TLE6368_06 Datasheet, PDF (51/59 Pages) Infineon Technologies AG – Multi-Voltage Processor Power Supply

TLE 6368 / SONIC
5.2 Buck converter circuit
A typical choice of external components for the buck converter is given in figure 14. For
basic operation of the buck converter the input capacitor C_I2, the bootstrap capacitor
C_BTP, the catch diode D_B, the inductance L_B, the output capacitor C_B and the charge
pump capacitors C_FLY and C_CCP are necessary. A Zener Diode at the FB/L_IN input is
recommended as a protection against overvoltage spikes.
The additional components shown on top of the circuit lower the electromagnetic
emission (L_I, C_I1, C_I3, R_Slew) and the switching losses (R_Boost, C_Boost, D_Boost). For 12V
battery systems the switching loss minimization feature might not be used. The Boost pin
(33) is connected directly to the IN pins (32, 30) in that case and the components R_Boost,
C_Boost and D_Boost are left away.
5.2.1 Buck inductance (L_B) selection:
The inductance value determines together with the input voltage, the output voltage and
the switching frequency the current ripple which occurs during normal operation of the
step down converter. This current ripple is important for the all over ripple at the output
of the switching converter.
As a rule of thumb this current ripple ∆I is chosen between 10% and 50% of the load
current.
L = ⁽---V----^I----–f--_S--V-_W----^O--_â‹-^U--V--^T--_I-⁾--_â‹--^â‹-_∆--V--I--^O----^U----^T-
For optimum operation of the control loop of the Buck converter the inductance value
should be in the range indicated in section 3.3, recommended operation range.
When picking finally the inductance of a certain supplier (Epcos, Coilcraft etc.) the
saturation current has to be considered. With a maximum current limit of the Buck
converter of 3.2A an inductance with a minimum saturation current of 3.2A has to be
chosen.
Data Sheet
51
Rev. 2.2, 2006-12-01