Datasheet

LTC2974
81
2974fc
For more information www.linear.com/LTC2974
applicaTions inForMaTion
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 oc
-
curs, the channel can be configured to disable the DC/DC
converter and propagate the fault to other channels using
the bidirectional FAUL
TB pins. Figure 25 shows a typical
on-sequence using the CONTROL pin. Note that overvolt-
age faults are checked against the VOUT_OV_FAULT_LIMIT
value at all times the device is powered up and not in a
reset state nor margining while ignoring OVs.
On State Operation
Once a channel has reached the ON state, the OPERA
-
TION 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
DACn
output
Figure 25. Typical ON Sequence Using Control Pin
VOUT_OV_FAULT_LIMIT
VOUT_UV_FAULT_LIMIT
V
CONTROL
V
OUT_EN
V
OUT
TON_DELAY
TON_RISE
2974 F25
V
DC(NOM)
V
OUT_COMMAND
TON_MAX_FAULT_LIMIT
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_LTC2974 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/non-continuous trim mode: MFR_
CONFIG_LTC2974 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5. In non-continuous trim mode, the servo will
drive the DAC until the ADC measures the output voltage
desired and then stop updating the DAC.
Non-continuous servo on warn mode: MFR_CONFIG_
LTC2974 b[7] = 0, b[6] = 1. When in non-continuous
mode, the LTC2974 will re-trim (re-servo) the output if
the output drifts beyond the OV or UV warn limits.