PA10 • PA10A
OPERATING
CONSIDERATIONS
GENERAL
Please read Application Note 1 "General Operating Consider-
ations" which covers stability, 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.
SAFE OPERATING AREA (SOA)
The output stage of most power amplifiers has three distinct
limitations:
1. The current handling capability of the transistor geometry and
the wire bonds.
2. The second breakdown effect which occurs whenever the
simultaneous collector current and collector-emitter voltage
exceeds specified limits.
3. The junction temperature of the output transistors.
5.0
4.0
3.0
2.0
1.5
1.0
.8
Tc = 85°C steady
TTcH=ER12M5A°CL
state
t = 5ms
SECOND
t
=
t=
1ms
0.5ms
BREAKDOWN
.6
.4
.3
.2
10 15 20 25 30 35 40 50 60 70 80 100
SUPPLY TO OUTPUT DIFFERENTIAL VOLTAGE VS – VO (V)
The SOA curves combine the effect of these limits. For a given
application, the direction and magnitude of the output current
should be calculated or measured and checked against the SOA
curves. This is simple for resistive loads but more complex for
reactive and EMF generating loads.
1. For DC outputs, especially those resulting from fault condi-
tions, check worst case stress levels against the new SOA
graph.
For sine wave outputs, use Power Design1 to plot a load line.
Make sure the load line does not cross the 0.5ms limit and that
excursions beyond any other second breakdown line do not
exceed the time label, and have a duty cycle of no more than
10%.
For other waveform outputs, manual load line plotting is
recommended. Applications Note 22, SOA AND LOAD LINES,
will be helpful. A Spice type analysis can be very useful in that
a hardware setup often calls for instruments or amplifiers with
wide common mode rejection ranges.
2. The amplifier can handle any EMF generating or reactive load
and short circuits to the supply rail or shorts to common if the
current limits are set as follows at TC = 85°C:
1 Note 1. Power Design is a self-extracting Excel spreadsheet
available free from www.apexmicrotech.com
SHORT TO ±VS
SHORT TO
±VS
C, L, OR EMF LOAD
COMMON
50V .21A
.61A
40V .3A
35V .36A
30V .46A
25V .61A
20V .87A
15V 1.4A
.87A
1.0A
1.4A
1.7A
2.2A
2.9A
CURRENT LIMITING
Refer to Application Note 9, "Current Limiting", for details of both
fixed and foldover current limit operation. Visit the Apex web site
at www.apexmicrotech.com for a copy of the Power Design
spreadsheet (Excel) which plots current limits vs. steady state
SOA. Beware that current limit should be thought of as a +/–20%
function initially and varies about 2:1 over the range of –55°C to
125°C.
For fixed current limit, leave pin 7 open and use equations 1 and 2.
RCL = 0.65/LCL
ICL = 0.65/RCL
(1)
(2)
Where:
ICL is the current limit in amperes.
RCL is the current limit resistor in ohms.
For certain applications, foldover current limit adds a slope to
the current limit which allows more power to be delivered to the
load without violating the SOA. For maximum foldover slope,
ground pin 7 and use equations 3 and 4.
ICL =
0.65 + (Vo * 0.014)
RCL
(3)
RCL =
0.65 + (Vo * 0.014)
ICL
(4)
Where:
Vo is the output voltage in volts.
Most designers start with either equation 1 to set RCL for the
desired current at 0v out, or with equation 4 to set RCL at the
maximum output voltage. Equation 3 should then be used to plot
the resulting foldover limits on the SOA graph. If equation 3 results
in a negative current limit, foldover slope must be reduced. This
can happen when the output voltage is the opposite polarity of the
supply conducting the current.
In applications where a reduced foldover slope is desired, this
can be achieved by adding a resistor (RFO) between pin 7 and
ground. Use equations 4 and 5 with this new resistor in the circuit.
0.65 + Vo * 0.14
ICL =
10.14 + RFO
RCL
(5)
RCL =
0.65 + Vo * 0.14
10.14 + RFO
ICL
Where:
RFO is in K ohms.
(6)
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PA10U REV. M FEBRURAY 2001 © 2001 Apex Microtechnology Corp.