Datasheet

LTC2978A
64
2978afa
For more information www.linear.com/LTC2978A
Figure 16. Typical On Sequence Using Control Pin
APPLICATIONS INFORMATION
Some examples of typical ON/OFF configurations are:
1. A DC/DC converter may be configured to turn on anytime
V
IN
exceeds VIN_ON.
2. A DC/DC converter may be configured to turn on only
when it receives an OPERATION command.
3. A DC/DC converter may be configured to turn on only
via the CONTROL pin.
4. A DC/DC converter may be configured to turn on only
when it receives an OPERATION command and the
CONTROL pin is asserted.
On Sequencing
The TON_DELAY command sets the amount of time that
a channel will wait following the start of an ON sequence
before its V
OUT_EN
pin will enable a DC/DC converter. Once
the DC/DC converter has been enabled, the TON_RISE value
determines the time at which the device soft-connects
the DAC and servos the DC/DC converter output to the
VOUT_COMMAND value. The TON_MAX_FAULT_LIMIT
value determines the time at which the device checks for
an undervoltage condition. If a TON_MAX_FAULT occurs,
the channel can be configured to disable the DC/DC
converter and propagate the fault to other channels using
the bidirectional FAULTB pins. Note that overvoltage faults
are checked against the VOUT_OV_FAULT_LIMIT at all
times the device is powered up and not in a reset state nor
margining while ignoring OVs. Figure 16 shows a typical
on-sequence using the CONTROL pin.
On State Operation
Once a channel has reached the ON state, the OPERATION
command can be used to command the DC/DC converter’s
output to margin high, margin low, or return to a nominal
output voltage indicated by VOUT_COMMAND. The
user also has the option of configuring a channel to
continuously trim the output of the DC/DC converter to the
VOUT_COMMAND voltage, or the channel’s V
DACPn
output
can be placed in a high impedance state thus allowing the
DC/DC converter output voltage to go to its nominal value,
V
DCn (NOM)
. Refer to the MFR_CONFIG_LT C 2978 command
for details on how to configure the output voltage servo.
Servo Modes
The ADC, DAC and internal processor comprise a digital
servo loop that can be configured to operate in several
useful modes. The servo target refers to the desired output
voltage.
Continuous/noncontinuous trim mode. MFR_CONFIG_
LTC2978A b[7]. In continuous trim mode, the servo will
update the DAC in a closed loop fashion each time it
takes a V
OUT
reading. The update rate is determined by
the time it takes to step through the ADC MUX, which is
no more than t
UPDATE_ADC
. See Electrical Characteristics
Table Note4. In noncontinuous trim mode, the servo will
drive the DAC until the ADC measures the output voltage
desired and then stop updating the DAC.
VOUT_0V_FAULT_LIMIT
DAC SOFT-CONNECTS
AND BEGINS
ADJUSTING OUTPUT
VOUT_UV_FAULT_LIMIT
V
CONTROL
V
OUT_EN
V
OUT
TON_DELAY
TON_RISE
2978a F16
V
DC(NOM)
V
OUT_COMMAND
TON_MAX_FAULT_LIMIT