MP2354 – 2A, 23V, 380KHz STEP-DOWN CONVERTER

MP2354 Rev. 1.4

www.MonolithicPower.com

5

1/6/2006

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© 2006 MPS. All Rights Reserved.

TM

OPERATION

The MP2354 is a current mode regulator. 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 (refer to Figure 1), 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 380KHz

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

MP2354 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 1.23V bandgap reference. The polarity is

such that an FB pin voltage lower than 1.222V

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.

MP2354_BD01

LOCKOUT

COMPARATOR

ERROR

AMPLIFIER

FREQUENCY

FOLDBACK

COMPARATOR

INTERNAL

REGULATORS

1.8V

SLOPE

COMP

CLK

CURRENT

COMPARATOR

CURRENT

SENSE

AMPLIFIER

SHUTDOWN

COMPARATOR

COMP

7

VIN 3

SYNC 1

RUN 8

GND

5

OSCILLATOR

35/380kHz

S

R

Q

LX

4

BST

2

5V

+

Q

0.7V

+

+

2.50V/

2.29V

+

1.22V

0.7V

+

+

FB

6

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Figure 1—Functional Block Diagram