LT3596IUHG View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LT3596IUHG

60V Step-Down LED Driver

Linear Technology 

LT3596

Applications Information

VREF

R2 LT3596

CTRLM

R1

(OPTION A TO D)

RNTC

(8a)

(8b)

RX

RX

RNTC

(8c)

RX

RNTC RX

3596 F08

(8d)

Figure 8. Programming the CTRLM Pin

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.

Table 7 shows a list of manufacturers/distributors of NTC

resistors. There are several other manufacturers available.

The chosen supplier should be contacted for more detailed

information.

Table 7. NTC Resistor Manufacturers/Distributors

Murata

www.murata.com

TDK Corporation

www.tdk.com

Digi-Key

www.digikey.com

If an NTC resistor is used to indicate LED temperature, it

is effective only if the resistor is placed as close as pos-

sible 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 dependence of an

NTC resistor is nonlinear as a function of temperature, it

is important to obtain its temperature characteristics from

the manufacturer. Hand calculations of the CTRLM voltage

are then performed at each given temperature using the

following equation:

VCTRLM

=

VREF

•





R1 

R1+ R2 

This produces a plot of VCTRLM versus temperature. From

this curve, the LED current is found using Figure 9.

Several iterations of resistor value calculations may be

necessary to achieve the desired breakpoint and slope of

the LED current derating curve.

14

100

RISETn = 20k

75

50

25

0

0 0.25 0.5 0.75 1 1.25 1.5

CTRLM VOLTAGE (V)

3596 F04

Figure 9. LED Current vs CTRLM Voltage

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.

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 on to www.murata.com and

download the software followed by instructions for creat-

ing an output voltage VOUT (CTRLM) from a specified VCC

supply (VREF).

The CTRLM pin must be tied to VREF if the temperature

derating function is not desired.

Programming Switching Frequency

The switching frequency of the LT3596 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.

Selecting the optimum switching frequency depends on

several factors. Inductor size is reduced with higher fre-

quency, but efficiency drops slightly due to higher switching

3596fa

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