Datasheet

LM3224
SNVS277C –DECEMBER 2004–REVISED MARCH 2013 www.ti.com
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 24 by using the resistor R
SET
to determine the amount of current that will flow through the LED using the
equation:
I
LED
= V
FB
/R
SET
(23)
If the flash and torch modes will both be used the resistor R
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) current. 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 25 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 reducing 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 ensured maximum output current for this
circuit is the same as for Figure 24.
Table 1. Maximum LED Drive current
(F
SW
=1.25MHz, L=4.7µH, LED V
FMAX
=4V (V
OUT
=5.26V)
V
IN
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
Table 2. 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
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