PA340

6

PA340U

3.

APPLICATION INFORMATION

Please read Application Note 1 "General Operating Considerations" which covers stability, power supplies, heat

sinking, mounting, current limit, SOA interpretation, and specification interpretation. Visit www.apexanalog.com

for design tools that help automate tasks such as calculations for stability, internal power dissipation, current limit,

heat sink selection, Apex Microtechnology's complete Application Notes library, Technical Seminar Workbook and

Evaluation Kits.

3.1 PHASE COMPENSATION

Open loop gain and phase shift both increase with increasing 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 applica-

tion (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.

3.2 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 R

IN = 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 ca-

pacitance, multiply by R

IN /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.

3.3 SAFE OPERATING AREA

The MOSFET output stage of the PA340 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 metallization.

3. Temperature of the output MOSFETS.

These limitations can be seen in the SOA (see Safe Operating Area

graphs). Note that each pulse capability line shows a constant power

level (unlike second breakdown limitations where power varies with

voltage stress). These lines are shown for a case temperature of

25°C. Pulse stress levels for other case temperatures can be calcu-

lated in the same manner as DC power levels at different tempera-

tures. The output stage is protected against transient flyback by the

parasitic diodes of the output stage MOSFET structure. However,

for protection against sustained high energy flyback external fast-

recovery diodes must be used.

10

20 30 50

100

200 300 500

0.001

0.002

0.003

0.005

0.01

0.02

0.03

0.05

0.1

0.2

SOA

200mS

0.3

0.5

300mS

SUPPLY TO OUTPUT DIFFERENTIAL, V

S - VO, (V)

DC, T

C = 25°C

DC, T

C = 85°C

1K

1.0

FIGURE 4. Safe Operating Area