Datasheet

Application information L7986
16/45 DocID022450 Rev 7
In this case the equation of C
IN
as a function of the target V
PP
can be written as follows:
Equation 10
neglecting the small ESR of ceramic capacitors.
Considering η = 1, this function has its maximum in D = 0.5, therefore, given the maximum
peak-to-peak input voltage (V
PP_MAX
), the minimum input capacitor (C
IN_MIN
) value is:
Equation 11
Typically C
IN
is dimensioned to keep the maximum peak-to-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, must be selected.
The rule to fix the current ripple value is to have a ripple at 20% - 40% of the output current.
In the continuous current mode (CCM), the inductance value can be calculated by the
following equation:
Equation 12
where T
ON
is the conduction time of the internal high-side switch and T
OFF
is the conduction
time of the external diode [in CCM, F
SW
= 1 / (T
ON
+ T
OFF
)]. The maximum current ripple, at
fixed V
OUT
, is obtained at maximum T
OFF
which is at minimum duty cycle (see Section 6.1 to
calculate minimum duty). So, by fixing ΔI
L
= 20% to 30% of the maximum output current, the
minimum inductance value can be calculated:
Table 6. Input MLCC capacitors
Manufacture Series Cap value (µF) Rated voltage (V)
Taiyo Yuden
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
⋅⋅
----------------------------------------------- -=
ΔI
L
V
IN
V
OUT
–
L
------------------------------ T
ON
⋅
V
OUT
V
F
+
L
---------------------------- T
OFF
⋅==