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| 型號： | APPLICATIONNOTE22 |
| 英文描述： | MICRF001 Theory of Operation |
| 中文描述： | MICRF001操作原理 |
| 文件頁數(shù)： | 2/4頁 |
| 文件大?。?/td> | 39K |
| 代理商： | APPLICATIONNOTE22 |
1997
7-71
Application Note 2
Micrel
7
Unlike LEDs, incandescent lamps require more current and
voltage than 5V digital logic circuits can deliver. In particu-
lar, lamps draw an appreciable inrush current because the
filament resistance is much lower when cold than when hot.
Inrush currents of 10 times rated operating current are not
uncommon. This impacts both the current rating of the
driver and the lifetime of the lamp. Among other contributing
factors, lamp lifetime is limited by the severe thermal shock
experienced at turn-on.
Display Driver
Figure 2 shows a practical display driver circuit using the
MIC4807. #1835 miniature lamps were selected for use on
a loosely regulated “48V” system supply, which normally
ran about 110% rated voltage. The #1835 lamp is specified
at 55V and 50mA, and it can easily withstand
±
15% varia-
tions in a 48V supply without loss of rated life. The lamps are
housed in #31099 (GTE/Sylvania) indicator assemblies.
Output current limit precludes the possibility of chip destruc-
tion from short circuit conditions such as arise when a lamp
socket is “tested” for power with the conductive end of a
screwdriver. Long-term short circuits (wiring faults) are
handled by the MIC4807’s thermal shutdown circuit.
When the MIC4807 cold-starts a #1835 lamp, the output is
immediately driven into current limit since it cannot deliver
the full inrush current. The cold resistance of a #1835 lamp
is approximately 94
; an initial current of 585mA would flow
if connected directly to 55V. The MIC4807 current limit is
typically 200mA at room temperature, which reduces the
thermal shock at turn-on and increases lamp lifetime. Note
that applying 200mA to the cold filament is equivalent to an
initial lamp voltage of only 18.8V.
4
OE /PWM
5
15V
15
14
13
16
6
7
A
B
C
DATA
CLEAR
CS
D0
D1
D2
D3
D4
D5
PARALLEL
PRINTER
PORT
#1835 LAMPS
(55V, 50 mA)
MIC4807
48V BUS
15V
+
-
+
-
100pF
100nF
10nF
10k
100k
100k
100k
100k
100k
100k
1/2 LM358
1/2 LM358
15V
BRIGHTNESS
ADJUST
Figure 2. MIC4807 Display Controller
with PWM Dimming
11
10
9
8
2
1
18
17
#1
#2
#3
#4
#5
#6
#7
#8
1
2
3
4
6
7
8
5
12
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