2000/05
VER.A
1-6
he
LM2931 positive voltage regulator features a very low
quiescent current of 1mA or less when supplying 10mA
loads. This unique characteristic and the extremely low in-put-
output differential required for proper regulation (0.2V for
output currents of 10mA) make the
LM2931 the ideal regulator
for standby power systems. Applications include memory
standby circuits, CMOS and other low power processor power
supplies as well as systems demanding as much as 100mA of
output current.
Designed originally for automotive applications, the
LM2931 and all regulated circuitry are protected from reverse
battery installations or 2 battery jumps. During line transients,
such as a load dump (60V) when the input voltage to the
regulator can momentarily exceed the specified maximum
operating voltage, the regulator will automatically shut down
to protect both internal circuits and the load. The
LM2931 can
not be harmed by temporary mirror-image insertion. Familiar
regulator features such as short circuit and thermal overload
protection are also provided.
Fixed output of 5V is available in the plastic
the popular TO-92 package.
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LM2931CT-5.0 5V TO-92
TO-92
Pin : 1. Output
2. Ground
3. Input
LM2931-5.0
LM2931ACT-5.0
5V
TO-92
2-6
Input Voltage
26V
Internal Power Dissipation(Note 1)
Internally Limited
Operating Range Operating Temperature Range 0
Overvoltage Protection
Maximum Junction Temperature
125
Storage Temperature Range to +150
PJ2931
50V
Lead Temp.(Soldering, 10 seconds)
210
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(
)
5
5.19
4.81
5.25
4.75
V
V
Output Voltage
6.0V
V
26V, Io 100mA
-40
Tj125
5.25
4.75
5.5
4.5
V
V
Line Regulation
9V
V
16V
6V
V
26V
2
4
10
30
2
4
10
30
mV
mV
Load Regulation
5mA
100mA 14
50
14
50
mV
Output Impedance 100mA
and 10mArms,
100Hz-10KHz
200 600
200
m
!
Quiescent Current Io
mA 6VV
26V
-40
Tj125
Io=100mA,V
=14V,Tj=25
0.4
15
1.0 1.0
30
5
0.4
15
1.0 1.0
mA
mA
mA
mA
Output Noise
Voltage
10Hz-100KHz, C
=100
F 500
1000
500
Long Term
Stability
20 50
20
mV
/1000hr
Ripple Rejection
fo=120Hz
80
55
80
dB
Dropout Voltage
Io=10mA
Io=100mA
,
0.3
0.2
0.6
,
0.3
0.2
0.6
V
V
Maximum
Operational Input
Voltage
33
26
33
26
V
V
Maximum Line
Transient
R
=500
5.5V,100ms 70
60
70
50
V
Reverse Polarity
Input Voltage,DC
Vo
-0.3V, R
=500
!
-30 -15
-30 -15
V
Reverse Polarity
Input Voltage,
Transient
1% Duty Cycle, 100ms
R
=500
!
-80
-50
-80
-50
V
Note 1:To ensure constant junction temperature, low duty cycle pulse testing is used.
Note 2:Guaranteed and 100% production tested.
Note 3:Guaranteed (but not 100% production tested)over the operating temperature and input current ranges. These limits are
not used to calculate outgoing quality levels.
Note 4:Thermal resistance junction-to-case (
jc) is 3/W; case-to-ambient is 50/W.
(V
=14V,Io=10mA, T
=25
(Note 1), C2=100F(unless otherwise specified)
LM2931-5.0
70
LM
LM
5
++,3
One of the distinguishing factors of the
LM2931 series
regulators is the requirement of an output capacitor for device
stability. The value required varies greatly depending upon the
application circuit and other factors. Thus some comments on
the characteristics of both capacitors and the regulator are in
order.
