MAX8688ALETG+ Datasheet MAX8688AHETG+, MAX8688ALETG+, MAX8688BHETG+T | P25-P27

MAX8688

Digital Power-Supply Controller/Monitor

with PMBus Interface

______________________________________________________________________________________ 25

Sending a DIRECT Format Value

To send a value, the host must use the equation below

to solve for Y:

Y = (mX + b) x 10

where:

Y is the 2-byte, two’s complement integer to be sent

to the unit;

the slope coefficient, is the 2-byte, two’s comple-

ment integer;

X is a real world value, in units such as amperes or

volts, to be converted for transmission;

b, the offset, is the 2-byte, two’s complement integer;

and

R, the exponent, is the decimal value equivalent to

the 1-byte, two’s complement integer.

The following example demonstrates how the host can

send and retrieve values from the MAX8688.

From Table 4, the coefficients used in the following

parameters are:

VOUT_COMMAND: m = 19995, b = 0, R = -1

READ_VOUT: m = 19995, b = 0, R = -1

If a host wants to request the POL to output a voltage of

3.0V, the corresponding VOUT_COMMAND value is:

Y = (mX + b) x 10

Y = (19995 x 3.0 + 0) x 10

-1

= 5998.5 (decimal)

= 176Eh (hex)

Conversely, if the host received a value of 176Eh on a

READ_VOUT command, this is equivalent to:

X =

1 (176Eh x 10

-(-1)

- 0)

19995

= 59980/19995 = 2.999750

which is within 0.0083% of 3.0V.

Power supplies and power converters generally have no

way of knowing how their outputs are connected to

ground. Within the power supply, all output voltages are

most commonly treated as positive. Accordingly, all out-

put voltages and output voltage-related parameters of

PMBus devices are commanded and reported as posi-

tive values. It is up to the system to know that a particu-

lar output is negative, if that is of interest to the system.

All output voltage-related commands use 2 data bytes.

Fault Management and Reporting

For reporting faults/warnings to the host on a real-time

basis, the MAX8688 asserts the open-drain FLT pin and

sets the appropriate bit in the STATUS_BYTE and

MFR_FAULT_STATUS registers, respectively. On

recognition of the FLT assertion, the host or system

manager is expected to poll multiple MAX8688s and

retrieves fault/warning information. The Manufacturer

Fault Status Register, MFR_FAULT_STATUS, provides

more detailed information on fault/warning. Faults/warn-

ings are cleared when any one of the following condi-

tions occurs:

• A CLEAR_FAULTS command is received.

• ENOUT is commanded through the OPERATION

command or A3/ONOFF to turn off and then turn

on.

•A RST signal is asserted for longer than t

RST

where

the MAX8688 is internally reset.

• Bias power to the MAX8688 is removed and then

reapplied.

The MAX8688 responds to fault conditions according to

the Manufacturer Fault Response command

(MFR_FAULT_RESPONSE). This command byte deter-

mines how the MAX8688 should respond to each par-

ticular fault.

In addition, the MAX8688 responds to the following

error conditions.

1) If the internal reference fails to operate, FLT is

asserted. To clear this fault, the MAX8688 has to go

through a device reset.

2) The MAX8688 responds to unsupported commands

with a NACK.

3) When the host sends insufficient data (too few

bytes), the MAX8688 sets the CML bit and asserts

FLT.

4) When the host sends too much data (too many

bytes), the MAX8688 sets the CML bit and asserts

FLT.

When a read request is issued to a write-only com-

mand, the read operation is aborted and no warning is

issued.

=×−

()

−

MAX8688

Digital Power-Supply Controller/Monitor

with PMBus Interface

26 ______________________________________________________________________________________

PMBus Commands

A summary of the PMBus commands supported by the

MAX8688 is described in Table 4.

OPERATION (01h)

The OPERATION command is used to turn the POL on

and off in conjunction with ENOUT according to the

ENOUT polarity select setup. The OPERATION com-

mand is also used to cause the POL to set the output

voltage to the upper or lower margin voltages. The POL

stays in the commanded operating mode until a subse-

quent OPERATION command or change in the state of

A3/ONOFF (if enabled) instructs the POL to change to

another state.

