PA241 ETC1 [List of Unclassifed Manufacturers], PA241 Datasheet - Page 5

• Manufacturer Part Number: PA241
• Description: High Voltage Power Operational Amplifiers
• Manufacturer: ETC1 [List of Unclassifed Manufacturers]

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OPERATING
CONSIDERATIONS
GENERAL
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
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
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
2. Including a feedback capacitor changes the feedback factor
(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.
Please read Application Note 1 "General Operating Consid-
Open loop gain and phase shift both increase with increas-
There are two important concepts about closed loop gain
As a general rule the DC summing junction impedance
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).
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.
CURRENT LIMIT
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:
graph.
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:
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
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.
ing 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 and correspond to thermal
resistances of 5.2°C/W for the PA241CE and DF and 10.4°C/W
for the PA241DW respectively. Pulse stress levels for other
case temperatures can be calculated in the same manner as
DC power levels at different temperatures. 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.
HEATSINKING
copper heatslug to which the monolithic amplifier is directly
attached. The solder connection of the heatslug to a minimum
of 1 square inch foil area on the printed circuit board will result
in thermal performance of 25°C/W junction to air rating of
the PA241DF. Solder connection to an area of 1 to 2 square
For proper operation, the current limit resistor, Rcl, must be
Note that +Vbe should be used to predict current through
When the amplifier is current limiting, there may be spurious
The MOSFET output stage of the PA241 is not limited by
These limitations can be seen in the SOA (see Safe Operat-
The PA241DF package has a large exposed integrated
Where Vbe is shown in the CURRENT LIMIT typical
Ilimit = Vbe
Rcl = 0.7
Ilimit
Rcl
PA241
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