MAXQ3183-RAN+ datasheet(71/102 Pages) MAXIM

71 / 102 page

Low-Power, Multifunction, Polyphase AFE

with Harmonics and Tamper Detect

______________________________________________________________________________________

Low-Power Measurement Mode (LOWPM)

This mode enables a subset of metering functions while

operating from the lower frequency internal RC oscilla-

tor to conserve power. The actual system clock fre-

quency used is the RC oscillator output frequency

divided by 8, which results in a system clock frequency

of approximately 1MHz.

The parameters provided in the LOWPM are:

• Voltage RMS

• Current RMS

• Ampere-Hour

The ampere-hour value is readable from the X.ESF reg-

isters (X = A/B/C). Entry to LOWPM mode only occurs

at the request of the master. The master must set the

LOWPM_E bit (register address 0xC03) to 1 to place

the MAXQ3183 into LOWPM mode. Entering LOWPM

mode changes the clock frequency, thereby invalidat-

ing a number of configuration registers. As a result, the

master must immediately reload the configuration regis-

ters and filter with new, updated values before metering

measurement operations can continue.

The master instructs the MAXQ3183 to exit LOWPM

mode by reading the LOWPM_X bit (register address

0xC04).

Temperature

The MAXQ3183 contains a temperature sensor that can

be used by host software for any purpose, including

compensating power readings for temperature effects.

Use the virtual register command (TEMP_C, address

0xC01) to perform a temperature conversion. The

MAXQ3183 returns the value in degrees Celsius, with a

resolution of 1°C. Internally, the MAXQ3183 performs a

temperature sensor read and scales the sensor output

by the TEMP_CC (address 0x060) coefficient to provide

an output in degrees Celsius. The conversion formula is:

TEMP_C = RAW_TEMP x TEMP_CC/216 - 273.15

Where RAW_TEMP is raw digitized temperature sensor

output that is proportional to absolute temperature.

Conversion coefficient TEMP_CC must be calibrated

and set by the host; the default value is 0x0000.

Advanced Calibrations

Calibrating Current Offset

Ideal hardware should produce a current reading lin-

early proportional to the input current. However, due to

noise or other factors, the RMS current read by the

meter might not be precisely linear. The current offset

(X.OFFS_HI, X = A/B/C) can be used to compensate

the current channel nonlinearity.

Since the MAXQ3183 tracks the input current to deter-

mine what linearity compensation factors to use, the

user must choose two points (ilo and ihi) comfortably

above the low current threshold, and get the X.IRMS

current readings (rlo and rhi). Then calculate the Y-inter-

cept of the line drawn between the two points, that is,

the offset. To calculate the value for the offset register,

use the following formula:

In this equation, ihi and rhi are the applied current and

the current reading, respectively, in meter units at the

higher of the two reference currents; ilo and rlo are the

applied current and the current reading, respectively, in

meter units at the lower of the two reference currents.

The gain (X.I_GAIN) may require recalibration after the

offset register updated.

Calibrating Linearity

The current channel includes a variable-gain amplifier

that introduces a gain of 32 when the current falls

below the low current threshold (about 1/32 of full-scale

current IFS). Because the gain of the amplifier cannot

be controlled with arbitrary precision, and because

high gain implies increased noise, it may be necessary

to calibrate the MAXQ3183 to maintain linearity at the

lowest inputs.

There are two settings that manage low-current lineari-

ty: an offset setting, OFFS_LO; and a gain setting,

GAIN_LO. Setting the offset is simple. Ensure no cur-

rent is flowing in the current circuit. Read X.IRMS. To

calculate offset use following formula:

offs = -X.IRMS

Program the offs into the OFFS_LO register.

So, if the user reads 0x0113 from the X.IRMS register,

program 0xFEED into the X.OFFS_LO register. Setting

the X.GAIN_LO register means applying a current below

the low-current threshold, reading the value from the

MAXQ3183, and adjusting the gain accordingly. Note

that the low-end gain is applied in addition to the overall

gain provided in the X.I_GAIN register.

Apply a known current with peak value less than the

low-current threshold. Ensure that there is no previous

value in the low-current gain register, A.GAIN_LO, by

offs

i r

hi lo

−

()

2 4

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