HEAT SINKING
To select the correct heat sink for your application, refer to the
thermal model and governing equation below.
CURRENT LIMIT
Thermal Model:
The MSK 0041 has an on-board current limit scheme de-
signed 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.
0.7
___
I
SC
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
See "Application Circuits" in this data sheet for additional
information on current limit connections.
Example:
(TO-8 PACKAGE)
In our example the amplifier application requires the output to drive
a 10 volt peak sine wave across a 100 ohm load for 0.1 amp of
output current. For a worst case analysis we will treat the 0.1 amp
peak output current as a D.C. output current. 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
]
=(3.5 mA) X (30V) + (5V) X (0.1A)
=0.1W + 0.5W
=0.6W
2.) For conservative design, set T
J
= +150°C.
3.) For this example, worst case T
A
= +25°C.
4.) R
θJC
= 85°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) / 0.6W - (85°C/W) - (0.15°C/W)
= 123°C/W
The heat sink in this example must have a thermal resistance of no
more than 123°C/W to maintain a junction temperature of less than
+150°C. This calculation assumes a case to sink thermal resistance
of 0.15°C/W.
3
POWER SUPPLY BYPASSING
Both the negative and the positive power supplies must
be effectively decoupled with a high and low frequency by-
pass circuit to avoid power supply induced oscillation. An
effective decoupling scheme consists of a 0.1 microfarad
ceramic capacitor in parallel with a 4.7 microfarad tantalum
capacitor from each power supply pin to ground.This ca-
pacitor 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.
SAFE OPERATING AREA
The safe operating area curve is a graphical representation
of the power handling capability of the amplifier under vari-
ous conditions. The wire bond current carrying capability,
transistor junction temperature and secondary breakdown
limitations are all incorporated into the safe operating area
curves. All applications should be checked against the curves
to ensure high M.T.B.F.
Rev. - 4/02
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○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○
APPLICATION NOTES
R
SC
=
MSK [ M.S. KENNEDY CORPORATION ]
