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LT3597

3597f

APPLICATIONS INFORMATION

Figure 4. LED Current vs CTRL1-3 Voltage

Figure 5. LED Current Using PWM Dimming

CTRL1-3 (V)

120

100

0.25

1.25

0.75

3597 F04

1.5

0.5

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 VREF.

For True Color PWM dimming, the LT3597 provides up to

10,000:1 PWM dimming range at 100Hz. This is achieved

byallowingthedutycycleofthePWM1-3pintobereduced

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 TSET 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

LT3597willautomaticallyprotectitselfandtheLEDstrings

under worst-case conditions.

Astheambienttemperatureincreases,sodoestheinternal

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-

grammedjunctiontemperature.Iftheambienttemperature

continues to rise above the programmed maximum junc-

tion temperature, the LED current will reduce to less than

20% of the full current.

AresistordividerfromtheVREFpinprogramsthemaximum

part junction temperature as shown in Figure 6.

LED1-3

CURRENT

PWM1-3

3597 F06

tPWM

tON(PWM)

MAX ILED

Figure 6. Programming the TSET Pin

LT3597

3597 F07

TSET

VREF

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