LT3755 Datasheet, PDF (7/24 Pages) Linear Technology – 40VIN, 75VOUT LED Controller

LT3755/LT3755-1
PIN FUNCTIONS (MSOP/QFN)
PWMOUT (Pin 1/Pin 11): Buffered Version of PWM Signal
for Driving LED Load Disconnect NMOS or Level Shift. This
pin also serves a protection function for the FB overvoltage
condition—will toggle if the FB input is greater than the FB
regulation voltage (V_FB) plus 60mV (typical). The PWMOUT
pin is driven from INTV_CC. Use of a FET with gate cut-off
voltage higher than 1V is recommended.
FB (Pin 2/Pin 12): Voltage Loop Feedback Pin. FB is
intended for constant-voltage regulation or for LED protec-
tion/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 is regulating
the loop, the OPENLED pull-down is asserted. This ac-
tion may signal an open LED fault. If FB is driven above
the FB threshold (by an external power supply spike, for
example), the OPENLED pull-down will be de-asserted and
the PWMOUT pin 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/Pin 13): Connection Point for the Negative
Terminal of the Current Feedback Resistor. The LED cur-
rent can be programmed by I_LED = 100mV/R_LED when
V_CTRL > 1.2V or I_LED = V_CTRL –100mV/(10 • R_LED). If ISN
is greater than 2.9V, input bias current is typically 20μA.
Below 3V, ISN is an input to the short-circuit protection
feature that forces GATE to 0V if ISN is more than 150mV
(typ) below ISP.
ISP (Pin 4/Pin 14): Connection Point for the Positive Termi-
nal of the Current Feedback Resistor. Input bias current for
this pin is typically 30μA. ISP is an input to the short-circuit
protection feature when ISP is less than 3.1V.
VC (Pin 5/Pin 15): Transconductance Error Amplifier
Output Pin Used to Stabilize the Voltage 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.
CTRL (Pin 6/Pin 16): Current Sense Threshold Adjustment
Pin. Regulating threshold V_ISP – V_ISN is 1/10th V_CTRL plus
an offset. CTRL linear range is from GND to 1.1V. Connect
CTRL to V_REF for the 100mV default threshold. Do not
leave this pin open.
V_REF (Pin 7/Pin 1): 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. Can supply up to 100μA.
PWM (Pin 8/Pin 2): A signal low turns off switcher, idles
oscillator and disconnects VC pin from all internal loads.
PWMOUT pin follows PWM pin. PWM has an internal
pull-down resistor. If not used, connect to INTV_CC.
OPENLED (Pin 9/Pin 3, LT3755 Only): An open-drain
pull-down on OPENLED asserts if the FB input is greater
than the FB regulation threshold minus 50mV (typical).
To function, the pin requires an external pull-up resistor.
When the PWM input is low and the DC/DC converter is
idle, the OPENLED condition is latched to the last valid
state when the PWM input was high. When PWM input
goes high again, the OPENLED pin will be updated. This
pin may be used to report an open LED fault.
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