Linear Technology LTC2977 Demo Board: Octal Power Supply Manager [Requires DC1613] DC2028A DC2028A Data Sheet
Product codes
DC2028A
LTC2977
77
2977fa
applicaTions inForMaTion
On Sequencing
The TON_DELAY command sets the amount of time that
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 17 shows
a typical on-sequence using the CONTROL pin.
On State Operation
Once a channel has reached the ON state, the 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_LTC2977 command
for details on how to configure the output voltage servo.
Servo Modes
The ADC, DAC and internal processor comprise a digital
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_
Continuous/noncontinuous trim mode. MFR_CONFIG_
LTC2977 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.
As part of continuous/noncontinuous trim mode, fast servo
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_
Noncontinuous servo on warn mode. MFR_CONFIG_
LTC2977 b[7] = 0, b[6] = 1. When in noncontinuous mode,
the LTC2977 will retrim (reservo) the output if the output
drifts beyond the OV or UV warn limits.
DAC Modes
The DACs that drive the V
The DACs that drive the V
DACn
pins can operate in several
useful modes. See MFR_CONFIG_LTC2977.
• Soft-connect. Using the LTC patented soft-connect
• 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 LTC2977 waits until TON_RISE has expired
before connecting the DAC. This is the most common
operating mode.
• Disconnected. DAC output is high Z.
Figure 17. Typical On Sequence Using Control Pin
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
2977 F17
V
DC(NOM)
V
OUT_COMMAND
TON_MAX_FAULT_LIMIT