APPLICATION NOTES
HEAT SINKING
To determine if a heat sink is necessary for your application and
if so, what type, refer to the thermal model and governing equation
below.
CURRENT LIMIT
The MSK 161 has an on-board current limit scheme designed to
shut off the output drivers anytime output current exceeds a prede-
termined limit. The following formula may be used to determine the
value of current limit resistance necessary to establish the desired
current limit.
R
CL
=(OHMs)=(0.65 volts/current limit in amps) - 0.01OHM
The 0.01 ohm term takes into account any wire bond and lead
resistance. Since the 0.65 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 tem-
perature has on current limit may be seen on the Current Limit vs.
Case Temperature Curve in the Typical Performance Curves.
Thermal Model:
CURRENT LIMIT CONNECTION
Governing Equation:
T
J=
P
D
x
(R
θJC
+
R
θCS
+
R
θSA
)
+
T
A
Where
T
J=
Junction Temperature
P
D=
Total Power Dissipation
R
θJC=
Junction to Case Thermal Resistance
R
θCS=
Case to Heat Sink Thermal Resistance
R
θSA=
Heat Sink to Ambient Thermal Resistance
T
C=
Case Temperature
T
A=
Ambient Temperature
T
S=
Sink Temperature
Example
:
In our example the amplifier application requires the output to
drive a 20 volt peak sine wave across a 400Ω load for 50mA of
peak output current. For a worst case analysis we will treat the
50mA peak output current as a D.C. output current. The power
supplies are ±40 VDC.
1.) Find Power Dissipation
P
D
=[(quiescent current) x (V
S
-(V
S
))]+[(+V
S
-V
O
) x I
OUT
]
=(3.0mA) x (80V)+(20V) x (1A)
=0.24W+20W
=20.24W
2.) For conservative design, set T
J
=+125°C
3.) For this example, worst case T
A
=+50°C
4.) R
θJC
=1.8°C/W from MSK 161 Data Sheet
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
)
=((125°C -50°C)/20.24W) - (1.8°C/W) - (0.15°C/W)
=1.76°C/W
The heat sink in this example must have a thermal resistance of
no more than 1.76°C/W to maintain a junction temperature of no
more than +125°C.
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 capacitor 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 161, to place a 30-50 microfarad
non-electrolytic 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 capacitors should be placed as close to the
package power supply pins as possible (pins 7 and 12).
3
8548-92 Rev. C 2/13
MSK [ M.S. KENNEDY CORPORATION ]
