AMS2931
TYPICAL PERFORMANCE CHARACTERISTICS (Continued)
Output Capacitor ESR
Maximum Power Dissipation (TO-263)
On/Off Treshold
100
10
4.0
3.8
AMS2931C ADJUSTABLE
COUT= 100mF
VO = 5V
qJA = 32° C/W
qJA = 37° C/W
4
3
2
3.6
3.4
3.2
OFF
ON
3.0
2.8
1
qJA = 50° C/W
qJA = 73° C/W
STABLE
REGION
2.6
2.4
0.1
0.01
1
0
2.2
2.0
0
10 20 30 40 50 60 70 80 90 100
AMBIENT TEMPERATURE (° C)
0
3
6
9
12 15 18 21 24
0
20
40
60
80
100
OUTPUT VOLTAGE (V)
OUTPUT CURRENT (mA)
APPLICATION HINTS
The AMS2931 series require an output capacitor for device
stability. The value required depends on the application circuit
and other factors.
With higher output currents, the stability of AMS2931 decreases.
Considering the fact that in many applications the AMS2931 is
operated at only a few milliamps (or less) of output current, the
output capacitor value can be reduced even further. For example,
a circuit that is required to deliver a maximum of 10mA of output
current from the regulator output will need an output capacitor of
only half the value compared to the same regulator required to
deliver the full output current of 100mA.
In the case of AMS2931C (adjustable), the minimum value of
output capacitance is a function of the output voltage. As a
general rule, with higher output voltages the value of the output
capacitance decreases, since the internal loop gain is reduced.
In order to determine the minimum value of the output capacitor,
for an application circuit, the entire circuit including the capacitor
should be bench tested at minimum operating temperatures and
maximum operating currents. To maintain internal power
dissipation and die heating to a minimum, the input voltage
should be maintain at 0.6V above the output. Worst-case occurs
just after input power is applied and before the die had the
chance to heat up. After the minimum capacitance value has been
found for the specific brand and type of electrolytic capacitor, the
value should be doubled for actual use to cover for production
variations both in the regulator and the capacitor.
Because high frequency characteristics of electrolytic capacitors
depend greatly on the type and even the manufacturer, the value
of capacitance that works well with AMS2931 for one brand or
type may not necessary be sufficient with an electrolytic of
different origin. Sometimes actual bench testing will be the only
means to determine the proper capacitor type and value. To
obtain stability in all general applications a high quality 100mF
aluminum electrolytic or a 47mF tantalum electrolytic can be
used.
A critical characteristic of the electrolytic capacitors is their
performance over temperature. The AMS2931 is designed to
operate to -40°C, but some electrolytics will freeze around -30°C
therefore becoming ineffective. In such case the result is
oscillation 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. In applications
where the regulator junction temperature will never be lower than
25°C the output capacitor value can be reduced by a factor of two
over the value required for the entire temperature range (47mF for
a high quality aluminum or 22mF for a tantalum electrolytic
capacitor).
Advanced Monolithic Systems, Inc. 6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140
ADMOS [ ADVANCED MONOLITHIC SYSTEMS ]
