LT3797
16
3797fa
For more information www.linear.com/LT3797
applicaTions inForMaTion
For applications where the EN/UVLO pin is to be used
only as a logic input, the EN/UVLO pin can be connected
directly to the input voltage, V
IN
, for “always on” operation.
Programming Overvoltage Lockout Threshold
with the OVLO Pin
The L
T3797 provides an OVLO pin that allows user
-pro-
grammable overvoltage lockout. A 1.25V (typical) rising
threshold with 125mV hysteresis detects the over
voltage
condition. The OVLO pin can be used to monitor V
IN
or
other voltages against overvoltage conditions.
Figure 1 shows OVLO connecting to V
IN
through a volt-
age divider to protect against V
IN
overvoltage. The rising
threshold voltage and falling threshold voltage can be
calculated by the following equations:
V
OV(RISING)
= 1.25V •
R4
V
OV(FALLING)
= 1.125V •
R3+R4
An overvoltage condition turns off all three channels
(including pulling the GATE pins to GND and TG pins to
ISP) and resets the soft-starts.
Loop Compensation
Loop compensation determines the stability and transient
performance. The LT3797 uses current mode control to
regulate the output which simplifies loop compensation.
The optimum values depend on the converter topology, the
component values and the operating conditions (includ
-
ing the input voltage, LED current switching frequency).
To compensate the feedback loop of the LT3797, a series
resistor-capacitor network is usually connected from the VC
pin to GND. Figure 1 shows the typical VC compensation
network. For most applications, the capacitor should be
in the range of 2.2nF to 22nF, and the resistor should be
in the range of 2k to 25k. A practical approach to design
-
ing the compensation network is to start with one of the
circuits in this data sheet that is similar to your applica-
tion, and tune the compensation network to optimize the
per
formance. Stability should then be checked across all
operating conditions, including LED current, input voltage
and temperature. Application Note 76 is a good reference
for loop compensation.
Soft-Start and Fault Protection
The L
T3797 has identical soft-start and fault protection
functions for each channel. The soft-start feature is de
-
signed to limit peak switch currents and output voltage
(V
OUT
) overshoot during start-up or recovery from a fault
condition. Figure 4 shows the state diagram of the soft-start
and fault protection of channel 1. Also refer to Figure 1
for channel 1 operation. In soft-start mode, the soft-start
capacitor is charged up by the 25µA current source. The
SS1 pin gradually increases the peak switch current al
-
lowed in M1 by clamping the VC1 voltage through Q4. In
this way the SS1 pin allows the output capacitor, C
OUT
,
voltage to be charged gradually toward its final value while
limiting M1 current overshoot. The soft-start interval is
set by the soft-start capacitor selection according to the
following equation:
t
SS
=
25µA
• C
SS
The discharge time of the soft-start capacitor is controlled
by a 2.5µA current source. Therefore, the SS1 pin is also
used as an adjustable timer in the FAULT2 protection modes
(see Figure5) to prevent thermal runaway problems on the
external components and/or the LEDs. In some fault condi-
tions, the soft-start capacitor is charged and discharged
repetitively, referred to as the hiccup mode operation. A
typical hiccup mode operation occurs when an LT3797 LED
driver has an output short-circuit fault. Figure 5 shows that
if an output short-circuit fault causes LT3797 overcurrent
(sensed by ISP1-ISN1) in the normal operation mode, the
LT3797 moves to FAULT2 protection mode, where TG1 is
pulled high, turning off the external PMOS and isolating
the output. As a result, the overcurrent condition is cleared.
When SS1 is discharged below 200mV, the LT3797 moves
to soft-start mode, where TG1 is pulled low to turn on
the external PMOS. If the short-circuit fault still exists,
the LT3797 senses an overcurrent fault again and moves
to FAULT2 protection mode: SS1 charged up and a new
cycle starts. In this manner, the soft-start capacitor is kept
charging and discharging between 200mV and 1.7V until
the short-circuit fault is cleared.
