APPLICATION NOTES
HEAT SINKING
POWER SUPPLY CONNECTIONS
To select the correct heat sink for your application, refer to the
thermal model and governing equation below.
The MSK2541 maximum supply voltage is specified as ±40V.
However, single sided or unbalanced power supply operation is
permissible as long as the total power supply voltage does not exceed
80V. Caution should be exercised when routing high current printed
circuit paths. Generally, these paths should not be placed near low
level, high impedance input circuitry to avoid oscillations.
Thermal Model:
During prototype evaluation, power supply current limiting
is strongly advised to avoid damaging the device. See the
application note entitled "Current Limit" for an explanation
of the limitations of the MSK2541 on board current limit.
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 MSK2541, 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 3 and 6).
Governing Equation:
TJ = PD X (RθJC + RθCS + RθSA) + TA
Where
TJ
PD
= Junction Temperature
= Total Power Dissipation
RθJC = Junction to Case Thermal Resistance
RθCS = Case to Heat Sink Thermal Resistance
RθSA = Heat Sink to Ambient Thermal Resistance
TC
TA
TS
= Case Temperature
= Ambient Temperature
= Sink Temperature
CURRENT LIMIT
The internal current limit should not be used as a short circuit
protection scheme. When the output is directly shorted to ground,
the power supply voltage is applied across the output transistor
that is conducting. If the power supplies were set to ±40V and
the output was shorted to ground, the transistor that is conducting
current would see 40V from its emitter to its collector. Referring to
the safe operating area curve shows when [VCC-VOUT]=40V, the
maximum safe output current (IO) at TC=25°C is 1.5A. In this case
the amplifier would not be protected by the internal current limit and
would probably be damaged. The internal current limit is provided as
a protection against unintentional load conditions which may require
larger amounts of load current than the amplifier is rated for.
Example:
In our example the amplifier application requires each output to
drive a 20 volt peak sine wave across a 10 ohm load for 2 amps of
output current. For a worst case analysis we will treat the 2 amps
peak output current as a D.C. output current. The power supplies
are ±35 VDC.
1.) Find Power Dissipation
PD = [(quiescent current) X (+VCC - (-VCC))] +
[(VCC - VO) X IOUT]
=
=
=
(30 mA) X (70V) + (15V) X (2A)+(15V)x(2A)
2.1W + 60W
62.1W
SAFE OPERATING AREA
The safe operating area curve is a graphical representation of the
powerhandlingcapabilityoftheamplifierundervariousconditions.The
wire bond current carrying capability, transistor junction temperature
and secondary breakdown limitations are all incorporated into the
safe operating area curves. All applications should be checked
against the S.O.A. curves to ensure high M.T.T.F.
2.) For conservative design, set TJ = +150°C
3.) For this example, worst case TA = +25°C
4.) RθJC = 1.2°C/W typically
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
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. Refer to the "Heat Sinking Options"
application note offered by MSK.
3
8548-128 Rev. K 9/21
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