Datasheet

Application informations L7981
18/44 Doc ID 15182 Rev 3
6.3 Output capacitor selection
The current in the capacitor has a triangular waveform which generates a voltage ripple
across it. This ripple is due to the capacitive component (charge or discharge of the output
capacitor) and the resistive component (due to the voltage drop across its ESR). So the
output capacitor has to be selected in order to have a voltage ripple compliant with the
application requirements.
The amount of the voltage ripple can be calculated starting from the current ripple obtained
by the inductor selection.
Equation 12
Usually the resistive component of the ripple is much higher than the capacitive one, if the
output capacitor adopted is not a multi layer ceramic capacitor (MLCC) with very low ESR
value.
The output capacitor is important also for loop stability: it fixes the double LC filter pole and
the zero due to its ESR. In Chapter 6.4, it will be illustrated how to consider its effect in the
system stability.
For example with V
OUT
=5 V, V
IN
=24 V, ΔI
L
=0.9 A (resulting by the inductor value), in order to
have a ΔV
OUT
=0.01·V
OUT
, if the multi layer ceramic capacitor are adopted, 10uF are needed
and the ESR effect on the output voltage ripple can be neglected. In case of not negligible
ESR (electrolytic or tantalum capacitors), the capacitor is chosen taking into account its
ESR value. So in case of 330 µF with ESR=30 mΩ, the resistive component of the drop
dominates and the voltage ripple is 28 mV.
The output capacitor is also important to sustain the output voltage when a load transient
with high slew rate is required by the load. When the load transient slew rate exceeds the
system bandwidth the output capacitor provides the current to the load. So if the high slew
rate load transient is required by the application the output capacitor and system bandwidth
have to be chosen in order to sustain the load transient.
In the table below some capacitor series are listed.
Table 8. Output capacitors
Manufacturer Series Cap value (μF) Rated voltage (V) ESR (mΩ)
MURATA
GRM32 22 to 100 6.3 to 25 < 5
GRM31 10 to 47 6.3 to 25 < 5
PANASONIC
ECJ 10 to 22 6.3 < 5
EEFCD 10 to 68 6.3 15 to 55
SANYO TPA/B/C 100 to 470 4 to 16 40 to 80
TDK C3225 22 to 100 6.3 < 5
ΔV
OUT
ESR ΔI
MAX
⋅
ΔI
MAX
8C
OUT
f
SW
⋅⋅
-------------------------------------+=