ST1S14 Datasheet, PDF (23/46 Pages) STMicroelectronics – Power good open collector output

ST1S14
Application information
7.1.2
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 suggested to avoid this type of capacitor for the input filter of the device
as they could be stressed by a high surge current when connected to the power supply.
Manufacturer
TAIYO YUDEN
MURATA
Table 8. List of ceramic capacitors for the ST1S14
Series
Capacitor value (µ) Rated voltage (V)
UMK325BJ106MM-T
10
50
GRM42-2 X7R 475K 50
4.7
50
If the selected capacitor is ceramic (so neglecting the ESR contribution), the input voltage
ripple can be calculated as:
Equation 20
V_{IN PP} = -_C----_I--_N----^I-_â‹-^O----_f--_S----_W--- ⋅
⎛
⎝
1
–
D_{--η-⎠⎞}
⋅
D + ^D--_η- ⋅
(1 – D)
Output capacitor
The output capacitor is very important to meet the output voltage ripple requirement.
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.
Ceramic capacitors
Ceramic capacitors and very low ESR capacitors that introduce a zero outside the
designed bandwidth (f_Z=1/(2*pi*ESR*C_OUT, see Section 6: Closing the loop) in general
should be avoided. A leading network across the upper resistor of the voltage divider is
useful to increase the phase margin and compensate the system (see Section 6.3:
Voltage divider). The effectiveness of the leading network increases at high output
voltage because the singularities become more split.
High ESR capacitors
The “high ESR capacitor” definition stands for a capacitor having an ESR value able to
introduce a zero into the designed system bandwidth, which can be, as a general rule,
up to f_SW/5 at maximum. Tantalum or electrolytic capacitors belong to this group.
Equation 21
f_Z
=
2^-----⋅^------π^-----⋅^-----E^----S^---1--R^-----⋅^------C^-----O^-----U^-----T^--
<
BW
<
-^f-^S----^W--
5
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