LT3795
pin FuncTions
RAMP (Pin 13):
The RAMP pin is used for spread spectrum
frequency modulation. The internal switching frequency
is spread out to 70% of the original value, where the
modulation frequency is set by 12µA/(2
•
1V
•
C
RAMP
). If
not used, tie this pin to GND.
PWM (Pin 14):
PWM Input Signal Pin. A low signal turns
off switching, idles the oscillator, disconnects the V
C
pin
from all internal loads, and drives TG to the ISP level.
PWM has an internal 500k pull-down resistor. If not used,
connect to V
REF
.
SHORTLED
(Pin 15):
An open-collector pull-down on
SHORTLED
asserts when any of the following conditions
happen:
1. FB < 0.3V after SS pin reaches 1.7V at start-up.
2. LED overcurrent (V
(ISP-ISN)
> 375mV).
To function, the pin requires an external pull-up resistor.
SHORTLED
status is only updated during PWM high state
and latched during PWM low state.
SHORTLED
remains
asserted until the SS pin is discharged below 0.2V.
OPENLED
(Pin 16):
An open-collector pull-down on
OPENLED
asserts if the FB input is above 1.20V (typical),
and V
(ISP-ISN)
is less than 25mV (typical). To function, the
pin requires an external pull-up resistor.
OPENLED
status
is updated only during PWM high state and latched during
PWM low state.
SENSE (Pin 18):
The current sense input for the control
loop. Kelvin connect this pin to the positive terminal of
the switch current sense resistor, R
SENSE
, in the source
of the N-channel MOSFET. The negative terminal of the
current sense resistor should be Kelvin connected to the
GND plane of the IC.
GATE (Pin 19):
N-Channel MOSFET Gate Driver Output.
Switches between INTV
CC
and GND. It is driven to GND
during shutdown, fault or idle states.
INTV
CC
(Pin 20):
Regulated Supply for Internal Loads
and GATE Driver. Supplied from V
IN
and regulates to 7.7V
(typical). INTV
CC
must be bypassed with a 4.7μF capacitor
placed close to the pin. Connect INTV
CC
directly to V
IN
if
V
IN
is always less than or equal to 8V.
V
IN
(Pin 23):
Input Supply Pin. Must be locally bypassed
with a 0.22μF (or larger) capacitor placed close to the
IC.
EN/UVLO (Pin 24):
Enable and Undervoltage Lockout
Pin. An accurate 1.22V falling threshold with externally
programmable hysteresis detects when power is OK to
enable switching. Rising hysteresis is generated by the
external resistor divider and an accurate internal 3μA
pull-down current. Above the threshold, EN/UVLO input
bias current is sub-μA. Below the falling threshold, a 3μA
pull-down current is enabled so the user can define the
hysteresis with the external resistor selection. An under-
voltage condition resets soft-start. Tie to 0.4V, or less, to
disable the device.
OVLO (Pin 25):
Input Overvoltage Lockout Pin. An ac-
curate 1.25V rising threshold detects when power is OK
to enable switching.
IVINN (Pin 26):
Connection Point for the Negative Terminal
of the Input Current Sense Resistor (R
INSNS
). The input
current can be programmed by I
IN
= 60mV/R
INSNS
.
IVINP (Pin 27):
Connection Point for the Positive Terminal
of the Input Current Sense Resistor.
IVINCOMP (Pin 28):
Input Current Sense Amplifier Output
Pin. The voltage at IVINCOMP pin is proportional to I
IN
as
V
IVINCOMP
= I
IN
•
R
INSNS
•
20. A 10nF or larger capacitor
to GND is required at this pin to compensate the input
current loop. Do not leave this pin open, and do not load
this pin with a current.
10
3795f
For more information
LINEAR_DIMENSIONS [ LINEAR DIMENSIONS SEMICONDUCTOR ]
