PAC1941T-1ESLASH4MX. datasheet(25/96 Pages) MICROCHIP

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DS20006543A-page 25

PAC194X

Whenever the SLOW pin changes the state, a limited

REFRESH or REFRESH_V command may be exe-

cuted by the chip hardware (default is REFRESH).

Like any other REFRESH command, this resets the

accumulators and accumulator count for a REFRESH

command and updates the readable registers for

either REFRESH or REFRESH_V. These are limited

REFRESH commands because no programmed

changes to the Control or Status registers take effect

(Control and Status registers are registers 01h, 1Ch,

1Dh and 20h-26h). The readable registers are stable

with the new values within 1 ms of the SLOW pin

transition.

The Slow register allows a selection of REFRESH or

REFRESH_V commands on the SLOW pin transitions,

allows this function to be disabled for either edge and

also tracks both the state of the SLOW pin and the

transitions on the SLOW pin (see Register 7-14).

5.8

Voltage Measurement

The VBUS voltage for each channel is measured by the

SENSE+ pin for each channel. A high-voltage multi-

plexer is connected to each SENSE+ pin and the mul-

tiplexer sequentially connects each SENSE+ pin to an

ADC input for conversion. The result is stored in a

16-bit VBUS results register and the 14 MSBs are mul-

tiplied by the VSENSE number for the VPOWER results

value. The VPOWER results are accumulated in the

accumulator.

For the PAC194X, the default FSR is 9V. The device

may be programmed for bipolar VBUS measurements.

In this Bipolar mode, the mathematical range for VBUS

numbers is +9V, the actual range is limited to about

-200 mV due to the impact of the ESD structures. 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 5-1. The FSR value stays the same but

the maximum range is divided in half.

EQUATION 5-1:

BUS VOLTAGE

5.9

Current Measurement

The PAC194X 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 PAC194X current sensing operates with a FSR of

100 mV in Unidirectional mode (default).

When sensing unidirectional currents (the default

mode), the ADC results are presented in unsigned

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.

5.10

Selecting RSENSE Values

RSENSE can easily be calculated if the maximum

current sensed is known, as shown in Equation 5-2.

Consider the need to select a value for IMax that

includes current peaks well beyond your nominal

current.

EQUATION 5-2:

CALCULATING RSENSE

Full-Scale Current (FSC) can be calculated with

Equation 5-3.

EQUATION 5-3:

FULL-SCALE CURRENT

The actual current through RSENSE can then be

calculated using Equation 5-4.

VSOURCE

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

=216 for FSR/2 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

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