LT3597EUHG#TRPBF Datasheet LT3597IUHG#PBF, LT3597EUHG#PBF, LT3597EUHG#TRPBF | P13-P15

LT3597

3597fa

applicaTions inForMaTion

Figure 4. LED Current vs CTRL1-3 Voltage

Figure 5. LED Current Using PWM Dimming

CTRL1-3 (V)

LED CURRENT (mA)

120

100

0.25 1.2510.75

3597 F04

1.50.5

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 V

REF

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.

LED1-3

CURRENT

PWM1-3

3597 F06

PWM

ON(PWM)

MAX I

LED

Figure 6. Programming the T

SET

LT3597

3597 F07

SET

REF

LT3597

3597fa

applicaTions inForMaTion

Table 6 shows commonly used values for R1 and R2.

Choose the ratio of R1 and R2 for the desired junction

temperature limit as described in Figure 7.

Table 6. T

SET

Programmed Junction Temperature

(°C) R1 (kΩ) R2 (kΩ)

85 49.9 97.6

100 49.9 90.9

115 49.9 84.5

The T

SET

pin must be tied to V

REF

if the temperature pro-

tection feature is not desired.

Figure 8. Programming the CTRLM Pin

LED Current Derating Using the CTRLM Pin

Another feature of the LT3597 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

LT3597 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

will fall below the internal 1V voltage reference, causing

LED currents to be controlled by the CTRLM pin voltage.

The LED current curve breakpoint and slope versus tem-

perature are deﬁned by the choice of resistor ratios and

use of temperature-dependent resistance in the divider

for the CTRLM pin.

LT3597

3597 F08

CTRLM

(OPTION

A TO D)

REF

NTC

–50

SET

VOLTAGE (V)

0.4

0.5

75 125

3597 F07

0.3

0.8

0.6

0.7

50250–25 100

TEMPERATURE (°C)

Figure 7. T

SET

Voltage for Temperature Derating

LT3597

3597fa

applicaTions inForMaTion

CTRLM (V)

LED CURRENT (mA)

120

100

0.25 1.2510.75

3597 F09

1.50.5

Table 7 shows a list of manufacturers/distributors of NTC

resistors. There are several other manufacturers avail-

able and the chosen supplier should be contacted for

more detailed information. If an NTC resistor is used to

indicate LED temperature, it is effective only if the resis-

tor is placed as closely as possible to the LED strings.

LED derating curves shown by manufacturers are listed

for ambient temperature. The NTC resistor should have

the same ambient temperature as the LEDs. Since the

temperature dependency of an NTC resistor can be non-

linear over a wide range of temperatures, it is important to

obtain a resistor’s exact value over temperature from the

manufacturer. Hand calculations of the CTRLM voltage can

then be performed at each given temperature, resulting in

the CTRLM versus temperature plotted curve. Iterations of

resistor value calculations may be necessary to achieve the

desired break point and slope of the LED current derating

curve. From the CTRLM voltage, the LED current can be

found using the curve shown in Figure 9.

Table 7. NTC Resistor Manufacturers/Distributors

Murata www.murata.com

TDK Corporation www.tdk.com

Digi-Key www.digikey.com

If calculating the CTRLM voltage at various temperatures

gives a downward slope that is too strong, use alternative

resistor networks (B, C, D in Figure 8). They use tempera-

ture independent resistance to reduce the effects of the

NTC resistor over temperature.

Figure 10. Programming Switching Frequency

Murata Electronics provides a selection of NTC resistors

with complete data over a wide range of temperatures.

In addition, a software tool is available which allows the

user to select from different resistor networks and NTC

resistor values, and then simulate the exact output volt-

age curve (CTRLM behavior) over temperature. Referred

to as the “Murata Chip NTC Thermistor Output Voltage

Simulator,” users can log onto www.murata.com and

download the software followed by instructions for creat-

ing an output voltage V

OUT

(CTRLM) from a speciﬁed V

supply (V

REF

The CTRLM pin must be tied to V

REF

if the temperature

derating function is not desired.

Programming Switching Frequency

The switching frequency of the LT3597 can be programmed

between 200kHz and 1MHz by an external resistor con-

nected between the RT pin and ground. Do not leave this

pin open. See Table 8 and Figure 10 for resistor values

and corresponding frequencies.

Table 8. R

Resistor Selection

SWITCHING FREQUENCY (MHz) R

VALUE (kΩ)

1.0 33.2

0.5 80

0.2 220

(kΩ)

25 50

SWITCHING FREQUENCY (MHz)

1.2

1.0

0.8

0.4

0.6

0.2

17515012510075

3597 F10

225200

Figure 9. LED Current vs CTRLM Voltage

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