LTM4677 datasheet(63/110 Pages) AD

63 / 110 page

LTC3888-1

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

PMBus communication rate, the resistor pull-up on the

SDA and SCL pins with the time constant set to one-third

the required rise time equals

RPULLUP =

tRISE

3 • 100pF

= 1k

The closest 1% resistor value is 1kΩ. Limit to 1.1kΩ for

best noise margin.

Be careful to minimize parasitic capacitance on the SDA

and SCL lines to avoid communication problems. To esti-

mate the loading capacitance, monitor the signal in ques-

tion and measure how long it takes for the desired signal

to reach approximately 63% of the output value. This is

one time constant.

The SYNC pin has an on-chip pull-down transistor with

the output held low for nominally 500ns when driven by

the LTC3888-1. If the internal oscillator is set for 500kHz

and the load is 100pF with a one-third rise time required,

the resistor calculation is as follows:

RPULLUP =

2µs – 500ns

3 • 100pF

= 5.0k

The closest 1% resistor is 4.99kΩ .

If timing errors are occurring or if the SYNC amplitude is not

as large as required, monitor the waveform and determine

if the RC time constant is too long for the application. If

possible reduce the parasitic capacitance. Otherwise reduce

the pull-up resistor sufficiently to assure proper operation.

Similar results (R < 5kΩ) should be applied to the

PGOOD0 output when configured as CLKOUT to ensure

proper clocking of the slave IC.

The SHARE_CLK output has a nominal period of 10μs

and is pulled low for about 1μs. If the system load on this

shared line is 100pF, the resistor calculation for this line

with a one-third rise time is:

RPULLUP =

9µs

3 • 100pF

= 30k

The closest 1% resistor is 30.1kΩ .

PMBUS COMMUNICATION AND COMMAND

PROCESSING

The LTC3888-1 has a one deep buffer to hold the last data

written for each supported command prior to processing,

as shown in Figure 43. Two distinct parallel sections of the

LTC3888-1 manage command buffering and command

processing to ensure the last data written to any com-

mand is never lost. When the part receives a new com-

mand from the bus, command data buffering copies the

data into the write command data buffer and indicates to

the internal processor that data for that command should

be handled. The internal processor runs in parallel and

performs the sometimes slower task of fetching, convert-

ing (to internal format) and executing commands marked

for processing.

Some computationally intensive commands (e.g., timing

parameters, temperatures, voltages and currents) have

internal processor execution times that may be long rela-

tive to PMBus timing. If the part is busy processing a

command, and a new command(s) arrives, execution may

be delayed or processed in a different order than received.

The part indicates when internal calculations are in pro-

cess with bit 5 of MFR_COMMON (Internal Calculations

Not Pending). When the internal processor is busy cal-

culating, bit 5 is cleared. When this bit is set, the part is

ready for another command. An example polling loop is

provided in Figure 44, which ensures that commands are

processed in order while simplifying error handling rou-

tines. MFR_COMMON always returns valid data at PMBus

speeds between 10kHz and 400kHz.

When the part receives a new command while it is busy,

it will communicate this condition using standard PMBus

protocol. Depending on device configuration and state it

may either NACK the command or return all ones (0xFF)

for reads. It may also generate a BUSY fault and ALERT

notification, or stretch the SCL clock low. For more infor-

mation refer to PMBus Specification V1.2, Part II, Section

10.8.7 and SMBus V2.0 section 4.3.3. Clock stretching

can be enabled by asserting bit 1 of MFR_CONFIG_ALL.

Clock stretching will only occur if enabled and the bus

communication speed exceeds 100kHz.

PMBus protocols for busy devices are well accepted

standards but can make writing system level software

LTM4677 Datasheet(PDF) 63 Page - Analog Devices

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