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UC1903 Folha de dados(PDF) 6 Page - Texas Instruments |
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UC1903 Folha de dados(HTML) 6 Page - Texas Instruments |
6 / 8 page UC1903 UC2903 UC3903 Sensing a Negative Voltage Level The UC1903 has a dedicated inverter coupled to the sense 4 input. With this inverter, a negative voltage level can be sensed as shown in Figure 6. The output of the in- verter is an unbiased emitter follower. By tying the invert- ing input, Pin 5, high the output emitter follower will be reverse biased, leaving the sense 4 input in a high imped- ance state. In this manner, the sense 4 input can be used, as the remaining sense inputs would be, for sensing posi- tive voltage levels. Using The Line/Switcher Sense Output The line switcher sense input to the UC1903 can be used for early detection of line, switcher, or other power source, failures. Internally referenced to 2.0V, the line sense com- parator will cause the POWER OK output to indicate a fault (active low) condition when the LINE/SWITCHER SENSE input goes from above to below 2.0V. The line sense comparator has approximately 175mV of hystere- sis requiring the line/switcher input to reach 2.175V be- fore the POWER OK output device can be turned off, allowing a no-fault indication. In Figure 7 an example showing the use of the LINE/SWITCHER SENSE input for early switcher-fault detection is detailed. A sample sig- nal is taken from the output of the power transformer, rec- tified and filtered, and used at the line/switcher input. By adjusting the R2C time constant with respect to the switching frequency of the supply and the hold up time of the output capacitor, switcher faults can be detected be- fore supply outputs are significantly affected. OV and UV Comparators Maintain Accurate Thresholds The structure of the OV and UV comparators, shown in Figure 8 results in accurate fault thresholds even in the case where multiple sense inputs cross a fault threshold simultaneously. Unused sense inputs can be tied either to the 2.5V reference, or to another, utilized, sense input. The four under- and over-voltage sense inputs on the UC1903 are clamped as detailed on the Sense 1 input in Figure 8. The series 2k resistor, R1, and zener diode Z1, prevent extreme under- and over-voltage conditions from inverting the outputs of the fault comparators. A parasitic diode, D1, is present at the inputs as well. Under normal operation it is advisable to insure that voltage levels at all of the sense inputs stay above -0.3V. The same type of input protection exists at the line sense input, Pin 15, ex- cept a 5k series resistor is used. The fault delay circuitry on the UC1903 is also shown in Figure 8. In the case of an over-voltage condition at one of the sense inputs Q20 is turned off, allowing the internal 60 µA current source to charge the user-selected delay capacitor. When the capacitor voltage reaches 1.8V, the OV and POWER OK outputs become active low. When the fault condition goes away Q20 is turned back on, rap- idly discharging the delay capacitor. Operation of the un- der-voltage delay is, with appropriate substitutions, the same. Figure 5. The G.P. op-amp on the UC1903 has a high source current ( ≥20mA) capability and enhanced transient sinking capa- bility through substrate device Q3. Figure 7. The line/switcher sense input can be used for an early line or switcher fault indication. Figure 6. Inverting the sense 4 input for monitoring a negative supply is accommodated with the dedicated inverter. Note: A similar scheme w/the G.P. op-amp will allow a sec- ond negative supply to be monitored. OPERATION AND APPLICATION INFORMATION (continued) 6 |
Nº de peça semelhante - UC1903 |
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Descrição semelhante - UC1903 |
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