MP2351 – 1.5A, 23V, 1.4MHz STEP-DOWN CONVERTER

MP2351 Rev. 1.5

www.MonolithicPower.com

4

1/6/2006

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© 2006 MPS. All Rights Reserved.

TM

OPERATION

The MP2351 is a current mode regulator. That

is, the COMP pin voltage is proportional to the

peak inductor current. At the beginning of a

cycle: the upper transistor M1 is off; the lower

transistor M2 is on; the COMP pin 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 off M2 and 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 Current Sense Amplifier output and

compared to the Error Amplifier output by the

Current Comparator. When the Current Sense

Amplifier plus Slope Compensation signal

exceeds the COMP pin voltage, the RS

Flip-Flop is reset and the MP2351 reverts to its

initial M1 off, M2 on state. If the Current Sense

Amplifier plus Slope Compensation signal does

not exceed the COMP voltage, 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 1.230V bandgap reference. The polarity is

such that an FB pin voltage lower than 1.230V

increases the COMP pin voltage. Since the

COMP pin voltage is proportional to the peak

inductor current an increase in its voltage

increases current delivered to the output. The

lower 10Ω switch ensures that the bootstrap

capacitor voltage is charged during light load

conditions. External Schottky Diode D1 carries

the inductor current when M1 is off.

MP2351_BD01

LOCKOUT

COMPARATOR

ERROR

AMPLIFIER

FREQUENCY

FOLDBACK

COMPARATOR

INTERNAL

REGULATORS

1.8V

SLOPE

COMP

CLK

CURRENT

COMPARATOR

CURRENT

SENSE

AMPLIFIER

SHUTDOWN

COMPARATOR

COMP

8

IN 4

EN 9

SYNC 10

GND

6

OSCILLATOR

180KHz/

1.4MHz

S

R

Q

SW

5

BS

2

5V

+

Q

0.7V

+

+

2.37V/

2.62V

+

1.23V

0.7V

+

+

FB

7

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Figure 1—Functional Block Diagram