STBB1XX

Application information

Doc ID 15474 Rev 5

13/21

8

Application information

8.1

Programming the output voltage

The STBB1 is available in two versions: fixed output voltage (STBB1PURXX) and adjustable

output voltage (STBB1PUR).

In the first case the device integrates the resistor divider needed to set the correct output

voltage. This allows the saving of 2 external components. The FB pin must be connected

and the middle point of the divider must be connected to FB as shown in Figure 3.

Equation 1

A suggested value for R2 is 100 k

Ω. To reduce the power consumption a maximum value of

500 k

Ω can be used.

8.2

Inductor selection

The inductor is the key passive component for switching converters. With a buck-boost

device, the inductor selection must take into consideration 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:

Equation 2

Equation 3

where:

fs: minimum switching frequency

ΔI

can be set at 10 % - 20 % of the output current.

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

=

1

V

V

2

R

1

R

FB

OUT

L

MAX

MAX

BUCK

MIN

I

fs

VIN

)

VOUT

VIN

(

VOUT

L

Δ

×

×

−

×

=

−

L

MIN

MIN

BOOST

MIN

I

fs

VOUT

)

VIN

VOUT

(

VIN

L

Δ

×

×

−

×

=

−