LT3486 datasheet(9/26 Pages) AD

9 / 26 page

LT3756/LT3756-1/LT3756-2

Rev. C

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CTRL(Pin6/Pin16):CurrentSenseThresholdAdjustment

Pin. Regulating threshold V(ISP – ISN) is 1/10th VCTRL plus

an offset for 0V < VCTRL < 1V. For VCTRL > 1.2V the current

sensethresholdisconstantatthefull-scalevalueof100mV.

For 1V < VCTRL < 1.2V, the dependence of current sense

threshold upon VCTRL transitions from a linear function

to a constant value, reaching 98% of full-scale value by

VCTRL = 1.1V. Do not leave this pin open.

VREF(Pin7/Pin1):VoltageReferenceOutputPin,Typically

2V.ThispindrivesaresistordividerfortheCTRLpin,either

foranalogdimmingorfortemperaturelimit/compensation

of LED load. Can supply up to 100μA.

PWM (Pin 8/Pin 2): A signal low turns off switcher, idles

oscillator and disconnects VC pin from all internal loads.

PWMOUT pin follows PWM pin. PWM has an internal

pull-down resistor. If not used, connect to INTVCC.

OPENLED (Pin 9/Pin 3, LT3756 and LT3756-2): An open-

collector pull-down on OPENLED asserts if the FB input

is greater than the FB regulation threshold minus 50mV

(typical). To function, the pin requires an external pull-up

current less than 1mA. When the PWM input is low and

the DC/DC converter is idle, the OPENLED condition is

latched to the last valid state when the PWM input was

high. When PWM input goes high again, the OPENLED

pin will be updated. This pin may be used to report an

open LED fault.

SYNC (Pin 9/Pin 3, LT3756-1 Only): The SYNC pin is used

to synchronize the internal oscillator to an external logic

level signal. The RT resistor should be chosen to program

aninternalswitchingfrequency20%slowerthantheSYNC

pulse frequency. Gate turn-on occurs a fixed delay after

the rising edge of SYNC. For best PWM performance, the

PWM rising edge should occur at least 200ns before the

SYNC rising edge. Use a 50% duty cycle waveform to drive

this pin. This pin replaces OPENLED on LT3756-1 option

parts. If not used, tie this pin to GND.

SS (Pin 10/Pin 4): Soft-Start Pin. This pin modulates

oscillator frequency and compensation pin voltage (VC)

clamp.Thesoft-startintervalissetwithanexternalcapaci-

tor. The pin has a 10µA (typical) pull-up current source

to an internal 2.5V rail. The soft-start pin is reset to GND

by an undervoltage condition (detected by SHDN/UVLO

pin) or thermal limit.

RT (Pin 11/Pin 5): Switching Frequency Adjustment Pin.

Set the frequency using a resistor to GND (for resistor

values, see the Typical Performance curve or Table 1).

Do not leave the RT pin open.

SHDN/UVLO (Pin 12/Pin 6): Shutdown and Undervoltage

Detect Pin. An accurate 1.22V falling threshold with ex-

ternally programmable hysteresis detects when power is

OK to enable switching. Rising hysteresis is generated

by the external resistor divider and an accurate internal

2.1µA pull-down current. Above the threshold (but below

6V), SHDN/UVLO input bias current is sub-µA. Below the

falling threshold, a 2.1µA pull-down current is enabled so

the user can define the hysteresis with the external resis-

tor selection. An undervoltage condition resets soft-start.

Tie to 0.4V, or less, to disable the device and reduce VIN

quiescent current below 1µA.

INTVCC (Pin 13/Pin 7): Regulated Supply for Internal

Loads, GATE Driver and PWMOUT Driver. Supplied from

VIN and regulates to 7.15V (typical). INTVCC must be

bypassed with a 4.7µF capacitor placed close to the pin.

Connect INTVCC directly to VIN if VIN is always less than

or equal to 8V.

VIN (Pin 14/Pin 8): Input Supply Pin. Must be locally

bypassed with a 0.22µF (or larger) capacitor placed close

to the IC.

SENSE (Pin 15/Pin 9): The current sense input for the

control loop. Kelvin connect this pin to the positive ter-

minal of the switch current sense resistor, RSENSE, in the

source of the NFET. The negative terminal of the current

sense resistor should be connected to the GND plane

close to the IC.

GATE (Pin 16/Pin 10): N-channel FET Gate Driver Output.

Switches between INTVCC and GND. Driven to GND during

shutdown, fault or idle states.

GND (Pin 17/Pin 17): Ground. This pin also serves as

current sense input for control loop, sensing negative

terminal of current sense resistor. Solder the exposed pad

directly to ground plane.

PIN FUNCTIONS

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