Datasheet

LTC2978A
65
2978afa
For more information www.linear.com/LTC2978A
As part of continuous/noncontinuous trim mode, fast servo
mode can be used to speed up large output transitions,
such as margin commands, or ON events. To use, set
Mfr_config_fast_servo_off=0. When enabled, fast servo
is started by a change to the target voltage or a new soft-
connect. The DAC is ramped one lsb every t
S_VDACP
period
until it is near the new target voltage, at which point slow
servo mode is entered to avoid overshoot.
Noncontinuous servo on warn mode. MFR_CONFIG_
LTC2978A b[7] = 0,
b[6] = 1. When in noncontinuous
mode, the LTC2978A will retrim (reservo) the output if
the output drifts beyond the OV or UV warn limits.
DAC Modes
The DACs that drive the V
DACn
pins can operate in several
useful modes. See MFR_CONFIG_LT C 2978.
• 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 connecting
to avoid introducing transients on the output. This mode
is used when servoing the output voltage. During start-
up, the LTC2978A 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
current value in MFR_DAC. After connection, the DAC is
moved by writing DAC codes to the MFR_DAC register.
Margining
The LTC2978A 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 from a non-global OPERATION command that
occur when the DAC is disconnected will force the DAC
to soft-connect. If a global (PAGE=0xFF) OPERATION
command is used to margin, the DACs must already
be
connected using
MFR_CONFIG_LTC2978 commands.
When in the margin high/low state, the DAC cannot be
disconnected. The DAC can only be disconnected from
the ON state.
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
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