LT3756 LINER [Linear Technology], LT3756 Datasheet - Page 7

• Manufacturer Part Number: LT3756
• Description: 100VIN, 100VOUT LED Controller
• Manufacturer: LINER [Linear Technology]

PIN FUNCTIONS
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
pin is driven from INTV
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
amplifi er 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. If ISN is
greater than 2.9V, the LED current can be programmed
by I
–100mV/(10 • R
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 Ter-
minal 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 Amplifi er
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
an offset. CTRL linear range is from GND to 1.1V. Connect
CTRL to V
leave this pin open.
LED
= 100mV/R
REF
for the 100mV default threshold. Do not
LED
LED
). Input bias current is typically 20μA.
FB
) plus 60mV (typical). The PWMOUT
when V
CC
. Use of a FET with gate cut-off
ISP
(MSOP/QFN)
CTRL
– V
ISN
> 1.2V or I
is 1/10th V
LED
CTRL
= V
CTRL
plus
V
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
OPENLED (Pin 9/Pin 3, LT3756 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.
SYNC (Pin 9/Pin 3, LT3756-1 Only): The SYNC pin is used
to synchronize the internal oscillator to an external logic
level signal. The R
an internal switching frequency 20% slower than the SYNC
pulse frequency. Gate turn-on occurs a fi xed delay after
the rising edge of SYNC. For best PWM performance, the
PWM rising edge should occur at least 200ns before the
SYNC rising edge. Use a 50% duty cycle waveform to drive
this pin. This pin replaces OPENLED on LT3756-1 option
parts. If not used, tie this pin to GND.
SS (Pin 10/Pin 4): Soft-Start Pin. This pin modulates
oscillator frequency and compensation pin voltage (VC)
clamp. The soft-start interval is set with an external capaci-
tor. The pin has a 10μA (typical) pull-up current source
to an internal 2.5V rail. The soft-start pin is reset to GND
by an undervoltage condition (detected by SHDN/UVLO
pin) or thermal limit.
RT (Pin 11/Pin 5): Switching Frequency Adjustment Pin.
Set the frequency using a resistor to GND (for resistor
values, see the Typical Performance curve or Table 1).
Do not leave the RT pin open.
SHDN/UVLO (Pin 12/Pin 6): Shutdown and Undervoltage
Detect Pin. An accurate 1.22V falling threshold with ex-
ternally programmable hysteresis detects when power is
OK to enable switching. Rising hysteresis is generated by
the external resistor divider and an accurate internal 2μA
REF
(Pin 7/Pin 1): Voltage Reference Output Pin, Typically
T
resistor should be chosen to program
LT3756/LT3756-1
CC
.
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