LM27341_15 datasheet(18/44 Pages) TI

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D =

VOUT + VD1

VIN + VD1 - VDS

K =

POUT

POUT + PLOSS

K =

POUT

CFF <=

VOUT x COUT

IOUT x R1

1 =

VOUT

- 1

VREF

x R2

LM27341, LM27342, LM27341-Q1, LM27342-Q1

SNVS497E – NOVEMBER 2008 – REVISED APRIL 2013

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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:

ID1 = IOUT x (1-D)

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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 VIN < 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 VOUT and the FB pin. A good starting value for R2 is 1 kΩ.

(22)

FEED FORWARD CAPACITOR (OPTIONAL)

A feed forward capacitor CFF can improve the transient response of the converter. Place CFF in parallel with R1.

The value of CFF should place a zero in the loop response at, or above, the pole of the output capacitor and

RLOAD. The CFF capacitor will increase the crossover frequency of the design, thus a larger minimum output

capacitance is required for designs using CFF. CFF should only be used with an output capacitance greater than

or equal to 44 uF.

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Calculating Efficiency, and Junction Temperature

The complete LM27341/LM27342 DC-DC converter efficiency can be calculated in the following manner.

(24)

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Calculations for determining the most significant power losses are shown below. Other losses totaling less than

2% are not discussed.

Power loss (PLOSS) 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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