Datasheet

Application information L5987
18/41 Doc ID 14972 Rev 3
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:
Equation 10
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 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:
Equation 11
where F
SW
is the switching frequency, 1/(T
ON
+ T
OFF
).
For example for V
OUT
= 3.3 V, V
IN
= 12 V, I
O
= 3 A and F
SW
= 250 kHz the minimum
inductance value to have ΔI
L
= 30% of I
O
is about 10 μH.
The peak current through the inductor is given by:
Equation 12
So if the inductor value decreases, the peak current (that has to be lower than the current
limit of the device) increases. The higher is the inductor value, the higher is the average
output current that can be delivered, without reaching the current limit.
In the table below some inductor part numbers are listed.
Table 7. Inductors
Manufacturer Series Inductor value (μH) Saturation current (A)
Coilcraft
MSS1038 3.8 to 10 3.9 to 6.5
MSS1048 12 to 22 3.84 to 5.34
Wurth
PD Type L 8.2 to 15 3.75 to 6.25
PD Type M 2.2 to 4.7 4 to 6
SUMIDA
CDRH6D226/HP 1.5 to 3.3 3.6 to 5.2
CDR10D48MN 6.6 to 12 4.1 to 5.7
ΔI
L
V
IN
V
OUT
–
L
----------------------------- -
T
ON
⋅
V
OUT
V
F
+
L
----------------------------
T
OFF
⋅==
L
MIN
V
OUT
V
F
+
ΔI
MAX
----------------------------
1D
MIN
–
F
SW
---------------------- -
⋅=
I
LPK,
I
O
ΔI
L
2
--------+=