Datasheet
D =
V
OUT
+ V
D1
V
IN
+ V
D1
- V
DS
K =
P
OUT
P
OUT
+ P
LOSS
K =
P
OUT
P
IN
C
FF
<=
V
OUT
x C
OUT
I
OUT
x R1
R1
=
V
OUT
- 1
V
REF
x R2
LM27341, LM27342, LM27341-Q1, LM27342-Q1
SNVS497E –NOVEMBER 2008–REVISED APRIL 2013
www.ti.com
CATCH DIODE
The catch diode (D1) conducts during the switch off-time. A Schottky diode is recommended for its fast switching
times and low forward voltage drop. The catch diode should be chosen so that its current rating is greater than:
I
D1
= I
OUT
x (1-D) (21)
The reverse breakdown rating of the diode must be at least the maximum input voltage plus appropriate margin.
To improve efficiency choose a Schottky diode with a low forward voltage drop.
BOOST DIODE (OPTIONAL)
For circuits with input voltages V
IN
< 5V and duty cycles (D) >0.75V. a small-signal Schottky diode is
recommended. A good choice is the BAT54 small signal diode. The cathode of the diode is connected to the
BOOST pin and the anode to a 5V voltage rail.
BOOST CAPACITOR
A ceramic 0.1 µF capacitor with a voltage rating of at least 6.3V is sufficient. The X7R and X5R MLCCs provide
the best performance.
OUTPUT VOLTAGE
The output voltage is set using the following equation where R2 is connected between the FB pin and GND, and
R1 is connected between V
OUT
and the FB pin. A good starting value for R2 is 1 kΩ.
(22)
FEED FORWARD CAPACITOR (OPTIONAL)
A feed forward capacitor C
FF
can improve the transient response of the converter. Place C
FF
in parallel with R1.
The value of C
FF
should place a zero in the loop response at, or above, the pole of the output capacitor and
R
LOAD
. The C
FF
capacitor will increase the crossover frequency of the design, thus a larger minimum output
capacitance is required for designs using C
FF
. C
FF
should only be used with an output capacitance greater than
or equal to 44 uF.
(23)
Calculating Efficiency, and Junction Temperature
The complete LM27341/LM27342 DC-DC converter efficiency can be calculated in the following manner.
(24)
Or
(25)
Calculations for determining the most significant power losses are shown below. Other losses totaling less than
2% are not discussed.
Power loss (P
LOSS
) is the sum of two basic types of losses in the converter, switching and conduction.
Conduction losses usually dominate at higher output loads, where as switching losses remain relatively fixed and
dominate at lower output loads. The first step in determining the losses is to calculate the duty cycle (D).
(26)
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