Datasheet

LT3796/LT3796-1
14
3796fa
For more information www.linear.com/3796
INTV
CC
Regulator Bypassing and Operation
The INTV
CC
pin requires a capacitor for stable operation
and to store the charge for the large GATE switching cur-
rents. Choose a 10V rated low ESR, X7R or X5R ceramic
capacitor for best performance. A 4.7μF ceramic capacitor
is 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 LT3796/LT3796-1 from excessive on-chip power dis-
sipation. The minimum value of this current limit should
be considered when choosing the switching N-channel
MOSFET and the operating frequency. I
INTVCC
can be
calculated from the following equation:
I
INTVCC
= Q
G
• f
OSC
Careful choice of a lower Q
G
MOSFET allows higher
switching frequencies, leading to smaller magnetics. The
INTV
CC
pin has its own undervoltage disable (UVLO) set
to 4V (typical) to protect the external FETs from excessive
power dissipation caused by not being fully enhanced.
If the INTV
CC
pin drops below the UVLO threshold, the
GATE pin is forced to 0V, TG pin is pulled high and the
soft-start pin will be reset. If the input voltage, V
IN
, will
not exceed 7V, then the INTV
CC
pin should be connected
to the input supply. Be aware that a small current (typically
10μA) loads the INTV
CC
in shutdown. If V
IN
is normally
above, but occasionally drops below the INTV
CC
regula-
tion voltage, then the minimum operating V
IN
is close to
6V. This value is determined by the dropout voltage of the
linear regulator and the 4V 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 3μA pull-down current is active when EN/
UVLO is below the threshold. The purpose of this current
APPLICATIONS INFORMATION
Figure 1.
LT3796/LT3796-1
37961 F01
EN/UVLO
R1
R2
V
IN
is to allow the user to program the rising hysteresis. The
following equations should be used to determine the
values of the resistors:
V
IN(FALLING)
= 1.22 •
R1+ R2
R2
V
IN(RISING)
= V
IN(FALLING)
+ 3µA • R
1
LED Current Programming
The LED current is programmed by placing an appropriate
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.
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
LED
=
V
CTRL
– 100mV
R
LED
• 4
, 0.1V < V
CTRL
≤ 1V
I
LED
= 0, V
CTRL
= 0V
When the CTRL pin voltage is between 1V and 1.2V, the
LED current varies with CTRL, but departs from the previ-
ous equation by an increasing amount as the CTRL volt-
age increases. Ultimately above 1.2V, the LED current no