Advanced Monolithic Systems

6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140

AMS1503

APPLICATION HINTS

Protection Diodes

Unlike older regulators, the AMS1503 family does not need any

protection diodes between the adjustment pin and the output and

from the output to the input to prevent die over-stress. Internal

resistors are limiting the internal current paths on the AMS1503

adjustment pin, therefore even with bypass capacitors on the

adjust pin no protection diode is needed to ensure device safety

under short-circuit conditions. The Adjust pin can be driven on a

transient basis

±7V with respect to the output without any device

degradation.

needed. Microsecond surge currents of 25A to50A can be handled

the device. In normal operations it is difficult to get those values

of surge currents even with the use of large output capacitances. If

high value output capacitors are used, such as 1000

µF to 5000µF

can occur. A diode from output to input is recommended, when a

crowbar circuit at the input of the AMS1503 is used (Figure 6).

Normal power supply cycling or even plugging and unplugging in

the system will not generate current large enough to do any

damage.

CONTROL

POWER

OUTPUT

AMS1503

SENSE

ADJ

R1

R2

V

OUT

V

POWER

+

+

+

D2*

D1*

V

CONTROL

Figure 6. Optional Clamp Diodes Protect Against

Input Crowbar Circuits

If the AMS1503 is connected as a single supply device with the

control and power input pins shorted together the internal diode

between the output and the power input pin will protect the

control input pin. As with any IC regulator, none the protection

circuitry will be functional and the internal transistors will break

down if the maximum input to output voltage differential is

exceeded.

Thermal Considerations

The AMS1503 series have internal power and thermal limiting

circuitry designed to protect the device under overload conditions.

However maximum junction temperature ratings should not be

exceeded under continuous normal load conditions. Careful

consideration must be given to all sources of thermal resistance

from junction to ambient, including junction-to-case, case-to-heat

sink interface and heat sink resistance itself.

Thermal resistance specification for both the Control Section and

the Power Transistor are given in the electrical characteristics.

The thermal resistance of the Control section is given as

0.65

°C/W and junction temperature of the Control section can

run up to 125

°C. The thermal resistance of the Power section is

given as 2.7

°C/W and junction temperature of the Power section

can run up to 150

°C. Due to the thermal gradients between the

power transistor and the control circuitry there is a significant

difference in thermal resistance between the Control and Power

sections.

Virtually all the power dissipated by the device is dissipated in

the power transistor. The temperature rise in the power transistor

will be greater than the temperature rise in the Control section

making the thermal resistance lower in the Control section. At

power levels below 12W the temperature gradient will be less

than 25

°C and the maximum ambient temperature will be

determined by the junction temperature of the Control section.

This is due to the lower maximum junction temperature in the

Control section. At power levels above 12W the temperature

gradient will be greater than 25

°C and the maximum ambient

temperature will be determined by the Power section. In both

cases the junction temperature is determined by the total power

dissipated in the device. For most low dropout applications the

power dissipation will be less than 12W.

The power in the device is made up of two components: the

power in the output transistor and the power in the drive circuit.

The power in the control circuit is negligible.

The power in the drive circuit is equal to:

where

is

equal

to

between

and

The power in the output transistor is equal to:

The total power is equal to:

Junction-to-case thermal resistance is specified from the IC

junction to the bottom of the case directly below the die. This is

the lowest resistance path for the heat flow. In order to ensure the

best possible thermal flow from this area of the package to the

heat sink proper mounting is required. Thermal compound at the

case-to-heat

sink

interface

is

recommended.

A

thermally

conductive spacer can be used, if the case of the device must be

electrically isolated, but its added contribution to thermal

resistance has to be considered.