FS7755 datasheet(21/22 Pages) FORTUNE

21 / 22 page

FS7755

Rev. 1.1

21/22

2.1 Frequency Outputs

Fig4 shows a timing diagram for the various frequency outputs. The outputs F1 and F2 are the low

frequency outputs that can be used to directly drive a stepper motor or electromechanical impulse counter.

The F1 and F2 outputs provide two alternating low going pulses. The pulse width (t1) is set at 275ms and

the time between the falling edges of F1 and F2 (t3) is approximately half the period of F1 (t2). If, however,

the period of F1 and F2 falls below 550ms (1.81Hz), the pulse width of F1 and F2 is set to half of their

period. The maximum output frequencies for F1 and F2 are shown in Table

Ⅲ

The high frequency CF output is intended to be used for communications and calibration purposes. CF

produces a 90ms wide active high pulse (t4) at a frequency proportional to active power. The CF output

frequencies are given in Table Ⅳ. As in the case of F1 and F2, if the period of CF (t5) falls below 180 ms,

the CF pulse width is set to half the period. For example, if the CF frequency is 20Hz, the CF pulse width is

25ms.

Note:

When the high frequency mode is selected, (i.e., SCF = 0, S1 = S0 = 1), the CF pulse width is fixed at 18μs. Therefore,

t4 will always be 18μs, regardless of the output frequency on CF.

No Load Threshold

The FS7755 also includes a “no load threshold” and “startup current” feature that will eliminate any creep

effects in the meter. The FS7755 is designed to issue a minimum output frequency on all modes except

when SCF = 0 and S1 = S0 = 1. The no load detection threshold is disabled on this output mode to

accommodate specialized application of the FS7755. Any load generating a frequency lowers than this

minimum frequency will not cause a pulse to be issued on F1, F2, or CF. The minimum output frequency is

given as 0.0014% of the full scale output frequency for each of the F1–4 frequency selections (see Table II).

For example, an energy meter with a meter constant of 100imp/kWhr on F1 and F2, using F2 (3.4Hz), the

maximum output frequency at F1 or F2 would be 0.0014% of 3.4Hz or 4.76×10

–5Hz. This would be

3.05×10

–3Hz at CF (64 ×F1 Hz). In this example, the no-load threshold is equivalent to 1.7W of load or a

start up current of 8mA at 220V. IEC1036 states that the meter must start up with a load current equal to or

less than 0.4% Ib. For a 5A (Ib) meter, 0.4% Ib is equivalent to 20mA. The start up current of this design

therefore satisfies the IEC requirement. As illustrated from this example, the choice of F1–4 and the ratio of

the stepper motor display will determine the start up current.

FS7755 Datasheet(PDF) 21 Page - Fortune Semiconductor Corp.