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參數資料
| 型號： | MSK1461B |
| 廠商： | Electronic Theatre Controls, Inc. |
| 英文描述： | IC REG LDO 500MA 3.3V SOT23-5 |
| 中文描述： | 高速/電壓運算放大器 |
| 文件頁數： | 3/5頁 |
| 文件大?。?/td> | 302K |
| 代理商： | MSK1461B |
1.) Find Driver Power Dissipation
P
D
= [(quiescent current) x (+V
S
- (-V
S
))] +
[(+V
S
-V
O
) x I
OUT
]
= [(50mA) x (80V)] + [(20V) x (0.05A)]
= 4W + 1.0W
= 5Watts
2.) For conservative design, set T
J
=+125°C.
3.) For this example, worst case T
A
=+50°C
4.) R
θ
JC
= 12°C/W from MSK 1461B Data Sheet
5.) R
θ
CS
= 0.15°C/W for most thermal greases
6.) Rearrange governing equation to solve for R
θ
SA
R
θ
SA
= ((T
J
- T
A
)/P
D
) - (R
θ
JC
) - (R
θ
CS
)
= ((125°C - 50°C) / 5W) - (12°C/W) - (.15°C/W)
2.85°C/W
The heat sink in this example must have a thermal
resistance of no more than 2.85°C/W to maintain a junc-
tion temperature of no more than +125°C
.
APPLICATION NOTES
HEAT SINKING
To select the correct heat sink for your application,
refer to the thermal model and governing equation below.
Thermal Model:
Governing Equation:
CURRENT LIMIT
The output current of the MSK 1461 is internally lim-
ited to approximately ±750mA by two 0.8
internal cur-
rent limit resistors. Additional current limit can be achieved
through the use of two external current limit resistors.
One resistor (+R
SC
) limits the positive output current and
the other (-R
SC
) limits the negative output current. The
value of the current limit resistors can be determined as
follows:
±R
SC
= [(0.65V/±I
LIM
) - 0.8
]
Since the 0.65V term is obtained from the base to
emitter voltage drop of a bipolar transistor, the equation
only holds true for +25°C operation. As case tempera-
ture increases, the 0.65V term will decrease making the
actual current limit set point decrease slightly.
T
J
=P
D
x (R
θ
JC
+ R
θ
CS
+ R
θ
SA
) + T
A
Where
T
J
= Junction Temperature
P
D
= Total Power Dissipation
R
θ
JC
= Junction to Case Thermal Resistance
R
θ
CS
= Case to Heat Sink Thermal Resistance
R
θ
SA
= Heat Sink to Ambient Thermal Resistance
T
C
= Case Temperature
T
A
= Ambient Temperature
T
S
= Sink Temperature
Example:
In our example the amplifier application requires the
output to drive a 20 volt peak sine wave across a 400
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.
The following schematic illustrates how to connect
each current limit resistor:
IN
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
transistor output op-amp.
1. Wire Bond Current Carrying Capability
2. Transistor Junction Temperature
3. Secondary Breakdown Limitations
Since the MSK 1461 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 tran-
sistors.
Whenever possible, junction temperature should be
kept below 150°C to ensure high reliability. See "Heat
Sinking" for more information involving junction tempera-
ture calculations.
SAFE OPERATING AREA
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 0.1μF ce-
ramic capacitor in parallel with a 4.7μF tantalum capaci-
tor from each power supply pin to ground.
POWER SUPPLY BYPASSING
INPUT OFFSET ADJUST CONNECTION
Rev. B 8/00
3
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