LT3757 datasheet(16/22 Pages) AD

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LT3935

Rev. 0

For more information www.analog.com

Figure 7. ISMON Filter Configuration

LT3935

3935 F07

ISMON

RMON

CMON

APPLICATIONS INFORMATION

Figure 6. Placement of Input Capacitors

as possible to each of the pairs of VIN pins (Pins 15 and

22) and their adjacent GND pins as shown in Figure 6.

These two capacitors should be at least 0.1μF if possible.

The VIN pins can be jointed together using a trace on the

second layer of the circuit board.

The resistor should be 1MΩ. The capacitance can be as

large or small as needed without affecting the stability

of the internal amplifier. For example, when the PWM

frequency is 200Hz, a 100nF capacitor combined with

the 1MΩ resistor would limit the ripple on ISMON to 1%.

Selecting the FB Resistors

Tworesistorsshouldbeselectedtoformanetworkbetween

the output voltage and the FB pin as shown in Figure 8.

15 16 17 18 19

8 7 6 5 4

20 21 22

3 2 1

BST

100nF

3935 F06

GND

0.1µF

CERAMIC

VIN

0.1µF

CERAMIC

VIN

TOP LAYER

BOTTOM LAYER

Monitoring LED Current

TheISMONpinprovidesanamplifiedandbufferedmonitor

of the voltage between the ISP and ISN pins. The gain of

the internal amplifier is ten, and the speed is fast enough

to track the pulse-width modulated LED current. However,

as shown in Figure 7, the ISMON voltage can be filtered

with a resistor-capacitor network to monitor the average

LED current instead.

This network forms part of a voltage regulation loop when

FB is nearly 1V. In this case, the LT3935 will override the

programmed LED current to lower the output voltage

and limit FB to 1V. This resistor configuration therefore

determines the maximum output voltage.

Note that this voltage limit may be reached inadvertently

if it is set too close to the typical output voltage and the

output capacitor is too small. To avoid interference with

the current regulation, the feedback resistors should be

chosen such that FB is about 700mV when the LEDs are

conducting.

For a 12V string of LEDs, design for a maximum output

voltage of about 17V. Start with a 10k resistor for RFB1.

To calculate the value of RFB2, add 10k for every volt of

difference between FB (1V) and the maximum output

voltage. In this case, the nearest standard 1% value for

RFB2 would be 162k.

In this way, the LT3935 can also be configured as a volt-

age regulator instead of an LED driver. It will regulate the

output voltage near the programmed maximum as long as

the load current is less than the current level programmed

by CTRL.

Figure 8. FB Resistor Configuration

LT3935

3935 F08

VOUT

RFB2

RFB1

VOUT(MAX) = 1V • 1+

RFB2

RFB1

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