EMC1701 Datasheet, PDF (22/54 Pages) SMSC Corporation – High-Side Current-Sense and Internal 1°C Temperature Monitor

4.1.2
4.1.3
High-Side Current-Sense and Internal 1°C Temperature Monitor
Datasheet
For a negative voltage the Sense Voltage Registers are read as 96_8h, also ignoring the lower four
bits since they are always zero. To calculate source current the binary value is first converted from
two’s complement by inverting the bits and adding one:
96_8h = 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.
Voltage Measurement
Source voltage is measured on the supply side of the R_SENSE resistor (SENSE+) and stored as an
unsigned 11-bit number in the Source Voltage Registers as V_SOURCE (see Section 5.19).
Each V_SOURCE measurement is averaged over a user programmable time (see Section 5.6 and
Section 5.15). The measurement is delayed by the programmed conversion rate. V_SOURCE is
compared against programmable high, low, and critical limits (see Section 5.12, Section 5.13, and
Section 5.14). If the value meets or exceeds the high limits or drops below the low limits, the ALERT
pin may be asserted (default is to enable this function). If the value meets or exceeds the critical limit,
the THERM pin will be asserted (see Section 5.23).
Full Scale Voltage (FSV) is given by the maximum value of the Source Voltage Registers:
FSV = 23.9883V
where:
FSV is the Full-Scale Voltage (a
[3]
constant)
Actual source voltage at the SENSE+ pin can be calculated using:
where:
Source Voltage is the voltage at the
SENSE+ pin
Source Voltage =
FSV
×
V_{-----S---O----U----R----C---E--}
4, 094
FSV is the Full-Scale Voltage (from
Equation [3])
[4]
V_SOURCE is the digital value read from
the Source Voltage Registers. Note that
the lowest five bits are always zero (see
Section 5.19)
For example: Suppose that the actual source voltage is 10.65V. The Source Voltage Registers are read
as V_SOURCE = 71_Ah (0111_0001_1010b or 1818d) which is 44.4% of the full scale source voltage.
This results in a calculated source voltage of 10.65V using Equation [4].
Note that the actual source voltage may also be determined by scaling each bit set by the indicated
bit weighting as described in Section 5.19.
Power Calculation
The EMC1701 may be used to determine the average power provided at the source side of R_SENSE
(SENSE+) using the value, P_RATIO, contained in the Power Ratio Registers (see Section 5.20). The
value represents the % of maximum calculable power.
P_RATIO is mathematically generated by multiplying the absolute values of V_SENSE and V_SOURCE (see
Section 4.1.1 and Section 4.1.2) and stored as a shifted 16-bit unsigned number. P_RATIO is updated
whenever either V_SENSE or V_SOURCE is updated.
Revision 1.2 (09-27-10)
22
DATASHEET
SMSC EMC1701