AT9919 datasheet(8/16 Pages) MICROCHIP

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AT9919

DS20005595A-page 8

 2016 Microchip Technology Inc.

FIGURE 3-2:

Inductor Current Waveform.

LED

DIM

RS(HI)

SENSE

RS(LO)

SENSE

DPDL

DPDH

S =

ΔI

Equation 3-3 shows how to determine the inductor

value for a desired operating frequency (fS).

EQUATION 3-3:

VIN VOUT

–

 V

OUT



fSVIN IO



------------------------------------------------------

VIN VOUT

–

 t

DPDL





-------------------------------------------------------

–

VOUTtDPDH



------------------------------

–

Where:



VRS HI



VRS LO



–

RSENSE

--------------------------------------------

and tDPDL and tDPDH are the propagation delays. 

Note that the current ripple (∆I) in the inductor (L) is greater than ∆IO.

The current ripple in the inductor (L) can be calculated

with Equation 3-4.

EQUATION 3-4:



VIN VOUT

–

 t

DPDL



-------------------------------------------------------

VOUTtDPDH

------------------------------

For proper inductor selection, note that the maximum

switching frequency occurs at the highest VIN and

VOUT = VIN/2.

3.8

MOSFET Selection

MOSFET selection is based on the maximum input

operating voltage VIN, output current ILED and

operating switching frequency. Choose a MOSFET that

has a higher breakdown voltage than the maximum

operation voltage, low RDS(ON) and low total charge for

better efficiency. MOSFET threshold voltage must be

adequate when operated at the low end of the input

voltage operating range.

3.9

Freewheeling Diode Selection

The forward voltage of the freewheeling diode should

be as low as possible for better efficiency. A Schottky

diode is a good choice as long as the breakdown

voltage is high enough to withstand the maximum

operating voltage. The forward current rating of the

diode must be at least equal to the maximum LED

current.

3.10

LED Current Ripple

The LED current ripple is equal to the inductor current

ripple. In cases when a lower LED current ripple is

needed, a capacitor can be placed across the LED

terminals.

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