MP8869SGL datasheet(18/39 Pages) MPS

18 / 39 page

MP8869S

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

MP8869S Rev. 1.03

www.MonolithicPower.com

1/18/2018

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

OPERATION

Pulse-Width Modulation (PWM) Operation

The

MP8869S

fully

integrated,

synchronous, rectified, step-down, switch-mode

converter. The MP8869S 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 below the error amplifier

output

voltage (VEAO),

which

indicates

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 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 on at the same time, causing a

dead short between input and GND, reducing

efficiency dramatically. The MP8869S 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 MP8869S enters either heavy-load

operation or light-load operation depending on

the amplitude of the output current.

VOUT

Cout

RESR

On Timer

Logic

Control

REF

RAMP

FB/VOUT

driver

VIN

Figure 3: Simplified Compensation Block

Switching Frequency

The MP8869S uses constant-on-time (COT)

control, and 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 MP8869S

’s switching frequency can be

adjusted by setting the two bits D[5:4] in

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 switching frequency decreases. Table 1

shows the maximum switching frequency vs.

the output voltage when VIN = 12V and VIN =

5V.

Table 1: Maximum Frequency Selecting vs.

Output Voltage

Maximum Frequency Selecting

Vo (V)

VIN = 12V

VIN = 5V

1.25MHz

3.3

1.25MHz

2.5

1.25MHz

1.8

1.25MHz

1.5

1.25MHz

1.2

1MHz

1.25MHz

750kHz

1.25MHz

0.9

750kHz

1.25MHz

0.6

500kHz

1.25MHz

Forced PWM Operation

When the MP8869S works in forced pulse-

width modulation (PWM) mode, the MP8869S

enters continuous conduction mode (CCM),

where the HS-FET and LS-FET repeat the

on/off operation, even if the inductor current is

zero

a negative value.

The switching

frequency (fSW) is fairly constant (see Figure 4).

Figure 4: Forced PWM Operation

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