Datasheet

L6730 - L6730B Application details
Doc ID 11938 Rev 3 33/52
6 Application details
6.1 Inductor design
The inductance value is defined by a compromise between the transient response time, the
efficiency, the cost and the size. The inductor has to be calculated to maintain the ripple
current (ΔI
L
) between 20% and 30% of the maximum output current. 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 28 shows the ripple current vs. the output voltage for different values of the
inductor, with V
IN
= 5 V and V
IN
= 12 V at a switching frequency of 400 kHz.
Increasing the value of the inductance reduces the current ripple but, at the same time,
increases the converter response time to a load transient. If the compensation network is
well designed, during a load transient the device is able to set the duty cycle to 100% or to
0%. When one of these conditions is reached, the response time is limited by the time
required to change the inductor current. During this time the output current is supplied by
the output capacitors. Minimizing the response time can minimize the output capacitor size.
Figure 28. Inductor current ripple
Vin
Vout
IFsw
VoutVin
L
L
⋅
Δ⋅
−
≅
(6)
0
1
2
3
4
5
6
7
8
01234
OUTPUT VOLTAGE (V)
INDUCTOR CURRENT RIPPL
V
in=5V
,
L=500n
H
V
in=5V, L=1.5uH
V
in=12V, L=2uH
V
in=12V, L=1uH