MP3213 – 700KHZ/1.3MHZ BOOST CONVERTER WITH A 3.5A SWITCH

MP3213 Rev. 1.1

www.MonolithicPower.com

6

5/12/2006

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2006 MPS. All Rights Reserved.

At the beginning of each cycle, the N-Channel

MOSFET switch is turned on, forcing the

inductor current to rise. The current at the

source of the switch is internally measured and

converted to a voltage by the current sense

amplifier. That voltage is compared to the error

voltage at COMP. The voltage at the output of

the error amplifier is an amplified version of the

difference between the 1.25V reference voltage

and the feedback voltage. When these two

voltages are equal, the PWM comparator turns

off the switch forcing the inductor current to the

output capacitor through the external rectifier.

This causes the inductor current to decrease.

The peak inductor current is controlled by the

voltage at COMP, which in turn is controlled by

the output voltage. Thus the output voltage is

regulated by the inductor current to satisfy the

load. The use of current mode regulation

improves transient response and control loop

stability.

APPLICATION INFORMATION

Components referenced below apply to Typical Application Circuit on page 1.

Selecting the Soft-Start Capacitor

The MP3213 includes a soft-start timer that

limits the voltage at COMP during startup to

prevent excessive current at the input. This

prevents fault tripping of the input voltage at

startup due to input current overshoot. When

power is applied to the MP3213, and enable is

asserted, a 6µA internal current source charges

the external capacitor at SS. As the SS

capacitor is charged, the voltage at SS rises.

The MP3213 internally clamps the voltage at

COMP to 700mV above the voltage at SS. The

soft-start ends when the voltage at SS reaches

0.45V. This limits the inductor current at startup,

forcing the input current to rise slowly to the

current required to regulate the output voltage.

The soft-start period is determined by the

equation:

SS

SS

C

5

7

t

×

=

period.

Determine the capacitor required for a given

soft-start period by the equation:

SS

SS

t

0133

.

0

C

×

=

Setting the Output Voltage

Set the output voltage by selecting the resistive

voltage divider ratio. Use 10kΩ for the low-side

resistor R2 of the voltage divider. Determine the

high-side resistor R1 by the equation:

FB

FB

OUT

V

)

V

V

(

2

R

1

R

−

=

Selecting the Input Capacitor

An input capacitor (C1) is required to supply the

AC ripple current to the inductor, while limiting

noise at the input source. A low ESR capacitor

is required to keep the noise at the IC to a

minimum. Ceramic capacitors are preferred, but

tantalum or low-ESR electrolytic capacitors may

also suffice.

Use an input capacitor value greater than

4.7µF. The capacitor can be electrolytic,

tantalum or ceramic. However since it absorbs

the input switching current it requires an

adequate ripple current rating. Use a capacitor

with RMS current rating greater than the

inductor ripple current (see Selecting The

Inductor to determine the inductor ripple

current).

To ensure stable operation, place the input

capacitor as close to the IC as possible.

Alternately a smaller high quality ceramic 0.1µF

capacitor may be placed closer to the IC with

the larger capacitor placed further away. If

using this technique, the larger capacitor can be

a tantalum or electrolytic type. All ceramic

capacitors should be placed close to the

MP3213.