APPLICATION NOTES
SAFE OPERATING AREA (SOA)
AMPLIFIER STABILITY
Since both output transistors in this amplifier are NPN, consid-
eration must be taken when stabilizing the output. A one ohm
resistor, 0.1uF capacitor snubber network should be added ex-
ternally from the output to -Vcc on each amplifier. This con-
figuration minimizes local output stage oscillations. As always,
adequate power supply bypassing is a necessity for ampli-
fier stability. A parallel combination of a 4.7uF electrolytic (for
every amp of output current) and a 0.01uF ceramic disc capaci-
tor should be connected as close as possible to the package
power supply pins to ground.
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 transis-
tor junction temperature and secondary breakdown effects. To
see if your application is meeting or exceeding the limitations
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 usually safe.
PARALLEL CONNECTION
yields single 4A amplifier
±Vcc
20V
15V
10V
5V
Capacitive Load
200uF
500uF
5mF
50mF
Inductive Load
7.5mH
25mH
35mH
150mH
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.
3
BIDIRECTIONAL MOTOR DRIVE
Rev. H 8/00
MSK [ M.S. KENNEDY CORPORATION ]
