LT3599(RevD) データシートの表示（PDF） - Linear Technology

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LT3599
(Rev.:RevD)

4-Channel 120mA LED Driver with ±1.5% Current Matching

Linear Technology 

LT3599

Applications Information

Using the TSET Pin for Thermal Protection

The LT3599 contains a special programmable thermal

regulation loop that limits the internal junction temperature

of the part. Since the LT3599 topology consists of a single

boost converter with four linear current sources, any LED

string voltage mismatch will cause additional power to be

dissipated in the package. This topology provides excellent

current matching between LED strings and allows a single

power stage to drive a large number of LEDs, but at the

price of additional power dissipation inside the part (which

means a higher junction temperature). Being able to limit

the maximum junction temperature allows the benefits of

this topology to be fully realized. This thermal regulation

feature provides important protection at high ambient tem-

peratures, and allows a given application to be optimized

for typical, not worst case, ambient temperatures with the

assurance that the LT3599 will automatically protect itself

and the LED strings under worst-case conditions.

The operation of the thermal loop is simple. As the ambi-

ent temperature increases, so does the internal junction

temperature of the part. Once the programmed maximum

junction temperature is reached, the LT3599 begins to

linearly reduce the LED current, as needed, to try and

maintain this temperature. This can only be achieved

when the ambient temperature stays below the desired

maximum junction temperature. If the ambient tempera-

ture continues to rise past the programmed maximum

junction temperature, the LEDs’ current will be reduced

to approximately 5% of the full LED current.

While this feature is intended to directly protect the LT3599,

it can also be used to derate the LED current at high tem-

peratures. Since there is a direct relationship between the

LED temperature and LT3599 junction temperature, the

TSET function also provides some LED current derating

at high temperatures.

VREF

R2

LT3599

TSET

R1

3599 F08

Figure 8. Programming the TSET Pin

14

Two external resistors program the maximum IC junction

temperature using a resistor divider from the VREF pin, as

shown in Figure 8. Choose the ratio of R1 and R2 for the

desired junction temperature. Table 6 shows commonly

used values for R1 and R2 (see TSET graph).

Table 6. Resistor Values to Program Maximum IC Junction

Temperature

TJ (°C)

100

R1 (k)

80.6

R2 (k)

53.6

105

82.5

53.6

110

82.5

51.1

115

84.5

51.1

120

84.5

49.9

135

84.5

44.2

145

90.9

44.2

Programming Switching Frequency

The switching frequency of the LT3599 is set between

200kHz and 2.1MHz by an external resistor connected

between the RT pin and ground (see Table 7). Do not

leave this pin open.

Selecting the optimum switching frequency depends

on several factors. Inductor size is reduced with higher

frequency, but efficiency drops due to higher switching

losses. In addition, some applications require very high duty

cycles to drive a large number of LEDs from a low supply.

Low switching frequency allows a greater operational duty

cycle and, hence, a greater number of LEDs to be driven.

In each case, the switching frequency can be tailored to

provide the optimum solution. When programming the

switching frequency, the total power losses within the IC

should be considered.

Table 7. Switching Frequency

SWITCHING FREQUENCY (MHz)

2.1

2.0

1.5

1.0

0.5

0.4

0.3

0.2

RT (k)

20

21.5

31.6

53.6

121

154

210

324

3599fd

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