10 Power Supply Recommendations

Figure 10-1 System Reset Controller With Undervoltage Sensing

When the TL770xB SENSE terminal is used to monitor V_CC, TI recommends a current-limiting resistor in series with C_T. During normal operation, the timing capacitor is charged by the onboard current source to approximately V_CC or an internal voltage clamp (≈7.1-V Zener), whichever is less. When the circuit then is subjected to an undervoltage condition during which V_CC is rapidly slewed down, the voltage on CT exceeds that on V_CC. This forward biases a secondary path internally, which falsely activates the outputs. A fault is indicated when V_CC drops below V_(CT), not when V_SENSE falls below V_T–.

Adding the external resistor, R_T, prevents false triggering. Its value is calculated as follows:

where

• V_(CT) = V_CC or 7.1 V, whichever is less
• V_T– = 4.55 V (nom)
• R_T = value of series resistor required

For V_CC = 5 V

Therefore,

Using a 20%-tolerance resistor, R_T should be greater than 560 Ω.

Adding this series resistor changes the duration of the reset pulse by no more than 10%. R_T extends the discharge of C_T, 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 must 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.