LT3756 datasheet(10/31 Pages) AD

10 / 31 page

LT8355-1

Rev. A

For more information www.analog.com

PIN FUNCTIONS

PWMTG1, PWMTG2 (Pins 1, 21): High Side Gate Driver

for the External Series PMOS Switch of Each Channel.

Use these pins to disconnect the load for PWM dimming

as well as fault events. PWMTG drives the PMOS gate

between ISP – 8.5V and ISP (typical) to cut off the flow of

residual charge from the output capacitor when the LED

load should be disconnected, as well as to prevent the

long transient that would result from needing to recharge

the output capacitor at the end of a long PWM off period.

Leave open if unused. The voltage at PWMTG is limited

to 8.5V (typical) below the voltage at ISP to protect the

gate of the PMOS switch.

ISN1, ISN2 (Pins 2, 20): Kelvin connect this pin to the low

side of the LED current feedback sense resistors RILED1,2

of each channel. Current through sense resistor, RILED2

is 250mV/RILED2 while CTRL2 > 1.5V and IADJ2 > 1.5V.

It varies as [(CTRL2 – 0.48V) • (IADJ2 – 0.5V) –0.02V]/

(4RILED2) if the voltage at the CTRL2 and IAJD2 pins vary

between 0.5V to 1.5V. The current through sense resistor

RILED1 is 250mV/RILED1 while CTRL1 > 1.5V. It varies as

(CTRL1 − 500mV)/(4RILED1) if the voltage at the CTRL1

pin is between 500mV and 1.5V.

ISP1, ISP2 (Pins 3, 19): Kelvin connect this pin to the

high side of the LED current feedback sense resistor

RILED of each channel, see ISN1, ISN2 for more details.

If the voltage difference VISP – VISN ever exceeds around

670mV (typical), switching stops, PWMTG goes high

to disconnect the load, and soft-start resets. After a

cooldown period, the part will re-enter soft-start. See the

Applications Information section for more details.

FB1, FB2 (Pins 4, 18): Output Voltage Feedback Pin for

Each Channel. This pin is used for output voltage regula-

tion and limiting. Tie this pin to a resistive voltage divider

from the output voltage. When the voltage at FB reaches

1.2V (typical), the control loop will reduce the switch cur-

rent to regulate the output voltage such that FB remains

around 1.2V. If the voltage at FB exceeds 1.26V (typical),

PWMTG is driven high and switching briefly stops. If the

voltage at FB falls below 300mV (typical) after soft-start

has finished, PWMTG is driven high, soft-start resets,

and switching stops. After a cooldown period, the part

will re-enter soft-start. See the Applications Information

section for more info on the use of the FB pin for general

applications.

VC1, VC2 (Pins 5, 17): An Internal Error Amplifier Node

Used for Compensation. Stabilize the loop by connecting

a capacitor or RC network between this pin and ground for

each channel. Read more in the Applications Information

section for details about compensation.

CTRL1, CTRL2, IADJ2 (Pins 6, 8, 9): Two-Pin Analog

Alternative to PWM Dimming. Tie CTRL1,2 and IADJ2 to

VREF or INTVCC to set RILED current to full-scale for each

channel. The product of the offset CTRL2 and IADJ2 pin

voltages sets the current in RILED2 as [(CTRL2 – 0.48V) •

(IADJ2 – 0.5V) – 0.02V]/(4RILED2) when the voltages at

CTRL2 and IADJ2 pins vary between 0.5V to 1.5V. The

pin voltage of CTRL1 linearly sets the current in RILED1

as (CTRL1 – 0.5V)/(4RILED1) when the voltage at CTRL1

pin varies between 0.5V and 1.5V.

FAULT1, FAULT2 (Pins 7, 13): Open-Drain Fault Indication

Pin Indicating Short LED, Open LED, Overvoltage and

Overcurrent Faults for Each Channel. Tie these pins

through 100k resistors to INTVCC, or any supply less than

15V or use them as open-drain signals. LT8355-1 pulls

these pins low to signal all reported fault events from each

channel independently.

PWM1, PWM2 (Pins 10, 11): Pulse Width Modulation

(PWM) Dimming Generation Control Pin for Each Channel.

Connect an analog signal to this pin to use the internal

exponential PWM dimming generator. When the voltages

at these pins remain between 0.5V and 1.5V, the duty ratio

of the internal dimming PWM generator will vary with the

pin voltage. A linear ramp of voltage on the PWM pins

between 0.5V and 1.5V lasting many PWMTG cycles at

fSW/1000 will result in an exponentially increasing PWM

duty ratio. An external PWM signal can also drive this pin

directly if the ON and OFF voltages are above 1.6V and

below 0.4V, respectively. If PWM dimming is not used,

tie these pins to INTVCC or VREF.

RT (Pin 12): Connect a resistor between this pin and

ground to set the switching frequency and PWM dimming

frequency. Do not connect anything but a resistor to this

pin or the device may not function correctly.

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