LTC2978CUP View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2978CUP' PDF : 64 Pages View PDF
LTC2978
APPLICATIONS INFORMATION
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 VOUT_EN pin will enable a DC/DC converter. Once
the DC/DC converter has been enabled, the TON_RISE
command determines the amount of time the LTC2978
waits before soft connecting the VDACPn output and
servoing the DC/DC converter output to VOUT_COMMAND.
The TON_MAX_FAULT_LIMIT command determines the
amount of time after the DC/DC converter has been enabled
that an undervoltage condition will be tolerated before a
fault occurs. 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. 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 VDACPn output can be placed in a
high impedance state thus allowing the DC/DC converter
output voltage to go to its nominal value, VDCn (NOM). Refer
VCONTROL
VOUT_EN
VOUT
VOUT_0V_FAULT_LIMIT
DAC SOFT-CONNECTS
AND BEGINS
ADJUSTING OUTPUT
TON_DELAY
VDC(NOM)
VOUT_COMMAND
VOUT_UV_FAULT_LIMIT
TON_RISE
2978 F16
TON_MAX_FAULT_LIMIT
to the MFR_CONFIG_LTC2978 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_
LTC2978 b[7]. In continuous trim mode, the servo will
update the DAC in a closed loop fashion each time it takes
a VOUT reading. The update rate is determined by the time
it takes to step through the ADC MUX which is typically
100ms. See Electrical Characteristics Table Note 6. In
noncontinuous trim mode, the servo will drive the DAC
until the ADC measures the output voltage desired and
then stop updating the DAC.
Noncontinuous servo on warn mode. MFR_CONFIG_
LTC2978 b[7] = 0, b[6] = 1. When in noncontinuous mode,
the LTC2978 can additionally retrim (reservo) the output
if the output drifts beyond the OV or UV warn limits.
DAC Modes
The DACs that drive the VDACn pins can operate in several
useful modes. See MFR_CONFIG_LTC2978.
• 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 LTC2978 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 . The DAC code
is driven to match the voltage at the feedback node. After
connection, the DAC is moved by writing DAC codes to
the device.
Figure 16. Typical On Sequence Using Control Pin
For more information www.linear.com/LTC2978
2978fd
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