IXYS
Features:
•
•
•
•
1.1V to 5.5V Input Range
2 Amp Peak Switch Current
High Efficiency > 80%
20V Maximum Output with Over-Voltage
Protection
•
LED Intensity Control
•
1.0 MHz Fixed Frequency Switching
•
8 Lead SOIC Package
MX841
White LED Step-Up Converter
General Description
The MX841 is a fixed frequency, constant current
source step-up DC/DC converter. The output current
is directly regulated making the MX841 ideal for
driving series connected white light emitting diodes
(LED’s) in backlight applications. The MX841
powers up to 3 series white LED’s at 1.1V, and 20
series/parallel white LED’s at 5.0V.
The MX841 features a 1.0MHz switching frequency
to accommodate the use of small capacitors and a
small inductor necessary in size sensitive portable
applications. Light intensity and shutdown are
conveniently controlled by a single analog voltage.
Power efficiency and battery life are extended
through the use of a high voltage, low R
DSON
N-
channel MOSFET switch.
The MX841 is designed to operate over a
temperature range of -40°C to +85°C, and is
available in an SOIC-8 Package, with or without an
Exposed Pad in Tube or on Tape and Reel.
(Alternate package types available upon request).
Applications:
•
White LED Display Backlighting
-
Low Voltage: Mobile Phones, PDA’s,
MP3 Players, Digital Cameras
-
High Drive Current: Vehicle
Instrumentation Panels
Ordering Information
Part No.
17000-00TU
17000-00TR
17043-00TU
17043-00TR
Description
SOIC-8
Standard Tube
SOIC-8
Standard T&R
SOIC-8
Exposed Pad Tube
SOIC-8
Exposed Pad T&R
Qty
100
2500
100
2500
Functional Block Diagram
Typical Application
VDD
SW
SW
Vbattery
Vboost
osc
CTL/SLEEP
err amp/
comparator
driver
5
1
4
VDD
SW
SW
OVD
6
Analog
Gnd
band gap
+
compare
OVD
CTRL
Isense
7
CTL/
SLEEP
MX841
Isense
8
Switch
Gnd
sleep det
bias gen
Analog
GND
2
Switch
GND
3
Rsense
MX841
Drawing No. 17023
Preliminary
1
03/10/05
www.ixys.net
MX841
Absolute Maximum Ratings
(Voltages with respect to GND=0V)
Parameter
Min
V
DD
Supply Voltage
-0.4
CTRL Voltage
G
ND
-0.4
I
SENSE
Voltage
G
ND
-0.4
SW Voltage
-0.4
OVD Voltage
-0.4
Operating Ambient Temperature Range
-40
Storage Temperature Range
-55
Operating Junction Temperature Range
-40
Power Dissipation
Standard SOIC
θ
JA
= 114°C/W
T
A
= 27°C
Exposed Pad SOIC
θ
JA
= 50°C/W
ESD Warning
Max
+6.5
V
DD
+0.4
V
DD
+0.4
+24.0
+24.0
+85
+150
+125
775
1.75
Unit
V
V
V
V
V
C
o
C
o
C
o
mW
W
IXYS
Absolute Maximum Ratings are
stress ratings. Stresses in excess of
these ratings can cause permanent
damage to the device. Functional
operation of the device at these or
any other conditions beyond those
indicated in the operational sections
of this data sheet is not implied.
Exposure of the device to the
absolute maximum ratings for an
extended period may degrade the
device and affect its reliability.
ESD (electrostatic discharge) sensitive device. Although the MX841 features proprietary ESD protection circuitry,
permanent damage may be sustained if subjected to high energy electrostatic discharges. Proper ESD precautions
are recommended to avoid performance degradation or loss of functionality.
