LT3956
0
3956f
applicaTions inForMaTion
INTV
CC
Regulator Bypassing and Operation
The INTV
CC
pin requires a capacitor for stable operation
and to store the charge for the switch driver and PWMOUT
pin switching currents. Choose a 10V rated low ESR, X7R
or X5R ceramic capacitor for best performance. A 4.7µF
capacitor will be adequate for many applications. Place
the capacitor close to the IC to minimize the trace length
to the INTV
CC
pin and also to the IC ground.
An internal current limit on the INTV
CC
output protects
the LT3956 from excessive on-chip power dissipation.
The INTV
CC
pin has its own undervoltage disable (UVLO)
set to 4.1V (typical) to protect the internal MOSFET from
excessive power dissipation caused by not being fully en-
hanced. If the INTV
CC
pin drops below the UVLO threshold,
the PWMOUT pin will be forced to 0V, the power switch
turned off and the soft-start pin will be reset.
If the input voltage, V
IN
, will not exceed 7V, then the INTV
CC
pin could be connected to the input supply. This action
allows the LT3956 to operate from as low as 4.5V. Be aware
that a small current (less than 12μA) will load the INTV
CC
in shutdown. Otherwise, the minimum operating V
IN
value
is determined by the dropout voltage of the linear regulator
and the 4.4V (4.1V typical) INTV
CC
undervoltage lockout
threshold mentioned above.
Programming the Turn-On and Turn-Off Thresholds
With the EN/UVLO Pin
The falling UVLO value can be accurately set by the resistor
divider. A small 2.1µA pull-down current is active when
EN/UVLO is below the falling threshold. The purpose of
this current is to allow the user to program the rising
hysteresis. The following equations should be used to
determine the values of the resistors:
V
R R
R
IN FALLING
,
. •
=
+
1 22
1
2
2
V
µA R
V
IN RISING
IN FALLING
,
,
.
•
=
+
2 1
1
EN/UVLO
LT3956
V
IN
R2
3956 F01
R1
Figure 1
LED Current Programming
The LED current is programmed by placing an appropri-
ate value current sense resistor, R
LED
, between the ISP
and ISN pins. Typically, sensing of the current should
be done at the top of the LED string. If this option is not
available, then the current may be sensed at the bottom
of the string, but take caution that the minimum ISN value
does not fall below 3V, which is the lower limit of the LED
current regulation function. The CTRL pin should be tied
to a voltage higher than 1.2V to get the full-scale 250mV
(typical) threshold across the sense resistor. The CTRL pin
can also be used to dim the LED current to zero, although
relative accuracy decreases with the decreasing voltage
sense threshold. When the CTRL pin voltage is less than
1V, the LED current is:
I
V
mV
R
LED
CTRL
LED
=
−
100
4
•
When the CTRL pin voltage is between 1V and 1.2V
the LED current varies with CTRL, but departs from the
previous equation by an increasing amount as the CTRL
voltage increases. Ultimately, above CTRL = 1.2V, the LED
current no longer varies with CTRL. At CTRL = 1.1V, the
actual value of I
LED
is ~98% of the equation’s estimate.
When V
CTRL
is higher than 1.2V, the LED current is regu-
lated to:
I
mV
R
LED
LED
=
250
The CTRL pin should not be left open (tie to V
REF
if not
used). The CTRL pin can also be used in conjunction with
a thermistor to provide overtemperature protection for
the LED load, or with a resistor divider to V
IN
to reduce
output power and switching current when V
IN
is low.
The presence of a time varying differential voltage signal
(ripple) across ISP and ISN at the switching frequency
is expected. The amplitude of this signal is increased by
high LED load current, low switching frequency and/or a
smaller value output filter capacitor. Some level of ripple
signal is acceptable: the compensation capacitor on the
V
C
pin filters the signal so the average difference between
ISP and ISN is regulated to the user-programmed value.
Ripple voltage amplitude (peak-to-peak) in excess of