Datasheet

DocID18279 Rev 5 23/37
ST1CC40 Application information
37
where V
F
is the freewheeling 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.
I
RMS
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:
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 recommended to avoid this type of capacitor for the input filter of the
device as they may be stressed by a high surge current when connected to the power
supply.
In case the selected capacitor is ceramic (so neglecting the ESR contribution), the input
voltage ripple can be calculated as:
Equation 27
7.2 Layout considerations
The layout of switching DC-DC converters is very important to minimize noise and
interference. Power-generating portions of the layout are the main cause of noise and so
high switching current loop areas should be kept as small as possible and lead lengths as
short as possible.
High impedance paths (in particular the feedback connections) are susceptible to
interference, so they should be as far as possible from the high current paths. A layout
example is provided in Figure 13.
The input and output loops are minimized to avoid radiation and high frequency resonance
problems. The feedback pin to the sensing resistor path must be designed as short as
possible to avoid pick-up noise. Another important issue is the ground plane of the board.
Since the package has an exposed pad, it is very important to connect it to an extended
ground plane in order to reduce the thermal resistance junction-to-ambient.
Table 7. List of ceramic capacitors for the ST1CC40
Manufacturer Series Capacitor value (µF) Rated voltage (V)
TAIYO YUDEN UMK325BJ106MM-T 10 50
MURATA GRM42-2 X7R 475K 50 4.7 50
V
IN PP
I
O
C
IN
f
SW

----------------------- 1
D

----–


D
D

---- 1D–+=