Advanced Monolithic Systems, Inc.

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

AMS2942

APPLICATION HINTS

External Capacitors

A 1.0

µF or greater capacitor is required between output and

ground for stability at output voltages of 5V or more. At lower

output voltages, more capacitance is required. Without this

capacitor the part will oscillate.

Most types of tantalum or

aluminum electrolytic works fine here; even film types work but

are not recommended for reasons of cost. Many aluminum types

have electrolytes that freeze at about -30

°C, so solid tantalums

are recommended for operation below -25

°C. The important

parameters of the capacitor are an ESR of about 5

Ω or less and

resonant frequency above 500 kHz parameters in the value of the

capacitor. The value of this capacitor may be increased without

limit.

At lower values of output current, less output capacitance is

required for stability. The capacitor can be reduced to 0.33

µF for

currents below 10 mA or 0.1

µF for currents below 1 mA. At

voltages below 5V the error amplifier operates at lower gains so

that more output capacitance is needed. For the worst-case

situation of a 100mA load at 1.23V output (Output shorted to

Feedback) a 3.3

µF (or greater) capacitor should be used.

A 1

µF tantalum or aluminum electrolytic capacitor should be

placed between input to ground if there is more than 10 inches of

wire between the input and the AC filter capacitor or if a battery

is used as the input.

Stray capacitance to Feedback terminal can cause instability.

This may especially be a problem when using a higher value of

external resistors to set the output voltage. Adding a 100 pF

capacitor between Output and Feedback and increasing the output

capacitor to at least 3.3

µF will fix this problem.

Error Detection Comparator Output

The comparator produces a logic low output whenever the output

falls out of regulation by more than approximately 5%. This

figure is the comparator’s built-in offset of about 60 mV divided

by the 1.235 reference voltage. This trip level remains “5% below

normal” regardless of the programmed output voltage. For

example, the error flag trip level is typically 4.75V for a 5V

output or 11.4V for a 12V output. The out of regulation condition

may be due either to low input voltage, current limiting, or

thermal limiting.

Figure 1 gives a timing diagram depicting the ERROR signal and

the regulator output voltage as the AMS2942 input is ramped up

and down.

Since the dropout voltage is load dependent the input voltage trip

point will vary with the load current. The output voltage trip point

does not vary with load. The error comparator has an open-

collector output which requires an external pullup resistor. This

resistor may be returned to the output or some other supply

voltage depending on system requirements.

In determining a

value for this resistor, note that the output is rated to sink 400

µA.

Suggested values range from 100K to 1M

Ω. The resistor is not

required if error flag terminal is unused.

Setting the Output Voltage

The AMS2942 it may be programmed for any output voltage

between its 1.235V reference and its 42V maximum rating. As

seen in Figure 2, an external pair of resistors is required.

The complete equation for the output voltage is:

× (1 + R

feedback pin bias current, nominally -20 nA.

The minimum

recommended load current of 1

µA forces an upper limit of 1.2

M

Ω on value of R

100k reduces this error to 0.17%.

Reducing Output Noise

In reference applications it may be an advantageous to reduce the

AC noise present at the output. One method is to reduce the

regulator bandwidth by increasing the size of the output

capacitor. Noise could be reduced fourfold by a bypass capacitor

unity. Pick

≅ 1 / 2πR

1

× 200 Hz

or about 0.01

µF. When doing this, the output capacitor must be

increased to 3.3

µF to maintain stability. These changes reduce

the output noise from 430

µV to 100 µV rms for a 100 kHz

bandwidth at 5V output. With the bypass capacitor added, noise

no longer scales with output voltage so that improvements are

more dramatic at higher output voltages.