STBB2 Datasheet, PDF (19/25 Pages) STMicroelectronics – 800 mA 2.5 MHz, high efficiency dual mode buck-boost DC-DC converter

STBB2
Equation 2
Application information
L _{MIN − BUCK}
= ^{VOUT × (VIN MAX − VOUT )}
VIN _MAX × fs × Δ I_L
Equation 3
L _{MIN −BOOST}
= ^{VIN MIN × (VOUT − VIN MIN )}
VOUT × fs × Δ I_L
where fs is the minimum value of the switching frequency and ΔI_L is the peak-to-peak
inductor ripple current. The peak-to-peak ripple can be set at 10% or 20% of the output
current.
The minimum inductor value for the application is the highest between Equation 2 and
Equation 3. In addition to the inductance value, the maximum current, which the inductor
can handle, must be calculated in order to avoid saturation.
Equation 4
I_{PEAK −BUCK}
= (I_OUT
/ η) +
VOUT × (VIN_MAX − VOUT )
2 × VIN_MAX × fs × L
Equation 5
I_{PEAK −BOOST}
=
VOUT × I_OUT
η × VIN_MIN
+
VIN_MIN × (VOUT − VIN_MIN )
2 × VOUT × fs × L
where η is the estimated efficiency. The maximum of the two above values must be
considered when the inductor is selected.
8.3
Input and output capacitor selection
It is recommended ceramic capacitors to be used 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 are needed to achieve good behavior of the
device. The input capacitor must be placed as closer as possible to the device.
8.4
Layout guidelines
Due to the high switching frequency and peak current, the layout is an important design step
for all switching power supplies. If the layout is not fulfilled carefully, important parameters
such as efficiency and output voltage ripple may be compromised.
DocID022745 Rev 9
19/25