STBB3J Datasheet, PDF (18/25 Pages) STMicroelectronics – 2 A, high efficiency single inductor buck-boost DC-DC converter

Application information
STBB3J
Equation 2
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 3
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 4
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 5
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.
8.3
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 26) to assure a clean input voltage to the
internal logic block.
18/25
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