www.advanced-monolithic.com

updated April 24, 2009

AMS2931

Maximum Power Dissipation (TO-263)

1

2

3

4

AMBIENT TEMPERATURE (° C)

10 20 30 40 50 60 70 80 90 100

0

0

θ

0.01

0.1

100

0

20

40

60

100

OUTPUT CURRENT (mA)

Output Capacitor ESR

1

10

V

C

80

3.0

4.0

On/Off Treshold

2.2

2.4

2.6

2.8

3.2

3.4

3.6

3.8

3

6

9

12

15

18

21

24

0

2.0

ON

OFF

OUTPUT VOLTAGE (V)

STABLE

REGION

θ

θ

θ

AMS2931C ADJUSTABLE

APPLICATION HINTS

The AMS2931 series require an output capacitor for device

stability. The value required depends on the application circuit and

other factors.

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 100

µF aluminum

electrolytic or a 47

µF 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 (47

µF for

a high quality aluminum or 22

µF for a tantalum electrolytic

capacitor).

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.