LTM8042/LTM8042-1
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For more information www.linear.com/LTM8042
APPLICATIONS INFORMATION
For most applications, the design process is straight
forward, summarized as follows:
1. Decide whether the LTM8042/LTM8042-1 should oper
-
ate in boost, buck, or buck-boost mode.
2. Look at Tables 1 through 6 and find the line that best
matches the input and output conditions of the system
under consideration.
3. Connect C
IN
, C
OUT
, C
VCC
and R
T
as indicated in the
appropriate table.
4. Connect the remaining pins as needed by the system
requirements.
While these component combinations have been tested
for proper operation, it is incumbent upon the user to
verify proper operation over the intended system’s line,
load and environmental conditions.
If the desired LED current is not listed in Tables 1 through 6,
set it by applying the proper voltage the CTL pin. Graphs
of the LTM8042/LTM8042-1 LED current scaling vs CTL
voltage are given in the Typical Performance Character
-
istics section. If a voltage source is not available to drive
the CTL pin, a resistor may be applied from the CTL pin
to GND. The CTL pin is internally pulled up to a 2V refer
-
ence voltage through a 20k resistor (please see the Block
Diagram for details).
Open LED Protection
The
LTM8042/LTM8042-1 has internal open LED circuit
protection. If the LED is absent or fails open, the LTM8042/
LTM8042-1 clamps the voltage on the LED+ and BSTOUT/
BKIN pin to protect the output against overvoltage. The
internal boost switching converter then regulates its
output to 36V. In buck mode, the full open LED voltage
is stood off by the internal power Schottky diode. At high
operating temperatures, the power Schottky reverse leak
-
age current
will rise. This increases the power dissipation
within
the diode, which raises the junction temperature.
This temperature rise can be large, so care needs to be
taken at high operating temperatures.
Setting the Switching Frequency
The LTM8042/LTM8042-1 uses a constant frequency
architecture that can be programmed over a 250kHz to
2MHz range with a single external timing resistor from the
RT pin to ground. Table 7 shows suggested R
T
selections
for a variety of switching frequencies.
Table 7. Switching Frequency vs R
T
SWITCHING FREQUENCY (kHz) R
T
(kΩ)
250 86.6
500 37.4
800 21.0
1000 15.8
1500 9.09
2000 6.04
The other way to set the operating frequency of the
LTM8042/LTM8042-1 is to drive the SYNC pin with an
external signal. For proper operation, a resistor should be
connected at the RT pin and be able to generate a switch
-
ing frequency 20%
lower than the external clock when the
external clock is absent.
In general, a lower switching frequency should be used
where either very high or very low switching duty cycle
operation is required, or high efficiency is desired. Selection
of a higher switching frequency will allow use of smaller
value external components and yield a smaller solution
size and profile.
Operating Modes
The LTM8042/LTM8042-1 employs a ground referred
power switch to implement a boost power switching
circuit. As such, it can be used to implement the three
most popular LED driving topologies: boost, buck mode,
and buck-boost mode. Example layouts of each operating
mode are given in Figures 2 through 4 and schematics are
shown in the Typical Applications section.
