LTC2977 Datasheet, PDF (78/92 Pages) Linear Technology – 8-Channel PMBus Power System Manager Featuring Accurate Output Voltage Measurement

LTC2977
APPLICATIONS INFORMATION
• DAC manual with soft-connect. Non servo mode. The
DAC soft-connects to the feedback node. Soft-connect
drives the DAC code to match the voltage at the feedback
node. After connection, the DAC is moved by writing
DAC codes to the MFR_DAC register.
• DAC manual with hard-connect. Non servo mode. The
DAC hard-connects to the feedback node using the
current value in MFR_DAC. After connection, the DAC is
moved by writing DAC codes to the MFR_DAC register.
Margining
The LTC2977 margins and trims the output of a DC/DC
converter by forcing a voltage across an external resistor
connected between the DAC output and the feedback node
or the trim pin. Preset limits for margining are stored in
the VOUT_MARGIN_HIGH/LOW registers. Margining is
actuated by writing the appropriate bits to the OPERA-
TION register.
Margining requires the DAC to be connected. Margin
requests that occur when the DAC is disconnected will
be ignored.
Automatic Restart Via MFR_RESTART_DELAY
Command and CONTROLn pin
An automatic restart sequence can be initiated by driving
the CONTROL pin to the off state for >10μs then releasing
it. The automatic restart disables all V_{OUT_EN} pins that are
mapped to a particular CONTROL pin for a time period
= MFR_RESTART_DELAY and then starts all DC-DC
Converters according to their respective TON_DELAYs.
(See Figure 18). V_{OUT_ENn} pins are mapped to one of the
CONTROL pins by the MFR_CONFIG_LTC2977 command.
This feature allows a host that is about to reset to restart
the power in a controlled manner after it has recovered.
V_CONTROL
CONTROL
PIN BOUNCE
V_{OUT_END}
2977 F18
TOFF_DELAY0
MFR_RESTART_DELAY TON_DELAY0
Figure 18. Off Sequence with Automatic Restart
Off Sequencing
An off sequence is initiated using the CONTROL pin or the
OPERATION command. The TOFF_DELAY value determines
the amount of time that elapses from the beginning of the
off sequence until each channel’s V_{OUT_EN} pin is pulled
low, thus disabling its DC/DC converter.
V_OUT Off Threshold Voltage
The MFR_VOUT_DISCHARGE_THRESHOLD command
register allows the user to specify the OFF threshold that
the output voltage must decay below before the channel
can enter/re-enter the ON state. The OFF threshold voltage
is specified by multiplying MFR_VOUT_DISCHARGE_
THRESHOLD and VOUT_COMMAND. In the event that an
output voltage has not decayed below its OFF threshold
before attempting to enter the ON state, the channel will
continue to be held off, the appropriate bit is set in the
STATUS_MFR_SPECIFIC register, and the ALERTB pin
will be asserted low. When the output voltage has decayed
below its OFF threshold, the channel can enter the ON state.
Fault Management
Output Overvoltage and Undervoltage Faults
The high speed voltage supervisor OV and UV fault
thresholds are configured using the VOUT_OV_FAULT_
LIMIT and VOUT_UV_FAULT_LIMIT commands,
respectively. The VOUT_OV_FAULT_RESPONSE and
VOUT_UV_FAULT_RESPONSE commands determine the
responses to OV/UV faults. Fault responses can range from
disabling the DC/DC converter immediately, waiting to
see if the fault condition persists for some interval before
disabling the DC/DC converter, or allowing the DC/DC
converter to continue operating in spite of the fault. If a DC/
DC converter is disabled, the LTC2977 can be configured
to retry one to six times, retry continuously without
limitation, or latch-off. The retry interval is specified using
the MFR_RETRY_DELAY command. Latched faults are
reset by toggling the CONTROL pin, using the OPERATION
command, or removing and reapplying the bias voltage to
the V_{IN_SNS} pin. All fault and warning conditions result in
2977f
78
For more information www.linear.com/LTC2977