APPLICATION NOTES
HEAT SINKING
CURRENT LIMIT
To select the correct heat sink for your application, refer to the
thermal model and governing equation below.
The MSK541 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 de-
termine the value of the current limit resistance necessary to
establish the desired current limit.
Thermal Model:
RCL (OHMs) = (0.809 volts / current limit in amps) - 0.057 OHM
The 0.057 OHM term takes into account any wire bond and
lead resistance. Since the 0.809 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 temperature has on current limit may be seen on the
Current Limit vs. Case Temperature Curve in the Typical Perfor-
mance Curves.
Current Limit Connection
Governing Equation:
TJ = PD X (RθJC + RθCS + RθSA) + TA
Where
TJ
PD
= Junction Temperature
= Total Power Dissipation
RθJC
RθCS
= Junction to Case Thermal Resistance
= Case to Heat Sink Thermal Resistance
RθSA = Heat Sink to Ambient Thermal Resistance
TC
TA
TS
= Case Temperature
= Ambient Temperature
= Sink Temperature
See "Application Circuits" in this data sheet for additional
information on current limit connections.
Example: (TO-3 PACKAGE)
POWER SUPPLY BYPASSING
In our example the amplifier application requires the output to
drive a 20 volt peak sine wave across a 5 ohm load for 4 amps of
output current. For a worst case analysis we will treat the 4 amps
peak output current as a D.C. output current. The power supplies
are ±35 VDC.
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 ca-
pacitor 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 MSK541, to place
a 30-50 microfarad nonelectrolytic capacitor with a low effec-
tive series resistance in parallel with the other two power sup-
ply 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 (pins 3 and 6 for the MSK541).
1.) Find Power Dissipation
PD = [(quiescent current) X (+VCC - (VCC))] + [(VS - VO) X IOUT]
= (30 mA) X (70V) + (15V) X (4A)
= 2.1W + 60W
= 62.1W
2.) For conservative design, set TJ = +150°C
3.) For this example, worst case TA = +25°C
4.) RθJC = 1.2°C/W typically for the TO-3 package
5.) RθCS = 0.15°C/W for most thermal greases
6.) Rearrange governing equation to solve for RθSA
RθSA
= (TJ - TA) / PD - (RθJC) - (RθCS)
= (150°C - 25°C) / 62.1W - (1.2°C/W) - (0.15°C/W)
= 0.66°C/W
SAFE OPERATING AREA
The safe operating area curve is a graphical representation
of the power handling capability of the amplifier under various
conditions. The wire bond current carrying capability, transis-
tor junction temperature and secondary breakdown limitations
are all incorporated into the safe operating area curves. All ap-
plications should be checked against the S.O.A. curves to
ensure high M.T.B.F.
The heat sink in this example must have a thermal resistance of
no more than 0.66°C/W to maintain a junction temperature of no
more than +150°C. Since this value of thermal resistance may be
difficult to find, other measures may have to be taken to decrease
the overall power dissipation.
8548-29 Rev. L 6/14
3
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