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LM196 データシート(PDF) 5 Page - National Semiconductor (TI) |
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LM196 データシート(HTML) 5 Page - National Semiconductor (TI) |
5 / 14 page Application Hints (Continued) Although it may not be immediately obvious best load regu- lation is obtained when the top of the divider is connected directly to the output pin not to the load This is illustrated in Figure 2 If R1 were connected to the load the effective resistance between the regulator and the load would be (Rw) c R2aR1 R1 J Rw e Line Resistance Connected as shown Rw is not multiplied by the divider ratio Rw is about 0004X per foot using 16 gauge wire This translates to 40 mVft at 10A load current so it is important to keep the positive lead between regulator and load as short as possible TLH9059 – 2 FIGURE 2 Proper Divider Connection The input resistance of the sense pin is typically 6 kX mod- eled as a resistor between the sense pin and the output pin Load regulation will start to degrade if a resistance higher than 10X is inserted in series with the sense This assumes a worst-case condition of 05V between output and sense pins Lower differential voltage will allow higher sense series resistance Thermal Load Regulation Thermal as well as electrical load regulation must be con- sidered with IC regulators Electrical load regulation occurs in microseconds thermal regulation due to die thermal gra- dients occurs in the 02 ms-20 ms time frame and regula- tion due to overall temperature changes in the die occurs over a 20 ms to 20 minute period depending on the time constant of the heat sink used Gradient induced load regu- lation is calculated from D VOUT e (VIN b VOUT) c (DIOUT) c (b) b e Thermal regulation specified on data sheet For VIN e 9V VOUT e 5V DIOUT e 10A and b e 0005%W this yields a 02% change in output voltage Changes in output voltage due to overall temperature rise are calculated from VOUT e (VINb VOUT) c (DIOUT) c (TC) c (ijA) TC e Temperature coefficient of output voltage ijA e Thermal resistance from junction to ambient ijA is approximately 05 CW a i of heat sink For the same conditions as before with TC e 0003% C and ijA e 15 CW the change in output voltage will be 018% Because these two thermal terms can have either polarity they may subtract from or add to electrical load regulation For worst-case analysis they must be assumed to add If the output of the regulator is trimmed under load only that portion of the load that changes need be used in the previous calculations significantly improving output ac- curacy Line Regulation Electrical line regulation is very good on the LM196typi- cally less than 0005% change in output voltage for a 1V change in input This level of regulation is achieved only for very low load currents however because of thermal ef- fects Even with a thermal regulation of 0002%W and a temperature coefficient of 0003% C DC line regulation will be dominated by thermal effects as shown by the follow- ing example Assume VOUT e 5V VIN e 9V IOUT e 8A Following a 10% change in input voltage (09) the output will change quickly (s100 ms) due to electrical effects by (0005%V) c (09V) e 00045% In the next 20 ms the output will change an additional (0002%W) c (8A) c (09V) e 00144% due to thermal gradients across the die After a much longer time determined by the time constant of the heat sink the output will change an additional (0003% C) c (8A) c (09V) c (2 CW) e 0043% due to the temperature coefficient of output voltage and the ther- mal resistance from die to ambient (2 CW was chosen for this calculation) The sign of these last two terms varies from part to part so no assumptions can be made about any cancelling effects All three terms must be added for a prop- er analysis This yields 00045 a 00144 a 0043 e 0062% using typical values for thermal regulation and tem- perature coefficient For worst-case analysis the maximum data sheet specifications for thermal regulation and temper- ature coefficient should be used along with the actual ther- mal resistance of the heat sink being used Paralleling Regulators Direct paralleling of regulators is not normally recommend- ed because they do not share currents equally The regula- tor with the highest reference voltage will supply all the cur- rent to the load until it current limits With an 18A load for instance one regulator might be operating in current limit at 16A while the second device is only carrying 2A Power dis- sipation in the high current regulator is extremely high with attendant high junction temperatures Long term reliability cannot be guaranteed under these conditions Quasi-paralleling may be accomplished if load regulation is not critical The connection shown in Figure 5a will typically share to within 1A with a worst-case of about 3A Load regulation is degraded by 150 mV at 20A loads An external op amp may be used as in Figure 5b to improve load regula- tion and provide remote sensing 5 |
同様の部品番号 - LM196 |
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同様の説明 - LM196 |
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