LT3477 datasheet(12/26 Pages) AD

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LT3950

Rev. 0

For more information www.analog.com

Introduction

LT3950 provides a variety of features to enable a wide

range of application options. Due to the small package

size and pin count, many pins perform more than one

function. The simplest LT3950 application is realizable in

only a few off-chip components and with very few design

choices on the part of the user. To fully utilize the capabil-

ity of the part, however, requires a good understanding

of how the individual features play together. The detailed

explanations of each feature below along with several

example application circuits will help with developing a

good understanding of the part.

Programming the LED Current

Full-scale current through the LED string is easily pro-

grammed using a single resistor (RSENSE) connected in

series with the LED string. The sense resistor should be

placed on the high side of the LED string, and Kelvin con-

nected to sense pins ISP and ISN. The loop will regulate

a drop of 250mV across this sense resistor, scaled by

the voltage at the CTRL pin, discussed later. A half-Watt

resistor will usually be sufficient. Since the loop regulates

the voltage across RSENSE, typical full-scale load current

(IFS) is defined as

IFS =

4RSENSE

For the best performance and protection, sense at the

highest potential in the LED string, and tie the source of

the external PWM dimming PMOS to ISN. More details

on the PWM dimming PMOS will be discussed later.

Note that the loop is able to regulate LED current down

to ISP = 0V, but a ground sensing configuration is not

desirable. For more information see the Low Voltage

(SEPIC) Startup section.

Adjusting LED Current with the CTRL Pin

The CTRL pin provides an analog alternative to PWM dim-

ming. The value of the voltage regulated across the sense

resistor can be adjusted with the CTRL pin. As the voltage

present at this pin varies from 200mV to 1.2V, the regu-

lated voltage drop across the sense resistor varies from

0V to 250mV. The system will supply no current if the

APPLICATIONS INFORMATION

CTRL pin voltage falls below 200mV, and will continue to

supply full-scale current after the voltage at the CTRL pin

exceeds 1.2V, all the way up to INTVCC. Total LED current

(ILED), taking the effects of the CTRL pin into account is

then:

ILED =

VCTRL – 0.2V

4RSENSE

A, 0.2V < VCTRL ≤ 1.2V

Note the CTRL pin can also be used for PWM by driving it

with a digital signal whose low value is below 100mV and

whose high value is above 1.3V. This technique for PWM

dimming will not allow very high frequency PWM signals.

The frequency of PWM driving the CTRL pin should be

below 1kHz. For higher frequency PWM, use the PWM pin,

discussed later. A graphical depiction of the full range of

CTRL pin voltage appears below.

Figure 1. VISP-ISN vs CTRL Pin Voltage

VCTRL (V)

0.4

0.8

1.2

1.6

2.0

–50

100

150

200

250

300

3950 F01

Setting the Output Regulation Voltage

In addition to output current regulation, LT3950 can also

provide output voltage regulation. The loop will go into

voltage regulation mode when the voltage at the FB pin

approaches 1.2V. Regulating output voltage will reduce

LED current below its programmed value. Voltage regula-

tion mode is primarily included as a safety feature, allow-

ing the part to gracefully manage Open LED and similar

events.

To set the output regulation voltage, connect a resistive

voltage divider from the output voltage to FB as shown

below. Choose RFB1 and RFB2 to set the output voltage:

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