DISCUSSION OF PERFORMANCE
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 resistors.
transistors.
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. The following guidelines may save extensive
analytical efforts.
1.Capacitive and dynamic* loads up to the following
maximums are safe with the current limits set as
specified.
2. The amplifier can handle any EMF generating or
reactive load and short circuits to the supply rail or
common if the current limits are set as follows at
T
c = 25
OC.
* 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.
These simplified limits may be exceeded with further
analysis using the operating conditions for a specific
application.
CURRENT LIMIT
For fixed current limit, leave pin 7 open and use the
equations in 1 and 2.
R
CL = 0.65/LCL
(1)
I
CL = 0.65/RCL
(2)
Where:
I
CL is the current limit in amperes.
R
CL is the current limit resistor in ohms.
For certain applications the 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
7and use equations 3 and 4.
I
CL= (0.65+(Vo*0.014)) / RCL
(3)
R
CL =( 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 output current at 0V out or with
equation 4 set to 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
(R
FO) between pin 7 and ground.
Use equations 5
and 6 with this new resistor in the circuit.
I
CL= ((0.65+(VO*0.014)/(10.14+RFO)) / RCL
(5)
R
CL= ((0.65+(VO*0.014)/(10.14+RFO)) / ICL
(6)
Where R
FO is in K ohms.
Capacitive Load
Inductive Load
±V
s
I
LIM = 5A
I
LIM = 10A
I
LIM = 5A
I
LIM= 10A
50V
200µF
125µF5mH
2.0mH
40V
500µF
350µF15mH
3.0mH
35V
2.0mF
850µF50mH
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
Short to ±V
S
Short to
±V
s
C,L or EMF Load
Common
50V
0.30A
2.4A
40V
0.58A
2.9A
35V
0.87A
3.7A
30V
1.50A
4.1A
25V
2.40A
4.9A
20V
2.90A
6.3A
15V
4.20A
8.0A
15
10
6.0
4.0
2.0
1.0
0.6
0.4
0.3
10
20
30
40
50
70
100
SUPPLY TO OUTPUT DIFFERENTIAL VOLTAGE (V)
SOA
SECOND BREAKDOWN
THER
MAL
T
C
= 23o
C
T
C
= 85o
C
T
C
= 125
oC
TPA12/12A Rev. B Oct. 2006