Advanced Monolithic Systems, Inc. 6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140

AMS112

APPLICATION HINTS

Package Power Dissipation

The package power dissipation is the level at which the thermal

sensor monitoring the junction temperature is activated. The

AMS112 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 the power dissipation for the SOT-23 package is

400mW. For operation above 25

°C derate the power dissipation

at 3.2 mW/

°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

(150

°C - T

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:

θ

The internal limit for junction temperature is 150

°C. If the

ambient temperature is 25

°C, then:

150

°C = θ

°C

θ

°C/ P

D

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

equilibrium is 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.

°C.

4.

°C.

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 AMS112 series require input and output decoupling

capacitors. The required value of these capacitors 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 AMS112 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 4.7

µF

aluminum electrolytic or a 2.2

µF tantalum electrolytic can be

used.

A critical characteristic of the electrolytic capacitors is their

performance over temperature. The AMS112 is designed to

operate to -30

°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 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. After the minimum capacitance

value has been found, the value should be doubled for actual use

to cover for production variations both in the regulator and the

capacitor. The recommended minimum capacitance for AMS112

is 2.2

µF. As a general rule, with higher output voltages the value

of the output capacitance decreases, since the internal loop gain

is reduced.

Noise Bypass Capacitor

The noise bypass capacitor should be connected as close as

possible to pin 3 and ground. The recommended value for this

capacitor is 0.01

µF. The noise bypass terminal is susceptible to

external noise, and oscillation can occur when the bypass

capacitor is not used and the solder pad for this pin is too large.

Because of the high impedance of the noise bypass terminal, care

should be taken if the bypass capacitor is not used.