L6910 - L6910A

22/29

10 APPLICATION IDEA 2: POSITIVE BUCK-BOOST REGULATOR 3V TO 13.2V

INPUT / 5V 2.5A OUTPUT

In some applications the input voltage changes in a very wide range while the output must be regulated to a

fixed value. In this case a Buck-Boost topology can be required in order to keep the output voltage in regulation.

The schematic below shows how to implement a Buck-Boost regulating 5V at the output from both 3.3V and 5V

and 12V input buses.

In a Buck-Boost topology the current is delivered to the output during the OFF phase only. So, for a given current

limit, the maximum output current depends strongly on the duty cycle. Assuming a 100% efficiency and neglect-

ing the current ripple across the inductor, the relationship betweent the current limit and the maximum output

current is the following:

Obviously, since the efficiency is lower than 100% and the ripple is usually not negligible, the maximum output

current is always lower than the value calculated in the above formula

Figure 30. Positive buck-boost regulator 3V to 13.2V input / 5V 2.5A Output Circuit

I

OMAX

I

LIM

1D

–

()

⋅

=

D

V

O

V

IN

V

O

+

-----------------------

=

L1

R1

C1- C2

C3

Q2

Q2

Q1

D2

C6

13

14

10

11

3

1

U1

L6910/A

VIN (3.3V-5V-12V BUSES)

VOUT ( 5V 2.5A )

6

12

5

4

2

15

7

9

VCC

GND

VREF

SS

OSC

OCSET

UGATE

PHASE

LGATE

PGND

PGOOD

C9

C8

C4

D1

-14

C13-14

C13

GNDOUT

R2

C12

C7

C7

C5

R7

C10

8

GNDIN

GNDIN

VCC (12V BUS)

R5

G1

R6

EAREF

BOOT

Q3

Q4

R3

R4

C11

GNDCC

GNDCC