ISL28025 Datasheet, PDF (39/47 Pages) Intersil Corporation – Precision Digital Power Monitor with Real Time Alerts

ISL28025
SIGNALS
S
FROM THE
T
MASTER ^A
R
T
MASTER
CODE
S
T ^{SLAVE ADDRESS}
ADDRESS
A ^{IDENTIFICATION}
BYTE
R
T
BYTE
WRITE/READ
DATA
BYTE
DATA
BYTE
TERMINATES HS
MODE
S
T
O
P
SIGNAL AT
SDA
00001xxx 100nnnnn
0000
SIGNALS FROM
THE ISL28025
fclk ≤ 400kHz
A
A
A
A
C
C fclk UP TO 3.4MHz C
C
K
K
K
K
FIGURE 75. BYTE TRANSACTION SEQUENCE FOR INITIATING DATA RATES ABOVE 400kbps
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 75
illustrates the data sequence for the high speed mode. The
minimum I²C supply voltage when operating at clock speeds
400kHz is 1.8V.
Signal Integrity
A filter stage should be considered to limit the effects of
common mode signals from bleeding into the measurement
made by the ADC. The filter attenuates the amplitude of the
unwanted signal to the noise level of the ISL28025. Figure 76 is
a simple filter example to attenuate unwanted signals.
Measuring large currents require 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 ISL28025. The connection is illustrated in
Figure 76. A single pole filter constructed of 2 resistors, R₁, and
R_SH will improve capacitor value selections for low frequency
filtering.
FROM
SOURCE
C₁
R₁
R₁
R₁ and C₁ at both shunt inputs are single-ended low pass filters.
The value of the series resistor to the ISL28025 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
ISL28025. The ISL28025 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 ISL28025 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%.
C₁
FIGURE 76. SIMPLIFIED FILTER DESIGN TO IMPROVE NOISE
PERFORMANCE TO THE ISL28025
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FN8388.3
June 17, 2015