Datasheet

LTC2974
82
2974fc
For more information www.linear.com/LTC2974
applicaTions inForMaTion
DAC Modes
The DACs that drive the V
DACn
pins can operate in several
useful modes. See MFR_CONFIG_LTC2974.
• Soft-connect. Using the LTC patented soft-connect
feature, the DAC output is driven to within 1 LSB of
the voltage at the DC/DC’s feedback node before con
-
necting, to avoid introducing transients on the output.
This mode is used when ser
voing the output voltage.
During startup, the LTC2974 waits until TON_RISE has
expired before connecting the DAC. This is the most
common operating mode.
•
Disconnected. DAC output is high Z.
• 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 cur
-
rent value in MFR_DAC. After connection, the DAC is
moved by writing DAC codes to the MFR_DAC register.
Margining
The LTC2974
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.
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.
Automatic Restart via MFR_RESTART_DELAY
Command and CONTROL pin
An automatic restart sequence can be initiated by driving
the CONTROL pin to the off state for >10μs and then re
-
leasing 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 26). V
OUT_EN
pins are mapped to one of the
CONTROL pins by the MFR_CONFIG_LTC2974 command.
This feature allows a host that is about to reset to restart
the power in a controlled manner after it has recovered.
Figure 26. Off Sequence with Automatic Restart
V
CONTROL
V
OUT_EN0
CONTROL
PIN BOUNCE
TOFF_DELAY0
TON_DELAY0
2974 F26
MFR_RESTART_DELAY
FAULT MANAGEMENT
Output Overvoltage, Undervoltage, Overcurrent, and
Undercurrent Faults
The high-speed voltage supervisor OV and UV fault thresh
-
olds 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 registers determine the responses to