Advanced Monolithic Systems, Inc.

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

AMS1085

APPLICATION HINTS

The AMS1085 series of adjustable and fixed regulators are easy

to use and have all the protection features expected in high

performance voltage regulators: short circuit protection and

thermal shutdown.

Pin compatible with older three terminal adjustable regulators,

these devices offer the advantage of a lower dropout voltage,

more precise reference tolerance and improved reference stability

with temperature.

Stability

The circuit design used in the AMS1085 series requires the use of

an output capacitor as part of the device frequency compensation.

µF aluminum electrolytic or a 22µF solid

tantalum on the output will ensure stability for all operating

conditions.

When the adjustment terminal is bypassed to improve the ripple

rejection, the requirement for an output capacitor increases. The

value of 22

bypassing the adjustment terminal. Without bypassing the

adjustment terminal smaller capacitors can be used with equally

good results.

To ensure good transient response with heavy load current

changes capacitor values on the order of 100

µF are used in the

output of many regulators. To further improve stability and

transient response of these devices larger values of output

capacitor can be used.

Protection Diodes

Unlike older regulators, the AMS1085 family does not need any

protection diodes between the adjustment pin and the output and

from the output to the input to prevent over-stressing the die.

Internal resistors are limiting the internal current paths on the

AMS1085 adjustment pin, therefore even with capacitors on the

adjustment pin no protection diode is needed to ensure device

safety under short-circuit conditions.

Diodes between the input and output are not usually needed.

Microsecond surge currents of 50A to 100A can be handled by the

internal diode between the input and output pins of 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 and

the input pin is instantaneously shorted to ground, damage can

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

crowbar circuit at the input of the AMS1085 is used. Normal

power supply cycling or even plugging and unplugging in the

system will not generate current large enough to do any damage.

The adjustment pin can be driven on a transient basis

±25V, with

respect to the output without any device degradation. 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.

AMS1085

IN

OUT

ADJ

+

C

OUT

150

µF

V

OUT

R

1

R

2

C

ADJ

10

µF

D1

V

IN

Overload Recovery

When the power is first turned on, as the input voltage rises, the

output follows the input, permitting the regulator to start up into

heavy loads. During the start-up, as the input voltage is rising,

the input-to-output voltage differential remains small, allowing

the regulator to supply large output currents. A problem can occur

with a heavy output load when the input voltage is high and the

output voltage is low, when the removal of an output short will

not permit the output voltage to recover. The load line for such a

load may intersect two points on the output current curve. In this

case, there are two stable output operating points for the

regulator. With this double intersection, the power supply may

need to be cycled down to zero and brought up again to make the

output recover.

Ripple Rejection

The ripple rejection values are measured with the adjustment pin

bypassed. The impedance of the adjust pin capacitor at the ripple

frequency should be less than the value of R1 (normally 100

Ω

to120

Ω) for a proper bypassing and ripple rejection approaching

the values shown. The size of the required adjust pin capacitor is

a function of the input ripple frequency. If R1=100

Ω at 120Hz

the adjust pin capacitor should be 25

µF. At 10kHz only 0.22µF is

needed.

The ripple rejection will be a function of output voltage, in

circuits without an adjust pin bypass capacitor. The output ripple

will increase directly as a ratio of the output voltage to the

Output Voltage

The AMS1085 series develops a 1.25V reference voltage

between the output and the adjust terminal. Placing a resistor

between these two terminals causes a constant current to flow

through R1 and down through R2 to set the overall output

voltage.