Preliminar
MX841 4 03/10/05
MX841
IXYS
Functional Description
The MX841 is specifically designed to be
operated from a single cell battery source. The
step-up voltage necessary for the constant
current source to drive up to 5 white LED’s in
series, or multiple strings of white LED’s, is
achieved with minimal external components. (See
Application Schematics on pages 1&4).
Setting the LED Current
The regulated output current is determined by the
selection of the sense Resistor connected from
the I
SENSE
input to ground, and the maximum user
defined voltage of the CTRL input. The desired
output current, (typically 15mA to 20mA for white
LEDs), is derived using the following formula for
the R
SENSE
value: R
SENSE Ω
= (V
CTRL
X 1/13) / I
LED
.
Current Regulation / Light Intensity Control
The I
SENSE
voltage is fed into an
ErrorAmp/Comparator. The user defined Control
Voltage applied at the CTRL input is scaled by a
gain factor of 1/13 and compared to the I
SENSE
voltage. If the scaled control voltage is greater
than the I
SENSE
voltage, the 1.0MHz oscillator
signal driving the N-Channel MOSFET will be
enabled. Once the I
SENSE
voltage reaches the
scaled CTRL voltage the oscillator signal is
modulated on and off by the
ErrorAmp/Comparator to maintain the desired
regulated output current.
Light intensity is controlled by reducing the
voltage at the CTRL input. As the CTRL input
voltage is reduced below the user defined
maximum value, the oscillator signal driving the
N-Channel MOSFET is interrupted by the
ErrorAmp/Comparator. The pulse skip modulating
of the N-Channel MOSFET causes a reduction in
the regulated current equal to the ratio of the
reduced CTRL voltage to the maximum CTRL
voltage.
Sleep Mode
The MX841 goes into sleep mode when the V
CTRL
input is less than 100mV. All circuits except V
CTRL
detection are shutdown and the supply current
falls to 6uA. Once the V
CTRL
input rises above
260mV the MX841 resumes normal operation.
Over Voltage Detection
The MX841 will go into a shutdown state to avoid
an over voltage condition if the current path
between the SW output and the R
SENSE
input is
interrupted. A voltage reference and comparator
circuit evaluates the voltage potential at the OVD
input and shuts down the oscillator and N-
Channel MOSFET if an over voltage condition is
detected. Normal operation is resumed once the
OVD input voltage falls below the reference
voltage, (20V typical).
Drawing No. 17023 www.ixys.net
