MP3209 – 1.4MHZ, 350mA BOOST CONVERTER

MP3209 Rev.0.941

www.MonolithicPower.com

6

11/30/2011

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PIN FUNCTIONS

TSOT23-5

Pin #

UTQFN8

Pin #

Name Function

1

8

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 22V.

2

1, 5

GND

Ground.

3

6

FB

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

4

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

2

IN

Input Supply Pin. Must be locally bypassed.

3, 7

NC

No Connect.

OPERATION

The MP3209 uses a fixed frequency, peak current

mode boost regulator architecture to regulate

voltage at the feedback pin. The operation of the

MP3209 can be understood by referring to the

block diagram of Figure 1. At the start of each

oscillator cycle the MOSFET 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 negative input of the PWM comparator. When

this voltage equals the output voltage of the error

amplifier the power MOSFET is turned off. The

voltage at the output of the error amplifier is an

amplified version of the difference between the

1.25V bandgap 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 to flow through the power MOSFET, thus

increasing the power delivered to the output.

The MP3209 has internal soft start to limit the

amount of input current at startup and to also

limit the amount of overshoot on the output. The

current limit is increased by a fourth every 40µs

giving a total soft start time of 120µs.

PWM

COMPARATOR

ERROR

AMPLIFIER

CONTROL

LOGIC

CURRENT

SENSE

AMPLIFIER

SW

M1

GND

+

-

FB

1.3MHz

OSC

+

-

-

+

+

1.25V

Figure 1—Functional Block Diagram