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LTM8042/LTM8042-1

80421fa

APPLICATIONS INFORMATION

For most applications, the design process is straight

forward, summarized as follows:

1. DecidewhethertheLTM8042/LTM8042-1shouldoperate

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 CIN, COUT, CVCC and RT 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 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

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 RT selections

for a variety of switching frequencies.

Table 7. Switching Frequency vs RT

SWITCHING FREQUENCY (kHz)

RT (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

TheLTM8042/LTM8042-1employsagroundreferredpower

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.

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