AMS116

APPLICATION HINTS

Package Power Dissipation

The package power dissipation is the level at which the thermal

sensor monitoring the junction temperature is activated. The

AMS116 shuts down when the junction temperature exceeds the

limit of 150

°C. The junction temperature rises as the difference

between the input power and output power increases. The

mounting pad configuration on the PCB, the board material, as

well as the ambient temperature affect the rate of temperature rise.

The junction temperature will be low, even if the power

dissipation is high, when the mounting of the device has good

thermal conductivity. When mounted on the recommended

mounting pad

(figure1) the power dissipation for the SOT-89

package is 600mW. For operation above 25

°C derate the power

dissipation at 4.8mW/

°C. To determine the power dissipation for

shutdown when mounted, attach the device on the PCB and

increase the input-to-output voltage until the thermal protection

circuit is activated. Calculate the power dissipation of the device

by subtracting the output voltage from the input voltage and

multiply by the output current. The measurements should allow

for the ambient temperature of the PCB. The value obtained from

should provide maximum thermal conductivity in order to

maintain low device temperatures. As a general rule, the lower the

temperature, the better the reliability of the device.

The thermal resistance when the device is mounted is equal to:

The internal limit for junction temperature is 150

°C. If the ambient

temperature is 25

°C, then:

150

°C = θ

θ

output is shorted. As the temperature rises, the input gradually will

reached, is the value that should be used.

The range of usable currents can be found from the graph in figure

2.

4

5

75

150

25

50

6

3

P

D

D

PD

T (°C)

(mW)

Figure 2

Procedure:

1.

2.

3.

4.

5.

Take a vertical line from the maximum operating temperature

(75

°C) to the derating curve.

6.

intersects the derating curve. This is the maximum power

The maximum operating current is:

External Capacitors

The AMS116 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 AMS116 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 AMS116 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 AMS116 decreases.

Considering the fact that in many applications the AMS116 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.

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.

Advanced Monolithic Systems, Inc.

www.advanced-monolithic.com

Phone (925) 443-0722

Fax (925) 443-0723