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4

OPERATING

CONSIDERATIONS

PA243

GENERAL

Please read Application Note 1 "General Operating Consid-

erations" which covers stability, power supplies, heat sinking,

mounting, current limit, SOA interpretation, and specification

interpretation. Visit www.apexmicrotech.com for design tools

that help automate tasks such as calculations for stability,

internal power dissipation, current limit, heat sink selection,

Apex's complete Application Notes library, Technical Seminar

Workbook and Evaluation Kits.

PHASE COMPENSATION

Open loop gain and phase shift both increase with increas-

ing temperature. The PHASE COMPENSATION typical graph

shows closed loop gain and phase compensation capacitor

value relationships for four case temperatures. The curves

are based on achieving a phase margin of 50°. Calculate

the highest case temperature for the application (maximum

ambient temperature and highest internal power dissipation)

before choosing the compensation. Keep in mind that when

working with small values of compensation, parasitics may

play a large role in performance of the finished circuit. The

compensation capacitor must be rated for at least the total

voltage applied to the amplifier and should be a temperature

stable type such as NPO or COG.

OTHER STABILITY CONCERNS

There are two important concepts about closed loop gain

when choosing compensation. They stem from the fact that

while "gain" is the most commonly used term,

β (the feedback

factor) is really what counts when designing for stability.

1. Gain must be calculated as a non-inverting circuit (equal

input and feedback resistors can provide a signal gain of

-1, but for calculating offset errors, noise, and stability, this

is a gain of 2).

2. Including a feedback capacitor changes the feedback factor

or gain of the circuit. Consider Rin=4.7k, Rf=47k for a gain

of 11. Compensation of 4.7 to 6.8pF would be reasonable.

Adding 33pF parallel to the 47k rolls off the circuit at 103kHz,

and at 2MHz has reduced gain from 11 to roughly 1.5 and

the circuit is likely to oscillate.

As a general rule the DC summing junction impedance

(parallel combination of the feedback resistor and all input

resistors) should be limited to 5k ohms or less. The amplifier

input capacitance of about 6pF, plus capacitance of connecting

traces or wires and (if used) a socket will cause undesirable

circuit performance and even oscillation if these resistances

are too high. In circuits requiring high resistances, measure or

estimate the total sum point capacitance, multiply by Rin/Rf, and

parallel Rf with this value. Capacitors included for this purpose

are usually in the single digit pF range. This technique results

in equal feedback factor calculations for AC and DC cases. It

does not produce a roll off, but merely keeps

β constant over

a wide frequency range. Paragraph 6 of Application Note 19

details suitable stability tests for the finished circuit.

CURRENT LIMIT

For proper operation, the current limit resistor, Rcl, must be

connected as shown in the external connection diagram. The

minimum value is 3.9 ohms, however for optimum reliability,

the resistor should be set as high as possible. The maximum

practical value is 110 ohms. Current limit values can be pre-

dicted as follows:

Ilimit = Vbe

Rcl

Where Vbe is shown in the CURRENT LIMIT typical

graph.

Note that +Vbe should be used to predict current through

the +Vs pin, -Vbe for current through the -Vs pin, and that they

vary with case temperature. Value of the current limit resistor

at a case temperature of 25° can be estimated as follows:

Rcl = 0.7

Ilimit

When the amplifier is current limiting, there may be spurious

oscillation present during the current limited portion of the nega-

tive half cycle. The frequency of the oscillation is not predictable

and depends on the compensation, gain of the amplifier, value

of the current limit resistor, and the load. The oscillation will

cease as the amplifier comes out of current limit.

SAFE OPERATING AREA

The MOSFET output stage of the PA243 is not limited by

second breakdown considerations as in bipolar output stages.

However there are still three distinct limitations:

1. Voltage withstand capability of the transistors.

2. Current handling capability of the die metalization.

3. Temperature of the output MOSFETS.