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TLE6389_07 Datasheet, PDF (33/38 Pages) Infineon Technologies AG – Step-Down DC/DC Controller
◁
TLE 6389
7.10.3 PFM and PWM thresholds
The crossover thresholds PFM to PWM and vice versa strongly depend on the input
voltage VIN, the Buck converter inductance L1, the sense resistor value RSENSE and the
turn on and turn off delays of the external PMOS.
For more details on the PFM to PWM and PWM to PFM thresholds please refer to the
application note “TLE6389 - Determining PFM/PWM current thresholds”.
7.10.4 Buck output capacitor (COUT) selection:
The choice of the output capacitor effects straight to the minimum achievable ripple
which is seen at the output of the buck converter. In continuous conduction mode the
ripple of the output voltage can be estimated by the following equation:
VRipple
=
∆I
⋅


RE
S
R
COUT
+
-8----⋅---f--S---W----1--⋅---C----O----U----T-
From the formula it is recognized that the ESR has a big influence in the total ripple at
the output, so low ESR tantalum capacitors are recommended for the application.
One other important thing to note are the requirements for the resonant frequency of the
output LC-combination. The choice of the components L and C have to meet also the
specified range given in section 3 otherwise instabilities of the regulation loop might
occur.
7.10.5 Input capacitor (CIN1) selection:
At high load currents, where the current through the inductance flows continuously, the
input capacitor is exposed to a square wave current with its duty cycle VOUT/VI. To
prevent a high ripple to the battery line a capacitor with low ESR should be used. The
maximum RMS current which the capacitor has to withstand is calculated to:
IRMS = ILOAD ⋅
V-----O----U----T- ⋅
VIN
1
+
1--
3
⋅


-2----⋅---I-∆-L---IO----A----D--
2
For low ESR an e.g. Al-electrolytic capacitance in parallel to an ceramic capacitance
could be used.
Datasheet Rev. 2.1
33
2007-08-13
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