LTM8040 데이터 시트보기 (PDF) - Linear Technology

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LTM8040

36V, 1A µModule LED Driver

Linear Technology 

LTM8040

OPERATION

The LTM8040 is a constant frequency, current mode

converter capable of generating a constant 1A output

intended to drive LEDs or other applications where a

constant current is required.

Operation can be best understood by referring to the

Block Diagram. The power stage is step down converter

that regulates the output current by reading the voltage

across a power sense resistor that is in series with the

output.

If the SHDN pin is tied to ground, the LTM8040 is shut

down and draws minimal current from the input source

tied to VIN. If the SHDN pin exceeds 1.5V, the internal bias

circuits turn on, including the internal regulator, reference,

and oscillator. When the SHDN pin exceeds 2.65V, the

switching regulator will begin to operate.

There are two means to dim a LED with the LTM8040.

The first is to adjust the current on the LEDA output via

a voltage on the ADJ pin. The ADJ pin is internally pulled

up to a precision 1.25V reference through a 1% 5.11K

resistor. Leaving the ADJ pin floating sets the LED pin

current to 1A. Reducing the voltage below 1.25V on the

ADJ pin proportionally reduces the current flowing out of

LEDA. This can be accomplished by connecting a resistor

from the ADJ pin to GND, forming a divider network with

the internal 5.11K resistor. LED pin current can also be

programmed by tying the ADJ pin directly to a voltage

source. For proper operation, make sure that LEDA is at

least 2.4V at the desired operating point.

The other means by which the LTM8040 can dim a LED

is with pulse width modulation using the PWM pin and

an optional external NFET. If the PWM pin is unconnected

or pulled high, the part operates nominally. If the PWM

pin is pulled low, the LTM8040 stops switching and the

internal control circuitry is held in its present state. Cir-

cuitry drawing current from the LPWR pin is also disabled.

This way, the LTM8040 “remembers” the current sourced

from the LEDA output until PWM is pulled high again.

This leads to a highly linear relationship between pulse

width and output light, allowing for a large and accurate

dimming range.

The RT pin allows programming of the switching fre-

quency. The LTM8040 is shipped with 80.6K on this pin

to GND, yielding a default switching frequency of 500KHz.

For applications requiring a faster switching frequency,

apply another resistor in parallel, from RT to GND. Refer to

table 2 for the frequencies that correspond to the applied

external resistor values.

An external voltage is required at the BIAS pin to power

internal circuitry. For proper operation, BIAS must be at

least 3.2V. For many applications, BIAS should be tied to

LWPR; if LWPR is below 3.2V, BIAS may be tied to VIN

or some other voltage source.

The switching regulator performs frequency foldback

during overload conditions. An amplifier senses when

LWPR is less than 2V and begins decreasing the oscillator

frequency down from full frequency to 20% of the nominal

frequency when VOUT = 0V. The LPWR pin is less than 2V

during startup, short circuit, and overload conditions, so

the BIAS pin will be below the specified limit for efficient

operation if the two pins are tied together. Frequency

foldback helps limit internal power and thermal stresses

under these conditions.

The LTM8040 is equipped with thermal protection that

reduces the output LED current if the internal operating

temperature is too high. This thermal protection is active

above the 125°C temperature rating of the LTM8040, so

continuous operation under this operating condition may

impair reliability.

8040p

7

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