L5987 Datasheet, PDF (18/42 Pages) STMicroelectronics – 3 A step-down switching regulator

Application information
L5987 - L5987A
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
ΔI_L
=
-^V----^I-^N-----^–-----^V----^O----^U----^T-
L
⋅
T_ON
=
V_{-----O----U---T-----}+_-----V_----F-
L
⋅
T_OFF
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
L_MIN
=
V_{-----O----U---T-----}+_-----V_----F-
ΔI_MAX
⋅
1_-----–_-----D_{----M-----I-N--}
F_SW
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
I_{L, PK}
=
I_O
+
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
Coilcraft
Series
MSS1038
MSS1048
Inductor value (μH)
3.8 to 10
12 to 22
Saturation current (A)
3.9 to 6.5
3.84 to 5.34
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