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LTM8042/LTM8042-1
APPLICATIONS INFORMATION
For most applications, the design process is straight
forward, summarized as follows:
1. Decide whether the LTM8042/LTM8042-1 should operate
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.
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
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
SWITCHING FREQUENCY (kHz)
250
500
800
1000
1500
2000
R T (kΩ)
86.6
37.4
21.0
15.8
9.09
6.04
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 cir-
cuit 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
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.
within the diode, which raises the junction temperature.
This temperature rise can be large, so care needs to be
taken at high operating temperatures.
80421fa
16
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