The valid OPERATION command byte values are shown

in Table 5.

The OPERATION command controls how the MAX8688

responds when commanded to change the output. When

the command byte is 00h, the MAX8688 turns the POL off

immediately and ignores any programmed turn-off delay

and fall time. When the command byte is set to 40h, the

MAX8688 powers down according to the programmed

turn-off delay and fall time.

In Table 5, Act On Fault means that if an output overvolt-

age warning or output overvoltage fault is detected when

the output is margined high, the MAX8688 treats this as a

warning or fault and responds as programmed by the

warning limit or fault response command. Similarly, if an

output undervoltage warning or output undervoltage fault

is detected when the output is margined low, the

MAX8688 treats this as a qualified warning/fault event

and responds as programmed by the warning/fault limit

or fault response command.

Any command value not shown in Table 5 is an invalid

command. If a MAX8688 receives a data byte that is not

listed in Table 5, then it may treat this as invalid data,

declare a communications fault (set CML bit and assert

FLT), and respond as described in the

Fault

Management and Reporting

section.

The default OPERATION value is 00h.

CLEAR_FAULTS (03h)

The CLEAR_FAULTS command is used to clear any

fault bits that have been set. This command clears all

bits in the STATUS_BYTE and MFR_FAULT_STATUS

registers simultaneously. It also deasserts FLT.

The CLEAR_FAULTS command does not cause a POL

that has latched off for a fault condition to restart. The

status of ENOUT under fault conditions is not affected

by this command and should change only if command-

ed through the OPERATION command or A3/ONOFF.

If the fault is still present after the CLEAR_FAULTS

command is executed, the fault bit shall be set again

and the host notified by the usual means.

This command is write-only. There is no data byte for

this command.

STORE_DEFAULT_ALL (11h)

The STORE_DEFAULT_ALL command instructs the

MAX8688 to store configuration information to an exter-

nal I

C EEPROM device attached to A1/SCLE and

A2/SDAE. If an error occurs during storing, FLT asserts

and the CML bit is set to 1.

It is permitted to use the STORE_DEFAULT_ALL com-

mand while the POL is operating. However, the

MAX8688 is unresponsive to PMBus commands while

storing the configuration. ENOUT maintains its state.

This command is write-only. There is no data byte for

this command.

For information on EEPROM contents, see the

External

EEPROM Interface section

RESTORE_DEFAULT_ALL (12h)

The RESTORE_DEFAULT_ALL command instructs the

MAX8688 to restore configuration information from an

external I

C EEPROM device attached to A1/SCLE and

A2/SDAE. The RESTORE_DEFAULT_ALL command

can only be executed when the POL is off. Otherwise, a

communication fault occurs (CML = 1) and FLT asserts.

If an error occurs during restoration, FLT asserts and

the CML bit is set to 1.

The STATUS_BYTE and MFR_FAULT_STATUS values are

not restored by the RESTORE_DEFAULT_ALL command.

This command is write-only. There is no data byte for

this command.

For information on EEPROM contents, see the

External

EEPROM Interface section.

COMMAND

BYTE

POL ON OR OFF MARGIN STATE

00h

Immediate off (no

sequencing)

—

40h

Soft-off (with

sequencing)

—

80h On Margin off (Nominal)

98h On

Margin low (Act On

Fault)

A8h On

Margin high (Act On

Fault)

Table 5. OPERATION Command Byte

MAX8688

Digital Power-Supply Controller/Monitor

with PMBus Interface

______________________________________________________________________________________ 27

VOUT_COMMAND (21h)

The VOUT_COMMAND command loads the MAX8688

with the voltage to which the POL output is to be

changed when it is commanded to turn on using the

OPERATION command or A3/ONOFF transition when

enabled. Once the POL is turned on, changing the

VOUT_COMMAND has no effect on the POL output

voltage. The MAX8688 only adjusts the POL to the new

VOUT_COMMAND voltage after receiving a new turn-

on command.

