LM3224 datasheet(15/18 Pages) NSC

15 / 18 page

Application Information (Continued)

The LM3224 can be configured to drive high current white

LEDs for the flash and torch functions of a digital camera,

camera phone, or any other similar light source. The flash/

torch can be set up with the circuit in Figure 4 by using the

resistor R

SET to determine the amount of current that will

flow through the LED using the equation:

LED =VFB/RSET

If the flash and torch modes will both be used the resistor

SET can be chosen for the higher current flash value. To

flash the circuit pull the SHDN high for the time duration

needed for the flash. To enable a lower current torch mode a

PWM signal can be applied to the SHDN pin. The torch

current would then be approximately the percent ON time of

the PWM signal multiplied by the flash (or maximum) cur-

rent. The optional disconnect FET can be used to eliminate

leakage current through the LEDs when the part is off and

also to disconnect the LED when the input voltage exceeds

the forward voltage drop of the LED. The maximum output

current the LM3224 can supply in this configuration is shown

in Table 1.

Figure 5 is another method of driving a high current white

LED. This circuit has a higher component count but allows

the switcher to remain on continuously for torch mode reduc-

ing stress on the supply. The two FETs also double for a

disconnect function as described above. In this circuit the

device and the torch enable FET are turned on setting a

lower current through the LED. When flash is needed the

flash enable FET is turned on to increase the current for the

amount of time desired. The minimum guaranteed maximum

output current for this circuit is the same as for Figure 4.

TABLE 1. Maximum LED Drive current

SW=1.25MHz, L=4.7µH, LED VFMAX=4V (VOUT=5.26V)

LED Drive Current (mA)

4.2

1077

4.1

1047

4.0

1017

3.9

987

3.8

958

3.7

929

3.6

900

3.5

871

3.4

842

3.3

814

3.2

785

3.1

757

3.0

729

2.9

701

2.8

673

2.7

646

Some Recommended Inductors (Others May Be Used)

Manufacturer

Inductor

Contact Information

Coilcraft

DO3316 and DT3316 series

www.coilcraft.com

800-3222645

TDK

SLF10145 series

www.component.tdk.com

847-803-6100

Pulse

P0751 and P0762 series

www.pulseeng.com

Sumida

CDRH8D28 and CDRH8D43 series

www.sumida.com

Some Recommended Input And Output Capacitors (Others May Be Used)

Manufacturer

Capacitor

Contact Information

Vishay Sprague

293D, 592D, and 595D series tantalum

www.vishay.com

407-324-4140

Taiyo Yuden

High capacitance MLCC ceramic

www.t-yuden.com

408-573-4150

Cornell Dubilier

ESRD seriec Polymer Aluminum Electrolytic

SPV and AFK series V-chip series

www.cde.com

MuRata

High capacitance MLCC ceramic

LM3224 Datasheet(PDF) 15 Page - National Semiconductor (TI)

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