10.2.1.1.2 Detailed Design Procedure

The main advantage of this unique application is being able to monitor a single AC source with a known

operating frequency AC source rail. Because the TPS37A is an over and undervoltage detector with delay

function, detecting faults either from a change of operating frequency range or voltage amplitude of the AC

source is achievable.

Figure 10-1 illustrates an example of how the TPS37A is monitoring an AC source. Input to SENSE1 of TPS37A

is monitoring a full wave rectifier DC signal. The DC signal is the result from the rectification of the 24 VAC

source and monitors the AC source for overvoltage events due to a change of voltage amplitude or an increase

to operating frequency. Input to SENSE2 of TPS37A will monitor the AC source by using a "resistive-drop"

supply topology circuit. The unique circuit resistively divides the AC voltage signal and provides only the positive

half wave Figure 10-2 into SENSE2 input. The half wave signal does not go through any output filter and hence

any change to the AC voltage or operating frequency can be rapidly detected. Knowing the operating frequency

of the AC source and converting to the time domain, the TPS37A SENSE2 delay can be programmed, by the

capacitor on CTS2 pin, to equal or be greater than one-half of the operating period (the frequency of the half

wave rectification signal) or the half cycle shown in Figure 10-2. When the half wave voltage amplitude falls

below the SENSE2 threshold voltage, the SENSE2 time delay counter begins to increment. If the next half wave

voltage amplitude exceeds the SENSE2 threshold voltage, the SENSE2 time delay counter will reset and the

TPS37A RESET2 pin will indicate no fault was detected. Conversely, if the voltage amplitude of the half wave

a fault can occur if the operating frequency from the AC source decreases, resulting in lower AC voltage

AC Early Fault Flag (RESET2)

t

Half Cycle

t

CTS

Half Wave (SENSE2)

Full Cycle

t

t

CTS

Figure 10-2. Design 2 Timing Diagram

The TPS37A, with its ability of having a wide VDD range from 2.7V to 65V and under and overvoltage detection,

offers a unique “window operating” AC power rail monitoring solution. Combining SENSE delay feature with the

"resistive-drop" supply circuitry, detecting an undervoltage event on the half cycle of the AC power rail provides a

fast power fault response. Likewise, the TPS37A provides an overvoltage monitoring and SENSE delay fault

detection for the same AC power rail. With under and overvoltage supervision of the AC power rail, applications

needing a specific operating DC range to protect its subsystems is achieve through TPS37A. Good design

practice recommends using a 0.1-µF capacitor on the VDD pin and this capacitance may need to increase if

using an adjustable version with a resistor divider.

Note that this design solution is not isolated and one must take into account when planning to use in high power

systems.

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TPS37B, TPS37F

SNVSBJ1 – OCTOBER 2020

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