The 2 data bytes are in DIRECT format. Valid values are

from 0 to 2.048V when the input range is 2.048V and 0

to 5.5V when the input range is 5.5V.

The default VOUT_COMMAND value is 00h.

VOUT_MARGIN_HIGH (25h)

The VOUT_MARGIN_HIGH command loads the

MAX8688 with the voltage to which the POL output is to

be changed when the OPERATION command is set to

margin high. If the POL is already operating at margin

high, changing VOUT_MARGIN_HIGH has no effect on

the output voltage. The MAX8688 only adjusts the POL

to the new VOUT_MARGIN_HIGH voltage after receiv-

ing a new margin high OPERATION command.

The 2 data bytes are in DIRECT format. Valid values are

the same as VOUT_COMMAND.

The default VOUT_MARGIN_HIGH value is 00h.

VOUT_MARGIN_LOW (26h)

The VOUT_MARGIN_LOW command loads the

MAX8688 with the voltage to which the POL output is to

be changed when the OPERATION command is set to

margin low. If the POL is already operating at margin

low, changing VOUT_MARGIN_LOW has no effect on

the output voltage. The MAX8688 only adjusts the POL

to the new VOUT_MARGIN_LOW voltage after receiv-

ing a new margin low OPERATION command.

The 2 data bytes are in DIRECT format. Valid values are

the same as VOUT_COMMAND.

The default VOUT_MARGIN_LOW value is 00h.

VOUT_TRANSITION_RATE (27h)

The VOUT_TRANSITION_RATE command sets the rate in

mV/µs at which the POL output voltage should change

when the POL is commanded to change between the

margin high, margin low, and margin off (ON) OPERA-

TION mode. This commanded rate of change does not

apply when the POL is commanded to turn on or turn off.

In that case, t

ON_RISE

and t

OFF_FALL

applies.

The 2 data bytes are in DIRECT format. Valid values are

from 0 to 128mV/µs.

The default VOUT_TRANSITION_RATE value is 0mV/µs.

When VOUT_TRANSITION_RATE is cleared to 0,

VOUT_TRANSITION_RATE is ignored and the voltage out-

put is not changed even when commanded by the OPER-

ATION commands. When VOUT_TRANSITION_RATE is

set to 07FFFh, the voltage output is adjusted as quickly

as possible. If a VOUT_TRANSITION_RATE parameter

results in the DAC outputting an out of range value (valid

range 0 to 4090) during transition, the CML flag is set

and FLT is asserted to issue a warning.

The VOUT_TRANSITION_RATE command applies to

REFIN mode only and is ignored in feedback mode.

VOUT_SCALE_LOOP (29h)

In a typical application, the output voltage of a power

converter is sensed through a resistive voltage-divider,

as illustrated in Figure 16. The resistive voltage-divider

reduces or scales the output voltage, V

OUT

The PMBus commands specify the actual POL output

voltages and not the input voltage to the control circuit.

To allow the MAX8688 to map between the command-

ed voltage (such as 3.3V) and the voltage at the control

circuit input (perhaps 3.3V divided down to match a

reference voltage of 2.0V), the VOUT_SCALE_LOOP

command is used.

The 2 data bytes are in DIRECT format. Valid values are

from 0 to 1.0. Note that due to m, b, R representation

restriction, the supplied value is rounded off to multi-

ples of 1/128. Therefore, to ensure optimum operation,

circuit design should choose a value as close to multi-

ples of 1/128 as possible to avoid rounding errors and

thus ensure the final accuracy of V

OUT

This value is dimensionless.

The default VOUT_SCALE_LOOP value is 00h.

The VOUT_SCALE_LOOP command is ignored in feed-

back mode.

VOUT SCALE LOOP

__=

MAX8688

RS-

RS+

POL

VO-

VO+

LOAD

Figure 16. VOUT_SCALE_LOOP

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MAX8688ALETG+

Mfr. #:

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Manufacturer:

Maxim Integrated

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Current & Power Monitors & Regulators Digital Power-Supply Controller/Monitor

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