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.
Thermal Model:
Rθ
SA
= ((T
J
- T
A
)/P
D
) - (Rθ
JC
) - (Rθ
CS
).
= ((125°C-100°C) /0.13W) - 187° C/W - 0.15°C/W
= 192.3 - 187.15
= 5.2°C/W
The heat sink in this example must have a thermal resistance
of no more than 5.2°C/W to maintain a junction temperature
of no more than+125°C.
SLEW RATE
VS
. SLEW RATE LIMIT
SLEW RATE
response of the amplifier and is calculated from the full power
bandwidth frequency.
SLEW RATE LIMIT
dv/dt: The slew rate limit is based upon the amplifier's res-
ponse to a step input and is measured between 10% and 90%.
MSK measures T
R
orT
F
, whichever is greater at±10Vou
T
,
RL=510Ω
SRL= V
O
-20%
T
R
or T
F
SR = 2πVp
F: Slew rate is based upon the sinusoidal linear
COMPENSATION
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
The MSK 032, can be frequency compensated by connecting
an R-C snubber circuit from pin 3 to pin 4 as shown below.
Example:
This 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 condition, maximum power
transfer occurs and the output is under maximum stress.
Conditions:
Vcc=±16VDC
Vo=±8Vp Sine Wave, Freq.= 1KHz
R
L
=510Ω
For a worst case analysis we treat the +8Vp sine wave as an 8
VDC output voltage.
1.) Find driver power dissipation
P
D
= (Vcc-Vo) (Vo/R
L
)
= (16V
- 8V) (8V/510Ω)
=
125.5mW
2.) For conservative design, set T
J
=+125°C
3.) For this example, worst caseT
A
=+100°C
4.) Rθ
JC=
187°C/W from MSK 032B Data Sheet
5.) Rθ
CS=
0.15°C/W for most thermal greases
6.) Rearrange governing equation to solve for Rθ
SA
The recommended capacitor value is 0.01µF and the resis-
tor value can range from 2Ω to 500Ω. The effects of this R-C
snubber can be seen on the typical performance curve labeled
Slew Rate
VS.
Compensation Resistance. The graph clearly illus-
trates the decrease in transition time as snubber resistance in-
creases. This occurs because the high frequency components
of the input square wave are above the corner frequency of the
R-C snubber and are applied common mode to the bases of the
second differential pair, (pins 3 and 4). There is no differential
gain for these higher frequencies since the input signal is ap-
plied common mode. Without the high frequency components
appearing at the output, the slew rate and bandwidth of the op-
amp are limited. However, at the cost of speed and bandwidth
the user gains circuit stability. A good design rule to follow is: as
closed loop gain decreases, circuit stability decreases, therefore
snubber resistance should decrease to maintain stability and avoid
oscillation. The MSK 032 can also be compensated using the
standard LH0032 techniques.
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
decoupling scheme consists of a 0.1 microfarad ceramic capa-
citor in parallel with a 4.7 microfarad tantalum capacitor from
each power supply pin to ground.
3
Rev. B 5/02
MSK [ M.S. KENNEDY CORPORATION ]
