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OPERATING
CONSIDERATIONS
PA98 PA98A
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.
CURRENT LIMIT
For proper operation, the current limit resistor (R
) must be
connected as shown in the external connection diagram. The
minimum value is 1.4 ohm, however for optimum reliability the
resistor value should be set as high as possible. The value is
calculated as follows; with the maximum practical value of 30
ohms.
.7
R
CL
=
I
LIM
- .016
SAFE OPERATING AREA (SOA)
The MOSFET output stage of this power operational ampli-
fier has two distinct limitations:
1. The current handling capability of the MOSFET geometry
and the wire bonds.
2. The junction temperature of the output MOSFETs.
NOTE: The output stage is protected against transient flyback.
However, for protection against sustained, high energy flyback,
external fast-recovery diodes should be used.
SAFE OPERATING CURVES
The safe operating area curves define the maximum addi-
tional internal power dissipation the amplifier can tolerate
when it produces the necessary output to drive an external
SUPPLY TO OUTPUT DIFFERENTIAL, V
S
–V
O
(V)
25
50
75
100
250
500
O
S
S
,
10
20
30
50
100
200
300
500
DC T
C
=125°C
DC T
C
=85C
200mS
PULSE CURVES @ 10% DUTY CYCLE MAX
DC T
C
=25°C
100mS
125
load. This is not the same as the absolute maximum internal
power dissipation listed elsewhere in the specification since
the quiescent power dissipation is significant compared to the
total.
INPUT PROTECTION
Although the PA98 can withstand differential voltages up to
±
25V, additional external protection is recommended. Since
the PA98 is a high speed amplifier, low leakage, low capaci-
tance JFETs connected as diodes are recommended (e.g.
2N4416, Q1-Q4 in Figure 1). The differential input voltage will
be clamped to
±
1.4V. This is sufficient overdrive to produce
maximum power bandwidth.
POWER SUPPLY PROTECTION
Unidirectional zener diode transient suppressors are recom-
mended as protection on the supply pins. 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.
Conditions which can cause open circuits or polarity rever-
sals on either power supply rail should be avoided or protected
against. Reversals or opens on the negative supply rail is
known to induce input stage failure. Unidirectional transzorbs
prevent this, and it is desirable that they be both electrically and
physically as close to the amplifier as possible.
STABILITY
The PA98 is externally compensated and performance can
be tailored to the application. Use the graphs of small signal
response and power response as a guide. The compensation
capacitor C
must be rated at 500V working voltage. An NPO
capacitor is recommended. The compensation network C
R
C
must be mounted closely to the amplifier pins 7 and 8 to avoid
spurious oscillation.
+V
S
–IN
+IN
Z1
Z2
11, 12
7, 8
1
2
6
PA98
Q1
Q2
–V
S
Q3
Q4
FIGURE 1.
OVERVOLTAGE
PROTECTION
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.
PA98 PA98AU REV.C JULY 2001 2001 Apex Microtechnology Corp.