APPLICATION NOTES
HEAT SINKING
To select the correct heat sink for your application, refer to the
thermal model and governing equation below.
CURRENT LIMIT
The MSK 541 has an on-board current limit scheme designed
to limit the output drivers anytime output current exceeds a
predetermined limit. The following formula may be used to
determine the value of the current limit resistance necessary to
establish the desired current limit.
R
CL
(OHMs) = (0.809 volts / current limit in amps) - 0.057 OHM
The 0.057 OHM term takes into account any wire bond and
lead resistance. Since the 0.809 volt term is obtained from the
base emitter voltage drop of a bipolar transistor, the equation
only holds true for operation at +25°C case temperature. The
effect that temperature has on current limit may be seen on the
Current Limit vs. Case Temperature Curve in the Typical Perfor-
mance Curves.
Thermal Model:
Current Limit Connection
Governing Equation:
T
J
= P
D
X (R
θJC
+ R
θCS
+ R
θSA
) + T
A
Where
T
J
P
D
R
θJC
R
θCS
R
θSA
T
C
T
A
T
S
=
=
=
=
=
=
=
=
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
Example:
(TO-3 PACKAGE)
In our example the amplifier application requires the output to
drive a 20 volt peak sine wave across a 5 ohm load for 4 amps of
output current. For a worst case analysis we will treat the 4 amps
peak output current as a D.C. output current. The power supplies
are ±35 VDC.
1.) Find Power Dissipation
P
D
= [(quiescent current) X (+V
CC
- (V
CC
))] + [(V
S
- V
O
) X I
OUT
]
= (30 mA) X (70V) + (15V) X (4A)
= 2.1W + 60W
= 62.1W
2.) For conservative design, set T
J
= +150°C
3.) For this example, worst case T
A
= +25°C
4.) R
θJC
= 1.2°C/W typically for the TO-3 package
5.) R
θCS
= 0.15°C/W for most thermal greases
6.) Rearrange governing equation to solve for R
θSA
R
θSA
= (T
J
- T
A
) / P
D
- (R
θJC
) - (R
θCS
)
= (150°C - 25°C) / 62.1W - (1.2°C/W) - (0.15°C/W)
= 0.66°C/W
The heat sink in this example must have a thermal resistance of
no more than 0.66°C/W to maintain a junction temperature of no
more than +150°C. Since this value of thermal resistance may be
difficult to find, other measures may have to be taken to decrease
the overall power dissipation.
3
See "Application Circuits" in this data sheet for additional
information on current limit connections.
POWER SUPPLY BYPASSING
Both the negative and the positive power supplies 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 0.1 microfarad ceramic ca-
pacitor in parallel with a 4.7 microfarad tantalum capacitor from
each power supply pin to ground. It is also a good practice
with very high power op-amps, such as the MSK 541, to place
a 30-50 microfarad nonelectrolytic capacitor with a low effec-
tive series resistance in parallel with the other two power sup-
ply decoupling capacitors. This capacitor will eliminate any
peak output voltage clipping which may occur due to poor
power supply load regulation. All power supply decoupling
capacitors should be placed as close to the package power
supply pins as possible (pins 3 and 6 for the MSK 541).
SAFE OPERATING AREA
The safe operating area curve is a graphical representation of
the power handling capability of the amplifier under various
conditions. The wire bond current carrying capability, transis-
tor junction temperature and secondary 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.
Rev. C 3/01
MSK [ M.S. KENNEDY CORPORATION ]
