LTM8042/LTM8042-1
15
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For more information www.linear.com/LTM8042
OPERATION
The LTM8042/LTM8042-1 is a complete, full featured,
current mode regulator specifically designed to drive light
emitting diodes (LEDs) or other loads where a constant
current up to 1A (350mA for the LTM8042-1) is required.
The LTM8042/LTM8042-1 can operate in any of three LED
drive topologies: boost, buck mode and buck-boost mode.
The device features both analog and PWM dimming, a PWM
P-channel MOSFET driver, and a suite of control functions:
RUN control, soft-start, user programmable switching
frequency, and external frequency synchronization.
Operation can be best understood by referring to the
Block Diagram. The power stage is a boost converter that
regulates the output current by reading the voltage across
a power sense resistor that is in series with the output.
As with any boost topology, there is an uninterrupted cur
-
rent path between the input and output terminals. Current
between these two terminals is not limited, so the device
is intolerant to a short-circuit or overload from any of the
output terminals (LED
+
, BSTOUT/BKIN) to GND.
There are two ways to dim a LED with the LTM8042/
LTM8042-1. One way is to adjust the current on the
LED
array
by setting the analog voltage on the CTL pin. The
CTL pin is internally pulled up to a precision 2V reference
through a 1% 20k resistor. Leaving the CTL pin floating
sets the LED pin current to 1A. Reducing the voltage below
1.1V on the CTL pin proportionally reduces the current
flowing out of LED
+
. This can be accomplished by con-
necting a resistor from the CTL pin to GND, forming a
divider
network with the internal 20k resistor, or by driving
the CTL pin directly to a voltage source, such as a DAC.
The other way the LTM8042/LTM8042-1 can dim a LED
array is by pulse width modulation using the PWM pin
and an optional external P-channel MOSFET. The external
P-channel MOSFET can be conveniently operated by the
integrated gate driver at pin TG. The gate drive function
can be enabled or disabled by the TGEN pin.
If the PWM pin is pulled high, the part operates normally. If
the PWM pin is unconnected or pulled low, the LTM8042/
LTM8042-1 stops switching and the internal control cir
-
cuitry is held in its present state. This way, the LTM8042/
LTM8042-1 “remembers” the current sourced from the
LED
+
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 RUN pin is used to deactivate the LTM8042/LTM8042-1.
When the RUN pin is pulled to a logic low state, the device
is
shut down and draws typically less than 1µA of current.
The SS pin is used to limit inrush current during start-up.
The LTM8042/LTM8042-1 integrates a current source with
this function, so only a capacitor is necessary to establish
the soft-start characteristics of the output current.
The switching frequency is set by applying a single resistor
from the RT pin to GND, allowing operation anywhere from
250kHz to 2MHz, and the SYNC pin allows synchroniza
-
tion to an external source between 300kHz and 2.5MHz.
