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LT3597
APPLICATIONS INFORMATION
LED Current Dimming
Two different types of dimming control are available with
the LT3597. The LED current can be dimmed using the
CTRL1-3 pin or the PWM1-3 pin.
For some applications, a variable DC voltage that adjusts
the LED current is the preferred method for brightness
control. In that case, the CTRL1-3 pin can be modulated
to set the LED dimming (see Figure 4). As the CTRL1-3 pin
voltage rises from 0V to 1.0V, the LED current increases
from 0mA to the maximum programmed LED current in a
linear fashion. As the CTRL1-3 pin continues to increase
past 1.0V, the maximum programmed LED current is
maintained. If this type of dimming control is not desired,
the CTRL1-3 pin can be tied to V REF .
120
100
80
60
40
20
0
0 0.25 0.5 0.75 1 1.25 1.5
CTRL1-3 (V)
3597 F04
Figure 4. LED Current vs CTRL1-3 Voltage
t PWM
For True Color PWM dimming, the LT3597 provides up to
10,000:1 PWM dimming range at 100Hz. This is achieved
by allowing the duty cycle of the PWM1-3 pin to be reduced
from 100% to 0.01% for a PWM frequency of 100Hz (see
Figure 5), hence a minimum on-time of 1μs and a maxi-
mum period of 100ms. PWM duty cycle dimming allows
for constant LED color to be maintained over the entire
dimming range.
Using the T SET Pin for Thermal Protection
The LT3597 contains a special programmable thermal
regulation loop that limits the internal junction tempera-
ture. This thermal regulation feature provides important
protection at high ambient temperatures, and allows a
given application to be optimized for typical, not worst-
case, ambient temperatures with the assurance that the
LT3597 will automatically protect itself and the LED strings
under worst-case conditions.
As the ambient temperature increases, so does the internal
junction temperature of the part. Once the programmed
maximum junction temperature is reached, the LT3597
linearly reduces the LED current, as needed, to maintain
this junction temperature. This can only be achieved when
the ambient temperature stays below the maximum pro-
grammed junction temperature. If the ambient temperature
continues to rise above the programmed maximum junc-
tion temperature, the LED current will reduce to less than
20% of the full current.
A resistor divider from the V REF pin programs the maximum
part junction temperature as shown in Figure 6.
t ON(PWM)
LT3597
PWM1-3
V REF
LED1-3
CURRENT
MAX I LED
R2
T SET
3597 F06
Figure 5. LED Current Using PWM Dimming
R1
3597 F07
Figure 6. Programming the T SET Pin
3597fa
13
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