Operation - Analog Devices LINEAR LTM 8005 Manual

LTM8005

OPERATION

The LTM8005 is a stand-alone non-isolated switching DC/
DC regulator intended for LED driver applications. This
µModule power converter provides a regulated current
from an input voltage range of 5V to 38V, and up to 38V
output. A simplified block diagram is provided in the pre-
vious section.
The LTM8005 is equipped with a ground referred power
switch and a power rectifier. Connect external power
devices, such as an inductor, to these pins to implement
boost, buck-boost mode, buck mode or SEPIC topologies
to drive a string of LEDs.
The LTM8005 features an integrated sense resistor to
control the LED current. The maximum regulated current
is 1.6A, and this can be reduced by applying a voltage less
than 1.2V to the CTRL1 or CTRL2 pins. The output current
is reported by the voltage on the ISMON pin.
The LTM8005 also features an integrated PMOS discon-
nect switch to implement PWM dimming that is controlled
by a signal on the PWM pin. The PMOS also disconnects
the LEDs during fault conditions.
If input current limiting is desired, apply an external sense
resistor to the IVINP and IVINN pins. The full input current
will flow through this external sense resistor, so choose a
resistor with an appropriate power rating. The LTM8005
will start to decrease the power if the voltage between the
IVINP and INVINN pins exceeds 60mV. A 10nF capacitor
is provided internally to compensate the input current
regulation loop, but additional capacitance may be added
externally to further filter the voltage at the IVINCOMP pin.
10
The LTM8005 features spread spectrum frequency
modulation, which causes the switching frequency to
modulate to a frequency that is approximately 70% of
the programmed value set by the RT resistor. This modu-
lation decreases the energy emitted at a single frequency,
reducing the EMI amplitude. The modulation behavior is
set by a capacitor on the RAMP pin to GND.
Input voltage turn-on and turn-off thresholds are set
by resistor networks at the EN/UVLO and OVLO pins.
Applying a voltage of greater than 1.24V to the EN/UVLO
pin enables the part.
OPENLED and SHORTLED are active low open drain sta-
tus bits that indicate an open LED or shorted LED condi-
tion. OPENLED transitions to a logic low when the FB pin
rises above 1.2V and the LED current decreases below
160mA. SHORTLED transitions to a logic low when the FB
pin falls below 300mV or the LED current exceeds 2.4A.
Further details on these and other functions are given in
the Applications Information section.
An external soft start capacitor at the SS pin minimizes
the current spike that occurs at start up and the SS pin
also programs hiccup or latchoff mode fault protection.
The LTM8005 is equipped with a thermal shutdown that
inhibits power switching at high junction temperatures.
The activation threshold of this function is above the abso-
lute maximum temperature rating to avoid interfering with
normal operation, so prolonged or repetitive operation
under a condition in which the thermal shutdown activates
may damage or impair the reliability of the device.
For more information www.analog.com
Rev. B