LT3795
pin FuncTions
ISP (Pin 1):
Connection Point for the Positive Terminal
of the Current Feedback Resistor (R
LED
). Also serves as
positive rail for TG pin driver.
ISN (Pin 2):
Connection Point for the Negative Terminal
of the Current Feedback Resistor (R
LED
).
TG (Pin 3):
Top Gate Driver Output. An inverted PWM
signal drives the gate of a series PMOS device between
V
ISP
and (V
ISP
– 7V) if V
ISP
> 7V. An internal 7V clamp
protects the PMOS gate by limiting VGS. Leave TG uncon-
nected 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 the negative terminal of the current sense
resistor in the source of the N-channel MOSFET. Solder
the exposed pad directly to the ground plane.
ISMON (Pin 5):
ISP/ISN Current Report Pin. The LED
current sensed by ISP/ISN inputs is reported as V
ISMON
=
I
LED
• R
LED
• 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.
CTRL2 (Pin 6):
Current Sense Threshold Adjustment
Pin 2. This pin has identical functions as CTRL1. The
V
(ISP-ISN)
threshold is regulated by the internal 1.1V refer-
ence voltage, CTRL1 or CTRL2. Whichever is the lowest
takes precedence. Regulating threshold V
(ISP-ISN)
is 0.25 •
V
CTRLX
less an offset for 0.1V < V
CTRLX
< 1V. For V
CTRLX
>
1.2V the current sense threshold is constant at the full-scale
value of 250mV. For 1V < V
CTRLX
< 1.2V, the dependence
of the current sense threshold upon V
CTRLX
transitions
from a linear function to a constant value, reaching 98%
of full-scale value by V
CTRLX
= 1.1V. Connect CTRL1 and
CTRL2 to V
REF
for the 250mV default current threshold.
Do not leave this pin open. Connect either CTRL pin to
GND for zero LED current.
FB (Pin 7):
Voltage Loop Feedback Pin. FB is intended for
constant-voltage regulation or for LED protection/open
LED detection. The internal transconductance amplifier
with output V
C
regulates FB to 1.25V (nominal) through
the DC/DC converter. If the FB input is regulating the loop,
and V
(ISP-ISN)
is less than 25mV (typical), the
OPENLED
pull-down is asserted. This action may signal an open LED
fault. If FB is driven above the 1.3V (by an external power
supply spike, for example), 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 tie this pin to GND as the
SHORTLED
will
be asserted and the part will be shut down.
V
C
(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.
CTRL1 (Pin 9):
Current Sense Threshold Adjustment
Pin 1. This pin has an identical function as CTRL2. Please
refer to the CTRL2 pin description.
V
REF
(Pin 10):
Voltage Reference Output Pin. Typically
2.015V. This pin drives a resistor divider for the CTRL
pins, either for analog dimming or for temperature limit/
compensation of the LED load. It can supply up to 100μA.
SS (Pin 11):
Soft-Start Pin. This pin modulates oscillator
frequency and compensation pin voltage (V
C
) 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 to complete a blanking period
at start-up, 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 (ISP-ISN > 0.375V)
2. INTV
CC
undervoltage
3. Output short (FB < 0.3V after start-up)
4. Thermal limit
The SS pin must be discharged below 0.2V to reinitiate a
soft-start cycle. Switching is disabled until SS begins to
recharge. It is important to select a capacitor large enough
that FB can exceed 0.3V under normal load conditions
before SS exceeds 1.7V. Do not leave this pin open.
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.
For more information
9
3795f
LINEAR_DIMENSIONS [ LINEAR DIMENSIONS SEMICONDUCTOR ]
