Advanced Monolithic Systems, Inc.

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

AMS2950/AMS2951

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 (2.2

µ or more is

recommended for 3V and 3.3V versions). 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. Using

the adjustable versions at voltages below 5V runs the error

amplifier at lower gains so that more output capacitance is needed.

For the worst-case situation of a 200mA load at 1.23V output

(Output shorted to Feedback) a 3.3

µF (or greater) capacitor should

be used.

Unlike many other regulators, the AMS2950, will remain stable

and in regulation with no load in addition to the internal voltage

divider. This is especially important in CMOS RAM keep-alive

applications. When setting the output voltage of the AMS2951

version with external resistors, a minimum load of 1

µA is

recommended.

A 1

µF tantalum or aluminum electrolytic capacitor should be

placed from the AMS2950/AMS2951 input to the 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 the AMS2951 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

AMS2951

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 (Refer to

the block diagram). This trip level remains “5% below normal”

regardless of the programmed output voltage of the 2951. 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 AMS2951 input is ramped up

and down. For 5V versions the ERROR signal becomes valid

(low) at about 1.3V input. It goes high at about 5V input (the input

voltage at which Vout = 4.75 ).

Since the AMS2951’s dropout voltage is load dependent (see

curve in typical performance characteristics), the input voltage trip

point (about 5V) will vary with the load current. The output

voltage trip point (approx. 4.75V) does not vary with load.

The error comparator has an open-collector output which requires

an external pull-up 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, this sink current adds to battery

drain in a low battery condition. Suggested values range from

100K to 1M

Ω. The resistor is not required if this output is unused.

Programming the Output Voltage (AMS2951)

The AMS2951 may be pin-strapped for the nominal fixed output

voltage using its internal voltage divider by tying the output and

together.

Alternatively, it may be programmed for any output

voltage between its 1.235V reference and its 30V maximum

rating.

As seen in Figure 2, an external pair of resistors is

required.

The complete equation for the output voltage is:

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% while increasing the resistor

program current by 12

µA. Since the AMS2951 typically draws 60

µA at no load with Pin 2 open-circuited, this is a small price to

pay.

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.

This is the only way that noise can be reduced on the 3 lead

AMS2950 but is relatively inefficient, as increasing the capacitor

from 1

µF to 220 µF only decreases the noise from 430 µV to 160

µV rms for a 100 kHz bandwidth at 5V output.

Noise could also be reduced fourfold by a bypass capacitor across

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.