LT3755(RevA) Ver la hoja de datos (PDF) - Linear Technology

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LT3755
(Rev.:RevA)

40VIN, 75VOUT LED Controller

Linear Technology 

LT3755/LT3755-1

PIN FUNCTIONS (MSOP/QFN)

SYNC (Pin 9/Pin 3, LT3755-1 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

an internal switching frequency 20% slower than the SYNC

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 LT3755-1

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. The soft-start interval is set with an external capaci-

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 2μA

pull-down current. Above the 1.24V (nominal) threshold

(but below 6V), SHDN/UVLO input bias current is sub-μA.

Below the falling threshold, a 2μA pull-down current is

enabled so the user can define the hysteresis with the

external resistor selection. An undervoltage condition

resets soft-start. Tie to 0.4V, or less, to disable the device

and reduce VIN quiescent current below 1μA. Do not tie

SHDN/UVLO to a voltage higher than VIN.

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

GATE Driver and PWMOUT Driver. Supplied from VIN and

regulates to 7V (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 7V.

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.

Exposed Pad (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.

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