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LT3596IUHG

60V Step-Down LED Driver

Linear Technology 

LT3596

Applications Information

For True Color PWM dimming, the LT3596 provides up

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

by reducing the duty cycle of the PWMn pin from 100%

to 0.01% for a PWM frequency of 100Hz (see Figure 5).

This equates to a minimum on time of 1µs and a maxi-

mum period of 10ms. PWM duty cycle dimming allows

for constant LED color to be maintained over the entire

dimming range.

tPWM

tON(PWM)

LED1-3

CURRENT

MAX ILED

3596 F05

Figure 5. LED Current Using PWM Dimming

Using the TSET Pin for Thermal Protection

The LT3596 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. It allows an

application to be optimized for typical, not worst-case,

ambient temperatures with the assurance that the LT3596

automatically protects 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 LT3596

linearly reduces the LED current, as needed, to maintain

this junction temperature. This is only 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

10% of the full current.

A resistor divider from the VREF pin programs the maximum

IC junction temperature as shown in Figure 6.

Table 6 shows commonly used values for R1 and R2.

Choose the ratio of R1 and R2 for the desired junction

temperature limit as shown in Figure 7.

VREF

R2 LT3596

TSET

R1

3596 F06

Figure 6. Programming the TSET Pin

Table 6. TSET Programmed Junction Temperature

TJ (°C)

85

R1 (kΩ)

49.9

R2 (kΩ)

97.6

100

49.9

90.9

115

49.9

84.5

The TSET pin must be tied to VREF if the temperature

protection feature is not desired.

0.8

0.7

0.6

0.5

0.4

0.3

–50 –25

0 25 50 75 100 125

TEMPERATURE (°C)

3596 F07

Figure 7. VTSET for LED Current Derating

LED Current Derating Using the CTRLM Pin

A useful feature of the LT3596 is its ability to program

a derating curve for maximum LED current versus tem-

perature. LED data sheets provide curves of maximum

allowable LED current versus temperature to warn against

exceeding this current limit and damaging the LED. The

LT3596 allows the output LEDs to be programmed for

maximum allowable current while still protecting the

LEDs from excessive currents at high temperature. This

is achieved by programming a voltage at the CTRLM pin

with a negative temperature coefficient using a resistor

divider with temperature-dependent resistance (Figure 8).

As ambient temperature increases, the CTRLM voltage

falls below the internal 1V voltage reference, causing

3596fa

13

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