MSK1461_15 Datasheet, PDF (3/6 Pages) M.S. Kennedy Corporation – Large Gain-Bandwidth Product

APPLICATION NOTES
HEAT SINKING
CURRENT LIMIT
To select the correct heat sink for your application, refer
to the thermal model and governing equation below.
Thermal Model:
The output current of the MSK1461 is internally limited
to approximately ±750mA by two 0.8Ω internal current limit
resistors. Additional current limit can be achieved through
the use of two external current limit resistors. One resistor
(+RSC) limits the positive output current and the other (-
RSC) limits the negative output current. The value of the cur-
rent limit resistors can be determined as follows:
±RSC = [(0.65V/±ILIM) - 0.8Ω]
Since the 0.65V term is obtained from the base to emit-
ter voltage drop of a bipolar transistor, the equation only
holds true for +25°C operation. As case temperature in-
creases, the 0.65V term will decrease making the actual
current limit set point decrease slightly.
Governing Equation:
The following schematic illustrates how to connect
each current limit resistor:
TJ=PD x (RθJC + RθCS + RθSA) + TA
Where
TJ = Junction Temperature
PD = Total Power Dissipation
RθJC = Junction to Case Thermal Resistance
RθCS = Case to Heat Sink Thermal Resistance
RθSA = Heat Sink to Ambient Thermal Resistance
TC = Case Temperature
TA = Ambient Temperature
TS = Sink Temperature
Example:
INPUT OFFSET ADJUST CONNECTION
IN
In our example the amplifier application requires the out-
put 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 shall be set to ±40VDC.
1.) Find Driver Power Dissipation
PD = [(quiescent current) x (+VS - (-VS))] +
[(+VS-VO) x IOUT]
= [(50mA) x (80V)] + [(20V) x (0.05A)]
= 4W + 1.0W
= 5Watts
2.) For conservative design, set TJ=+125°C.
3.) For this example, TA=+25°C
4.) RθJC = 16°C/W from MSK 1461B Data Sheet
5.) RθCS = 0.15°C/W for most thermal greases
6.) Rearrange governing equation to solve for RθSA
RθSA = ((TJ - TA)/PD) - (RθJC) - (RθCS)
= ((125°C - 25°C) / 5W) - (16°C/W) - (0.15°C/W)
≅ 3.85°C/W
The heat sink in this example must have a thermal resis-
tance of no more than 3.85°C/W to maintain a junction tem-
perature 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 by-
pass circuit to avoid power supply induced oscillation. An
effective decoupling scheme consists of a 0.1μF ceramic
capacitor in parallel with a 4.7μF tantalum capacitor from
each power supply pin to ground.
SAFE OPERATING AREA
Any designer who has worked with power operational
amplifiers is familiar with Safe Operating Area (S.O.A.)
curves. S.O.A. curves are a graphical representation of the
following three power limiting factors of any bipolar transis-
tor output op-amp.
1. Wire Bond Current Carrying Capability
2. Transistor Junction Temperature
3. Secondary Breakdown Limitations
Since the MSK1461 utilizes a MOSFET output, there are
no secondary breakdown limitations and therefore no need
for S.O.A. curves. The only limitation on output power is the
junction temperature of the output drive transistors.
Whenever possible, junction temperature should be kept
below 150°C to ensure high reliability. See "Heat Sinking"
for more information involving junction temperature calcula-
tions.
3
8548-137 Rev. E 10/14