MP2155 –HIGH EFFICIENCY SINGLE INDUCTOR BUCK-BOOST CONVERTER WITH 2.2A SWITCHES

MP2155 Rev. 1.0

www.MonolithicPower.com

15

1/26/2014

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© 2014 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the Output Voltage

To use MP2155 correctly, A resistor divider must

be connected between Vout and GND, and the

middle point of the divider connected to FB pin as

shown in Typically Application on page 1.

2

R

)

1

V

V

(

1

R

FB

OUT

×

−

=

(1)

High R2 resistance (eg. 100kΩ) can reduce the

resistance is recommended for R1 for good

output accuracy.

Inductor Selection

The inductor is the key passive component for

switching converters. With a buck-boost device,

the inductor selection affects the boundary

conditions in which the converter works, as buck

at the maximum input voltage and as a boost at

the minimum input voltage.

Two critical inductance values are then obtained

according to the following formulas.

L

REQ

)

MAX

(

IN

OUT

)

MAX

(

IN

OUT

BUCK

MIN

I

F

V

)

V

V

(

V

L

Δ

×

×

−

×

=

−

(2)

L

REQ

OUT

)

MIN

(

IN

OUT

)

MIN

(

IN

BOOST

MIN

I

F

V

)

V

V

(

V

L

Δ

×

×

−

×

=

−

(3)

Where:

current. As a rule of thumb, the peak-to-peak

ripple can be set at 10%-20% of the output

current.

The minimum inductor value for the application is

the higher one between Equation 2 and Equation

3. In addition to the inductance value the

maximum current the inductor can handle must

be calculated in order to avoid saturation.

L

F

V

2

)

V

V

(

V

I

I

REQ

)

MAX

(

IN

OUT

)

MAX

(

IN

OUT

OUT

BUCK

PEAK

×

×

×

−

×

+

η

=

−

(4)

L

F

V

)

V

V

(

V

V

I

V

I

REQ

OUT

)

MIN

(

IN

OUT

)

MIN

(

IN

)

MIN

(

IN

OUT

OUT

BOOST

PEAK

×

×

−

×

+

×

η

×

=

−

(5)

Where η is the estimated efficiency of MP2155.

The maximum of the two values above must be

considered when selecting the inductor.

Input and Output Capacitor Selection

It is recommended to use ceramic capacitors with

low ESR as input and output capacitors in order

to filter any disturbance present in the input line

and to obtain stable operation.

Minimum values of 10uF for both capacitors are

needed to achieve good behavior of the device.

The input capacitor must be placed as close as

possible to the device.

Other Consideration

MP2155 employs the classic hiccup mode for

SCP. This method has an inherited drawback: if

the output short is released at a time closed to

SS end, then Vo would has overshoot. To

attenuate Vo overshoot at SCP recovery, a

forward RC series can be connected in parallel

with high side resistor of FB divider, as R3 and

C5 in Figure 5 shows. The RC acts as a soft

startup when Vo short is released at the time of

internal SS’s end.

PCB Layout Guide

1. Input and output capacitors should be close to

MP2155’s Vin, Vout and PGND pins.

2. The wire connecting input capacitor to Vcc pin

should be as short as possible. For better

performance in noisy environment, an additional

capacitor very close to Vcc pin can be used to

bypass noise for Vcc.