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LTC2945

2945f

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.

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

5mA

≤R4≤

VS(MIN) – 6.9V

0.5mA

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.

Figure 12. LTC2945 Serial Bus SDA Alert Response Protocol

ALERT

RESPONSE

ADDRESS

0 0 0 1 1 0 0

DEVICE

ADDRESS

a7:a0

2945 F12

A P

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:

ILOAD(MAX)=

6.7V

2• R1

+R3

(2)

(3)

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