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UC3903DWTRG4 Datasheet(PDF) 10 Page - Texas Instruments |
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UC3903DWTRG4 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 16 page OV and UV Comparators Maintain Accurate Thresholds 6.4V Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8 Q9 Q10 Q11 Q12 Q13 Q14 Q15 Q16 Q17 Q18 OVCOMPARATOR UVCOMPARATOR SENSE1 2kW R1 Z1 5.7V D1 SENSE2 SENSE3 SENSE4 100 A m 6.4V 50 A m TOUV THRESHOLD VOLTAGE OV HYSTERESIS CONTROL 6.4V 60 A m Q20 Q19 1.8V TOOV THRESHOLD VOLTAGE UV HYSTERESIS CONTROL 6.4V 60 A m 1.8V EXT. OVDELAY CAPACITOR OVFAULT INDICATION TOOUTPUT LOGIC V EXT. UVDELAY CAPACITOR REF UVFAULT INDICATION TOOUTPUT LOGIC Start Latch and Supply Under-Voltage Sense Allow Predictable Power-Up UC1903 UC2903 UC3903 SLUS233A – OCTOBER 1999 – REVISED SEPTEMBER 2008 ........................................................................................................................................ www.ti.com 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 2.5-V 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 2-k Ω 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 –0.3 V. The same type of input protection exists at the line sense input, Pin 15, except a 5-k Ω 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-mA current source to charge the user-selected delay capacitor. When the capacitor voltage reaches 1.8 V, the OV and POWER OK outputs become active low. When the fault condition goes away Q20 is turned back on, rapidly discharging the delay capacitor. Operation of the under- voltage delay is, with appropriate substitutions, the same. A. The OV and UV comparators on the UC1903 trigger respective fault delay circuits when one or more of the sense inputs move outside the fault window. Input clamps insure proper operation under extreme fault conditions. Terminating the UV delay capacitor to VREF assures correct logic at power up. Figure 8. OV and UV Comparators on the UC1903 At power-Up, while the +VIN input supply is below 3 V, all open collector outputs are off. With +VIN greater than 3 V the POWER OK output will be driven low and the UV OV FAULT outputs are disabled. Once +VIN rises above the VSUV threshold of 7 V the fault outputs will be enabled. As would be expected, the SENSE 1-4 voltages at power up may be below the UVFAULT window and the UVFAULT output may be driven low. 10 Submit Documentation Feedback Copyright © 1999–2008, Texas Instruments Incorporated Product Folder Link(s): UC1903 UC2903 UC3903 |
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