APPLICATION NOTES
POWER SUPPLIES:
The MSK 181 maximum total supply voltage is specified as
60V. However, dual and unbalanced power supply operation is
permissible as long as total supply voltage does not exceed
60V.
THERMAL PROTECTION:
The MSK 181 is equipped with thermal protection circuitry
that protects the amplifier from damage caused by excessive
junction temperature. The output is disabled when the junction
temperature reaches approximately 160°C. After the junction
temperature cools to approximately 140°C, the output is again
enabled. The thermal protection may cycle on and off depend-
ing on the output load and signal conditions; this may have an
undesirable effect on the load.
It should be noted that even though this internal protection
circuitry does protect against overload conditions, it does not
take the place of proper heat sinking. For reliable operation,
junction temperature should be limited to 150°C, maximum.
POWER SUPPLY BYPASSING:
Power supply terminals must be effectively decoupled with a
high and low frequency bypass circuit to avoid power supply
induced oscillation. An effective decoupling scheme consists
of a 0.1µF ceramic capacitor in parallel with a 10µF tantalum
capacitor for each power supply pin to ground. In addition, it is
recommended that a 0.01µF capacitor be placed between ±Vcc
as close to the MSK 181 as possible.
ENABLE/STATUS PIN:
This pin actually has a dual function. First, when the pin is
forced low, the output stage is disabled. Second, it can be
monitored to determine if the device is in thermal shutdown.
These functions can be used on the same device with either
single or dual supplies. For normal operation, the E/S pin must
be left open or pulled at least 2.4 volts above the negative rail.
In noisy applications, a small value capacitor between the E/S
pin and -Vcc may be required.
To disable the output, the user must pull the E/S pin low, no
greater than 0.8V above -Vcc. To once again enable the de-
vice, the E/S pin must be brought at least 2.4 volts above -Vcc
or disconnected. It should be noted that when the E/S pin is
high, the internal thermal shutdown is still active.
If the E/S pin is used to monitor thermal shutdown, during
normal operation the voltage on the E/S pin is typically 3.5V
above -Vcc. Once shutdown has occurred this voltage will drop
to approximately 350mV above -Vcc.
CURRENT LIMIT:
The MSK 181 offers accurate, user-selectable current limit.
Unlike typical designs that use a power resistor in series with
the output to sense load, the MSK 181 senses the load indi-
rectly and therefore does not require a resistor to handle the full
output current. Current limit is selected by controlling the input
to the I
LIM
pin.
The easiest method is to use a resistor or potentiometer con-
nected between-Vcc and the I
LIM
pin. Use the following equa-
tion to select proper resistor value:
71,250
-13.75KΩ
Rcc=
I
LIM
A low level control signal (0-330µA) can also be used to con-
trol the current level digitally. If the pin is left open, the current
is programmed to OA, while connecting I
LIM
directly to -Vcc
sets the output current to it's maximum, typically 5A.
SAFE OPERATING AREA:
The safe operating area curve is a graphical representation of
the power handling capability of the amplifier under various
conditions. Power dissipation of the device is equal to the prod-
uct of the voltage across the output transistor times the output
current. As can be seen in the curve, safe operating current
decreases with an increase in temperature as well as an in-
crease in the voltage across the output transistor. Therefore,
for maximum amplifier performance it is important to keep case
temperature as low as possible and to keep ±Vcc as close to
the output rail as achievable.
For normal operation output compensation is not typically
required. However, if the MSK 181 is intended to be driven into
current limit the user may find that an R/C network is required.
A snubber network from the output to ground will provide sta-
bility. If driving large capacitive or inductive loads, a snubber
network will also enhance stability. Typically 3Ω to 10Ω in se-
ries with 0.01µF is acceptable.
COMPENSATION:
TYPICAL CONNECTION DIAGRAM
3
PRELIMINARY Rev. C 8/01
MSK [ M.S. KENNEDY CORPORATION ]
