L6728 Datasheet, PDF (19/32 Pages) STMicroelectronics – Single phase PWM controller with PowerGOOD

L6728
11 Application information
Application information
11.1
Inductor design
The inductance value is defined by a compromise between the dynamic response time, the
efficiency, the cost and the size. The inductor has to be calculated to maintain the ripple
current (∆I_L) between 20% and 30% of the maximum output current (typ). The inductance
value can be calculated with the following relationship:
L
=
-^V----^I-^N-----–-----^V----^O----^U----^T-
F_SW ⋅ ∆I_L
⋅
-^V----^O----^U---^T--
V_IN
Where F_SW is the switching frequency, V_IN is the input voltage and V_OUT is the output
voltage. Figure 9 shows the ripple current vs. the output voltage for different values of the
inductor, with V_IN = 5V and V_IN = 12V.
Increasing the value of the inductance reduces the current ripple but, at the same time,
increases the converter response time to a dynamic load change. The response time is the
time required by the inductor to change its current from initial to final value. Until the inductor
has not finished its charging time, the output current is supplied by the output capacitors.
Minimizing the response time can minimize the output capacitance required. If the
compensation network is well designed, during a load variation the device is able to set a
duty cycle value very different (0% or 80%) from steady state one. When this condition is
reached, the response time is limited by the time required to change the inductor current.
Figure 9. Inductor current ripple vs. output voltage
10
8
6
4
2
0
0
1
2
3
4
Output voltage [V]
Vin=12V, L=1uH
Vin=12V, L=2uH
Vin=5V, L=500nH
Vin=5V, L=1.5uH
5
19/32