APPLICATION NOTES
POWER SUPPLY BYPASSING
Both the negative and positive power supplies must
be effectively decoupled with a high and low frequency
bypass circuit to avoid power supply induced oscillation.
An effective deecoupling scheme consists of a 0.1
microfarad ceramic capacitor in parallel with a 4.7 mi-
crofarad tantalum capacitor from each power supply pin
to ground.
Conditions:
V
CC
=±40VDC
V
O
=±20Vp Sine Wave, Freq.=1KHz
R
L
=1KΩ
For a worst case analysis we treat the ±20Vp sine wave
as an 8 VDC output voltage.
1.) Find driver power dissipation
P
D
=(V
CC
-V
O
) (V
O
/R
L
)
=(40V-20V) (20V/1KΩ)
=400mW
2.) For conservative design, set T
J
=+125°C.
3.) For this example, worst case T
A
=+100°C.
4.) R
θJC
= 16°C/W.
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 - 100°C)/0.4W - 16°C/W - 0.15°C/W
=62.5 - 16.15
=46.4°C/W
The heat sink in this example must have a thermal
resistance of no more than 46.4°C/W to maintain a junc-
tion temperature of less than +125°C.
HEAT SINKING
To determine if a heat sink is necessary for your appli-
cation and if so, what type, refer to the thermal model
and governing equation below.
Thermal Model:
TYPICAL APPLICATION CIRCUIT
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
REPLACING THE LH0004
The MSK 0004 is not an exact copy of the LM0004
but it is only slightly different. The MSK 0004 is inter-
nally compensated and is lower cost. Pins 1,5,6,7 and
10 are not connected internally in the MSK0004.
Example:
The example demonstrates a worst case analysis for
the op-amp output stage. This occurs when the output
voltage is 1/2 the power supply voltage. Under this con-
dition, maximum power transfer occurs and the output
is under maximum stress.
3
Rev. - 8/02
MSK [ M.S. KENNEDY CORPORATION ]
