Datasheet

L =
V
OUT
+ V
D1
I
OUT
x r x f
SW
x (1-D
MIN
)
D =
V
OUT
+ V
D1
V
IN
+ V
D1
- V
DS
D =
V
OUT
V
IN
LM27341, LM27342, LM27341-Q1, LM27342-Q1
www.ti.com SNVS497E –NOVEMBER 2008–REVISED APRIL 2013
Figure 36. Recommended Ripple Ratio Vs. Duty Cycle
The Duty Cycle (D) can be approximated quickly using the ratio of output voltage (V
OUT
) to input voltage (V
IN
):
(10)
The application's lowest input voltage should be used to calculate the ripple ratio. The catch diode forward
voltage drop (V
D1
) and the voltage drop across the internal NFET (V
DS
) must be included to calculate a more
accurate duty cycle. Calculate D by using the following formula:
(11)
V
DS
can be approximated by:
V
DS
= I
OUT
x R
DS(ON)
(12)
The diode forward drop (V
D1
) can range from 0.3V to 0.5V depending on the quality of the diode. The lower V
D1
is, the higher the operating efficiency of the converter.
Now that the ripple current or ripple ratio is determined, the required inductance is calculated by:
where
• D
MIN
is the duty cycle calculated with the maximum input voltage
• f
sw
is the switching frequency
• I
OUT
is the maximum output current of 2A (13)
Using I
OUT
= 2A will minimize the inductor's physical size.
INDUCTOR CALCULATION EXAMPLE
Operating conditions for the LM27342 are:
V
IN
= 7 - 16V V
OUT
= 3.3V I
OUT
= 2A
f
SW
= 2 MHz V
D1
= 0.5V
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