L7986 STMicroelectronics, L7986 Datasheet - Page 18

• Manufacturer Part Number: L7986
• Description: 3A step-down switching regulator
• Manufacturer: STMicroelectronics

Specifications of L7986

• Low Dropout Operation: 100% duty cycle

Application information
6.3
18/44
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 must 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.
stability.
For example, with V
to have a ΔV
needed and the ESR effect on the output voltage ripple can be neglected. In the case of
non-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
must be chosen in order to sustain the load transient.
In
Table 8.
Manufacturer
Table 8
PANASONIC
MURATA
SANYO
TDK
some capacitor series are listed.
OUT
Output capacitors
=0.01·V
OUT
TPA/B/C
GRM32
GRM31
EEFCD
=5 V, V
Series
C3225
OUT
Chapter 6.4
ECJ
ΔV
, if the multi-layer ceramic capacitors are adopted, 10 µF are
Doc ID 022450 Rev 1
IN
OUT
=24 V, ΔI
=
illustrates how to consider its effect in the system
ESR ΔI
Cap value (μF)
100 to 470
L
22 to 100
22 to 100
10 to 47
10 to 22
10 to 68
=0.9 A (resulting from the inductor value), in order
⋅
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
L7986