MP3910 –PEAK CURRENT MODE BOOST CONTROLLER

MP3910 Rev. 1.11

www.MonolithicPower.com

16

11/6/2014

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APPLICATION INFORMATION

COMPONENT SELECTION

MP3910 can be used for topologies including

Flyback, Boost and Sepic. Refer to figure 5 and

below

introduction

for

typical

external

component selection of boost converter.

Setting the Output Voltage

Set the output voltage by selecting the resistive

voltage divider ratio. If we use 10kΩ for the low-

the equation:

FBL

OUT

REF

FBH

REF

R(V

V

)

R

V





Selecting the Soft-start Capacitor

MP3910 ramps external capacitor voltage on

SS pin to control COMP voltage, which

determines inductor peak current. The SS pin

voltage can be calculated by blew equation:

ss

ss

54 A

Vss

T

C





When OLP, SCP, OVP occurs, the SS acts as a

timer. Once the protection occurs, the 1.66uA

current discharges SS cap for hiccup protection.

Selecting the Input Capacitor

An input capacitor 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 to the IC at a

minimum. Ceramic capacitors are preferred, but

tantalum or low-ESR electrolytic capacitors may

also suffice. When using tantalum or electrolytic

capacitors, a small high quality ceramic

capacitor, i.e. 0.1uF, should be place close to

IC. The capacitance for boost input can be

calculated as:

IN

IN

SW

I

C

8V

F









Where ΔI is the peak-to-peak inductor ripple

Selecting the Output Capacitor

The output capacitor maintains the DC output

voltage.

For

best

results,

use

low-ESR

capacitors to minimize the output voltage ripple.

The output capacitor’s characteristics also

affect system stability. For best results, use

ceramic, tantalum, or low-ESR electrolytic

capacitors.

For

ceramic

capacitors,

the

capacitance dominates the impedance at the

switching frequency, and so the output voltage

ripple is mostly independent of the ESR. The

output voltage ripple is estimated as:

IN

OUT

OUT

LOAD

OUT

SW

V

1

V

VI

CF









the output capacitor.

For tantalum or low-ESR electrolytic capacitors,

the ESR dominates the impedance at the

switching frequency, so the output ripple is

estimated as:

IN

OUT

LOAD

ESR

OUT

OUT

LOAD

OUT

SW

IN

V

1

VI

R

V

VI

CF

V













of the output capacitors. Choose an output

capacitor that satisfies the output ripple and

load transient requirements of the design.

Selecting the Inductor and Current Sensing

Resistor

The inductor is required to transfer the energy

between the input source and the output

capacitors. A larger value inductor results in

less ripple current that results in lower peak

inductor current, and therefore reduces the

stress on the power MOSFET. However, the

larger value inductor has a larger physical size,

higher series resistance, and lower saturation

current.

A good rule of thumb is to allow the peak-to-

peak ripple current to be approximately 30-50%

of the maximum input current. Make sure that

the peak inductor current is below 80% of the

IC’s maximum current limit at the operating duty

cycle to prevent loss of regulation. Make sure

that the inductor does not saturate under the

worst-case load transient and startup conditions.

The

required

inductance

value

can

be

calculated by: