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LTM8040 Datasheet(PDF) 8 Page - Linear Technology |
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LTM8040 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 20 page  LTM8040 8 8040fa 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 a 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 LEDA 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. LEDA 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 2.5V 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 at its nominal rating. If the PWM pin is pulled low, the LTM8040 stops switch- ing and the internal control circuitry is held in its present state. Circuitry 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 ac- curate 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 1 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 2.6V. For many applications, BIAS should be tied to LPWR; if LPWR is below 2.6V, BIAS may be tied to VIN or some other voltage source. The switching regulator performs frequency foldback dur- ing overload conditions. Frequency foldback helps limit internal power and thermal stresses. The LTM8040 is equipped with thermal protection that reduces the output LED current if the internal operating temperature is too high. The junction temperature will exceed the 125°C temperature rating of the LTM8040 when the thermal protection is active, so continuous operation under this operating condition may impair reliability. |
Similar Part No. - LTM8040_15 |
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Similar Description - LTM8040_15 |
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