PAC1934 datasheet(21/64 Pages) MICROCHIP

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DS20005850E-page 21

PAC1931/2/3/4

4.5

Voltage Measurement

The VBUS voltage for each channel is measured by the

SENSE+ pin for each channel. A high-voltage

multiplexer is connected to each SENSE+ pin, and the

multiplexer sequentially connects each SENSE+ input

to an ADC for conversion. The result is stored in a

16-bit VBUS results register and the 14 MSBs are

multiplied by the VSENSE number for the VPOWER

results value. The VPOWER results are accumulated in

the accumulator.

Full-Scale Voltage (FSV) is 32V by default. The device

may be programmed for bipolar VBUS measurements.

in this bipolar mode, the mathematical range for

negative VBUS numbers is -32V, the actual range is

limited to about -200mV due to physical factors. This

bipolar capability for VBUS enables accurate offset

measurement and correction. For bipolar operation,

the 16-bit VBUS result is a two’s complement (signed)

number.

The measured voltage at SENSE+ can be calculated

using Equation 4-1.

EQUATION 4-1:

BUS VOLTAGE

4.6

Current Measurement

The PAC1931/2/3/4 device family includes high-side

current sensing circuits. These circuits measure the

voltage (VSENSE) induced across a fixed external

current sense resistor (RSENSE) and store the voltage

as a 16-bit number in the VSENSE Results registers.

The PAC1931/2/3/4 current sensing operates with a

Full-Scale Range (FSR) of 100 mV in unidirectional

mode (default).

When sensing unidirectional currents (the default

mode), the ADC results are presented in straight binary

format. For bidirectional current sensing, the ADC

results are in two’s complement (signed) format. For

bipolar current measurements, the range is ±100 mV,

but use FSR = 100 mV in the equations that follow. For

best accuracy on current values near zero, it is

recommended to use the bidirectional current mode

and 8x average current results.

4.7

Selecting RSENSE Values

RSENSE can easily be calculated if you know the

maximum current you want to sense, as shown in

Equation 4-2.

Consider that you may need to select a value for IMax

that includes current peaks well beyond your nominal

current.

EQUATION 4-2:

CALCULATING RSENSE

Full-Scale Current (FSC) can be calculated from

Equation 4-3.

EQUATION 4-3:

FULL-SCALE CURRENT

The actual current through RSENSE can then be

calculated using Equation 4-4.

EQUATION 4-4:

SENSE CURRENT

VSource

32V

VBUS

Denominator

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



Where:

VSOURCE = The measured voltage on the

SENSE+ pin

VBUS = The value read from the VBUS

results registers

Denominator = 216 for unipolar measurements

=215 for bipolar measurements

Rsense

FSR

IMax

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

Where:

FSR = Full Scale VSENSE voltage input

RSENSE =External RSENSE resistor value

IMax = Maximum current to measure

FSC

100 mV

RSENSE

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

Where:

FSC = Full-scale current

RSENSE = External sense resistor value

SENSE

FSC

VSENSE

Denominator

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



Where:

ISENSE = Actual bus current

FSC = Full-scale current value (from

Equation 4-3)

VSENSE = The value read from the

VSENSE results registers

Denominator = 216 for unipolar measurements

=215 for bipolar measurements

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