PAC1721 datasheet(21/92 Pages) MICROCHIP

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DS-20007088A-page 21

PAC1721

Equation 4-8 shows how to calculate energy using the

accumulated power and the sample rate (sampling

frequency), ƒS. For this calculation, consider ƒS as

measured in Hz instead of sps.

EQUATION 4-8:

4.8.1

ADAPTIVE ACCUMULATOR

In Adaptive Accumulator (AA) mode, regardless of the

selected ƒS, PAC1721 accumulates samples at the

equivalent rate of 8,192 sps. For example, after the

SLOW pin is pulled high, if PAC1721 begins sampling

at a programmed rate of 8 sps (Slow mode), the

samples taken are shifted to the left by an appropriate

amount and accumulated. This is done to simulate

sampling as if ƒS is set to the maximum of 8,192 sps.

To achieve the simulated sample rate in AA mode, the

accumulator count is also incremented by the

appropriate amount for each sample taken while

PAC1721 operates in Slow mode.

AA mode provides a big reduction in host overhead

and bus traffic for applications that require to:

Use the SLOW pin for low power operation

during specific times

Have continuous accurate energy monitoring for

both the maximum and Slow sample rates

As shown in Equation 4-7 and Equation 4-8, it is

necessary to know the sample rate (ƒS) and sampling

period (tS) for each interval during which energy is

accumulated. If the SLOW pin is pulled high and tS

changes, the host must read accumulator data and

determine the accumulator value and accumulator

count every time ƒS changes. While operating in AA

mode, the host needs to read accumulator data only

once, just before the accumulator and/or accumulator

count overflows. Using AA mode, PAC1721 accurately

calculates the accumulated energy, independent of

how many times the SLOW pin was pulled high and

low during tS.

The SLOW register offers a variety of options to

monitor when the SLOW pin transitions or when the

Slow function is active. This includes issuing Refresh

(any) and Limited Refresh commands. This behavior

is controlled by setting the appropriate bits in the

SLOW register. For more details, see Register 6-23.

While AA mode is enabled, if ƒS is configured to any

other value than 8,192 sps, registers VACC and

ACC_COUNT shift the accumulator data and count to

mimic sample accumulation at the maximum sampling

rate of 8,192 sps. Adaptive Accumulator mode is

enabled by setting the AA bit in the CONTROL register

to 1b. For more details, see Register 6-2.

4.8.2

ADDITIONAL ACCUMULATOR

INFORMATION

Power calculation and accumulation inside PAC1721

are always done using 2's complement, regardless of

what the output registers are set to show. Internally,

VBUS and VSENSE are 12-bit 2's complement signed

numbers. VPOWER is calculated by multiplying VSENSE

and VBUS, resulting in a 24-bit 2's complement signed

number. In some cases, while the 24-bit VPOWER result

is not identical to the product of the VBUS register

multiplied by the VSENSE register, the VPOWER result

is more accurate.

The FSR for VBUS or VSENSE measurements is set in

the NEG_PWR_FSR register using the CFG_VB[1:0]

and CFG_VS[1:0] bits. For details, see Register 6-20.

If FSR for both VSENSE and VBUS measurements is set

to unipolar mode, these measurements are stored in

the VBUS and VSENSE registers as 12-bit unsigned

numbers.

If FSR for VBUS and VSENSE sampling is set to bipolar

mode, the measurement results are stored in the VBUS

and the VSENSE registers as 12-bit 2’s complement

signed numbers.

However, VPOWER calculations use 12-bit 2's comple-

ment signed values. Therefore, a mismatch is possible

between an externally calculated power value (where

VPOWER = VBUS × VSENSE) and the actual power value

determined by PAC1721. This internal value is more

accurate than the externally calculated power. This

also applies to FSR/2 modes that use bit shifting to

change the FSR. Therefore, accuracy is not lost during

power calculations (similar to unipolar mode).

Continuous power integration periods, also called

energy accumulation periods, range from ~ 1 ms to

several hours, depending on the programmed ƒS. The

total number of samples is limited by the size of the

ACC_COUNT register to 4,294,967,296 or 232. The

count corresponds to about 1,165 hours at 1,024 sps or

approximately 17 years at 8 sps.

Where:

E = Energy (J)

CONV(VACC) = Converted digital value, from

hexadecimal to decimal, read from

the VACC register

224 = VPOWER Register Size (bits)

FSRP = Full Scale Range for Power (W)

fS = Sample Rate (Hz)

CONV VACC



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FSRP

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