LT3791
9
3791fb
For more information www.linear.com/LT3791
pin FuncTions
CTRL (Pin 1): Current Sense Threshold Adjustment Pin for
Analog Dimming. Regulating threshold V
(ISP-ISN)
is 1/10th
of (V
CTRL
– 200mV). CTRL linear range is from 200mV
to 1.1V. For V
CTRL
> 1.3V, the current sense threshold is
constant at the full-scale value of 100mV. For 1.1V < V
CTRL
< 1.3V, the dependence of the current sense threshold
upon V
CTRL
transitions from a linear function to a con-
stant value, reaching 98% of full scale by V
CTRL
= 1.2V.
Connect CTRL to V
REF
for the 100mV default threshold.
Force less than 175mV (typical) to stop switching. Do not
leave this pin open.
SS (Pin 2): Soft-start reduces the input power sources
surge current by gradually increasing the controller’s cur
-
rent limit
. A minimum value of 10nF is recommended on
this
pin. SS is used as a timer when an open or shorted
LED condition occurs. A 500k resistor placed from SS to
V
REF
will latch the part off in the event of a fault. If left
open, a 1.4µA current source pulls down on SS and the
part restarts in a fault.
PWM (Pin 3): A signal low turns off switches, idles switch
-
ing and disconnects the V
C
pin from all external loads. The
PWMOUT pin follows the PWM pin. PWM has an internal
100k pull-down resistor. If not used, connect to INTV
CC
.
OPENLED (Pin 4): An open-drain pull-down on OPENLED
asserts if FB is greater than 1.15V (typical) and V
(ISP-ISN)
is less than 10mV (typical). To function, the pin requires
an external pull-up resistor.
SHORTLED (Pin 5): An open-drain pull-down on
SHORTLED asserts
if FB is less than 400mV
(typical)
.
To function, the pin requires an external pull-up resistor.
V
REF
(Pin 6): Voltage Reference Output Pin, Typically 2V.
This pin drives a resistor divider for the CTRL pin, either
for analog dimming or for temperature limit/compensa
-
tion of the LED load. Can supply up to 200µA of current.
ISMON (Pin 7): Monitor pin that produces a voltage that
is ten times the voltage V
(ISP-ISN)
. ISMON will equal 1V
when V
(ISP-ISN)
= 100mV.
IVINMON (Pin 8): Monitor pin that produces a voltage
that is twenty times the voltage V
(IVINP-IVINN)
. IVINMON
will equal 1V when V
(IVINP-IVINN)
= 50mV.
EN/UVLO (Pin 9): Enable Control Pin. Forcing an accurate
1.2V falling threshold with an externally programmable
hysteresis
is generated by the external resistor divider
and a 3µA pull-down current. Above the 1.2V (typical)
threshold (but below 6V), EN/UVLO input bias current is
sub-µA. Below the falling threshold, a 3µA pull-down cur
-
rent is
enabled so the user can define the hysteresis with
the
external resistor selection. An undervoltage condition
resets soft-start. Tie to 0.3V, or less, to disable the device
and reduce V
IN
quiescent current below 1µA.
IVINP (Pin 10): Positive Input for the Input Current Limit
and Monitor. Input bias current for this pin is typically 90µA.
IVINN (Pin 11): Negative Input for the Input Current Limit
and Monitor. The input bias current for this pin is typically
20µA.
V
IN
(Pin 12): Main Input Supply. Bypass this pin to PGND
with a capacitor.
INTV
CC
(Pin 13): Internal 5V Regulator Output. The driver
and control circuits are powered from this voltage. Bypass
this pin to PGND with a minimum 4.7µF ceramic capacitor.
TG1 (Pin 14): Top Gate Drive. Drives the top N-channel
MOSFET with a voltage equal to INTV
CC
superimposed on
the switch node voltage SW1.
BST1 (Pin 15): Bootstrapped Driver Supply. The BST
1 pin
swings
from a diode voltage below INTV
CC
up to a diode
voltage below V
IN
+ INTV
CC
.
SW1 (Pin 16): Switch Node. SW1 pin swings from a diode
voltage drop below ground up to V
IN
.
PGND (Pins 17, 20): Power Ground. Connect these pins
closely to the source of the bottom N-channel MOSFET.
BG1 (Pin 18): Bottom Gate Drive. Drives the gate of the
bottom N-channel MOSFET between ground and INTV
CC
.
BG2 (Pin 19): Bottom Gate Drive. Drives the gate of the
bottom N-channel MOSFET between ground and INTV
CC
.
SW2 (Pin 21): Switch Node. SW2 pin swings from a diode
voltage drop below ground up to V
OUT
.
BST2 (Pin 22): Bootstrapped Driver Supply. The BST2 pin
swings from a diode voltage below INTV
CC
up to a diode
voltage below V
OUT
+ INTV
CC
.
