LM27341_14 datasheet(15/42 Pages) TI1

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L =

VOUT + VD1

IOUT x r x fSW

x (1-DMIN)

D =

VOUT + VD1

VIN + VD1 - VDS

D =

VOUT

VIN

LM27341, LM27342, LM27341Q, LM27342Q

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SNVS497D – NOVEMBER 2008 – REVISED FEBRUARY 2010

Figure 36. Recommended Ripple Ratio Vs. Duty Cycle

The Duty Cycle (D) can be approximated quickly using the ratio of output voltage (VOUT) to input voltage (VIN):

(10)

The application's lowest input voltage should be used to calculate the ripple ratio. The catch diode forward

voltage drop (VD1) and the voltage drop across the internal NFET (VDS) must be included to calculate a more

accurate duty cycle. Calculate D by using the following formula:

(11)

VDS can be approximated by:

VDS = IOUT x RDS(ON)

(12)

The diode forward drop (VD1) can range from 0.3V to 0.5V depending on the quality of the diode. The lower VD1

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

•

DMIN is the duty cycle calculated with the maximum input voltage

•

fsw is the switching frequency

•

IOUT is the maximum output current of 2A

(13)

Using IOUT = 2A will minimize the inductor's physical size.

INDUCTOR CALCULATION EXAMPLE

Operating conditions for the LM27342 are:

VIN = 7 - 16V

VOUT = 3.3V

IOUT = 2A

fSW = 2 MHz

VD1 = 0.5V

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