AN-8023

APPLICATION NOTE

© 2009 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.0 • 7/30/09

2

Principle of Operation

To understand buck-boost topology, buck topolgy is briefly

compared below. When the MOSFET switch (Q1 in Figure 3)

transferred to the load with positive output voltage.

Meanwhile, in buck-boost topology, the inductor and

freewheeling diode switch positions. When the MOSFET

Therefore, it can generate negative output voltage.

Buck-boost circuit with buck regulators require several

design considerations. Table 1 summarizes the design

parameter comparison between buck and buck-boost circuit.

C

OUT

Load

PWM

Q1

D1

V

IN

C

IN

Load

Vo

PWM

D1

Q1

1

Q

I

1

D

I

L

I

∆

L

I

L

V

IN

V

OUT

V

t

OUT

V

−

Q1 ON

Q1 OFF

1

Q

I

1

D

I

L

I

∆

L

I

L

V

IN

OU T

VV

−

OUT

V

t

OUT

V

−

Q1 ON

Q1 OFF

Figure 2. Buck-Boost Topology

Figure 3. Buck Topology

Table 1.

Buck and Buck-Boost Design Parameters

First of all, inductor current is limited by (1–D); so attention

is needed to see that the maximum output current of buck

regulator is be always lower than the maximum current in

buck-boost circuit. Second, the switch node is a sum of input

voltage and output voltage in buck-boost. It also needs to be

limited to the maximum switch node voltage of buck

regulator. Since buck-boost is very noisy on input and

output compared to buck circuit, it requires good-quality

MLCC as input and output filters.

Topology

Duty Cycles

Buck-Boost

1

OUT

I

D

−

IN

OUT

VV

+

OUT

IN

OUT

V

VV

+

Buck

OUT

I

IN

V

OUT

IN

V

V