Datasheet

LT3796/LT3796-1
17
3796fa
For more information www.linear.com/3796
Figure 6. Short-circuit Current with PNP Helper
Figure 7. The Simplified LED Short-Circuit Protection Schematic
for Buck Mode Converter
Similar to boost, Schottky diodes D2, D3 and PNP transis-
tor Q1 are recommended to protect short-circuit event in
the buck mode.
PWM Dimming Control for Brightness
There are two methods to control the LED current for
dimming using the LT3796/LT3796-1. One method uses
the CTRL pin to adjust the current regulated in the LEDs.
A second method uses the PWM pin to modulate the LED
current between zero and full current to achieve a precisely
programmed average current, without the possibility of
color shift that occurs at low current in LEDs. To make
PWM dimming more accurate, the switch demand cur-
rent is stored on the V
C
node during the quiescent phase
when PWM is low. This feature minimizes recovery time
when the PWM signal goes high. To further improve
the recovery time, a disconnect switch should be used
in the LED current path to prevent the output capacitor
from discharging during the PWM signal low phase. The
minimum PWM on or off time depends on the choice of
operating frequency through the RT input. For best current
accuracy, the minimum PWM high time should be at least
three switching cycles (3μs for f
SW
= 1MHz).
A low duty cycle PWM signal can cause excessive start-up
times if it were allowed to interrupt the soft-start sequence.
Therefore, once start-up is initiated by PWM > 1V, it will
ignore a logical disable by the external PWM input signal.
The device will continue to soft-start with switching and
TG enabled until either the voltage at SS reaches the 1.0V
level, or the output current reaches one-fourth of the full-
scale current. At this point the device will begin following
the dimming control as designated by PWM. If at any time
an output overcurrent is detected, GATE and TG will be
disabled even as SS continues to charge.
Programming the Switching Frequency
The RT frequency adjust pin allows the user to program
the switching frequency from 100kHz to 1MHz to optimize
efficiency/performance or external component size. Higher
frequency operation yields smaller component size but
increases switching losses and gate driving current, and
may not allow sufficiently high or low duty cycle operation.
Lower frequency operation gives better performance at the
cost of larger external component size. For an appropriate
APPLICATIONS INFORMATION
I
M2
1A/DIV
FAULT
10V/DIV
37961 F06
1µs/DIV
LED
+
50V/DIV
LT3796/LT3796-1
37961 F07
LED
STRING
TGISP ISNV
IN
V
IN
R
LED
R
SNS
Q1
D3
GATE
SENSE
D1
D2
LED
+
LED
–
L1
M1
M2
C2
C1