Linear Technology LT3956 Series Manual page 10

LT3956
applicaTions inForMaTion
INTV Regulator Bypassing and Operation
CC
The INTV pin requires a capacitor for stable operation
CC
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 pin and also to the IC ground.
CC
An internal current limit on the INTV
the LT3956 from excessive on-chip power dissipation.
The INTV pin has its own undervoltage disable (UVLO)
CC
set to 4.1V (typical) to protect the internal MOSFET from
excessive power dissipation caused by not being fully en-
hanced. If the INTV pin drops below the UVLO threshold,
CC
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 , will not exceed 7V, then the INTV
IN
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
in shutdown. Otherwise, the minimum operating V
is determined by the dropout voltage of the linear regulator
and the 4.4V (4.1V typical) INTV
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:
R
V = 1 22 . •
IN FALLING ,
V = 2 1 . µA R • 1
IN RISING ,
LT3956
EN/UVLO
Figure 1
0
output protects
CC
IN
undervoltage lockout
CC
+
1 R 2
R 2
+ V
IN FALLING ,
V
IN
R1
R2
3956 F01
LED Current Programming
The LED current is programmed by placing an appropri-
ate value current sense resistor, R
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:
V
CTRL
=
I
LED
R
LED
CC
When the CTRL pin voltage is between 1V and 1.2V
the LED current varies with CTRL, but departs from the
CC
previous equation by an increasing amount as the CTRL
value
voltage increases. Ultimately, above CTRL = 1.2V, the LED
current no longer varies with CTRL. At CTRL = 1.1V, the
actual value of I
When V is higher than 1.2V, the LED current is regu-
CTRL
lated to:
250 mV
I =
LED
R
LED
The CTRL pin should not be left open (tie to V
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
output power and switching current when V
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 pin filters the signal so the average difference between
C
ISP and ISN is regulated to the user-programmed value.
Ripple voltage amplitude (peak-to-peak) in excess of
, between the ISP
LED
− 100 mV
4 •
is ~98% of the equation's estimate.
LED
if not
REF
to reduce
IN
is low.
IN
3956f