LTM4686BIVPBF datasheet(78/130 Pages) AD

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LTM4686B

Rev. 0

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APPENDIX B

PMBUS SERIAL DIGITAL INTERFACE

The LTM4686B communicates with a host (master) using

the standard PMBus serial bus interface. The Timing

Diagram, Figure 37, shows the timing relationship of the

signals on the bus. The two bus lines, SDA and SCL,

must be high when the bus is not in use. External pull-up

resistors or current sources are required on these lines.

The LTM4686B is a slave device. The master can com-

municate with the LTM4686B using the following formats:

Master transmitter, slave receiver

Master receiver, slave transmitter

The following PMBus protocols are supported:

Write Byte, Write Word, Send Byte, Block Write

Read Byte, Read Word, Block Read

Block Write – Block Read Process Call

Alert Response Address

Figure 39 to Figure 55 illustrate the aforementioned PMBus

protocols. All transactions support PEC (parity error check)

and GCP (group command protocol). The Block Read

supports 255 bytes of returned data. For this reason, the

PMBus timeout may be extended when reading the fault log.

Figure 38 is a key to the protocol diagrams in this section.

PEC is optional.

A value shown below a field in the following figures is a

mandatory value for that field.

The data formats implemented by PMBus are:

Master transmitter transmits to slave receiver. The

transfer direction in this case is not changed.

Master reads slave immediately after the first byte. At

the moment of the first acknowledgment (provided by

the slave receiver) the master transmitter becomes a

master receiver and the slave receiver becomes a slave

transmitter.

Combined format. During a change of direction within

a transfer, the master repeats both a start condition and

the slave address but with the R/W bit reversed. In this

case, the master receiver terminates the transfer by

generating a NACK on the last byte of the transfer and

a STOP condition.

Figure 37. Timing Diagram

SDA

SCL

tHD(STA)

tHD(DAT)

tSU(STA)

tSU(STO)

tSU(DAT)

tLOW

tHD(SDA)

tSP

tBUF

START

CONDITION

STOP

CONDITION

REPEATED START

CONDITION

START

CONDITION

tHIGH

4686B F37

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