LT3756 datasheet(8/26 Pages) AD

8 / 26 page

LT3756/LT3756-1/LT3756-2

Rev. C

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PIN FUNCTIONS

PWMOUT (Pin 1/Pin 11): Buffered Version of PWM Signal

for Driving LED Load Disconnect NMOS or Level Shift.

This pin also serves in a protection function for the FB

overvoltagecondition—willtoggleiftheFBinputisgreater

than the FB regulation voltage (VFB) plus 60mV (typical).

The PWMOUT pin is driven from INTVCC. Use of a FET with

gate cut-off voltage higher than 1V is recommended.

FB (Pin 2/Pin 12): Voltage Loop Feedback Pin. FB is

intendedforconstant-voltageregulationorforLEDprotec-

tion/open LED detection. The internal transconductance

amplifierwithoutputVCwillregulateFBto1.25V(nominal)

through the DC/DC converter. If the FB input is regulating

the loop, the OPENLED pull-down is asserted. This ac-

tion may signal an open LED fault. If FB is driven above

the FB threshold (by an external power supply spike, for

example),theOPENLEDpull-downwillbede-assertedand

the PWMOUT pin will be driven low to protect the LEDs

from an overcurrent event. Do not leave the FB pin open.

If not used, connect to GND.

ISN (Pin 3/Pin 13): Connection Point for the Negative

Terminal of the Current Feedback Resistor. If ISN is greater

than 2.9V, the LED current can be programmed by ILED =

100mV/RLEDwhenVCTRL>1.2VorILED=(VCTRL–100mV)/

(10 • RLED) when VCTRL ≤ 1V. Input bias current is typi-

cally 25µA. Below 3V, ISN is an input to the short-circuit

protection feature that forces GATE to 0V if ISP exceeds

ISN by more than 150mV (typ).

ISP (Pin 4/Pin 14): Connection Point for the Positive

Terminal of the Current Feedback Resistor. Input bias

current is dependent upon CTRL pin voltage as shown

in the TPC. ISP is an input to the short-circuit protection

feature when ISN is less than 3V.

VC (Pin 5/Pin 15): Transconductance Error Amplifier

Output Pin Used to Stabilize the Voltage Loop with an RC

Network. This pin is high impedance when PWM is low, a

feature that stores the demand current state variable for

thenextPWMhightransition.Connectacapacitorbetween

this pin and GND; a resistor in series with the capacitor is

recommended for fast transient response.

(MSOP/QFN)

TYPICAL PERFORMANCE CHARACTERISTICS T

A = 25°C, unless otherwise noted.

ISP/ISN Input Bias Current

vs CTRL Voltage

INTVCC Dropout Voltage

vs Current, Temperature

CTRL (V)

375612 G19

0.5

1.5

ISP

ISN

LDO CURRENT (mA)

375612 G20

–2.5

–1.0

–1.5

–2.0

–0.5

–45°C

125°C

VIN = 7V

25°C

150°C

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