fs7755 Fortune Semiconductor Corporation, fs7755 Datasheet - Page 21

• Manufacturer Part Number: fs7755
• Description: Energy Metering Ic With Impulse Output
• Manufacturer: Fortune Semiconductor Corporation

Rev. 1.0
2.1 Frequency Outputs
3.
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 (t
the time between the falling edges of F1 and F2 (t
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
The high frequency CF output is intended to be used for communications and calibration purposes. CF
produces a 90ms wide active high pulse (t
frequencies are given in Table Ⅳ. As in the case of F1 and F2, if the period of CF (t
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.
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 F
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
3.05×10
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% I
therefore satisfies the IEC requirement. As illustrated from this example, the choice of F
the stepper motor display will determine the start up current.
No Load Threshold
–3
Hz at CF (64 ×F1 Hz). In this example, the no-load threshold is equivalent to 1.7W of load or a
b
. For a 5A (I
b
) meter, 0.4% I
4
) at a frequency proportional to active power. The CF output
b
is equivalent to 20mA. The start up current of this design
3
) is approximately half the period of F1 (t
1–4
frequency selections (see Table II).
Ⅲ
.
1
5
) is set at 275ms and
–5
) falls below 180 ms,
Hz. This would be
1–4
and the ratio of
2
). If, however,
FS7755
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