Thermal Considerations And Output Current Derating - Analog Devices Linear ADI Power LTM4681 Manual

LTM4681
APPLICATIONS INFORMATION
When the part receives a new command while it is busy,
it will communicate this condition using standard PMBus
protocol. Depending on part configuration it may either
NACK the command or return all ones (0xFF) for reads.
It may also generate a BUSY fault and ALERT notifica-
tion, or stretch the SCL clock low. For more information
refer to PMBus Specification v1.1, 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 com-
munication speed exceeds 100kHz.
// wait until chip is not busy
do
{
mfrCommonValue = PMBUS_READ_BYTE(0xEF);
partReady = (mfrCommonValue & 0x68) == 0x68;
}while(!partReady)
// now the part is ready to receive the next
command
PMBUS_WRITE_WORD(0x21, 0x2000); //write VOUT_
COMMAND to 2V
Figure 33. Example of a Command Write of VOUT_COMMAND
PMBus busy protocols are well accepted standards, but
can make writing system level software somewhat com-
plex. The part provides three 'hand shaking' status bits
which reduce complexity while enabling robust system
level communication.
The three hand shaking status bits are in the MFR_
COMMON register. When the part is busy executing an
internal operation, it will clear bit 6 of MFR_COMMON
('chip not busy'). When the part is busy specifically
because it is in a transitional V
power off/on, moving to a new output voltage set point,
etc.) it will clear bit 4 of MFR_COMMON ('output not in
transition'). When internal calculations are in process, the
part will clear bit 5 of MFR_COMMON ('calculations not
pending'). These three status bits can be polled with a
PMBus read byte of the MFR_COMMON register until all
three bits are set. A command immediately following the
status bits being set will be accepted without NACKing or
generating a BUSY fault/ALERT notification. The part can
NACK commands for other reasons, however, as required
by the PMBus spec (for instance, an invalid command
68
state (margining hi/lo,
OUT
For more information
or data). An example of a robust command write algo-
rithm for the VOUT_COMMAND register is provided in
Figure 33.
It is recommended that all command writes (write byte,
write word, etc.) be preceded with a polling loop to avoid
the extra complexity of dealing with busy behavior and
unwanted ALERT notification. A simple way to achieve this
is to create a SAFE_WRITE_BYTE() and SAFE_WRITE_
WORD() subroutine. The above polling mechanism allows
your software to remain clean and simple while robustly
communicating with the part. For a detailed discussion
of these topics and other special cases please refer to the
Application Note section.
When communicating using bus speeds at or below
100kHz, the polling mechanism shown here provides a
simple solution that ensures robust communication with-
out clock stretching. At bus speeds in excess of 100kHz,
it is strongly recommended that the part be configured to
enable clock stretching. This requires a PMBus master that
supports clock stretching. System software that detects
and properly recovers from the standard PMBus NACK/
BUSY faults as described in the PMBus Specification v1.1,
Par II, Section 10.8.7 is required to communicate The
LTM4681 is not recommended in applications with bus
speeds in excess of 400kHz.
THERMAL CONSIDERATIONS AND OUTPUT
CURRENT DERATING
The thermal resistances reported in the Pin Configuration
section of this data sheet are consistent with those
parameters defined by JESD51-12 and are intended for
use with finite element analysis (FEA) software model-
ing tools that leverage the outcome of thermal modeling,
simulation, and correlation to hardware evaluation per-
formed on a µModule package mounted to a hardware
test board defined by JESD51-9 ("Test Boards for Area
Array Surface Mount Package Thermal Measurements").
The motivation for providing these thermal coefficients is
found in JESD51-12 ("Guidelines for Reporting and Using
Electronic Package Thermal Information").
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