V
DD
=3.0V, R
SENSE
=5Ω, V
CTRL
=1.0V, L=6.8µH, C
BYP
=1µF, T
A
= -40
o
C to +85
o
C, Typical Values at +25
o
C
Unless otherwise specified
DC Electrical Characteristics
Parameter
Supply Voltage
Supply Current Overvoltage
Supply Current Operating
Sleep Current
CTRL Bias Current
I
SENSE
Bias Current
OVD Bias Current
Over Voltage Threshold
CTRL / V
RSENSE
V
CTRL
Sleep Mode
SW On Resistance
Switch Current
SW Voltage Range
SW Leakage Current
V
CTRL
Condition
T
A
= 25 C
o
Min
1.1
Typ
0.5
6
7
Max
5.5
0.8
2
Unit
V
mA
µA
µA
µA
µA
µA
Not Switching: V
CTRL
= 260mV
V
RSENSE
> 13 X V
CTRL
Switching: V
CTRL
= V
DD,
V
RSENSE
=GND
T
A
= +25
o
C
V
CTRL
< 100mV
T
A
= +85
o
C
-10
OVD = 18V
OVD Rising
OVD Falling
V
CTRL
Rising
V
CTRL
Falling
V
SW
= 0.1V
V
SW
= 0.5V
V
DD
=1.2V, L=33µH
V
DD
=5.5V, L=2.5µH
V
CTRL
>260mV
V
CTRL
= GND
18.5
-7
20
0
22
13
260
100
1.9
2.2
30
2
V
SCHOTTKY
0.1
0.3
0
V
DD
V
DD
-
20
V
V
V/V
mV
mV
Ω
Ω
mA
A
V
µA
µA
V
OVD = V
SW
= 18V
T
A
= +25
o
C
T
A
= +85
o
C
MX841
Drawing No. 17023
Preliminary
2
03/10/05
www.ixys.net
MX841
AC Electrical Characteristics
Parameter
Oscillator Frequency
Duty Cycle
IXYS
Condition
Min
0.6
Typ
1.0
72.5
Max
1.3
Unit
MHz
%
V
DD
=3.0V, R
SENSE
=5Ω; V
CTRL
=1.0V, L=6.8µH; C
BYP
=1µF, T
A
= -40
o
C to +85
o
C, Typical Values at +25
o
C
Unless otherwise specified
Pin Description
Pin No.
1, 4
Pin Name
SW
Description
Power Switch. Connects the drain of the internal high voltage N-channel
MOSFET to the Inductor and Schottky Diode to generate the LED boost
voltage. This voltage is determined by the total forward bias voltage of the
LED series string and the Isense resistor value. (Note: Both pins must be
connected.)
Analog Ground (Note: Must be connected to a low noise ground source.)
Switch Ground (Note: Must be connected to a high current ground source.)
Input supply pin. Input range from 1.1 to 5.5 volts. Must be bypassed with a
low ESR capacitor to GND.
Over Voltage Detect input. Compares the LED boots voltage to the internal
voltage reference. If the boost voltage rises above 22V the OVD circuit
disables the oscillator and N-channel MOSFET until the boost voltage falls
below 18.5V. Must be bypassed with a low ESR capacitor to GND.
Shutdown and Intensity Control Input. A control voltage above 260mV
enables device operation. A control voltage below 100mV disables device
operation. LED intensity is linearly adjusted with voltages between 260mV
and V
DD
.
Current Sense Input. The LED bias current is set by connecting a carbon
composite or monolithic resistor from this input to Ground. I
SENSE
regulates
this current to 1/13 of the CTRL input voltage.
2
3
5
Analog GND
Switch GND
VDD
6
OVD
7
CTRL
8
Isense
MX841
Drawing No. 17023
Preliminary
3
03/10/05
www.ixys.net
MX841
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.
IXYS
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).
MX841
Drawing No. 17023
Preliminary
4
03/10/05
www.ixys.net
MX841
Circuit Application Driving 15 White LED’s
Vbattery 3.0V - 5.0V
6.8uH
I
pk
=1.7A
22uF 35V
0.1uF in Parallel
1N5817
V
F
=0.3V
Vboost
IXYS
22uF 35V
0.1uF in Parallel
26K Ohms @ 3.0V
57K Ohms @ 5.0V
5
1
4
SW
VDD
SW
OVD
6
20K Ohms
(Dimming Control)
CTRL
7
CTL/SLEEP
MX841
Isense
8
Analog
GND
2
Switch
GND
3
Rsense
5 Ohms
5 Ohms
5 Ohms
8-LEAD SOI
IXYS Corporation makes no representations or warranties with respect to the accuracy or completeness of the contents of this
publication and reserves the right to make changes to specifications and product descriptions at any time without notice. Neither
circuit patent licenses nor indemnity are expressed or implied. Except as set forth in IXYS’ Standard Terms and Conditions of Sale,
IXYS Corporation assumes no liability whatsoever, and disclaims any expressed or implied warranty, relating to its products
including, but not limited to, the implied warranty of merchantability, fitness for a particular purpose, or infringement of any
intellectual property right
.
MX841
Drawing No. 17023
Preliminary
5
03/10/05
www.ixys.net
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