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 V
IN
. 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 fi rst 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 fl oating sets the LEDA pin
current to 1A. Reducing the voltage below 1.25V on the
ADJ pin proportionally reduces the current fl owing 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 V
IN
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.
