on-time (COT)

control, so there is no dedicated oscillator in the

IC. The input voltage is fed into the one-shot

on-timer through the internal frequency resistor.

The duty ratio is VOUT/VIN, and the switching

frequency is fairly constant over the input

voltage

range.

The

MP8861’s

switching

frequency can be adjusted by setting the two

bits D[5:4] in register 02 through the I2C

communication. When the output voltage

setting is low and the input voltage is high, the

switching on-time may be limited by the internal

minimum on-time limit, and the switching

frequency decreases. Table 1 shows the

maximum switching frequency vs. the output

voltage when VIN = 12V and VIN = 5V.

Table 1: Maximum Frequency Selection vs.

Output Voltage

Maximum Frequency Selection

Vo (V)

VIN = 12V

VIN = 5V

5

1.25MHz

/

3.3

1.25MHz

1.25MHz

2.5

1.25MHz

1.25MHz

1.8

1.25MHz

1.25MHz

1.5

1.25MHz

1.25MHz

1.2

1MHz

1.25MHz

1

750kHz

1.25MHz

0.9

750kHz

1.25MHz

0.6

500kHz

1.25MHz

Forced PWM Operation

When the MP8861 works in forced pulse-width

modulation (PWM) mode, the MP8861 enters

continuous conduction mode (CCM), where the

HS-FET

and

LS-FET

repeat

the

on/off

operation, even if the inductor current is zero or

a negative value. The switching frequency (FSW)

is fairly constant. Figure 4 shows the timing

diagram during this operation.

Whenever V

RAMP drops

below V

EAO, the HS-FET

is turned ON

V

IN

T

ON is constant

V

RAMP

V

EAO

I

OUT

V

SW

HS-FET

Driver

LS-FET

Driver

I

L

Figure 4: Forced PWM Operation

Light-Load Operation

When the MP8861 works in auto-PWM or auto-

pulse-frequency modulation (PFM) mode in

light-load operation, the MP8861 reduces the

switching frequency automatically to maintain

high efficiency, and the inductor current drops