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MSK739 Datasheet(PDF) 3 Page - M.S. Kennedy Corporation |
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MSK739 Datasheet(HTML) 3 Page - M.S. Kennedy Corporation |
3 / 6 page APPLICATION NOTES HEAT SINKING To determine if a heat sink is necessary for your application and if so, what type, refer to the thermal model and governing equation below. Governing Equation: Example: This example demonstrates a worst case analysis for the op-amp output stage. This occurs when the output voltage is 1/2 the power supply voltage. Under this condition, maximum power transfer oc- curs and the output is under maximum stress. Conditions: VCC=±16VDC VO=±8Vp Sine Wave, Freq.=1KHz RL=100Ω For a worst case analysis we will treat the +8Vp sine wave as an 8VDC output voltage. 1.) Find Driver Power Dissapation PD=(VCC-VO) (VO/RL) =(16V-8V) (8V/100Ω) =0.64W 2.) For conservative design, set TJ=+125°C 3.) For this example, worst case TA=+90°C 4.) RθJC=45°C/W from MSK 739B 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 -90°C)/0.64W) - 45°C/W - 0.15°C/W =54.7 - 46.15 =9.5°C/W Rev. A 4/02 3 TJ=PD x (RθJC + RθCS + RθJC) + 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 The value of the short circuit current limit resistors (±RSC) can be calculated as follows. +RSC=VCC-0.7/+ISC -RSC=VCC+0.7/-ISC Short circuit current limit should be set at least 2X above the highest normal operating output current to keep the value of RSC low enough to ensure that the voltage dropped accross the short circuit current limit resistor doesn't adversely affect normal operation. INTERNAL FEEDBACK RESISTOR The MSK 739 is equipped with an internal 1.5KΩ feedback resis- tor. Bandwidth and slew rate can be optimized by connecting the MSK 739 as shown in Figure 2. Placing the feedback resistor inside the hybrid reduces printed circuit board trace length and its' asscociated capacitance which acts as a capacitive load to the op- amp output. Reducing the capacitive load allows the output to slew faster and greater bandwidths will be realized. Refer to Table 1 for recommended RIN values for various gains. APPROXIMATE DESIRED GAIN RIN VALUE 1.5KΩ TABLE 1 Whenever the internal resistor is not being used it is good practice to short pin 4 and 5 to avoid inadvertently picking up spurious sig- nals. APPROXIMATE DESIRED GAIN RI(+) RI(-) Rf(Ext) Cf 1 -1 249Ω 499Ω 499Ω 2 1 -2 160Ω 249Ω 499Ω 2 1 -5 169Ω 200Ω 1KΩ 2 1 -8 100Ω 124Ω 1KΩ 2 1 90.9Ω 100Ω 1KΩ 2 -20 -10 1 100Ω 100Ω 2KΩ 2 TABLE 2 1 The positive input resistor is selected to minimize any bias current induced offset voltage. 2 The feedback capacitor will help compensate for stray input capacitance. The value of -1 -2 -10 750Ω 150Ω Thermal Model: OUTPUT SHORT CIRCUIT PROTECTION The output section of the MSK 739 can be protected from direct shorts to ground by placing current limit resistors between pins 1 and 12 and pins 9 and 10 as shown in Figure 1. Recommended External Component Selection Guide Using External Rf this capacitor can be dependent on individual applications. A 0.5 to 5pF capacitor is usually optimum for most applications. 3 Effective load is RL in parallel with Rf. |
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