Datasheet

Application information ST1S41
16/27 DocID023654 Rev 2
Typically, C
IN
is dimensioned to keep the maximum peak-to-peak voltage ripple 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 as possible to the VINA 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 uF.
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%-40% of the output current.
In continuous current mode (CCM), the inductance value can be calculated by the following
equation:
Equation 19
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 (see previous section to
calculate minimum duty). So, fixing ΔI
L
=20% to 30% of the maximum output current, the
minimum inductance value can be calculated as:
Equation 20
where F
SWMIN
is the minimum switching frequency, according to Table 4.
The peak current through the inductor is given by:
Equation 21
Table 6. Input MLCC capacitors
Manufacturer Series Cap value (μF) Rated voltage (V)
Murata
GRM31 10 25
GRM55 10 25
TDK C3225 10 25
ΔI
L
V
IN
V
OUT
–
L
----------------------------- -
T
ON
⋅
V
OUT
L
--------------
T
OFF
⋅==
L
MIN
V
OUT
ΔI
MAX
----------------
1D
MIN
–
F
SWMIN
---------------------- -
⋅=
I
LPK,
I
O
ΔI
L
2
--------+=