Datasheet

Application information L5980
16/42 Doc ID 13003 Rev 6
5.2 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.
The inductance value can be calculated by the following equation:
Equation 6
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 40% of the maximum output current, the
minimum inductance value can be calculated:
Equation 7
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
= 0.7 A and F
SW
= 250 kHz the minimum
inductance value to have ΔI
L
= 30% of I
O
is about 45 μH.
The peak current through the inductor is given by:
Equation 8
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)
Wurth PD3 L 33 to 68 1.35 to 1.8
Coilcraft
MSS1038 39 to 68 1.62 to 2.04
LPS6235 12 to 33 1.4 to 2.2
Coiltronics
DRQ73 10 to 22 1.67 to 2.47
LD2 27 to 47 1.64 to 2.1
SUMIDA
CDR6D28MN 10 to 22 1.65 to 2.5
CDRH105RNP 27 to 56 1.9 to 2.7
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
--------+=