OPERATING
CONSIDERATIONS
PA41/PA42 • PA41A/PA42A
The PA41/42 is externally compensated and performance
can be tailored to the application. Use the graphs of small
signal gain and phase response as well as the graphs for slew
rate and power response as a guide. The compensation
capacitor CC must be rated at 350V working voltage. The
compensation capacitor and associated resistor RC must be
mounted closely to the amplifier pins to avoid spurious oscilla-
tion. An NPO capacitor is recommended for compensation.
GENERAL
Please read Application Note 1 "General Operating Consid-
erations" which covers stability, supplies, heat sinking, mount-
ing, current limit, SOA interpretation, and specification inter-
pretation. 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 Work-
book; and Evaluation Kits.
SAFE OPERATING AREA (SOA)
The MOSFET output stage of this power operational ampli-
fier has two distinct limitations:
CURRENT LIMIT
For proper operation the current limit resistor, RCL, must be
connected as shown in the external connection diagram. The
minimum value is 18 ohms, however for optimum reliability the
resistor value should be set as high as possible. The value can
be estimated as follows with the maximum practical value of
1. The current handling capability of the die metallization.
2. The temperature of the output MOSFETs.
NOTE: The output stage is protected against transient
flyback. However, for protection against sustained, high en-
ergy flyback, external fast-recovery diodes should be used.
500 ohms.
3
RCL
=
ILIM
Use the typical performance graphs as a guide for expected
variations in current limit value with a given RCL and variations
over temperature. The selected value of RCL must be added to
the specified typical value of output resistance to calculate the
total output resistance. Since the load current passes through
RCL the value selected also affects the output voltage swing
according to:
PA41 SOA
200
120
200mS
100
300mS
VR = IO RCL
*
50
40
where VR is the voltage swing reduction.
30
When the amplifier is current limiting, there may be small
signal spurious oscillation present during the current limited
portion of the negative half cycle. The frequency of the oscil-
lation is not predictable and depends on the compensation,
gain of the amplifier, and load. The oscillation will cease as the
amplifier comes out of current limit.
20
10
5
4
INPUT PROTECTION
3
The PA41/42 inputs are protected against common mode
voltages up the supply rails and differential voltages up to ±16
volts as well as static discharge. Differential voltages exceed-
ing 16 volts 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.
The protection circuitry includes 300 ohm current limiting
resistors at each input, but this may be insufficient for severe
overloads. It may be necessary to add external resistors to the
application circuit where severe overload conditions are ex-
pected. Limiting input current to 1mA will prevent damage.
PULSE CURVES @ 10% DUTY CYCLE MAX
2
10
20 30
50
100
200 300
500
SUPPLY TO OUTPUT DIFFERENTIAL, VS -VO (V)
PA42 SOA
200
120
100
50
40
30
STABILITY
20
The PA41/42 has sufficient phase margin when compen-
sated for unity gain to be stable with capacitive loads of at least
10 nF. However, the low pass circuit created by the sumpoint
(–in) capacitance and the feedback network may add phase
shift and cause instabilities. As a general rule, the sumpoint
load resistance (input and feedback resistors in parallel)
should be 5K ohm or less at low gain settings (up to 10).
Alternatively, use a bypass capacitor across the feedback
resistor. The time constant of the feedback resistor and
bypass capacitor combination should match the time constant
of the sumpoint resistance and sumpoint capacitance.
10
5
4
3
PULSE CURVES @ 10% DUTY CYCLE MAX
2
10
20 30
50
100
200 300
500
SUPPLY TO OUTPUT DIFFERENTIAL, VS -VO (V)
This data sheet has been carefully checked and is believed to be reliable, however, no responsibility is assumed for possible inaccuracies or omissions. All specifications are subject to change without notice.
PA41/42U REV. G JANUARY 1999 © 1999 Apex Microtechnology Corp.
APEX [ CIRRUS LOGIC ]
