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.

GAIN

The MSK 111, unlike most operational amplifiers, has

an internal feedback resistor. The value of this resistor

is 1.5K

Ω. 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 20K

Ω potentiometer may be placed

between pins 4 and 8 with the wiper arm connected to

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.

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.

IN

To select the correct heat sink for your application, refer

to the thermal model and governing equation below.

Thermal Model:

Governing Equation:

Where

= Junction Temperature

= Total Power Dissipation

R

θJC

= Junction to Case Thermal Resistance

R

R

= Case Temperature

= Ambient Temperature

= Sink Temperature

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

=(28 mA) x (30V) + (5V) x (0.2A)

=0.84W + 1W

=1.84W

4.) R

5.) Rearrange governing equation to solve for R

θSA:

R

= (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

HEAT SINKING