LT3757 datasheet(13/22 Pages) AD

13 / 22 page

LT3935

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Below 250mV, the CTRL pin commands zero LED current,

and above 1.25V, it commands the maximum. When an

independent voltage source is not available, the intermedi-

ate CTRL voltages may be derived from the 2V reference

at the VREF pin using a resistor network or potentiometer

as long as the total current drawn from the VREF pin is

less than 1mA.

Additionally, the LT3935 is capable of interpreting a pulse

at the CTRL pin. The high level of the pulse must be greater

than 1.6V. The low level must be less than 400mV. The

frequency must be greater than 100kHz and less than

1MHz.ThentheregulatedvoltagebetweenISPandISNwill

vary with the duty ratio of the pulse as shown in Figure 2.

In this case, the LED current is zero for duty ratios less

than 12.5% and reaches its maximum above 62.5%. The

LT3935 will cease switching if the duty ratio of the CTRL

pin pulse is less than 10%, and also for DC CTRL pin

voltages less than 200mV.

Figure 1. Analog CTRL Range

Figure 2. Duty Ratio CTRL Range

Figure 3. Setting CTRL with NTC Resistors

UsethefollowingguidetoconfiguretheLT3935according

to your application requirements and to select the

appropriate external components.

Programming LED Current with the CTRL Pin

TheprimaryfunctionoftheLT3935istoregulatethecurrent

for a string of LEDs. This current should pass through a

series current sense resistor that can be placed anywhere

in the string. Then the voltage across this resistor will be

sensed by the current regulation amplifier through the

ISP and ISN pins and regulated to a level programmed by

the CTRL pin. The maximum resistor voltage that can be

programmed is 100mV, which corresponds to 4A through

theLEDstringwhena25mΩcurrentsenseresistorisused.

To allow for this maximum current, the CTRL pin may

be connected directly to the VREF pin, which provides

an accurate 2V reference. Lower current levels can be

programmed by DC CTRL voltages between 250mV and

1.25V as shown in Figure 1.

0 0.25V

0.75V

ILED

VCTRL

1.25V 1.5V

3935 F01

VCTRL < 200mV

CTRL–OFF

100mV

RSNS

50mV

RSNS

LT3935

CTRL

VREF

RCTRL1

RCTRL2

RNTC

LT3935

3935 F03

CTRL

VREF

RCTRL1

RCTRL2

RNTC

0 12.5%

37.5%

62.5% 75%

3935 F02

DCTRL < 10%

CTRL–OFF

100mV

RSNS

50mV

RSNS

ILED

DCTRL

To reduce the LED current when the temperature of the

LEDs rises, use resistors with negative temperature coef-

ficient (NTC) in the network from VREF to CTRL as shown

in Figure 3.

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