APPLICATION NOTES
SAFE OPERATING AREA (SOA)
AMPLIFIER STABILITY
Since both output transistors in this amplifier are NPN, con-
sideration must be taken when stabilizing the output. A one
ohm resistor, 0.047uF capacitor snubber network has been
added internally from the output to -Vcc on each amplifier. This
configuration minimizes local output stage oscillations. As al-
ways, adequate power supply bypassing is a necessity for
amplifier stability. A parallel combination of a 4.7uF electro-
lytic (for every amp of output current) and a 0.01uF ceramic
disc capacitor should be connected as close as possible to the
package power supply pins to ground. The R-C snubber net-
works shown on the outputs of the amplifiers in the typical
circuits are internal and should not be added externally.
Safe operating area curves are a graphical representation of
all of the power limiting factors involved in the output stage of
an operational amplifier. Three major power limiting factors
are; output transistor wire bond carrying capability, output tran-
sistor junction temperature and secondary breakdown effects.
To see if your application is meeting or exceeding the limita-
tions of the safe operating area curves, perform the following
steps:
1.) Find the worst case output power dissipation. For a split
supply, purely resistive load application, this occurs when
V
OUT
=1/2 V
CC
.
2.) Take the values of (V
CC
-V
OUT
) and the corresponding out-
put current and find their intersection on the safe operating
area curves.
3.) Verify this point is below the safe operating area curves.
This is a simple task for purely resistive loads, for reactive loads
the following table will save extensive analysis. Under tran-
sient conditions, capacitive and inductive loads up to the fol-
lowing maximum are safe.
±Vcc
20V
15V
10V
5V
Capacitive Load
200uF
500uF
5mF
50mF
Inductive Load
7.5mH
25mH
35mH
150mH
PARALLEL CONNECTION
yields single 6A amplifier
If the inductive load is driven near steady state conditions al-
lowing the output to drop more than 6V below the supply rail
while the amplifier is current limiting, the inductor should be
capacitively coupled or the supply voltage must be lowered to
meet the SOA criteria. It is a good practice to also connect
reverse biased fast recovery diodes to the output for protection
against sustained high energy flyback.
BIDIRECTIONAL MOTOR DRIVE
3
Rev. D 10/01
MSK [ M.S. KENNEDY CORPORATION ]
