Datasheet
LTM8040
11
8040fa
Programming LED Current
The LED current can be set by adjusting the voltage on the
ADJ pin. The ADJ pin is internally pulled up to a precision
1.25V voltage source through a 5.11k 1% resistor. This
resistor makes it easy to adjust the LED current with a
single external resistor. For a 1A LED current, leave the
ADJ pin fl oating. For lower output currents, apply a re-
sistor from ADJ to GND as shown in Figure 3, using the
following formula:
R
ADJ
= 5.11k • I
LED
/(1A – I
LED
),
where I
LED
is the desired current out of LEDA.
In order to have accurate LED current, a precision 1%
resistor is recommended.
Dimming Control
There are several different types of dimming control
circuits. Analog dimming control (Figure 4) changes the
voltage on the ADJ pin by tying a low on-resistance FET to
the resistor. This allows the selection of two different LED
currents. For reliable operation, program an LED current of
no less than 35mA. The maximum current dimming ratio
(I
RATIO
) can be calculated from the maximum LED current
(I
MAX
) and the minimum LED current (I
MIN
) as follows:
I
MAX
I
MIN
= I
RATIO
Figure 4. Dimming with an NFET and Resistor
GND
LTM8040
8040 F04
R2
ADJ
DIM
APPLICATION INFORMATION
The LEDA voltage must be at least 2.5V for proper current
regulation. See Table 2 for recommended R
ADJ
values.
Figure 3. Setting ADJ with a Resistor
GND
LTM8040
8040 F03
R
ADJ
ADJ