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LT3761H

60 VIN LED Controller with Internal PWM Generator

Linear Technology 

LT3761

Pin Functions

PWMOUT (Pin 1): 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 over-

voltage condition—will toggle if the FB input is greater

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): Voltage Loop Feedback Pin. FB is intended for

constant-voltage regulation or for LED protection and open

LED detection. The internal transconductance amplifier with

output VC will regulate FB to 1.25V (nominal) through the

DC/DC converter. If the FB input exceeds the regulation

voltage, VFB, minus 50mV and the voltage between ISP

and ISN has dropped below the C/10 threshold of 25mV

(typical), the OPENLED pull-down is asserted. This action

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

overvoltage threshold, the PWMOUT and GATE pins 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): Connection Point for the Negative Terminal

of the Current Feedback Resistor. The constant output

current regulation can be programmed by ILED = 250mV/

RLED when CTRL > 1.2V or ILED = (CTRL – 100mV)/(4 •

RLED). If ISN is greater than INTVCC, input bias current

is typically 20μA flowing into the pin. Below INTVCC,

ISN bias current decreases until it flows out of the pin.

ISP (Pin 4): Connection Point for the Positive Terminal of

the Current Feedback Resistor. Input bias current depends

upon CTRL pin voltage. When it is greater than INTVCC

it flows into the pin. Below INTVCC, ISP bias current

decreases until it flows out of the pin. If the difference

between ISP and ISN exceeds 600mV (typical), then an

overcurrent event is detected. In response to this event,

the GATE and PWMOUT pins are driven low to protect

the switching regulator, a 1.5mA pulldown on PWM and

a 9mA pulldown on the DIM/SS pin are activated for 4µs.

VC (Pin 5): Transconductance Error Amplifier Output Pin

Used to Stabilize the Switching Regulator Control Loop

with an RC Network. The VC pin is high impedance when

PWM is low. This feature allows the VC pin to store the

demand current state variable for the next PWM high

transition. Connect a capacitor between this pin and GND;

a resistor in series with the capacitor is recommended for

fast transient response.

CTRL (Pin 6): Current Sense Threshold Adjustment Pin.

Constant current regulation point VISP-ISN is one-fourth

VCTRL plus an offset for 0V ≤ CTRL ≤ 1V. For CTRL >

1.2V the VISP-ISN current regulation point is constant at

the full-scale value of 250mV. For 1V ≤ CTRL ≤ 1.2V, the

dependence of VISP-ISN upon CTRL voltage transitions

from a linear function to a constant value, reaching 98% of

full-scale value by CTRL = 1.1V. Do not leave this pin open.

VREF (Pin 7): Voltage Reference Output Pin, Typically 2V.

This pin drives a resistor divider for the CTRL pin, either

for analog dimming or for temperature limit/compensation

of LED load. It can be bypassed with 10nF or greater, or

less than 50pF. Can supply up to 185µA (typical).

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

oscillator and disconnects the VC pin from all internal

loads. PWMOUT pin follows the PWM pin, except in fault

conditions. The PWM pin can be driven with a digital

signal to cause pulse width modulation (PWM) dimming

of an LED load. The digital signal should be capable of

sourcing or sinking 200μA at the high and low thresholds.

During start-up when DIM/SS is below 1V, the first rising

edge of PWM enables switching which continues until

VISP-ISN ≥ 25mV or SS ≥ 1V. Connecting a capacitor from

PWM pin to GND invokes a self-driving oscillator where

internal pull-up and pull-down currents set a duty ratio

for the PWMOUT pin for dimming LEDs. The magnitude

of the pull-up/down currents is set by the current in the

DIM/SS pin. The capacitor on PWM sets the frequency of

the dimming signal. For hiccup mode response to output

short-circuit faults, connect this pin as shown in the ap-

plication titled Boost LED Driver with Output Short-Circuit

Protection. If not used, connect the PWM pin to INTVCC.

OPENLED (Pin 9): An open-drain pull-down on this pin

asserts if the FB input is greater than the FB regulation

voltage (VFB) minus 50mV (typical) AND the difference

3761f

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