STBB1-AXX Datasheet, PDF (16/24 Pages) STMicroelectronics – Buck-boost DC-DC converter

Application information
STBB1-AXX
8
Application information
8.1
Programming the output voltage
The STBB1-AXX is available in two versions: fixed output voltage (STBB1-APURXX) and
adjustable output voltage (STBB1-APUR). The former integrates the resistor divider
needed to set the correct output voltage. This allows 2 external components to be saved.
The FB pin must be connected directly to VOUT.
Regarding to the adjustable version, the resistor divider must be connected between VOUT
and GND and the middle point of the divider has to be connected to FB as shown in Figure
3: "Application circuit (adjustable output version)"
Equation 1
( ) ^{R1 = R2 --V-}_V^--O--_F^--U-_B^{--T- – 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
L_{MIN – BUCK}
=
V_-----O_-----U_-----T_------------V_----I_--N_{-----M----A----X----}–_-----V_----O_-----U_-----T_----
VIN_MAX fs I_L
L_{MIN– BOOST} = ^V-----^I--^N----^M-----^I_V-^N----_O-----_U---^V-_T----^O-----^U-_f--_s-^T------^–----^V--_I--_L^I---^N----^M---^I--^N----
where:
fs: minimum switching frequency
DI_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. The minimum inductor value for the
application is the highest between the above equations. Furthermore, the maximum current
the inductor can handle must be calculated in order to avoid saturation.
equation 4
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DocID022197 Rev 4