L5980 데이터 시트보기 (PDF) - STMicroelectronics
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L5980 Datasheet PDF : 37 Pages
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Application information
L5980
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
ΔIL
=
-V----I-N-----–-----V----O----U----T-
L
⋅
TON
=
V-----O----U---T-----+-----V----F-
L
⋅
TOFF
Where TON is the conduction time of the internal high side switch and TOFF is the conduction
time of the external diode (in CCM, FSW = 1 / (TON + TOFF)).The maximum current ripple, at
fixed Vout, is obtained at maximum TOFF that is at minimum duty cycle (see previous section
to calculate minimum duty). So fixing ΔIL = 20 % to 40 % of the maximum output current, the
minimum inductance value can be calculated:
Equation 7
LMIN
=
V-----O----U---T-----+-----V----F-
ΔIMAX
⋅
1-----–-----D----M-----I-N--
FSW
where FSW is the switching frequency, 1/(TON + TOFF).
For example for VOUT = 3.3 V, VIN = 12 V, IO = 0.7 A and FSW = 250 kHz the minimum
inductance value to have ΔIL = 30 % of IO is about 45 μH.
The peak current through the inductor is given by:
Equation 8
IL, PK
=
IO
+
Δ-----I-L-
2
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
Wurth
Coilcraft
Coiltronics
SUMIDA
Series
PD3 L
MSS1038
LPS6235
DRQ73
LD2
CDR6D28MN
CDRH105RNP
Inductor value (μH)
33 to 68
39 to 68
12 to 33
10 to 22
27 to 47
10 to 22
27 to 56
Saturation current (A)
1.35 to 1.8
1.62 to 2.04
1.4 to 2.2
1.67 to 2.47
1.64 to 2.1
1.65 to 2.5
1.9 to 2.7
15/37
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