Below is a cache of http://www.allegromicro.com/en/Products/Design/an/an295012.pdf. It's a snapshot of the page taken as our search engine crawled the Web.
The web site itself may have changed. You can check the current page or check for previous versions at the Internet Archive. Yahoo! is not affiliated with the authors of this page or responsible for its content.
APPLICATIONS INFORMATION A PRIMER ON DRIVING
INCANDESCENT LAMPS
Figure 1 Lamp Warming Resistor
+V
WARMING
Dwg. EP-059
Tungsten-filament incandescent lamps exhibit a very-high
positive temperature coefficient of resistance with the cold filament
resistance being approximately 10% of the hot filament resistance.
When an incandescent lamp is initially turned on, the cold filament
is at minimum resistance and will normally allow a 10x to 12x peak
current. Within 3 to 5 ms the current falls to approximately 2x the
hot current. This high lamp turn-on current (commonly called
in-rush current), can contribute to poor lamp reliability and can
destroy semiconductor lamp drivers. Even if the active part of the
driver output could survive the short-duration peak current, the
internal bonding wires can fuse open. High-current drivers or
paralleled drivers, rated to handle the peak inrush current, may
prove cost prohibitive.
Two simple methods are shown here for controlling the lamp
in-rush current in standard power driver applications.
Warming resistors (Figure 1) protect both driver and lamp by
preheating the lamp to between the cold and hot values. A general
recommendation is to provide a warming current of 25% of the
rated operating current. This will reduce the in-rush current to
approximately 4x. Depending on the amount of warming current, a
dim lamp glow might be visible under low-light conditions. This
method also provides reduced filament breakage due to vibration
but adds significant power consumption when the lamp is off.
Alternatively, an inexpensive current-limiting resistor (Figure 2)
can be used to protect the lamp and driver, which will dissipate only
a small amount of power when the lamp is on. The minimum
resistor value is easily calculated as
R
S
V
CC
V
CE(sat)
V
L
I
cm
10 x I
L
where I
L
= rated lamp current
I
cm
= rated peak driver current
V
CC
= supply voltage
V
CE(sat)
= driver saturation voltage at rated peak current
V
L
= rated lamp voltage
Figure 2 Current-Limiting Resistor
Application Note
29501.2A
+V
LIMITING
Dwg. EP-059-1
APPLICATIONS
INFORMATION A PRIMER ON DRIVING
INCANDESCENT LAMPS
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
NORMAL LAMP IN-RUSH CURRENT
LAMP CURRENT
TIME
CURRENT LIMIT
Dwg. WH-001-1
NOT TO SCALE
Figure 3 Lamp Current vs Time
The required resistor power rating is determined by the hot filament
current and is usually
1
/
4
or
1
/
2
W (P
S
= I
L2
x R
S
). The slightly
reduced lamp voltage, caused by the series resistor voltage drop
(I
L
x R
S
), should have minimum effect on lamp brightness and will
prolong lamp life. If brightness is an important concern, the supply
voltage can be increased* to compensate for the driver saturation
voltage and the series resistor voltage drop or, alternatively, a
negative-temperature coefficient resistor can be used where R
S
is
its cold resistance.
Lamps can also be efficiently driven by current-limited power
drivers without the need for warming or current-limiting resistors.
Allegro current-limited drivers are listed below. With these drivers,
during turn-on, the high in-rush current is sensed by an internal low-
value resistor and/or a thermal-gradient sensing circuit. During this
transition period, the output stage is driven in a linear fashion,
limiting the current while the filament resistance increases to its hot
value (Figure 3). As the lamp current falls, the output stage goes
into saturation and applies full supply voltage to the lamp.
Allegro
Channels per
Maximum
Current Limit
Lamp Driver(s)
Driver
Continuous Current
Value
UDx2543B/EB
4
700 mA
1.0 A
UDx2547B/EB
4
600 mA
1.3 A
UDx2549B/EB
4
600 mA
1.0 A
A2557xEB/LB
4
300 mA
500 mA
UDx2559B/EB
4
600 mA
1.0 A
UDx2559LB
4
400 mA
1.0 A
UGQ5140K Hall
1
300 mA
900 mA
Some Allegro drivers, with over-current protection in the form of
a fault latch (as in the UDN2987A), cannot be used in lamp driver
applications except with stringent design efforts. In applications
controlling multiple lamp filaments, the internal clamp diodes may
be connected together through an appropriate current-limiting
resistor to a simple lamp test switch. A side-effect with any
current-limiting method is that lamp turn-on time will increase, but
generally this is unimportant and not noticeable.
* Multiplexed lamps must typically be operated at a voltage N
times the nominal dc operating voltage (where N is the number of
digits), to obtain sufficient brightness.
Copyright © 1994, 2000 Allegro MicroSystems, Inc. A PRIMER ON DRIVING
INCANDESCENT LAMPS
www.allegromicro.com
Figure 4 Lamp Characteristics at Reduced Voltage
APPLIED VOLTAGE/DESIGN VOLTAGE RATIO
Dwg. GP-048A
NOMALIZED CHARACTERISTIC
1.0
0.98
0.96
0.94
0.92
0.90
4.0
3.0
2.0
1.5
1.0
0.75
0.5
CANDLEPOWER
CURRENT
0.55
3.5
LIFE
-12
(V
APPLIED
/V
DESIGN
)
(V
APPLIED
/V
DESIGN
)
(V
APPLIED
/V
DESIGN
)
For reference, as approximations, the light output of an incan-
descent lamp (other than long life [>5000 hrs] or halogen-cycle
lamps) varies as the 3.5 power of the applied voltage-to-design
voltage ratio, the lamp current varies as the 0.55 power of the
voltage ratio, and lamp life varies inversely as the 12
th
power of the
voltage ratio (Figure 4). A PRIMER ON DRIVING
INCANDESCENT LAMPS
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
The products described here are manufactured under one or more
U.S. patents or U.S. patents pending.
Allegro MicroSystems, Inc. reserves the right to make, from time to
time, such departures from the detail specifications as may be required
to permit improvements in the performance, reliability, or
manufacturability of its products. Before placing an order, the user is
cautioned to verify that the information being relied upon is current.
Allegro products are not authorized for use as critical components
in life-support devices or systems without express written approval.
The information included herein is believed to be accurate and
reliable. However, Allegro MicroSystems, Inc. assumes no responsi-
bility for its use; nor for any infringement of patents or other rights of
third parties which may result from its use.