MP8642 — DUAL 3A, 23V, 600kHz STEP-DOWN CONVERTER

MP8642 Rev. 1.0

www.MonolithicPower.com

7

9/13/2012

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© 2012 MPS. All Rights Reserved.

OPERATION

IN1/IN2

EN/SYNC1/

EN/SYNC2

FB1/FB2

REGULATOR

OSCILLATOR

600KHz

DRIVER

CURRENT SENSE

AMPLIFIER

x40

ERROR

AMPLIFIER

CURRENT

LIMIT

COMPARATOR

PWM

COMPARATOR

DRIVER

REGULATOR

REFERENCE

--

+

--

+

--

+

S

R

R

Q

Q

C1

50pF

C1

1pF

POWER

GOOD

Figure 1—Functional Block Diagram (Diagram Portrays ½ of the MP8642)

The MP8642 is a dual channel, fixed frequency,

synchronous, step-down switching regulator with

integrated high-side power MOSFETs and gate

drivers for a low-side external MOSFETs. It

achieves 3A continuous output current for each

output over a wide input supply range with

excellent load and line regulation in each

channel. It provides a single highly efficient

solution with current mode control for fast loop

response and easy compensation.

The MP8642 operates in a fixed frequency, peak

current control mode to regulate the output

voltage. On each channel, a PWM cycle is

initiated by the internal clock. The integrated

high-side power MOSFET is turned on and

remains on until its current reaches the value set

by the COMP voltage. When the power switch is

off, it remains off until the next clock cycle starts.

If, in 90% of one PWM period, the current in the

power MOSFET does not reach the COMP set

current value, the power MOSFET will be forced

to turn off.

Error Amplifier

The error amplifier compares the FB pin voltage

with the internal 0.8V reference (REF) and outputs

a current proportional to the difference between the

two. This output current is then used to charge or

discharge the internal compensation network to

form the COMP voltage, which is used to control

the power MOSFET current. The optimized

internal compensation network minimizes the

external component counts and simplifies the

control loop design.

Internal Regulator

Most of the internal circuitries are powered from

the 5V internal regulator. This regulator takes the

VIN input and operates in the full VIN range. When

VIN is greater than 5.0V, the output of the regulator

is in full regulation. When VIN is lower than 5.0V,

the output decreases. Since this internal regulator

provides the bias current for the bottom gate driver

that requires significant amount of current

depending upon the external MOSFET selection, a

1uF ceramic capacitor for decoupling purpose is

required.