Datasheet

Application information STBB1-AXX
14/22 Doc ID 022197 Rev 2
8 Application information
8.1 Programming the output voltage
The STBB1-A is available in two versions: fixed output voltage (STBB1-APURXX) and
adjustable output voltage (STBB1-APUR).
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
directly to V
OUT
.
For the adjustable version, the resistor divider must be connected between V
OUT
and GND
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 kΩ. To reduce the power consumption a maximum value of
500 kΩ 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
L
= the peak-to-peak inductor ripple current. As a rule of thumb, the peak-to-peak ripple
can be set at 10% - 20% of the output current.
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−×= 1
V
V
2R1R
FB
OUT
LMAX
MAX
BUCKMIN
IfsVIN
)VOUTVIN(VOUT
L
Δ××
−×
=
−
L
MIN
MIN
BOOSTMIN
IfsVOUT
)VINVOUT(VIN
L
Δ××
−×
=
−