Datasheet

T
S
=
1
f
S
x (V
IN
- V
OUT
)
L =
2'i
L
DT
S
V
IN
- V
OUT
L
=
2'i
L
DT
S
t
L
i'
OUT
I
S
T
S
DT
L
V
OUT
L
- V
OUT
V
IN
D =
V
OUT
+ V
SW_BOT
V
IN
+ V
SW_BOT
± V
SW_TOP
D =
V
OUT
V
IN
LM26420
www.ti.com SNVS579F –FEBRUARY 2009–REVISED MARCH 2013
(1)
The voltage drop across the internal NMOS (SW_BOT) and PMOS (SW_TOP) must be included to calculate a
more accurate duty cycle. Calculate D by using the following formulas:
(2)
V
SW_TOP
and V
SW_BOT
can be approximated by:
V
SW_TOP
= I
OUT
x R
DSON_TOP
(3)
V
SW_BOT
= I
OUT
x R
DSON_BOT
(4)
The inductor value determines the output ripple current. Lower inductor values decrease the size of the inductor,
but increase the output ripple current. An increase in the inductor value will decrease the output ripple current.
One must ensure that the minimum current limit (2.4A) is not exceeded, so the peak current in the inductor must
be calculated. The peak current (I
LPK
) in the inductor is calculated by:
I
LPK
= I
OUT
+ Δi
L
(5)
Figure 35. Inductor Current
(6)
In general,
Δi
L
= 0.1 x (I
OUT
) → 0.2 x (I
OUT
) (7)
If Δi
L
= 20% of 2A, the peak current in the inductor will be 2.4A. The minimum ensured current limit over all
operating conditions is 2.4A. One can either reduce Δi
L
, or make the engineering judgment that zero margin will
be safe enough. The typical current limit is 3.3A.
The LM26420 operates at frequencies allowing the use of ceramic output capacitors without compromising
transient response. Ceramic capacitors allow higher inductor ripple without significantly increasing output ripple.
See the output capacitor section for more details on calculating output voltage ripple. Now that the ripple current
is determined, the inductance is calculated by:
(8)
Where
(9)
When selecting an inductor, make sure that it is capable of supporting the peak output current without saturating.
Inductor saturation will result in a sudden reduction in inductance and prevent the regulator from operating
correctly. The peak current of the inductor is used to specify the maximum output current of the inductor and
saturation is not a concern due to the exceptionally small delay of the internal current limit signal. For example, if
the designed maximum output current is 2.0A and the peak current is 2.3A, then the inductor should be specified
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