LTC2974 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2974' PDF : 32 Pages View PDF
LTC2945
APPLICATIONS INFORMATION
If two or more LTC2945s on the same bus are generating
alerts when the ARA is broadcasted, the bus master will
repeat the alert response protocol until the ALERT line
is released. The device with the highest priority (lowest
address) will reply first and the device with the lowest
priority (highest address) will reply last.
ALERT
S RESPONSE R A
ADDRESS
0001100 1 0
DEVICE
ADDRESS
a7:a0
AP
1
2945 F12
Figure 12. LTC2945 Serial Bus SDA Alert Response Protocol
Opto-Isolating the I2C Bus
Opto-isolating a standard I2C device is complicated
by the bidirectional SDA pin. The LTC2945/LTC2945-1
minimize this problem by splitting the standard I2C SDA
line into SDAI (input) and SDAO (output, LTC2945) or
SDAO (inverted output, LTC2945-1). The SCL is an input
only pin and does not require special circuitry to isolate.
For conventional non-isolated I2C applications, use the
LTC2945 and tie the SDAI and SDAO pins together to form
a standard I2C SDA pin.
Low speed isolated interfaces that use standard open-
drain opto-isolators typically use the LTC2945 with the
SDAI and SDAO pins separated as shown in Figure 13.
Connect SDAI to the output of the incoming opto-isolator
with a pullup resistor to INTVCC or a local 5V supply; con-
nect SDAO to the cathode of the outgoing opto-isolator
with a current-limiting resistor in series with the anode.
The input and output must be connected together on the
isolated side of the bus to allow the LTC2945 to participate
in I2C arbitration. Note that maximum I2C bus speed will
generally be limited by the speed of the opto-couplers
used in this application.
Both low and high side shunt regulators can supply up to
34mA of current to drive opto-isolator and pullup resis-
tors as shown in Figure 14 and 15. For identical SDAI/SCL
pullup resistors the maximum load is:
IL O AD(M AX )=
2
6.7V
• R1+R3
(2)
RSHUNT can then be calculated using Equation 1. Note that
both LTC2945 and LTC2945-1 can be used in the shunt
regulator applications mentioned.
Figure 16 shows an alternate connection for use with low-
speed opto-couplers and the LTC2945-1. This circuit uses
a limited-current pullup on the internally clamped SDAI
pin and clamps the SDAO pin with the input diode of the
outgoing opto-isolator, removing the need to use INTVCC
for biasing in the absence of an auxiliary low voltage sup-
ply. For proper clamping:
VS(MAX ) – 5.9V ≤R4 ≤ VS(MIN) – 6.9V
5mA
0.5mA
(3)
As an example, a supply that operates from 36V to 72V
would require the value of R4 to be between 13k and 58k.
The LTC2945-1 must be used in this application to ensure
that the SDAO signal polarity is correct.
The LTC2945-1 can also be used with high-speed opto-
couplers with push-pull outputs and inverted logic as
shown in Figure 17. The incoming opto-isolator draws
power from the INTVCC, and the data output is connected
directly to the SDAI pin with no pullup required. Ensure
the current drawn does not exceed the 10mA maximum
capability of the INTVCC pin. The SDAO pin is connected
to the cathode of the outgoing optocoupler with a current
limiting resistor connected back to INTVCC. An additional
discrete N-channel MOSFET is required at the output of
the outgoing optocoupler to provide the open-drain pull-
down that the I2C bus requires. Finally, the input of the
incoming opto-isolator is connected back to the output
as in the low-speed case.
Layout Considerations
A Kelvin connection between the sense resistor RSNS and
the LTC2945 is recommended to achieve accurate current
sensing (Figure 18). The recommended minimum trace
width for 1oz copper foil is 0.02” per amp to ensure the
trace stays at a reasonable temperature. Using 0.03” per
amp or wider is preferred. Note that 1oz copper exhibits
a sheet resistance of about 530μΩ per square.
2945f
17
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