MP3431

– 21A, HIGH-EFFICIENCY, SYNC, BOOST CONVERTER W/ PROG CURRENT LIMIT

MP3431 Rev. 1.0

www.MonolithicPower.com

21

6/8/2017

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© 2017 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider is used to set the

output voltage. Typically, choose R1 to be

between 300 - 800k

Ω. Then calculate R2 with

Equation (4):

1

2

R

V

V

V

R

REF

OUT

REF







(4)

and R2 is the bottom feedback resistor.

Selecting the Input Capacitor

The input capacitor (C1) is used to maintain the

DC input voltage. Low ESR ceramic capacitors

are recommended. The input voltage ripple can

be estimated with Equation (5):

(5)

inductor value.

Selecting the Output Capacitor

The output current of the boost converter is

discontinuous and therefore requires an output

capacitor (C2) to supply AC current to the load.

For the best performance, low ESR ceramic

capacitors

are

recommended.

The

output

voltage ripple can be estimated with Equation (6):

(6)

Ceramic capacitors with X5R or X7R dielectrics

are highly recommended because of their low

ESR and small temperature coefficients.

Selecting the Inductor

An inductor is required to transfer the energy

between the input source and the output

capacitors. An inductor with a larger value

results in less ripple current and a lower peak

inductor current, reducing stress on the power

MOSFET. However, the larger value inductor

has a larger physical size, a higher series

resistance, and a lower saturation current.

For most designs, the inductance value can be

calculated with Equation (7):

(7)

Choose the inductor ripple current to be

approximately 20 ~ 50% of the maximum

inductor

peak

current.

Typically,

a

1.5

μH

inductor is recommended. Ensure that the

inductor does not saturate under the worst-case

condition. The inductor should have a low series

resistance (DCR) to reduce the resistive power

loss.

Soft-Start (SS) Capacitor Selection

With the required output voltage rising time

Equation (8):

REF

SS

RISE

SS

V

I

T

C





(8)

rising time.

Input Current Limit Setting

The ILIM resistor (R5) is used to set the input

current

limit.

One

1

kΩ resistor in series

connected to ILIM prevents noise injection.

Calculate R5 with Equation (9):















IN _ LIM

IN

980

R5

1

3.45

I

(1.2

V

0.12)

(9)

For example, if the required current limit is 13A,

and VIN is 3.3V, then R5 is calculated to be

22.29k

Ω, so chose a 22kΩ value for a 13A input

current limit. Place a 4.7 ~ 10nF decoupling

capacitor parallel with R5.

If the input current limit function is not used,

connect ILIM to AGND.

VDD Capacitor Selection

The MP3431 integrates the VDD power at about

3.4V, which powers the internal MOSFET gate

driver and internal control circuit, typically. One

ceramic bypass capacitor 4.7µF or higher is

necessary for the internal regulator. Do not

connect the external load to the VDD power.

IN

IN

IN

2

s

OUT

VV

V1

8f

L C1

V







 







OUT

IN

OUT

s

L

OUT

V

V

V1

f

R

C2

V







 







IN

OUT

IN

s

OUT

L

V (V

V )

L

f

V

I







 