TPS37 datasheet(27/48 Pages) TI

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operating frequency that requires a full-bridge rectifier and capacitors to convert its signal to a DC voltage where

it can be monitored by a voltage supervisor. One drawback with the described conversion is the response time of

the DC voltage when the AC power rail experiences a change of operating frequency or voltage amplitude. Due

to the output filter of the full-bridge rectifier, the detection in the change of voltage or operating frequency may

require several AC cycles before the voltage supervisor outputs a fault condition. The direct monitoring of the

AC source by using a “Resistive-Drop” supply topology circuit provides the user a fast transient fault detection.

In this design example, the TPS37A is being highlighted with the ability to offer a unique “window operating”

solution by monitoring the output of the AC source for over or undervoltage operation.

DC Supply

(Rec

ed)

AC Early Fault Flag

24 VAC

TPS37A

2.7V to 65V

VPull-Up

DC Fault Flag

GND

UV SENSE2

OV SENSE1

VDD

RESET1

RESET2

CTS1

CTS2

* The circuit solu on is not isolated and one must take into account when planning to use in high power systems.

Pull-Up

"Resis ve-Drop"

supply topology circuit

Figure 10-4. Sensing an AC Signal for Power Fault Detection

10.2.1.1.1 Design Requirements

This design requires voltage supervision on an AC, with a known operating frequency, power supply rail. The

overvoltage fault sensing is achieved by monitoring the DC output of a full bridge rectifier while the undervoltage

fault is detected by inputting a half wave signal and its voltage frequency and magnitude are being monitored.

The target output of this TPS37A application is for 5 V reset logic.

PARAMETER

DESIGN REQUIREMENT

DESIGN RESULT

Power Rail Voltage Supervision

Monitor 24 VAC 800 Hz power supply for

undervoltage and overvoltage conditions. Trigger

undervoltage fault at 5 V and overvoltage fault at

24 V.

TPS37A provides voltage monitoring with 1.5%

max accuracy with adjustable/non-adjustable

variations.

Maximum Input Voltage

Operate with power supply input up to 34 V.

The TPS37A can support a VDD of up to 65 V.

Output logic voltage

Open-Drain Output Topology

An open-drain output is recommended to provide

the a 5 V reset signal.

SENSE Delay when a fault is

detected

RESET delay of at least 0.625 ms which is the time

between half wave cycles

CCTS2 = 5.6 nF sets 717 µs delay

Voltage Monitor Accuracy

Maximum voltage monitor accuracy of 1.5%.

The TPS37A has 1.5% maximum voltage monitor

accuracy.

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 10-4 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

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TPS37

SNVSBJ1D – OCTOBER 2020 – REVISED DECEMBER 2021

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