High frequency characteristics of electrolytic capacitors
depend greatly on the type and even the manufacturer. As a
result, a value of capacitance that works well with the
LM2931
for one brand or type may not necessary be sufficient with an
electrolytic of different origin. Sometimes actual bench
testing, as described later, will be the only means to determine
the proper capacitor and value. Experience has shown that, as
a rule of thumb, the more expensive and higher quality
electrolytics generally allow a smaller value for regulator
stability. As an example, while a high-quality 100
"F
aluminum electrolytic covers all general application circuits,
similar stability can be obtained with a tantalum electrolytic of
only 47
"F. This factor of two can generally be applied to any
special application circuit also.
Another critical characteristic of electrolytics is their
performance over temperature. While the
LM2931 is designed
to operate to -40
, the same is not always true with all
electrolytics(hot is generally not a problem). The electrolyte in
many aluminum types will freeze around -30
,reducing their
effective value to zero. Since the capacitance is needed for
regulator stability, the natural result is oscillation (and lots of
it)at the regulator output. For all application circuits where
cold operation is necessary, the output capacitor must be rated
to operate at the minimum temperature. By coincidence,
worst-case stability for the
LM2931 also occurs at minimum
temperatures. As a result, in applications where the regulator
junction temperature will never be less than 25
, the output
capacitor can be reduced approximately by a factor of two
over the value needed for the entire temperature range. To
continue our example with the tantalum electrolytic, a value of
only 22
"F would probably thus suffice. For high-quality
aluminum, 47
"F would be adequate in such an application.
Another regulator characteristic that is noteworthy is that
stability decreases with higher output currents. This sensible
fact has important connotations. In many applications, the
LM2931 is operated at only a few milliamps of output current
or less. In such a circuit, the output capacitor can be further
reduced in value. As a rough estimation, a circuit that is
required to deliver a maximum of 10mAof output current from
the regulator would need an output capacitor of only half the
value compared to the same regulator required to deliver the
full output current of 100mA. If the example of the tantalum
capacitor in the circuit rated at 25
junction temperature and
above were continued to include a maximum of 10 mA of
output current, then the 22
#"F output capacitor could be
reduced to only 10
#"F.
In the case of the
LM2931CT adjustable regulator, the
minimum
value of output capacitance is a function of the output voltage.
As a general rule, the value decreases with higher output
voltages, since internal loop gain is reduced.
At this point, the procedure for bench testing the minimum
value of an output capacitor in a special application circuit
should be clear. Since worst-case occurs at minimum
operating temperatures and maximum operating currents, the
entire circuit, including the electrolytic, should be cooled to
the minimum temperature. The input voltage to the regulator
should be maintained at 0.6V above the output to keep internal
power dissipation and die heating to a minimum. Worst-case
occurs just after input power is applied and before the die has
had a chance to heat up. Once the minimum value of
capacitance has been found for the brand and type of
electrolytic in question, the value should be doubled for actual
use to account for production variations both in the capacitor
and the regulator.(All the values in this section and the
remainder of the data sheet were determined in this fashion.)
--'
'+ 4 The input-output voltage differential at
which the circuit ceases to regulate against further reduction
in input voltage. Measured when the output voltage has
dropped 100 mV from the nominal value obtained at 14V
input, dropout voltage is dependent upon load current and
junction temperature.
+4The DC voltage applied to the input terminals
with respect to ground.
+#+ --'4 The voltage difference between
the unregulated input voltage and the regulated output voltage
for which the regulator will operate.
4 The change in output voltage for a change
in the input voltage. The measurement is made under
conditions of low dissipation or by using pulse techniques
such that the average chip temperature is not significantly
affected.
(4The change in output voltage for a change
in load current at constant chip temperature.
'
*4 Output voltage stability under
accelerated life-test conditions after 1000 hours with
maximum rated voltage and junction temperature.
+ 5 4 The rms AC voltage at the output,
with constant load and no input ripple, measured over a
specified frequency range.
6, ''4 That part of the positive input current
that does not contribute to the positive load current. The
regulator ground lead current.
++7,4The ratio of the peak-to-peak input ripple
voltage to the peak-to-peak output ripple voltage.
+''
* - 4 The percentage change in
output voltage for a thermal variation from room temperature
true to either temperature extreme.
LM2931-5.0
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