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.
5.0
OUTPUT CURRENT FROM +V
S
OR – V
S
(A)
CURRENT LIMITING
Refer to Application Note 9, "Current Limiting", for details of
both fixed and foldover current limit operation. Visit the Apex
Precision Power web site at www.Cirrus.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.
(1)
R
CL
= 0.65/L
CL
(2)
I
CL
= 0.65/R
CL
Where:
I
CL
is the current limit in amperes.
R
CL
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.
0.65 + (Vo * 0.014)
(3)
I
CL
=
R
CL
R
CL
=
0.65 + (Vo * 0.014)
I
CL
(4)
4.0
3.0
2.0
1.5
1.0
.8
.6
.4
.3
.2
T
C
=8
T
C
5°C
25
ste
ad
TH
=1
ys
t=
ta
te
SE
5m
s
ER
°C
MA
CO
t=
t=
1m
0 . 5
s
ms
BR
L
ND
EA
KD
OW
N
10
15
20
25 30 35 40
50 60 70 80 100
SUPPLY TO OUTPUT DIFFERENTIAL VOLTAGE V
S
– V
O
(V)
3. The junction temperature of the output transistors.
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 Design
1
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%.
1
Note 1. Power Design is a self-extracting Excel spread-
sheet available free from www.Cirrus.com
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 com-
mon if the current limits are set as follows at T
C
= 85°C:
4
Where:
Vo is the output voltage in volts.
Most designers start with either equation 1 to set R
CL
for the
desired current at 0v out, or with equation 4 to set R
CL
at the
maximum output voltage. Equation 3 should then be used to
plot the resulting foldover limits on the SOA graph. If equa-
tion 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 (R
FO
) between pin
7 and ground. Use equations 4 and 5 with this new resistor
in the circuit.
0.65 + Vo * 0.14
I
CL
=
0.65 +
R
CL
=
Where:
R
FO
is in K ohms.
10.14 + R
FO
R
CL
10.14 + R
FO
I
CL
Vo * 0.14
(5)
(6)
PA10U
P r o d u c t I n n o v a t i o nF r o m
PA10 • PA10A
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
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