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LTC2992

Rev A

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is not pulled low again until the fault registers indicate a

different fault has occurred or the original fault is cleared

and it occurs again. Note that this means repeated or

continuing faults will not generate additional alerts until

the associated fault register bits have been cleared.

Figure 15. Serial Bus SDA Alert Response Protocol

open-drain opto-isolators can use the LTC2992 with the

SDAI and SDAO pins separated, as shown in Figure 16.

Connect SDAI to the output of the incoming opto-isolator

with a pull-up 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 LTC2992 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.

Figure 17 shows an alternate connection for use with low

speed opto-couplers and the LTC2992-1. This circuit uses

a limited-current pull-up 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 a separate low voltage sup-

ply. For proper clamping:

VIN(MAX) – VSDA,SCL(MIN)

ISDA,SCL(MAX)

≤R4≤

VIN(MIN) – VSDA,SCL(MAX)

ISDA,SCL(MIN)

VIN(MAX) – 5.9V

5mA

≤R4≤

VIN(MIN) – 6.9V

0.5mA

(1)

As an example, a supply that operates from 36V to 72V

would require the value of R4 to be between 13k and

58k. The LTC2992-1 must be used in this application

to ensure that SDAO signal polarity is correct. R4 may

APPLICATIONS INFORMATION

SCL

SDAI

SDAO

GND

LTC2992

SCL

3.3V

VDD

SDA

GND

µP

1/2 MOCD207M

MOCD207M

2992 F16

4.7k

0.47k

R10

0.82k

Figure 16. Opto-Isolation of a 10kHz I2C Interface Between

LTC2992 and Microcontroller

Figure 17. Opto-Isolation of a 1.5kHz I2C Interface Between

LTC2992-1 and Microcontroller (SCL Omitted for Clarity)

VIN

48V

SDAI

SDAO

GND

LTC2992-1

3.3V

VDD

SDA

GND

µP

1/2 MOCD207M

2992 F17

20k

0.51k

5.1k

ALERT

RESPONSE

ADDRESS

R A

0001100

2992 F15

A P

a7:a0

DEVICE

ADDRESS

If two or more LTC2992s on the same bus are generat-

ing alerts when the ARA is broadcast, the bus master

will repeat the alert response protocol until the GPIO4

(ALERT) line is released. Standard I2C arbitration causes

the device with the highest priority (lowest address) to

reply first and the device with the lowest priority (highest

address) to reply last.

Opto-Isolating the I2C Bus

Opto-isolating a standard I2C device is complicated by the

bidirectional SDA pin. The LTC2992/LTC2992-1 minimize

thisproblembysplittingthestandardI2CSDAlineintoSDAI

(input) and SDAO (output, LTC2992) or SDAO (inverted

output,LTC2992-1).TheSCLisaninput-onlypinanddoes

not require special circuitry to isolate. For conventional

nonisolated I2C applications, use the LTC2992 and tie

the SDAI and SDAO pins together to form a standard I2C

SDA pin. Low speed isolated interfaces that use standard

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