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LT3796

100V Constant-Current and Constant-Voltage Controller with Dual Current Sense

Linear Technology 

LT3796

PIN FUNCTIONS

ISP (Pin 1): Connection Point for the Positive Terminal

of the Current Feedback Resistor (RLED). Also serves as

positive rail for TG pin driver.

ISN (Pin 2): Connection Point for the Negative Terminal

of the Current Feedback Resistor (RLED).

TG (Pin 3): Top Gate Driver Output. An inverted PWM signal

drives series PMOS device between VISP and (VISP – 7V)

if VISP > 7V. An internal 7V clamp protects the PMOS gate

by limiting VGS. Leave TG unconnected if not used.

GND (Pins 4, 17, 21, 22, Exposed Pad Pin 29): Ground.

These pins also serve as current sense input for control

loop, sensing negative terminal of current sense resistor in

the source of the N-channel MOSFET. Solder the exposed

pad directly to ground plane.

ISMON (Pin 5): ISP/ISN Current Report Pin. The LED

current sensed by ISP/ISN inputs is reported as VISMON =

ILED • RLED • 4. Leave ISMON pin unconnected if not used.

When PWM is low, ISMON is driven to ground. Bypass

with a 47nF capacitor or higher if needed.

FB2 (Pin 6): Voltage Loop Feedback 2 Pin. This pin is

connected to the internal transconductance amplifier posi-

tive input node. The internal transconductance amplifier

with output VC regulates FB2 to 1.25V through the DC/

DC converter. If FB2 is driven above 1.3V, the TG pin is

pulled high to turn off the external PMOS and GATE pin is

driven to GND to turn off the external N-channel MOSFET.

Connect to GND if not used.

FB1 (Pin 7): Voltage Loop Feedback 1 Pin. FB1 is intended

for constant-voltage regulation or for LED protection/open

LED detection. The internal transconductance amplifier

with output VC regulates FB1 to 1.25V (nominal) through

the DC/DC converter. If the FB1 input is regulating the loop

and V(ISP-ISN) is less than 25mV (normal), the VMODE

pull-down is asserted. This action may signal an open

LED fault. If FB1 is driven above the 1.3V (by an external

power supply spike, for example), the FAULT pull-down is

asserted, the GATE pin is pulled low to turn off the external

N-channel MOSFET and the TG pin is driven high to protect

the LEDs from an overcurrent event. Do not leave the FB1

pin open. If not used, connect FB1 to GND.

VC (Pin 8): Transconductance Error Amplifier Output Pin.

Used to stabilize the control loop with an RC network.

This pin is high impedance when PWM is low, a feature

that stores 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. Do not leave

this pin open.

CTRL (Pin 9): Current Sense Threshold Adjustment Pin.

Regulating threshold V(ISP-ISN) is 0.25 • VCTRL plus an offset

for 0.1V < VCTRL < 1V. For VCTRL > 1.2V the current sense

threshold is constant at the full-scale value of 250mV. For

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

threshold upon VCTRL transitions from a linear function

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

VCTRL = 1.1V. Connect CTRL to VREF for the 250mV default

current threshold. Do not leave this pin open. Pull CTRL

pin to GND for zero LED current.

VREF (Pin 10): Voltage Reference Output Pin. Typically

2.015V. This pin drives a resistor divider for the CTRL

pin, either for analog dimming or for temperature limit/

compensation of LED load. It can supply up to 100μA.

SS (Pin 11): Soft-Start Pin. This pin modulates oscillator

frequency and compensation pin voltage (VC) clamp. The

soft-start interval is set with an external capacitor. The pin

has a 28μA (typical) pull-up current source to an internal

2.5V rail. This pin can be used as fault timer. Provided the

SS pin has exceeded 1.7V, the pull-up current source is

disabled and a 2.8µA pull-down current enabled when any

one of the following fault conditions happen:

1. LED overcurrent

2. INTVCC undervoltage

3. Thermal limit

The SS pin must be discharged below 0.2V to reinitiate a

soft-start cycle. Switching is disabled until SS is recharged.

RT (Pin 12): Switching Frequency Adjustment Pin. Set the

frequency using a resistor to GND (for resistor values, see

the Typical Performance curve or Table 2). Do not leave

the RT pin open.

3796f

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