Datasheet

L
t
L
i
L
i'
S
T
S
DT
( )
tL
I
L
V
IN
VV
OUT
IN
-
K
=
c
D
V
OUT
V
IN
D =
V
OUT
-
IN
V
OUT
V
-
D1
1
1
=
c
D
=
V
OUT
V
IN
¸
¹
·
¨
©
§
LMR62421
www.ti.com SNVS734B –OCTOBER 2011–REVISED APRIL 2013
SOFT-START
This function forces V
OUT
to increase at a controlled rate during start up. During soft-start, the error amplifier’s
reference voltage ramps to its nominal value of 1.255V in approximately 4.0ms. This forces the regulator output
to ramp up in a more linear and controlled fashion, which helps reduce inrush current.
INDUCTOR SELECTION
The Duty Cycle (D) can be approximated quickly using the ratio of output voltage (V
O
) to input voltage (V
IN
):
(1)
Therefore:
(2)
Power losses due to the diode (D1) forward voltage drop, the voltage drop across the internal NMOS switch, the
voltage drop across the inductor resistance (R
DCR
) and switching losses must be included to calculate a more
accurate duty cycle (See Calculating Efficiency and Junction Temperature for a detailed explanation). A more
accurate formula for calculating the conversion ratio is:
where
• η equals the efficiency of the LMR62421 application. (3)
The inductor value determines the input ripple current. Lower inductor values decrease the size of the inductor,
but increase the input ripple current. An increase in the inductor value will decrease the input ripple current.
Figure 15. Inductor Current
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