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SI6862DQ Arkusz danych(PDF) 3 Page - Vishay Siliconix |
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SI6862DQ Arkusz danych(HTML) 3 Page - Vishay Siliconix |
3 / 4 page AN606 Vishay Siliconix Document Number: 71991 17-Dec-03 www.vishay.com 3 FIGURE 3. Normalized rm(on) for the Sense Die 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 −50 −25 0 25 50 75 100 125 150 175 On-Resistance vs. Junction Temperature VGS = 10 V ID = 25 A TJ − Junction Temperature (_C) DESIGN EQUATIONS The following three equations enable circuit design and analysis. ISENSE = x ID/r VDS = ISENSE x [rm(on) + RSENSE] or VDS = ID x rDS(on)/(rm(on) + RSENSE) VSENSE = ISENSE x RSENSE or VSENSE = VDS x RSENSE/(rm(on) + RSENSE) Where ISENSE Current flowing out of sense terminal r Current-sensing ratio ID Drain-source current VDS Drain-source voltage rm(on) Mirror active resistance RSENSE External current-sense resistor Application Aspects and Design Examples The current-sense ratio r, even though fixed by design, is dependent on manufacturing process variations. Furthermore, mirror active resistance rm(on) depends on circuit parameters VGS and ID and junction temperature TJ. As a result, a practical design can realize an accuracy of 15% — 20% for current sensing. Accordingly, the current-sensing MOSFET is most suitable for supervisory functions such as overcurrent and/or short-circuit protection. Three keys to a successful design are to: 1. have an adequate margin between the normal operating-current value and the trip-current value; 2. use a minimum value of the ISENSE signal at the maximum value of ID; and 3. use a fast comparator with hysterisis to control and protect the MOSFET. Typical schematic configurations for implementing the current sense are shown in Figure 4 and Figure 5. The Virtual Earth Sensing Scheme, Figure 4, is suitable for applications aiming at higher noise immunity and speed. This approach also improves measurement accuracy by eliminating the sense resistor. However, a dual power supply and inverted (negative) output signal are the price designers pay for deriving these benefits. The Resistor Sensing Scheme shown in Figure 5 is a quite simple and economical approach. The accuracy of current measurement is affected by the introduction of an external sense resistor RS. However, the latter aids in lowering the temperature sensitivity of the current-sense signal. V1 G VDD S Sense Kelvin Load R1 Stray Resistance R2 R3 − + D IL G VDD S Sense Kelvin Load − + V1 D IL −V +V FIGURE 4. Virtual Earth Sensing Scheme FIGURE 5. Resistor Sensing Scheme |
Podobny numer części - SI6862DQ |
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Podobny opis - SI6862DQ |
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