Datasheet

DocID17928 Rev 5 15/29
ST1S40 Application information
29
In Table 6 some multi layer ceramic capacitors suitable for this device are reported.
A ceramic bypass capacitor, as close as possible to the V
INA
pin, 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 330 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, to have the expected current ripple, must be selected. The rule
to fix the current ripple value is to have a ripple at 20% to 40% of the output current.
In continuous current mode (CCM), the inductance value can be calculated by Equation 18
Equation 18
where T
ON
is the conduction time of the high side switch and T
OFF
is the conduction time of
the low side switch (in CCM, F
SW
= 1/(T
ON
+ T
OFF
)). The maximum current ripple, given the
Vout, is obtained at maximum T
OFF
, that is at minimum duty cycle. So by fixing I
L
= 20% to
30% of the maximum output current, the minimum inductance value can be calculated:
Equation 19
where F
SWMIN
is the minimum switching frequency, according to Table 4
The peak current through the inductor is given by:
Equation 20
so if the inductor value decreases, the peak current (that must be lower than the current limit
of the device) increases. The higher the inductor value, the higher the average output
current that can be delivered, without reaching the current limit.
Table 6. Input MLCC capacitors
Manufacturer Series Cap value (F) Rated voltage (V)
Murata
GRM31 10 25
GRM55 10 25
TDK C3225 10 25
I
L
V
IN
V
OUT
–
L
----------------------------- - T
ON

V
OUT
L
-------------- T
OFF
==
L
MIN
V
OUT
I
MAX
----------------
1D
MIN
–
F
SWMIN
---------------------- -=
I
LPK
I
O
I
L
2
--------+=