LT3956
3956f
pin FuncTions
FB:
Voltage Loop Feedback Pin. FB is intended for con-
stant-voltage regulation or for LED protection/open LED
detection. The internal transconductance amplifier with
output V
C
will regulate FB to 1.25V (nominal) through the
DC/DC converter. If the FB input is regulating the loop, the
VMODE
pull-down is asserted. This action may signal an
open LED fault. If FB is driven above the FB threshold (by
an external power supply spike, for example), the
VMODE
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:
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
LED
= 250mV/R
LED
when V
CTRL
> 1.2V or I
LED
= (V
CTRL
–100mV)/(4 • R
LED
)
if V
CTRL
< 1V. Input bias current is typically 20µA. Below
3V, ISN is an input to the short-circuit protection feature
that forces GATE to 0V if ISP exceeds ISN by more than
350mV (typ).
ISP:
Connection point for the positive terminal of the current
feedback resistor. Input bias current for this pin depends
on CTRL pin voltage, as shown in the Typical Performance
Characteristics. ISP is an input to the short-circuit protec-
tion feature when ISN is less than 3V.
V
C
:
Transconductance Error Amplifier Output Pin. This pin
is 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:
Current Sense Threshold Adjustment Pin. Regula-
ting threshold V
(ISP – ISN)
is 0.25 • V
CTRL
plus an offset
for 0V < V
CTRL
< 1V. For V
CTRL
> 1.2V the current sense
threshold is constant at the full-scale value of 250mV. For
1V < V
CTRL
< 1.2V, the dependence of the current sense
threshold upon V
CTRL
transitions from a linear function
to a constant value, reaching 98% of full-scale value by
V
CTRL
= 1.1V. Connect CTRL to V
REF
for the 250mV default
threshold. Do not leave this pin open.
NC:
No Internal Connection. These pins may be left floating
or connected to an adjacent pin.
EN/UVLO:
Shutdown and Undervoltage Detect Pin. An
accurate 1.22V falling threshold with externally program-
mable hysteresis detects when power is OK to enable
switching. Rising hysteresis is generated by the external
resistor divider and an accurate internal 2.1µA pull-down
current. Above the 1.24V (nominal) threshold (but below
6V), EN/UVLO input bias current is sub-µA. Below the
falling threshold, a 2.1µA pull-down current is enabled so
the user can define the hysteresis with the external resis-
tor selection. An undervoltage condition resets soft-start.
Tie to 0.4V, or less, to disable the device and reduce V
IN
quiescent current below 1µA.
INTV
CC
:
Regulated supply for internal loads, GATE driver
and PWMOUT driver. Supplied from V
IN
and regulates to
7.15V (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 7V.
GND:
Ground. The exposed pad, Pin 37, is ground and
must be soldered directly to the ground plane.
V
IN
:
Input Supply Pin. Must be locally bypassed with
a 0.22µF (or larger) capacitor to PGND placed close to
the IC.
SW:
The exposed pad, Pin 38, is the drain of the switch-
ing N-channel MOSFET and must be connected to the
external inductor.
PGND:
Source terminal of switch and the GND input to
the switch current comparator. Kelvin connect to the GND
plane close to the IC using Pin 12. Pins 13 to 17 should be
connected externally to the PGND terminals of components
in the switching path. See the Board Layout section.
PWMOUT:
Buffered Version of the PWM Signal. This pin is
used to drive the LED load disconnect N-channel MOSFET
or level shift. This pin also serves in 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 MOSFET with gate cut-off voltage higher than
1V is recommended.