MP28164 – SINGLE-INDUCTOR, BUCK-BOOST CONVERTER WITH 4.2A SWITCHES

MP28164 Rev. 1.0

www.MonolithicPower.com

17

3/28/2016

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APPLICATION INFORMATION

Setting the Output Voltage

A resistor divider from VOUT to FB is necessary

to set the MP28164’s output voltage. The high-

side feedback resistor (R1) can be calculated

with Equation (1):

2

R

)

1

V

V

(

1

R

FB

OUT







(1)

Where R2 is the low-side feedback resistor with a

recommended value from 60kΩ through 360kΩ

to balance the stability and transient response.

Inductor Selection

With one buck-boost topology circuit, the inductor

must support the buck application with the

maximum input voltage and boost application

with the minimum input voltage. Two critical

inductance values can be determined according

to the buck mode and boost mode current ripple,

as shown in Equation (2) and Equation (3):

L

REQ

)

MAX

(

IN

OUT

)

MAX

(

IN

OUT

BUCK

MIN

I

F

V

)

V

V

(

V

L















(2)

L

REQ

OUT

)

MIN

(

IN

OUT

)

MIN

(

IN

BOOST

MIN

I

F

V

)

V

V

(

V

L















(3)

the peak-to-peak inductor current ripple. The

peak-to-peak ripple can be set to 10%-30% of

the inductor current. The minimum inductor value

for the application must be higher than the

calculated value from both Equation 2 and

Equation 3.

In addition to the inductance value, the inductor

must support the peak current based on Equation

(4) and Equation (5) to avoid saturation:

OUT

IN(MAX)

OUT

PEAK BUCK

OUT

IN(MAX)

REQ

V(V

V )

II

2V

F

L











(4)

IN(MIN)

OUT

IN(MIN)

OUT

OUT

PEAK BOOST

IN(MIN)

OUT

REQ

V(V

V

)

VI

I

V2 V

F

L

















(5)

Where η is the estimated efficiency.

Input and Output Capacitor Selection

It is recommended to use ceramic capacitors with

a low ESR as input and output capacitors to filter

any disturbance present in the input and output

line and to achieve stable operation.

Output capacitors with a minimum 10µF input

and 22µF output are required to achieve optimal

behavior from the device. The output capacitor

affects loop stability. The input and output

capacitors must be placed as close as possible

to the device. Refer to the Typical Application

Circuits section for optimized capacitor selection

details.