TLE6389_07 Datasheet, PDF (34/38 Pages) Infineon Technologies AG – Step-Down DC/DC Controller

TLE 6389
7.10.6 Freewheeling diode / catch diode (D1)
For lowest power loss in the freewheeling path Schottky diodes are recommended. With
those types the reverse recovery charge is negligible and a fast hand over from
freewheeling to forward conduction mode is possible. Depending on the application (12V
battery systems) 40V types could be also used instead of the 60V diodes. Also for high
temperature operation select a Schottky-diode with low reverse leakage.
A fast recovery diode with recovery times in the range of 30ns can be also used if smaller
junction capacitance values (smaller spikes) are desired.
7.10.7 Buck driver supply capacitor (C_BDS)
The voltage at the ceramic capacitor is clamped internally to 7V, a ceramic type with a
minimum of 220nF and voltage class 16V would be sufficient.
7.10.8 Input pi-filter components for reduced EME
At the input of Buck converters a square wave current is observed causing
electromagnetical interference on the battery line. The emission to the battery line
consists on one hand of components of the switching frequency (fundamental wave) and
its harmonics and on the other hand of the high frequency components derived from the
current slope. For proper attenuation of those interferers a π-type input filter structure is
recommended which is built up with inductive and capacitive components in addition to
the Input caps C_IN1 and C_IN2. The inductance can be chosen up to the value of the Buck
converter inductance, higher values might not be necessary, the additional capacitance
should be a ceramic type in the range up to 100nF.
Inexpensive input filters show due to their parasitrics a notch filter characteristic, which
means basically that the low pass filter acts from a certain frequency as a high pass filter
and means further that the high frequency components are not attenuated properly. To
slower down the slopes at the gate of the PMOS switch and get down the emission in the
high frequency range a small gate resistor can be put between GDRV and the PMOS
gate.
7.10.9 Frequency compensation
The external frequency compensation pin should be connected via a 2.2nF (>10V)
ceramic capacitor and a 680 Ω (1/8W) resistor to GND. This node should be kept free
from switching noise.
Datasheet Rev. 2.1
34
2007-08-13