The MOSFET output stage of the PA240 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.
OUTPUT CURRENT FROM +VS OR –VS, (A)
1.0
0.5
0.3
0.2
0.1
0.05
0.03
0.02
0.01
0.005
0.003
0.002
0.001
10
20 30
50
100
200 300 500
1K
DC, T
C
= 25°C
DC, T
C
= 85°C
200mS
300mS
SOA
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 high-
est 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.
SUPPLY TO OUTPUT DIFFERENTIAL, V S - V O, (V)
These limitations can be seen in the SOA (see Safe Operat-
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. 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
The PA240CC 7-pin DDPAK surface mountable package
has a large exposed integrated copper heatslug to which the
monolithic amplifier is directly attached. The PA240CC requires
surface mount techniques of heatsinking. A solder connection
to a copper foil area as defined in Note 5 of Page 2 is recom-
mended for circuit board layouts. This may be adequate heat-
sinking but the large number of variables suggests temperature
measurements to be made on the top of the package. Do not
allow the temperature to exceed 85°C.
4
PA240U
Product Innovation From
PA240
+Vs
Z1
-IN
Q1
+IN
Q2
+Vs
OUT
-Vs
FIGURE 1
OVERVOLTAGE PROTECTION
Although the PA240 can withstand differential input voltages
up to 16V, in some applications additional external protection
may be needed. Differential inputs exceeding 16V will be clipped
by the protection circuitry. However, if more than a few milliamps
of current is available from the overload source, the protection
circuitry could be destroyed. For differential sources above
16V, adding series resistance limiting input current to 1mA will
prevent damage.Alternatively, 1N4148 signal diodes connected
anti-parallel across the input pins is usually sufficient. In more
demanding applications where bias current is important, diode
-Vs
Z2
connected JFETs such as 2N4416 will be required. See Q1
and Q2 in Figure 1. In either case the differential input voltage
will be clamped to 0.7V. This is sufficient overdrive to produce
the maximum power bandwidth.
In the case of inverting circuits where the +IN pin is grounded,
the diodes mentioned above will also afford protection from
excessive common mode voltage. In the case of non-invert-
ing circuits, clamp diodes from each input to each supply will
provide protection. Note that these diodes will have substantial
reverse bias voltage under normal operation and diode leak-
age will produce errors.
Some applications will also need over-voltage protection
devices connected to the power supply rails. Unidirectional
zener diode transient suppressors are recommended. The
zeners clamp transients to voltages within the power supply
rating and also clamp power supply reversals to ground.
Whether the zeners are used or not the system power supply
should be evaluated for transient performance including power-
on overshoot and power-off polarity reversals as well as line
regulation. See Z1 and Z2 in Figure 1.
APPLICATION REFERENCES:
For additional technical information please refer to the fol-
lowing Application Notes:
AN01: General Operating Considerations
AN03: Bridge Circuit Drives
AN25: Driving Capacitive Loads
AN38: Loop Stability with Reactive Loads
CONTACTING CIRRUS LOGIC SUPPORT
For all Apex Precision Power product questions and inquiries, call toll free 800-546-2739 in North America.
For inquiries via email, please contact apex.support@cirrus.com.
International customers can also request support by contacting their local Cirrus Logic Sales Representative.
To find the one nearest to you, go to www.cirrus.com
IMPORTANT NOTICE
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to change without notice and is provided "AS IS" without warranty of any kind (express or implied). Customers are advised to obtain the latest version of relevant
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CUSTOMER OR CUSTOMER’S CUSTOMER USES OR PERMITS THE USE OF CIRRUS PRODUCTS IN CRITICAL APPLICATIONS, CUSTOMER AGREES,
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