APPLICATION NOTES
POWER SUPPLY BYPASSING
Both the negative and the positive supplies must be
effectively decoupled with a high and low frequency
bypass circuit to avoid power supply induced oscilla-
tion. An effective decoupling scheme consists of a 0.01
microfarad ceramic capacitor in parallel with a 4.7 mi-
crofarad tantalum capacitor from each power supply pin
to ground. All power supply decoupling capacitors should
be placed as close to the package power supply pins as
possible.
Output, power supply, and bypass leads should be kept
as short as possible. Long connections can add signifi-
cant inductance, raising impedance and limiting output
current slew rate. This is especially true in the video
frequency range.
The case of the MSK 111 is electrically isolated and
should be connected to a common ground plane. In ad-
dition to the case, the input signal and input resistors
should be connected to this common ground plane us-
ing a single point grounding scheme. This will help to
prevent undesired current feedback that can cause in-
stability in the circuit.
HEAT SINKING
To select the correct heat sink for your application, refer
to the thermal model and governing equation below.
Thermal Model:
Governing Equation:
T
J
= P
D
x (R
θ
JC
+ R
θ
CS
+ R
θ
SA
) + T
A
Where
T
J
P
D
R
θJC
R
θCS
R
θSA
T
C
T
A
T
S
GAIN
The MSK 111, unlike most operational amplifiers, has
an internal feedback resistor. The value of this resistor
is 1.5KΩ. Fewer external components are required to
configure the MSK 111 in either inverting or non-invert-
ing modes. Using an internal feedback resistor shortens
the feedback path, lowering summing node capacitance
to ground and stabilizing high frequency characteristics.
OUTPUT OFFSET NULL
Typically,the MSK 111 has an input offset voltage of
less than ±2mV. The input offset voltage is laser
trimmed to less than ±5mV, but in applications where
offset is critical, the balance pins may be used to null
the offset to zero. A 20KΩ potentiometer may be placed
between pins 4 and 8 with the wiper arm connected to
+V
CC
. If the balance function is not used pins 4 and 8
should not be connected (floating). However, if settling
time is extremely important, pin 8 should be tied to the
AC ground with a 100-150pF capacitor.
=
=
=
=
=
=
=
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Junction Temperature
Total Power Dissipation
Junction to Case Thermal Resistance
Case to Heat Sink Thermal Resistance
Heat Sink to Ambient Thermal Resistance
Case Temperature
Ambient Temperature
Sink Temperature
IN
Example:
In our example the amplifier application requires the output
to drive a 10 volt peak sine wave across a 50 ohm load for
0.2 amp of output current. For a worst case analysis we will
treat the 0.2 amp peak output current as a D.C. output cur-
rent. The power supplies are ±15 VDC.
1.) Find Power Dissipation
P
D
=[(quiescent current) x (+V
CC
-
(V
CC
))] + [(V
S
-
V
O
) x I
OUT
]
=(28 mA) x (30V) + (5V) x (0.2A)
=0.84W + 1W
=1.84W
2.) For conservative design, set T
J
= +150°C.
3.) For this example, worst case T
A
= +25°C.
4.) R
θJC
= 31°C/W
5.) Rearrange governing equation to solve for R
θSA:
R
θSA
= (T
J
- T
A
) / P
D
- (R
θJC
) - (R
θCS
)
= (150°C - 25°C) / 1.84W - (31°C/W) - (0.15°C/W)
= 36°C/W
The heat sink in this example must have a thermal resis-
tance of no more than 36°C/W to maintain a junction tempera-
ture of less than +150°C. This calculation assumes a case to
sink thermal resistance of 0.15°C/W.
3
Rev. B 10/05
SAFE OPERATING AREA-POWER DISSIPATION
The safe operating area curve is a graphical represen-
tation of the power handling capability of the amplifier
under various conditions. The wire bond current carry-
ing capability, transistor junction temperature and sec-
ondary breakdown limitations are all incorporated into
the safe operating area curves. All applications should
be checked against the S.O.A. curves to ensure high
M.T.B.F.
MSK [ M.S. KENNEDY CORPORATION ]
