ISL28023_15 Datasheet, PDF (47/55 Pages) Intersil Corporation – Precision Digital Power Monitor with Margining

ISL28023
Clock Speed
The device supports high-speed digital transactions up to
3.4Mbs. To access the high speed I²C feature, a master byte
code of 0000 1xxx is attached to the beginning of a standard
frequency read/write I²C protocol. The x in the master byte
signifies a “Do not care state”. X can either equal a 0 or a 1. The
master byte code should be clocked into the chip at frequencies
equal or less than 400kHz. The master code command
configures the internal filters of the ISL28023 to permit data bit
frequencies greater than 400kHz. Once the master code has
been clocked into the device, the protocol for a standard read/
write transaction is followed. The frequency at which the
standard protocol is clocked in at can be as great as 3.4MHz. A
stop bit at the end of a standard protocol will terminate the high
speed transaction mode. Appending another standard protocol
serial transaction to the data string without a stop bit, will
resume the high speed digital transaction mode. Figure 109
illustrates the data sequence for the high speed mode. The
minimum I²C supply voltage when operating at clock speeds of
400kHz is 1.8V.
Signal Integrity
The purity of the signal being measured by the ISL28023 is not
always ideal. Environmental noise or noise generated from a
regulator can degrade the measurement accuracy. The
ISL28023 maintains a high CMRR ratio from DC to
approximately 10kHz, as shown in Figure 110.
130
TIME = 2.048ms
125
TIME = 1.024ms
TIME = 0.512ms
120
115
110
105
TIME = 0.128ms
100
TIME = 0.256ms
95
90
10
TIME = 0.64ms
100
1k
10k
FREQUENCY (Hz)
FIGURE 110. CMRR vs FREQUENCY
100k
The CMRR vs Frequency graph best represents the response of
the ISL28023 when an aberrant signal is applied to the circuit.
The graph was generated by shorting the ISL28023 V_SHUNT
inputs without any filtering and applying a 0 to 20V sine wave to
the shunt inputs, VINP and VINM. A 0 to 3V sine wave was applied
to the auxiliary V_SHUNT inputs, AuxP and AuxM. The voltage range
from a 1024 sample set was recorded for each frequency
applied to shunt input. CMRR results prior to 10kHz are mostly a
result of the variability of the measurement due to the
programmed acquisition time. The input is not able to bleed
through the noise floor.
The CMRR can be improved by designing a filter stage before the
ISL28023. The purpose of the filter stage is to attenuate the
amplitude of the unwanted signal to the noise level of the
ISL28023. Figure 111 is a simple filter example to attenuate
unwanted signals.
Measuring large currents requires low value sense resistors. A
large valued capacitor is required to filter low frequencies if the
shunt capacitor, C_SH is connected directly in parallel to the sense
resistor, R_SH. For more manageable capacitor values, it may be
better to directly connect the shunt resistor across the shunt inputs
of the ISL28023. The connection is illustrated in Figure 111. A
single pole filter constructed of 2 resistors, R₁, and C_SH will
improve capacitor value selections for low frequency filtering.
FROM
SOURCE
C₁
R1
R₁
C1
FIGURE 111. SIMPLIFIED FILTER DESIGN TO IMPROVE NOISE
PERFORMANCE TO THE ISL28023
R₁ and C₁ at both shunt inputs are single ended low pass filters.
The value of the series resistor to the ISL28023 can be a larger
value than the shunt resistor, R_SH. A larger series resistor to the
input allows for a lower cutoff frequency filter design to the
ISL28023. The ISL28023 inputs can source up to 20µA of
transient current in the measurement mode. The transient or
switching offset current can be as large as 10µA. The switching
offset current combined with the series resistance, R₁, creates
an error offset voltage. A balance of the value of R₁ and the
shunt measurement error should be achieved for this filter
design.
The common mode voltage of the shunt input stage ranges from
0V to 60V. The capacitor voltage rating for C₁ and C_SH should
comply with the nominal voltage being applied to the input.
Fast Transients
An small isolation resistor placed between ISL28023 inputs and
the source is recommended. In hot swap or other fast transient
events, the amplitude of a signal can exceed the recommended
operating voltage of the part due to the line inductance. The
isolation resistor creates a low pass filter between the device and
the source. The value of the isolation resistor should not be too
large. A large value isolation resistor can effect the
measurement accuracy. The value of the isolation resistor
combined with the offset current creates an offset voltage error
at the shunt input. The input of the Bus channel is connected to
the top of a precision resistor divider. The accuracy of the resistor
divider determines the gain error of the Bus channel. The input
resistance of the Bus channel is 600kΩ. Placing an isolation
resistor of 10Ω will change the gain error of the Bus channel by
0.0016%.
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FN8389.4
June 17, 2015