LTM8040 Ver la hoja de datos (PDF) - Linear Technology

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Fabricante

LTM8040

36V, 1A µModule LED Driver

Linear Technology 

LTM8040

APPLICATION INFORMATION

For most applications, the design process is straight

forward, summarized as follows:

1. Look at Table 1 and find the row that has the desired

input voltage range LED string voltage range and output

current.

2. Apply the recommended CIN, RT and RADJ values.

3. Connect BIAS as indicated.

4. Connect LEDA to the anode of the LED string.

5. 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.

Choose resistors according to the following formula:

R2 =

VUVLO

2.65V

− 2.65V –

10.3µA

R1

where VUVLO is the desired UVLO Threshold

Suppose that the output needs to stay off until the input

is above 8V.

VTH = 8V

Let R1 = 100k

R2

=

8V

−

2.65V

2.65V –

10.3µA

=

61.9

100k

Open LED Protection

The LTM8040 has internal open LED circuit protection. If

the LED is absent or fails open, the LTM8040 clamps the

voltage on the LEDA pin to 14V. The switching regulator

then skips cycles to limit the input current. The input current

and output voltage during an open LED condition is shown

in the Typical Performance Characteristics section.

Undervoltage Lockout

Undervoltage lockout (UVLO) is typically used in situations

where the input supply is current limited, or has high source

resistance. A switching regulator draws constant power

from the source, so the source current increases as the

source voltage drops. This looks like a negative resistance

load to the source and can cause the source to current

limit or latch low under low source voltage conditions.

UVLO prevents the regulator from operating at source

voltages where this might occur. An internal comparator

will force the part into shutdown when VIN falls below 3.5V.

An adjustable UVLO threshold is also realized through

the SHDN pin, as the internal comparator that performs

this function has a 2.65V threshold. An internal resistor

pulls 10.3μA to ground from the SHDN pin at the UVLO

threshold in order to provide hysteresis.

VIN

VIN

R1

LTM8040

SHDN

C1 R2

GND

8040 F01

Figure 1. Undervoltage Lockout

Keep the connections from the resistors to the SHDN pin

short. If high resistance values are used, the SHDN pin

should be bypassed with a 1nF capacitor to prevent cou-

pling problems from switching nodes.

Setting the Switching Frequency

The LTM8040 uses a constant frequency architecture that

can be programmed over a 500kHz to 2MHz range with a

single external timing resistor from the RT pin to ground.

The current that flows into the timing resistor is used to

charge an internal oscillator capacitor. The LTM8040 is

configured such that the default frequency is 500KHz

without adding any resistor. Many applications use this

value. If another frequency is desired, a graph for selecting

8040p

8

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