MP3217 –36V, 670kHz STEP-UP CONVERTER WITH A 0.5A SWITCH

MP3217 Rev. 0.91

www.MonolithicPower.com

7

10/15/2009

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2009 MPS. All Rights Reserved.

OPERATION

The MP3217 uses a constant frequency, peak

current mode boost regulator architecture to

regulate the output voltage. The operation of

the MP3217 can be understood by referring to

the block diagram of Figure 1.

At the start of each oscillator cycle the FET is

turned on through the control circuitry. To

prevent sub-harmonic oscillations at duty cycles

greater than 50 percent, a stabilizing ramp is

added to the output of the current sense

amplifier and the result is fed into the positive

input of the PWM comparator. When this

voltage equals the output voltage of the error

amplifier the power FET is turned off.

The voltage at the output of the error amplifier

is an amplified version of the difference

between the 1.24V reference voltage and the

feedback voltage. In this way the peak current

level keeps the output in regulation.

If the feedback voltage starts to drop, the output

of the error amplifier increases. This results in

more current flowing through the power FET,

thus increasing the power delivered to the

output.

Over voltage protection shuts off the MP3217

if the output voltage goes too high. In some

cases, loose high side resistor or improper

divided resistor; this results in the feedback

voltage is always below reference. The part

runs at maximum duty cycle boosting the output

voltage higher and higher. If the output ever

exceeds 42V, the MP3217 shuts down. And It

does not switch again until the power is

recycled.

670kHz

Enable

AMPLIFIER

PWM

COMPARATOR

CONTROL

LOGIC

CURRENT

SENSE

AMPLIFIER

PGND

SW

GND

M1

ENABLE&

REFERENCE

CONTROL

INTERNAL

REGULATOR

EN

FB

IN

Figure 1—Functional Block Diagram