PAC1720 datasheet(21/47 Pages) SMSC

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Dual High-Side Current Sense Monitor with Power Calculation

Datasheet

SMSC PAC1720

Revision 1.1 (12-08-11)

SMSC CONFIDENTIAL DATASHEET

Actual current through RSENSE can then be calculated using:

As an example, suppose the system is drawing 1.65A through a 10m

Ω resistor, the FSR is set for

+20mV, and sample time is 80ms. Using Equation [1], the FSC is 2A. The measured VSENSE is 1.65A

* 10m

Ω = 16.5mV. This value of VSENSE is represented in the Sense Voltage Registers as 69_8h

(0110_1001_1000b or 1688d) ignoring the 4 lower bits of the low byte as these are always 0. This

value, when applied to Equation [2], results in an IBUS current of 1.649A.

For a negative voltage the Sense Voltage Registers are read as 96_0h (again ignoring the lower 4 bits

of the low byte as these are always 0). To calculate current, the binary value is first converted from

two’s complement by inverting the bits and adding one:

96_80h = 1001_0110_1000b. Inverting equals 0110_1001_0111b (69_7h) and adding one gives

0110_1001_1000b (69_8h).

This results in the same calculated value as in the positive voltage case.

4.4

Voltage Measurement

The pin voltage is measured on the supply side of SENSE1+ and SENSE2+ and stored as an unsigned

11-bit number in the VSOURCE Voltage Registers as VSOURCE (see Section 5.11).

Full Scale Voltage (FSV) is given by the maximum value of the VSENSE Voltage Registers:

Actual voltage at SENSE+ can be calculated using:

where:

[2]

IBUS is the actual bus current

FSC is the full scale current value (from Equation [1])

VSENSE is the value read from the Sense Voltage Registers

(in decimal), ignoring the four lowest bits which are always

zero (see Section 5.10)

Denominator is determined by the sample time, as shown in

Table 5.14, "Current Sensing Sampling Time Settings"

where:

[3]

FSV is the full scale voltage

Denominator is determined by the sample time, as shown in

Table 5.10, "Voltage Source Sampling Time Settings".

where:

[4]

VSOURCE is the actual voltage on the SENSEX+ pin

FSV is the full scale voltage (from Equation [3])

VSOURCE is the value read from the VSOURCE Voltage

Registers (in decimal), ignoring the lowest five bits which

are always zero. (see Section 5.11, "VSOURCE Voltage

Registers")

Denominator is determined by the sample time, as shown in

Table 5.10, "Voltage Source Sampling Time Settings".

BUS

FSC

SENSE

Denominator

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

FSV

Denominator

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

–

SOURCE

_PIN

FSV

SOURCE

Denominator

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

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