Datasheet

V
IN
I
L AVG
= I
OUT
'I
L
Time
Time
I
L
V
SW
L
MIN
=
(V
IN
- V
OUT
) x D
'i
L
x f
SW
D =
V
OUT
V
IN
C
IN
PVIN SW
GND
FB
PGOOD
R
FB1
R
FB2
C
OUT
EN
C
SS
SS/TRK
AVIN
C
F
C
C1
COMP
R
C1
V
IN
LM20123
L
R
F
VCC
C
VCC
V
OUT
PGND
V
IN
R
PG
V
PG
LM20123
www.ti.com SNVS524E –OCTOBER 2007–REVISED MARCH 2013
Figure 25. Typical Application Circuit
The first equation to calculate for any buck converter is duty-cycle. Ignoring conduction losses associated with
the FETs and parasitic resistances it can be approximated by:
(2)
INDUCTOR SELECTION (L)
The inductor value is determined based on the operating frequency, load current, ripple current, and duty cycle.
The inductor selected should have a saturation current rating greater than the peak current limit of the device.
Keep in mind the specified current limit does not account for delay of the current limit comparator, therefore the
current limit in the application may be higher than the specified value. To optimize the performance and prevent
the device from entering current limit at maximum load, the inductance is typically selected such that the ripple
current, Δi
L
, is less than 30% of the rated output current. Figure 26, shown below illustrates the switch and
inductor ripple current waveforms. Once the input voltage, output voltage, operating frequency, and desired ripple
current are known, the minimum value for the inductor can be calculated by the formula shown below:
(3)
Figure 26. Switch and Inductor Current Waveforms
If needed, slightly smaller value inductors can be used, however, the peak inductor current, I
OUT
+ Δi
L
/2, should
be kept below the peak current limit of the device. In general, the inductor ripple current, Δi
L
, should be greater
than 10% of the rated output current to provide adequate current sense information for the current mode control
loop. If the ripple current in the inductor is too low, the control loop will not have sufficient current sense
information and can be prone to instability.
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