PAC1721 datasheet(18/92 Pages) MICROCHIP

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PAC1721

DS-20007088A-page 18

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4.3

Voltage Measurement

PAC1721 measures the following voltages:

•VBUS:

◦ 8-Lead VDFN: across pin VSENSE+ to GND

◦ 10-Lead VDFN: across pin VBUS+ to GND

•VSENSE between the VSENSE+ and VSENSE– pins

Measurements can be unsigned full range, signed full

range or signed half range 12-bit values, depending on

NEG_PWR_FSR register settings. ADC internal values

for VBUS and VSENSE are multiplied together and result

in VPOWER. This VPOWER result is then summed into

the accumulator. VACC is the Accumulator register and

it can be programmed to accumulate different results

depending on the settings of the CONTROL register:

• Accumulate VBUS results → CONFIG[1:0] = 10b

• Accumulate VSENSE results → CONFIG[1:0] = 01b

• Accumulate VPOWER results → CONFIG[1:0] = 00b

By default, the VACC register accumulates VPOWER

results. For details, see Register 6-2 and Register 6-4.

The default Full Scale Range (FSR) for PAC1721 is

0V to 9V. The device can be programmed for bipolar

VBUS measurements. In this bipolar mode, the mathe-

matical range for VBUS results is ±9V, while the actual

range is limited to approximately -200 mV to +9V due

to physical factors. Bipolar mode enables accurate

VBUS offset measurements. While PAC1721 operates

in bipolar mode, 12-bit VBUS measurements are 2’s

complement signed numbers.

Calculate the actual value of VBUS using Equation 4-1.

EQUATION 4-1:

Note:

FSR/2 mode can be used to reduce FSR.

For more details, see Section 4.6.

4.4

Current Measurement

PAC1721 includes a high-side current sensing circuit

that measures the voltage (VSENSE) induced across a

fixed external current shunt resistor (RSHUNT). It then

stores the measured value as a 12-bit number in the

VSENSE register.

By default, PAC1721 operates in unipolar current mode

with a FSR of 0 to 100 mV. When sensing unipolar

currents, ADC results are in unsigned binary format.

For bipolar current mode, FSR changes to ±50mV or

±100 mV. When sensing bipolar currents, ADC results

are in 2’s complement signed format.

Note:

For best accuracy when sampling current

values near zero, Microchip recommends

using bipolar mode and 8x average current

results. For more details, see Register 6-2.

4.5

Selecting RSHUNT Values

RSHUNT can be calculated only when the maximum

current to sense, IMAX, is known. First determine IMAX

and then use Equation 4-2 to calculate RSHUNT.

EQUATION 4-2:

Determine the value of ISENSE using Equation 4-3.

EQUATION 4-3:

Where:

VBUS = Bus Voltage (V)

LSB(VBUS) = LSBs of the VBUS measurement

FSVBUS = Full Scale Voltage for VBUS (V):

• 9V for Unipolar mode

• 18V for Bipolar mode

• 9V for Bipolar FSR/2 mode

212 = 12-bit Resolution Denominator

CONV(VBUS) = Converted analog-to-digital value,

from hexadecimal to decimal, read

from the VBUS register

LSB VBUS



FSVBUS

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

VBUS

LSB VBUS

 CONV VBUS





Note:

Consider that the value selected for IMAX

needs to include current peaks beyond

nominal current.

Where:

RSHUNT = External Shunt Resistor (Ω)

|FSVSENSE| = Full Scale Voltage for VSENSE (V),

absolute value:

• 100 mV for Unipolar mode

• 100 mV for Bipolar mode

• 50 mV for Bipolar FSR/2 mode

IMAX = Maximum Current to Measure (A)

RSHUNT

FSVSENSE

IMAX

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

ISENSE

VSENSE

RSHUNT

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

Where:

ISENSE = Sense Current (mA)

VSENSE = Sense Voltage (mV)

RSHUNT = External Shunt Resistor (Ω)

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