Datasheet

Application information L6738A
26/32 Doc ID 18134 Rev 2
10 Application information
10.1 Inductor design
The inductance value is defined by a compromise between the dynamic response time, the
efficiency, the cost, and the size. The inductor must 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:
where F
SW
is the switching frequency, V
IN
is the input voltage and V
OUT
is the output
voltage. Figure 13 shows the ripple current vs. the output voltage for different values of the
inductor, with V
IN
= 5 V and V
IN
= 12 V.
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 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 the steady-state one. When this condition
is reached, the response time is limited by the time required to change the inductor current.
Figure 13. Inductor current ripple vs. output voltage
L
V
IN
V
OUT
–
F
SW
∆I
L
⋅
------------------------------
V
OUT
V
IN
--------------
⋅=