STBB3JCC Datasheet, PDF (19/27 Pages) STMicroelectronics – 2 A, high efficiency single inductor buck-boost DC-DC converter and high brightness WLED driver

STBB3JCC
Application information
8.3
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 following two conditions in
which the converter works:
• as buck region at the maximum input voltage
• as boost region at the minimum input voltage
Two critical inductance values are then obtained according to the following formulas:
Equation 3
L_{MIN_BUCK} = ^V-----^O----^U--_V--^T--_I--_N^×----_{_}-⁽-_M-^V---_A-^I--^NX----^_-_×-^M----_f^A-_S--^X---_×--–---_Δ--^V--_I--^OL----^U---^T----⁾
Equation 4
L_{MIN_BOOST} = ^V-----^I-^N----^_---^M----^I_V-^N---_O--^×--_U---⁽-_T-^V---_×-^O----_f^U--_S--^T--_×--–---_Δ--^V--_I--^IL--^N---^_---^M----^I-^N----⁾
where f_s is the minimum value of the switching frequency and ΔI_L is the inductor ripple
current. The amplitude of the inductor ripple current is typically set between 20% and 40%
of the maximum inductor current. To guarantee an inductor ripple current always lower than
the selected value ΔI_L, the higher value between L_{MIN_BUCK} and L_{MIN_BOOST} have to be
chosen. In addition to the inductance value, also the maximum current which the inductor
can handle must be calculated in order to avoid saturation.
Equation 5
I_{PEAK_BUCK} = -^I-^O---_η-^U----^T- + ^V-----^O----₂^U---^T--_×---^×-_V----⁽-_I-^V_N----_{_I}--^N-_M---^_A--^M--_X--^A--_×-^X----_f-–-_S---^V-_×----^O-_L---^U---^T----⁾
Equation 6
I _{P E A K _BOOST}
=


-^V-_η---^O--_×--^U---_V-^T---_I-^×-_N----_{_}^I--^OM----^UI-_N--^T--_
+ ^V-----^I-^N----^_---^M--₂--^I-^N-_×----^×-_V----⁽_O--^V--_U--^O--_T--^U---_×-^T---_f-–-_S----^V-_×---^I--^N_L---^_---^M----^I-^N----⁾
where η is the estimated efficiency. The maximum of the two values above must be
considered when selecting the inductor.
8.4
Input and output capacitor selection
It is recommended to use ceramic capacitors with low ESR as input and output capacitors in
order to filter any disturbance present in the input line and to obtain stable operation.
Minimum values of 10 µF for both capacitors, C_IN and C_OUT, are needed to achieve good
behavior of the device. The input capacitor must be placed as close as possible to the
device.
An R-C filter is added to VINA pin (R3-C5 Figure 30 and Figure 31) to assure a clean input
voltage to the internal logic block.
DocID025425 Rev 5
19/27