HEAT SINKING
To select the correct heat sink for your application, refer to the
thermal model and governing equation below.
CURRENT LIMIT
The MSK 106RH 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 resis-
tance necessary to establish the desired current limit.
0.7
___
I
SC
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:
In our example the amplifier application requires the output to
drive a 10 volt peak sine wave across a 10 ohm load for 1 amp of
output current. For a worst case analysis we will treat the 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 (1A)
=0.1W + 6W
=6.1W
2.) For conservative design, set T
J
= +125°C.
3.) For this example, worst case T
A
= +25°C.
4.) R
θJC
= 6.0°C/W
5.) Rearrange governing equation to solve for R
θSA:
R
θSA
=(T
J
- T
A
) / P
D
- (R
θJC
) - (R
θCS
)
= (125°C - 25°C) / 6.1W - (6.0°C/W) - (0.15°C/W)
= 10.2°C/W
The heat sink in this example must have a thermal resistance of
no more than 10.2°C/W to maintain a junction temperature of less
than +125°C.
3
Rev. D 1/05
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 effec-
tive 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. It is also a good
practice with high power op-amps, such as the MSK 106RH,
to place a 30-50 microfarad capacitor with a low effective
series resistance, in parallel with the other two power supply
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 capaci-
tors should be placed as close to the package power supply
pins as possible.
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○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○
APPLICATION NOTES
R
SC
=
MSK [ M.S. KENNEDY CORPORATION ]
