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TL7702BID 데이터시트(PDF) 9 Page - Texas Instruments |
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TL7702BID 데이터시트(HTML) 9 Page - Texas Instruments |
9 / 9 page TL7702B, TL7705B, TL7733B SUPPLY-VOLTAGE SUPERVISORS SLVS037M – SEPTEMBER 1989 – REVISED MAY 2003 9 POST OFFICE BOX 655303 • DALLAS, TEXAS 75265 APPLICATION INFORMATION VS 10 k Ω 0.1 µF SENSE RESIN GND REF RESET RESET VCC CT To System RESET RT CT System Supply Reset Input (from system) To System RESET (see text) 7 2 1 3 5 6 8 4 10 k Ω Figure 8. System Reset Controller With Undervoltage Sensing When the TL770xB SENSE terminal is used to monitor VCC, a current-limiting resistor in series with CT is recommended. During normal operation, the timing capacitor is charged by the onboard current source to approximately VCC or an internal voltage clamp (≈7.1-V Zener), whichever is less. When the circuit then is subjected to an undervoltage condition during which VCC is rapidly slewed down, the voltage on CT exceeds that on VCC. This forward biases a secondary path internally, which falsely activates the outputs. A fault is indicated when VCC drops below V(CT), not when VSENSE falls below VT–. Texas Instruments performs a 100% electrical screen to verify that the outputs do not switch with 1 mA forced into the CT terminal. Adding the external resistor, RT, prevents false triggering. Its value is calculated as follows: V (CT) * VT– R T Where: V(CT) = VCC or 7.1 V, whichever is less VT– = 4.55 V (nom) RT = value of series resistor required For VCC = 5 V: 5 * 4.55 R T t 1mA Therefore, R T u 450 W Using a 20%-tolerance resistor, RT should be greater than 560 Ω. Adding this series resistor changes the duration of the reset pulse by no more than 10%. RT extends the discharge of CT, but also skews the V(CT) threshold. These effects tend to cancel one another. The precise percentage change can be derived theoretically, but the equation is complicated by this interaction and is dependent upon the duration of the supply-voltage fault condition. Both outputs of the TL770xB should be terminated with similar value resistors, even when only one is being used. This prevents unwanted plateauing in either output waveform during switching, which may be interpreted as an undefined state or delay system reset. The TL7705BM is obsolete and no longer is supplied. |
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