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.
OUTPUT CURRENT FROM +V
S
OR -V
S
(A)
15
10
6.0
4.0
3.0
2.0
1.0
.6
load and short circuits to the supply rail or common if the
current limits are set as follows at T
C
= 25°C:
SHORT TO ±V
S
SHORT TO
±V
S
C, L, OR EMF LOAD
COMMON
50V
.30A
2.4A
40V
.58A
2.9A
35V
.87A
3.7A
30V
1.5A
4.1A
25V
2.4A
4.9A
20V
2.9A
6.3A
15V
4.2A
8.0A
These simplified limits may be exceeded with further analysis
using the operating conditions for a specific application.
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.
R
CL
= 0.65/L
CL
(1)
I
CL
= 0.65/R
CL
(2)
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)
I
CL
=
(3)
R
CL
0.65 + (Vo * 0.014)
(4)
R
CL
=
I
CL
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.
Vo * 0.14
0.65 +
10.14 + R
FO
I
CL
=
(5)
R
CL
Vo * 0.14
0.65 +
10.14 + R
FO
R
CL
=
(6)
I
CL
Where:
R
FO
is in K ohms.
SOA
t=
TH
ER
0.5
MA
L
T
s
1m
s
t=
5m
t=
C
=2
ms
T
T
5°C
C
=8
5°C
=1
25°
C
ad
ste
C
.4
SECOND BREAKDOWN
.3
10
20
30
40 50
70
100
SUPPLY TO OUTPUT DIFFERENTIAL VOLTAGE, V
S
- V
O
(V)
The SOA curves combine the effect of all limits for this Power
Op Amp. 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. However, the following guidelines may save extensive
analytical efforts.
1. Capacitive and dynamic* inductive loads up to the following
maximum are safe with the current limits set as specified.
CAPACITIVE LOAD
INDUCTIVE LOAD
±V
S
I
LIM
= 5A
I
LIM
= 10A I
LIM
= 5A
I
LIM
= 10A
50V
200µF
125µF
5mH
2.0mH
40V
500µF
350µF
15mH
3.0mH
35V
2.0mF
850µF
50mH
5.0mH
30V
7.0mF
2.5mF
150mH
10mH
25V
25mF
10mF
500mH
20mH
20V
60mF
20mF
1,000mH
30mH
15V
150mF
60mF
2,500mH
50mH
*If the inductive load is driven near steady state conditions,
allowing the output voltage to drop more than 8V below the
supply rail with I
LIM
= 15A or 25V below the supply rail with I
LIM
= 5A while the amplifier is current limiting, the inductor must
be capacitively coupled or the current limit must be lowered
to meet SOA criteria.
2. The amplifier can handle any EMF generating or reactive
ys
e
tat
4
PA12U
P r o d u c t I n n o v a t i o nF r o m
PA12 • PA12A
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
Cirrus Logic, Inc. and its subsidiaries ("Cirrus") believe that the information contained in this document is accurate and reliable. However, the information is subject
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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