Datasheet

Application informations L7981
16/44 Doc ID 15182 Rev 3
Given the physical dimension, ceramic capacitors can meet well the requirements of the
input filter sustaining an higher input RMS current than electrolytic / tantalum types. In this
case the equation of C
IN
as a function of the target V
PP
can be written as follows:
Equation 7
neglecting the small ESR of ceramic capacitors.
Considering η=1, this function has its maximum in D=0.5, thus, given the maximum peak to
peak input voltage (V
PP_MAX
), the minimum input capacitor (C
IN_MIN
) value is:
Equation 8
Typically C
IN
is dimensioned to keep the maximum peak-peak voltage in the order of 1% of
V
INMAX
In Table 6. some multi layer ceramic capacitors suitable for this device are reported
A ceramic bypass capacitor, as close to the VCC and GND pins as possible, so that
additional parasitic ESR and ESL are minimized, is suggested in order to prevent instability
on the output voltage due to noise. The value of the bypass capacitor can go from 100 nF to
1 µF.
6.2 Inductor selection
The inductance value fixes the current ripple flowing through the output capacitor. So the
minimum inductance value in order to have the expected current ripple has to be selected.
The rule to fix the current ripple value is to have a ripple at 20%-40% of the output current.
In the continuos current mode (CCM), the inductance value can be calculated by the
following equation:
Table 6. Input MLCC capacitors
Manufacture Series Cap value (μF) Rated voltage (V)
Ta i yo Yu d e n
UMK325BJ106MM-T 10 50
GMK325BJ106MN-T 10 35
Murata GRM32ER71H475K 4.7 50
C
IN
I
O
V
PP
F
SW
⋅
-------------------------- -
1
D
η
----–
⎝⎠
⎛⎞
D
D
η
----
1D–()⋅+⋅⋅=
C
IN_MIN
I
O
2V
PP_MAX
F
SW
⋅⋅
----------------------------------------------- -=