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參數(shù)資料
| 型號(hào)： | LM1946M |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 電源管理 |
| 英文描述： | LM1946 Over/Under Current Limit Diagnostic Circuit |
| 中文描述： | 1-CHANNEL POWER SUPPLY SUPPORT CKT, PDSO20 |
| 封裝： | 0.300 INCH, PLASTIC, SOP-20 |
| 文件頁數(shù)： | 6/12頁 |
| 文件大小： | 216K |
| 代理商： | LM1946M |
Application Hints
(Continued)
Typical values are:
R1
e
6.2k
g
5%
R2
e
1.2k
g
5%
Iset
e
20
m
A
@
25
§
C
Vthrshld
e
20
m
A (6.2k
b
1.2k)
e
100 mV
For values of sense voltages greater than 100 mV, the com-
parator output is off (low). Sense voltages less than 100 mV
turn the output on (high).
It’s also important that the output of the comparator be in
the ‘‘off’’ state when the inputs are taken to ground, i.e. S1
is opened and the lamp is turned ‘‘off’’. The input section of
LM1946 has been designed to turn ‘‘off’’ when the inputs
are grounded and therefore not deliver an erroneous bulb
out indication. The comparator is only activated when the
inputs are above ground by at least 3V.
R1 and R2 are necessary for another reason. These resis-
tors protect the input terminals of the IC from the many
transients in an automobile found on the battery line, some
of which can exceed a thousand volts for a few microsec-
onds. A minimum value of approximately 1 k
X
is therefore
recommended.
COMPENSATING FOR BATTERY VOLTAGE
The current through a typical automotive lamp, whether a
headlight or dashboard illumination lamp, will vary as battery
voltage changes. The change, however, is nonlinear. Dou-
bling the battery voltage does not double the lamp current.
TL/H/8707–21
FIGURE 5
This occurs since a higher voltage will heat the filament
more, increasing its resistance and allowing less current to
flow than expected. Figure 5 shows this effect. A best fit
straight line over the normal battery range of 9V to 16V for
this particular example can be given by:
I
L
(Amps)
e
0.62
a
0.069
#
Vbattery
TL/H/8707–10
Iset
e
V
CC
b
1.4
R4
a
Vref
b
1.4
R3
Iset
e
V
CC
R4
a
Vref
R3
FIGURE 6
b
1.4
#
1
R3
a
1
R4
J
Thus, in actual use, the LM1946 threshold voltage should
track the variations in bulb current with respect to battery
voltage. To accomplish this, Iset should have a component
that varies with the battery. As shown in the LM1946 circuit
schematic of Figure 18, the Iset pin is two diode drops
above ground, or approximately 1.4V. A resistor from this
pin to the 6.4V reference sets the fixed component of Iset; a
resistor to the battery line sets the variable component.
Thus, the best fit straight line in Figure 5 can be realized
exactly with only two resistors. The result is shown inFigure
6, giving a nominal Iset of 20
m
A that tracks the bulb current
as supply varies from 9V to 16V. The graph of Figure 7
shows the final result comparing a typical sense voltage
across Rs with the comparator threshold voltage as the
supply varies.
COMPENSATING FOR AMBIENT
TEMPERATURE VARIATION
If the sense resistors used in a system are perfect compo-
nents with no temperature coefficient, then the compensa-
tion to be subsequently detailed here is unnecessary. How-
ever, resistors of the very small values usually required in a
lamp monitoring system are sometimes difficult or expen-
sive to acquire. A convenient alternative is the wiring har-
ness, a length of wire, or even a trace on a printed circuit
board. All of these are of copper material and therefore can
vary by as much as 3900 ppm/
§
C. The LM1946 has been
designed to accommodate a wide range of temperature
compensation techniques. If the Iset current is designed to
increase or decrease with temperature, nearly any tempera-
ture coefficient can be produced in the threshold voltage of
the five input pairs.
TL/H/8707–22
FIGURE 7
6
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相關(guān)代理商/技術(shù)參數(shù) |
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|---|---|
| LM1946N | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:LM1946 Over/Under Current Limit Diagnostic Circuit |
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