Linear Technology DC1784A Demo Manual page 3

Demo circuit options
DC1784A can be adjusted for higher output voltage, dif-
ferent LED current, or different topology. The following
options are for simple changes to the demonstration
circuit. The data sheet gives more information regarding
designing with the LT3797. For more information, see
the data sheet for details or contact Linear Technology
technical support.
Maximum LED Voltage
DC1784A is set for 55V of overvoltage protection and the
maximum LED string voltage used on the standard build
should be 50V. However, the output can go up to 100V,
so OVP can be set at about 93V for maximum LED voltage
and the maximum LED string voltage is therefore about
87V. The limits are based upon open LED overshoot and
voltage and current regulation regions. If changes are
made in the FB resistors for higher voltage, the MOSFET
and catch diode should be switched out for higher voltage
devices. See the LT3797 data sheet for details.
Current or Voltage Regulation
The LT3797 can be used for constant current or voltage
regulation. If the load placed on the LED
allows V to climb high enough for V
OUT
the voltage regulation loop of the converter takes over. In
this case, the compensation for a given channel should be
adjusted for proper use as a constant voltage regulator.
The IC can be used as a boost or SEPIC constant voltage
regulator. Output voltage can be almost as high as 100V
when used as a constant voltage regulator.
LED Current and Switch Current
LED current on DC1784A is set for 1A with 0.25Ω resis-
tors R2, R3, R4. For a different maximum LED current,
change this resistor:
250mV/R = I .
LED LED
DEMO MANUAL DC1784A
A change in LED current or input voltage may lead to higher
or lower maximum switch current. R11, R12, R13 can be
changed to alter the maximum switch current for different
applications. 100mV/R
Overvoltage Protection
Overvoltage protection is set with resistor pairs R8 & R14,
R9 & R15, R10 & R16. The high side feedback method
allows simple changes for different topologies.
For buck mode and buck-boost mode, see the data sheet
for details how to set the feedback resistors. Note that
R35, R36, R37 are provided as optional placeholders on
the demo circuit for simple feedback resistor changes for
these topologies.
BOOST, BUCK MODE, BUCK-BOOST MODE, SEPIC
Any channel of the DC1784A can be changed to a differ-
ent topology than boost. A few simple changes including
feedback overvoltage protection feedback resistors, out-
put resistor, components, and connection to input can
be made. Please consult the LT3797 data sheet and the
factory for details.
+
to GND terminals
= 1.2V, then
Undervoltage and Overvoltage Lockout
ISP-FBH
UVLO can be adjusted by changing the values of R26
and R27.
The LT3797 has an internal 41V OVLO that protects the IC
from switching at high input voltage transients. However,
an additional OVLO pin can be used to set a lower OVLO
using resistors R28, R31, and/or R46.
= peak switch current.
SENSE
dc1784af
3