MP1540 – 1.3MHz, 18V STEP-UP CONVERTER

MP1540 Rev. 1.0

www.MonolithicPower.com

4

8/15/2005

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© 2005 MPS. All Rights Reserved.

TM

PIN FUNCTIONS

Pin #

Name

Pin Function

1 SW

Power Switch Output. SW is the drain of the internal MOSFET switch. Connect the power

inductor and output rectifier to SW. SW can swing between GND and 20V.

2

GND

Ground.

3

FB

Feedback Input. FB voltage is 1.25V. Connect a resistor divider to FB.

4 EN

Regulator On/Off Control Input. A high input at EN turns on the converter, and a low input

turns it off. When not used, connect EN to the input source for automatic startup. The EN

pin cannot be left floating.

5

IN

Input Supply Pin. Must be locally bypassed.

OPERATION

The MP15 40 use s a f ixed frequ ency, p eak

current mo de boost re gulator ar chitecture to

regulate vo ltage at

the fee dback pin . The

operation of the MP1540 can be understood by

referring to the block diagram of Figure 1.

At the start of each oscillator cycle the MOSFET

is tu rned on through t he con trol circuitry. To

prevent sub-harmonic oscillations at duty cycles

greater than 50 pe rcent, a stab ilizing ra mp is

added to t he out put of the

current sense

amplifier and the result is fed into the negative

input of the PWM comparator. When this voltage

equals the out put voltage of th e error amplifier

the power MOSFET is turned off.

The voltage at the output of the error amplifier is

an amplified version o f t he difference bet ween

the 1.25V bandgap re ference vo ltage an d the

feedback vol tage. I n this w ay th e pea k current

level ke eps the out put in reg ulation. I f the

feedback voltage starts to drop, the output of the

error amplifier increases. This re sults i n more

current to flow through the power MOSFET, thus

increasing the power delivered to the output.

The MP1540 has intern al soft start to limit th e

amount of input current at startup and to also

limit the amount of overshoot on t

he output .

The current limit is increased by a fourth every

40µs giving a total soft start time of 120µs.

PWM

COMPARATOR

ERROR

AMPLIFIER

CC

RC

CONTROL

LOGIC

CURRENT

SENSE

AMPLIFIER

SW

M1

GND

+

-

1

FB

3

2

1.3MHz

OSC

+

-

-

+

+

1.25V

MP1540_F01_BD01

Figure 1—Functional Block Diagram