MP28367 – 1A, 21V, 1.4MHz STEP-DOWN CONVERTER IN A TSOT23-6

MP28367 Rev. 0.9

www.MonolithicPower.com

6

12/13/2007

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© 2007 MPS. All Rights Reserved.

TM

OPERATION

The MP28367 is a current mode buck regulator.

That is, the EA output voltage is proportional to

the peak inductor current.

At the beginning of a cycle, M1 is off. The EA

output voltage is higher than the current sense

amplifier output, and the current comparator’s

output is low. The rising edge of the 1.4MHz

CLK signal sets the RS Flip-Flop. Its output

turns on M1 thus connecting the SW pin and

inductor to the input supply.

The increasing inductor current is sensed and

amplified by the Current Sense Amplifier. Ramp

compensation is summed to the Current Sense

Amplifier output and compared to the Error

Amplifier output by the PWM Comparator.

When the sum of the Current Sense Amplifier

output and the Slope Compensation signal

exceeds the EA output voltage, the RS Flip-

Flop is reset and M1 is turned off. The external

Schottky rectifier diode (D1) conducts the

inductor current.

If the sum of the Current Sense Amplifier output

and the Slope Compensation signal does not

exceed the EA output for a whole cycle, then

the falling edge of the CLK resets the Flip-Flop.

The output of the Error Amplifier integrates the

voltage difference between the feedback and

the 0.81V bandgap reference. The polarity is

such that a FB pin voltage lower than 0.81V

increases the EA output voltage. Since the EA

output voltage is proportional to the peak

inductor current, an increase in its voltage also

increases current delivered to the output.

5

IN

EN

FB

GND

SW

BST

4

3

2

1

6

REGULATOR

OSCILLATOR

1.4MHz

DRIVER

CURRENT SENSE

AMPLIFIER

ERROR

AMPLIFIER

CURRENT

LIMIT

COMPARATOR

PWM

COMPARATOR

D

x20

M1

REGULATOR

REFERENCE

EA

--

+

--

+

--

+

S

R

R

Q

27pF

1pF

Figure 1—Functional Block Diagram