R5975D Datasheet, PDF (25/47 Pages) STMicroelectronics – Up to 3 A step-down switching regulator for aerospace applications

R5975D
Application information
and
Equation 19
D_MIN = _V-----_I-^V_N----^OM----^UA----^TX----+–-----^VV----^FS----_W--
where V_F is the free-wheeling diode forward voltage and V_SW the voltage drop across the
internal PDMOS. Considering the range D_MIN to D_MAX, it is possible to determine the max.
IRMS going through the input capacitor. Capacitors that can be considered are:
 Electrolytic capacitors
These are widely used due to their low price and their availability in a wide range of RMS
current ratings.
The only drawback is that, considering ripple current rating requirements, they are
physically larger than other capacitors.
 Ceramic capacitors
If available for the required value and voltage rating, these capacitors usually have a higher
RMS current rating for a given physical dimension (due to very low ESR).
The drawback is the considerably high cost.
 Tantalum capacitors
Very good, small tantalum capacitors with very low ESR are becoming more available.
However, they can occasionally burn if subjected to very high current during charge.
Therefore, it is better to avoid this type of capacitor for the input filter of the device. They
can, however, be subjected to high surge current when connected to the power supply.
Table 6. List of ceramic capacitors for the R5975D
Manufacturer
Series
Capacitor value (µF) Rated voltage (V)
TAIYO YUDEN
UMK325BJ106MM-T
10
50
MURATA
GRM42-2 X7R 475K 50
4.7
50
 Output capacitor
The output capacitor is very important to meet the output voltage ripple requirements.
Using a small inductor value is useful to reduce the size of the choke but it increases the
current ripple. So, to reduce the output voltage ripple, a low ESR capacitor is required.
Nevertheless, the ESR of the output capacitor introduces a zero in the open loop gain,
which helps to increase the phase margin of the system. If the zero goes to a very high
frequency, its effect is negligible. For this reason, ceramic capacitors and very low ESR
capacitors in general should be avoided.
Tantalum and electrolytic capacitors are usually a good choice for this purpose. A list of
some tantalum capacitor manufacturers is provided in Table 7.
DocID026878 Rev 1
25/47
47