L7986TA STMicroelectronics, L7986TA Datasheet - Page 18

• Manufacturer Part Number: L7986TA
• Description: 3A step-down switching regulator
• Manufacturer: STMicroelectronics

Specifications of L7986TA

• Low Dropout Operation: 100% duty cycle

Application informations
6.3
18/43
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 15
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
system stability.
For example with V
have a ΔV
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.
Manufacturer
PANASONIC
MURATA
SANYO
TDK
OUT
Output capacitors
=0.01·V
OUT
OUT
TPA/B/C
=5 V, V
GRM32
GRM31
EEFCD
Series
C3225
ECJ
, if the multi layer ceramic capacitor are adopted, 10 µF are needed
Chapter
ΔV
Doc ID 022098 Rev 1
IN
OUT
=24 V, ΔI
=
6.4, it will be illustrated how to consider its effect in the
ESR ΔI
Cap value (μF)
L
100 to 470
22 to 100
22 to 100
=0.9 A (resulting by the inductor value), in order to
10 to 47
10 to 22
10 to 68
⋅
MAX
+
------------------------------------ -
8 C
⋅
ΔI
OUT
Rated voltage (V)
MAX
⋅
f
6.3 to 25
6.3 to 25
SW
4 to 16
6.3
6.3
6.3
ESR (mΩ)
15 to 55
40 to 80
< 5
< 5
< 5
< 5
L7986TA