Datasheet

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
D
V
IN
+ V
D
- V
SW
D =
V
OUT
V
IN
LMR10520
www.ti.com SNVS730B –OCTOBER 2011–REVISED APRIL 2013
Design Guide
Inductor Selection
The Duty Cycle (D) can be approximated quickly using the ratio of output voltage (V
O
) to input voltage (V
IN
):
(1)
The catch diode (D1) forward voltage drop and the voltage drop across the internal PMOS must be included to
calculate a more accurate duty cycle. Calculate D by using the following formula:
(2)
V
SW
can be approximated by:
V
SW
= I
OUT
x R
DSON
(3)
The diode forward drop (V
D
) can range from 0.3V to 0.7V depending on the quality of the diode. The lower the
V
D
, the higher the operating efficiency of the converter. 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
(4)
Figure 23. Inductor Current
(5)
In general,
Δi
L
= 0.1 x (I
OUT
) → 0.2 x (I
OUT
) (6)
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