Datasheet

L =
I
LR
x f
SW
x V
IN
V
OUT
x (V
IN
- V
OUT
)
1.3 x 10
-10
V
IN(MAX)
x 150 ns
R
ON
t
LMR24210
www.ti.com SNVS738G –OCTOBER 2011–REVISED APRIL 2013
APPLICATIONS INFORMATION
EXTERNAL COMPONENTS
The following guidelines can be used to select external components.
R
FB1
and R
FB2
: These resistors should be chosen from standard values in the range of 1.0 kΩ to 10 kΩ,
satisfying the following ratio:
R
FB1
/R
FB2
= (V
OUT
/0.8V) - 1 (9)
For V
OUT
= 0.8V, the FB pin can be connected to the output directly with a pre-load resistor drawing more than
20 µA. This is needed because the converter operation needs a minimum inductor current ripple to maintain
good regulation when no load is connected.
R
ON
: Equation 2 can be used to select R
ON
if a desired operating frequency is selected. But the minimum value
of R
ON
is determined by the minimum on-time. It can be calculated as follows:
(10)
If R
ON
calculated from Equation 2 is smaller than the minimum value determined in Equation 10, a lower
frequency should be selected to re-calculate R
ON
by Equation 2. Alternatively, V
IN(MAX)
can also be limited in
order to keep the frequency unchanged. The relationship of V
IN(MAX)
and R
ON
is shown in Figure 24.
On the other hand, the minimum off-time of 260 ns can limit the maximum duty ratio.
Figure 24. Maximum V
IN
for selected R
ON
L: The main parameter affected by the inductor is the amplitude of inductor current ripple (I
LR
). Once I
LR
is
selected, L can be determined by:
where
• V
IN
is the maximum input voltage and
• f
SW
is determined from Equation 2. (11)
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