MCP3906 datasheet(16/26 Pages) MICROCHIP

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MCP3905/06

DS21948C-page 16

The

high-frequency

output

HFOUT

has

lower

integration times and, thus, higher frequencies. The

output frequency value can be calculated with the

following equation:

EQUATION 4-2:

HFOUT FREQUENCY

OUTPUT EQUATION

The constant HFC depends on the FOUT0 and FOUT1

digital settings with the Table 4-4.

The detailed timings of the output pulses are described

in the Timing Characteristics table (see Section 1.0

“Electrical Characteristics” and Figure 1-1).

MINIMAL OUTPUT FREQUENCY FOR

NO-LOAD THRESHOLD

The MCP3905/06 also includes, on each output

frequency, a no-load threshold circuit that will eliminate

any creep effects in the meter. The outputs will not

show any pulse if the output frequency falls below the

no-load threshold. The minimum output frequency on

FOUT0/1 and HFOUT is equal to 0.0015% of the

maximum output frequency (respectively FC and HFC)

for each of the F2, F1 and F0 selections (see Table 4-3

and Table 4-4); except when F2, F1, F0 = 011. In this

last configuration, the no-load threshold feature is

disabled. The selection of FC will determine the start-up

current load. In order to respect the IEC standards

requirements, the meter will have to be designed to

allow start-up currents compatible with the standards

choosing

the

value

matching

these

requirements. For additional applications information

on no-load threshold, startup current and other meter

design points, refer to AN994, "IEC Compliant Active

Energy

Meter

Design

Using

The

MCP3905/6”,

(DS00994).

TABLE 4-4:

OUTPUT FREQUENCY CONSTANT HFC FOR HFOUT (VREF =2.4V)

OUT Hz

()

8.06

REF

()

----------------------------------------------------------------

Where:

V0 = the RMS differential voltage on Channel 0

V1 = the RMS differential voltage on Channel 1

G= the PGA gain on Channel 0

(current channel)

FC = the frequency constant selected

VREF = the voltage reference

HFC

HFC (Hz)

(MCLK = 3.58 MHz)

HFOUT Frequency (Hz) with

full-scale AC Inputs

64 x FC

MCLK/215

109.25

27.21

32 x FC

MCLK/215

109.25

27.21

16 x FC

MCLK/215

109.25

27.21

2048 x FC

MCLK/27

27968.75

6070.12

128 x FC

MCLK/216

219.51

47.42

64 x FC

MCLK/216

219.51

47.42

32 x FC

MCLK/216

219.51

47.42

16 x FC

MCLK/216

219.51

47.42

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