Datasheet LTM8040 (Analog Devices) - 8

LTM8040
OPERATION
The LTM8040 is a constant frequency, current mode or pulled high, the part operates at its nominal rating. If converter capable of generating a constant 1A output the PWM pin is pulled low, the LTM8040 stops switch- intended to drive LEDs or other applications where a ing and the internal control circuitry is held in its present constant current is required. state. Circuitry drawing current from the LPWR pin is also Operation can be best understood by referring to the Block disabled. This way, the LTM8040 “remembers” the current Diagram. The power stage is a step down converter that sourced from the LEDA output until PWM is pulled high regulates the output current by reading the voltage across again. This leads to a highly linear relationship between a power sense resistor that is in series with the output. pulse width and output light, allowing for a large and ac- curate dimming range. If the SHDN pin is tied to ground, the LTM8040 is shut down and draws minimal current from the input source The RT pin allows programming of the switching fre- tied to V quency. The LTM8040 is shipped with 80.6k on this pin IN. If the SHDN pin exceeds 1.5V, the internal bias circuits turn on, including the internal regulator, reference, to GND, yielding a default switching frequency of 500kHz. and oscillator. When the SHDN pin exceeds 2.65V, the For applications requiring a faster switching frequency, switching regulator will begin to operate. apply another resistor in parallel, from RT to GND. Refer to Table 1 for the frequencies that correspond to the applied There are two means to dim a LED with the LTM8040. external resistor values. The fi rst is to adjust the current on the LEDA output via a voltage on the ADJ pin. The ADJ pin is internally pulled An external voltage is required at the BIAS pin to power up to a precision 1.25V reference through a 1% 5.11k internal circuitry. For proper operation, BIAS must be at resistor. Leaving the ADJ pin fl oating sets the LEDA pin least 2.6V. For many applications, BIAS should be tied to current to 1A. Reducing the voltage below 1.25V on the LPWR; if LPWR is below 2.6V, BIAS may be tied to VIN ADJ pin proportionally reduces the current fl owing out of or some other voltage source. LEDA. This can be accomplished by connecting a resistor The switching regulator performs frequency foldback dur- from the ADJ pin to GND, forming a divider network with ing overload conditions. Frequency foldback helps limit the internal 5.11k resistor. LEDA pin current can also be internal power and thermal stresses. programmed by tying the ADJ pin directly to a voltage The LTM8040 is equipped with thermal protection that source. For proper operation, make sure that LEDA is at reduces the output LED current if the internal operating least 2.5V at the desired operating point. temperature is too high. The junction temperature will The other means by which the LTM8040 can dim a LED exceed the 125°C temperature rating of the LTM8040 when is with pulse width modulation using the PWM pin and the thermal protection is active, so continuous operation an optional external NFET. If the PWM pin is unconnected under this operating condition may impair reliability. 8040fa 8