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541 Scheda tecnica(PDF) 3 Page - M.S. Kennedy Corporation |
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541 Scheda tecnica(HTML) 3 Page - M.S. Kennedy Corporation |
3 / 7 page HEAT SINKING To select the correct heat sink for your application, refer to the thermal model and governing equation below. Thermal Model: Governing Equation: 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: (TO-3 PACKAGE) 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. 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 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. APPLICATION NOTES CURRENT LIMIT The MSK 541 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 determine the value of the current limit resistance necessary to establish the desired current limit. 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 See "Application Circuits" in this data sheet for additional information on current limit connections. 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 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 MSK 541, 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 capacitors should be placed as close to the package power supply pins as possible (pins 3 and 6 for the MSK 541). 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 applications should be checked against the S.O.A. curves to ensure high M.T.B.F. 3 Rev. C 3/01 |
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