LTC2972 datasheet(21/112 Pages) LINER

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LTC2972

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OPERATION

PMBus SERIAL DIGITAL INTERFACE

TheLTC2972communicateswithahost(master)usingthe

standard PMBus serial bus interface. The PMBus Timing

Diagram 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 LTC2972 is a slave device. The master can com-

municate with the LTC2972 using the following formats:

• Master transmitter, slave receiver

• Master receiver, slave transmitter

The following SMBus commands are supported:

• Write Byte, Write Word, Send Byte

• Read Byte, Read Word, Block Read

• Alert Response Address

Figures 1 to 13 illustrate the aforementioned SMBus pro-

tocols. All transactions support PEC (packet error check)

and GCP (group command protocol). The Block Read

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

SMBus timeout may be extended using the Mfr_config_

all_longer_pmbus_timeout setting.

PMBus

PMBus is an industry standard that defines a means

of communication with power conversion devices. It is

comprised of an industry standard SMBus serial interface

and the PMBus command language.

The PMBus two wire interface is an incremental extension

of the SMBus. SMBus is built upon I2C with some minor

differences in timing, DC parameters and protocol. The

SMBus protocols are more robust than simple I2C byte

commands because they provide timeouts to prevent

bus hangs and optional Packet Error Checking (PEC) to

ensure data integrity. In general, a master device that

can be configured for I2C communication can be used

for PMBus communication with little or no change to

hardware or firmware.

For a description of the minor extensions and exceptions

PMBusmakestoSMBus,refertoPMBusSpecificationPart

1 Revision 1.1: Section 5: Transport. This can be found at:

www.pmbus.org

ForadescriptionofthedifferencesbetweenSMBusandI2C,

refer to System Management Bus (SMBus) Specification

Version 2.0: Appendix B – Differences between SMBus

and I2C. This can be found at:

www.smbus.org

WhenusinganI2CcontrollertocommunicatewithaPMBus

part it is important that the controller be able to write a

byte of data without generating a stop. This will allow the

controller to properly form the repeated start of a PMBus

read command by concatenating a start command byte

write with an I2C read.

Device Address

The I2C/SMBus address of the LTC2972 equals the base

address + N where N is a number from 0 to 8. N can be

configured by setting the ASEL0 and ASEL1 pins to VDD33,

GND or FLOAT. See Table 1. Using one base address and

the nine values of N, nine LTC2972s can be connected

together to control eighteen outputs. The base address is

storedintheMFR_I2C_BASE_ADDRESSregister.Thebase

address can be written to any value, but generally should

not be changed unless the desired range of addresses

overlap existing addresses. Watch that the address range

does not overlap with other I2C/SMBus device or global

addresses, including I2C/SMBus multiplexers and bus

buffers. This will bring you great happiness.

TheLTC2972alwaysrespondstoitsglobaladdressandthe

SMBus Alert Response address regardless of the state of

itsASELpinsandtheMFR_I2C_BASE_ADDRESSregister.

Processing Commands

The LTC2972 uses a dedicated processing block to ensure

quick response to all of its commands. There are a few ex-

ceptions where the part will NACK a subsequent command

becauseitisstillprocessingthepreviouscommand.These

are summarized in the following tables. MFR_COMMON

is a special command that may always be read even when

the part is busy. This provides an alternate method for a

host to determine if the LTC2972 is busy.

LTC2972 Datasheet(PDF) 21 Page - Linear Technology

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