MP8869 datasheet(17/37 Pages) MPS

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MP8869

– 17V, 12A, SYNCHRONOUS STEP-DOWN CONVERTER

MP8869 Rev.1.0

www.MonolithicPower.com

6/8/2016

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

OPERATION

PWM Operation

The MP8869 is a fully integrated, synchronous,

rectified, step-down, switch-mode converter.

The MP9969 uses constant-on-time (COT)

control to provide fast transient response and

easy 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 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 turned on at the same time,

causing a dead short between input and GND

and

reducing

efficiency

dramatically.

The

MP8869 prevents this by generating a dead-

time (DT) internally between when the HS-FET

is off and the LS-FET is on, and when the LS-

FET is off and the HS-FET is on. The device

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 2: Simplified Compensation Block

Switching Frequency

The MP8869 uses constant-on-time (COT)

control, so there is no dedicated oscillator in the

IC. The input voltage is fed into the on-time

one-shot 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 MP8869

’s switching frequency can be

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

2C 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 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 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 MP8869 works in forced PWM mode,

the MP8869 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 3 shows the timing diagram during this

operation.

Figure 3: Forced PWM Operation

MP8869 Datasheet(PDF) 17 Page - Monolithic Power Systems

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