FS7755

Rev. 1.1

15/22

Fig10 shows two typical connections for Channel V2. The first option uses a PT (Potential Transformer) to

provide complete isolation from the power line. In the second option, the FS7755 is biased around the

neutral wire, and a resistor divider provides a voltage signal that is proportional to the line voltage. Adjusting

5

Power Supply Monitor

monitored by the FS7755. If the supply is less than 4V ± 5%, the FS7755 will be reset. This is useful to

ensure correct device startup at power-up and power-down. The power supply monitor has built in

hysteresis and filtering. This gives a high degree of immunity to false triggering due to noisy supplies.

In Fig11, the trigger level is nominally set at 4V. The tolerance on this trigger level is about ±5%. The

as specified for normal operation.

t

t

5V

4V

Active

Reset

Reset

0V

AV

DD

Fig11

On-Chip Power Supply Monitor

6

Power to Impulse Converter

As previous descriptions, the digital output of the low pass filter after multiplication contains the real power

information. However, since this LPF is not an ideal “brick wall” filter implementation, the output signal also

contains attenuated components at the line frequency and its harmonics, i.e., cos(hωt) where h = 1, 2, 3,

and so on. Fig12 shows the instantaneous real power signal at the output of the LPF, which still contains a

significant amount of instantaneous power information, i.e. cos(2ωt). This signal is then passed to the

digital to frequency converter where it is integrated (accumulated) over time to produce an output

frequency. This accumulation of the signal will suppress or average out any non-DC components in the

instantaneous real power signal. The average value of a sinusoidal signal is zero. Hence, the frequency

generated by the FS7755 is proportional to the average real power. Fig12 shows the digital to frequency

converter for steady load conditions, i.e., constant voltage and current.