LT3797
9
3797fa
For more information www.linear.com/LT3797
TG1, TG2, TG3 (Pins 14, 33, 35): Top Gate Driver Out-
put Pins for Driving LED Loads Disconnect P-Channel
MOSFETs (PMOSs). One for each channel. An inverted
PWM signal drives an external PMOS gate of the respec-
tive converter between V
ISP
and (V
ISP
– 6.5V). Leave TG
pins unconnected if not used.
ISN1, ISN2, ISN3 (Pins 15, 32, 36): Connection Points for
the Negative Terminals of the Current Feedback Resistors.
ISP1, ISP2, ISP3 (Pins 16, 31, 37): Connection Points
for the Positive Terminals of the Current Feedback Resis-
tors. Also serves as positive rails for TG pin drivers and
the reference point for FBH.
FBH1, FBH2, FBH3 (Pins 17, 30, 38): V
oltage Loop Feed-
back Pins. The output feedback voltage V
FB
is measured
between the ISP pin and the FBH pin (absolute value):
V
FB
= |ISP – FBH|. The FBH pin is intended for constant-
voltage regulation or for LED protection/open-LED detec-
tion for each channel. In an open-LED event, the internal
amplifier with output VC regulates V
FB
to 1.25V (typical)
through the respective converter. If V
FB
is above the over-
voltage threshold (typical 1.3V), the TG pin of the same
channel is driven high to disconnect the external PMOS to
protect the
LEDs from an overvoltage event. Either open-
LED or overvoltage event signals a fault condition. Do
not leave the FBH pins open. It requires ISP to be no less
than 4.5V to maintain an accurate V
FB1
voltage sense. If
ISP falls below 4.5V, the voltage regulation is deactivated
and the ISP-ISN current regulation dominates regardless
of the |ISP-FBH| value. If not used, connect the FBH pin
to the ISP pin of the same channel.
VC1, VC2, VC3 (Pins 19, 28, 40): Error Amplifier Com-
pensation Pins. Connect a series RC from each VC pin to
GND. In each channel, the VC pin is high impedance when
the PWM pin is low, or the CTRL pin is below 150mV. This
feature allows the VC pin to store the demand current
state variable for the next PWM or CTRL high transition.
SS1, SS2, SS3 (Pins 20, 27, 41): Soft-Start Pins. Each
SS pin modulates compensation VC pin voltage of the
respective channel. Each of the soft-start intervals is set
with an external capacitor.
SENSEN1, SENSEN2, SENSEN3 (Pins 21, 26, 42): The
Negative Current Sense Inputs for the Control Loops.
Kelvin connect the SENSEN pin to the negative terminal
of the switch current sense resistor (which connects to
the GND plane) of the respective converter.
SENSEP1, SENSEP2, SENSEP3 (Pins 22, 25, 43): The
Positive Current Sense Inputs for the Control Loops.
Kelvin connect the SENSEP pin to the positive terminal
of the switch current sense resistor in the source of the
external N-channel MOSFET (NMOS) switch of the respec-
tive converter.
GATE1, GATE2, GATE3 (Pins 23, 24, 44): N-Channel
MOSFET Gate Driver Outputs. Switch between INTV
CC
and
GND. Driven to GND during shutdown, fault or idle states.
INTV
CC
(Pins 45, 46): INTV
CC
pins are the integrated power
supply output voltage nodes that provide supply for control
circuits and NMOS gate drivers. The two INTV
CC
pins are
internally shorted. Must be bypassed with a 10µF ceramic
capacitor placed close to the pins.
SW2 (Pin 47): Integrated Power Supply Switch Node.
Connect this pin to one side of the integrated power sup-
ply inductor.
BOOST (Pin 48): Connect this pin to SW1 pin through a
0.1µF ceramic capacitor.
SW1 (Pin 49): Integrated Power Supply Switch Node.
Connect this pin to the other side of the integrated power
supply inductor, and to the BOOST pin with a 0.1µF ce-
ramic capacitor.
V
IN
(Pin 50): Input Supply Pin. If V
IN
is over 41V (typical),
the integrated INTV
CC
power supply is turned off. All three
channels are also turned off (including pulling the GATE
pins to GND and TG pins to ISP) and the soft-starts are
reset. Must be locally bypassed with low ESR capacitors
placed close to the pin.
pin FuncTions
