Datasheet

LTC3880/LTC3880-1
18
3880fc
For more information www.linear.com/LTC3880
OPERATION
2.5V, 3.3V and 5V. If V
IN
is below 6V, the INTV
CC
and V
IN
pins must be tied together. The controller configuration
is initialized by an internal threshold based UVLO where
V
IN
must be approximately 4V and the 5V, 3.3V and 2.5V
linear regulators must be within approximately 20% of
the regulated values. The LTC3880-1 does not have an
internal 5V linear regulators. The EXTV
CC
pin is driven by
an external regulator to improve efficiency of the circuit
and minimize power on the LTC3880. The EXTV
CC
pin
must exceed approximately 4V before the internal UVLO
is exceeded. To minimize application power, the EXTV
CC
pin can be supplied by a switching regulator.
During initialization, the external configuration resistors
are identified and/or contents of the NVM are read into
the controller’s commands. The GPIOn pins are in high
impedance (Hi-Z) mode. The TGn, BGn and RUNn pins
are held low. The LTC3880 will use the contents of Tables
12 to 15 to determine the resistor defined parameters.
See the Resistor Configuration section for more detail.
The resistor configuration pins only control some of the
preset values of the controller. The remaining values are
programmed in NVM either at the factory or by the user.
If the configuration resistors are not inserted or if the
ignore RCONFIG bit is asserted (bit 6 of the MFR_
CONFIG_ALL_LTC3880 configuration command), the
L
TC3880 will use only the contents of NVM to determine
the
DC/DC characteristics. The ASEL value read at power-
up or reset is always respected unless the pin is open.
The ASEL will use the MSB from NVM and the LSB from
the detected threshold. See the Applications Information
section for more detail.
After the part has initialized, an additional comparator
monitors V
IN
. The VIN_ON threshold must be exceeded
before the output power sequencing can begin. After V
IN
is initially applied, the part will typically require 130ms to
initialize and begin the TON_DELAY timer. The readback of
voltages and currents may require an additional 100ms.
SOFT-START
The part must enter the run state prior to soft-start.
The run pins are released by the LTC3880 after the part
initializes and V
IN
is greater than the VIN_ON threshold. If
multiple LTC3880s are used in an application, they all hold
their respective run pins low until all devices initialize and
V
IN
exceeds the VIN_ON threshold for every device. The
SHARE_CLK pin assures all the devices connected to the
signal use the same time base. The SHARE_CLK pin is held
low until the part has initialized after V
IN
is applied. The
LTC3880 can be set to turn off (or remain off) if SHARE_CLK
is low (set bit 2 of MFR_CHAN_CONFIG_LTC3880 to a 1).
This allows the user to assure synchronization across
numerous LTC ICs even if the RUN pins can not be con
-
nected together due to board constraints. In general, if
the user cares about synchronization between chips it is
best to connect all the respective RUN pins together and
to connect all the respective SHARE_CLK pins together
.
This
assures all chips begin sequencing at the same time
and use the same time base.
After the RUN pin releases and prior to entering a constant
output voltage regulation state, the LTC3880 performs a
monotonic initial ramp or “soft-start”. Soft-start is per
-
formed by actively regulating the load voltage while digitally
ramping the target voltage from 0V to the commanded
voltage set-point. Once the LTC3880 is commanded to turn
on, (after power up and initialization) the controller waits
for the user specified turn-on delay (TON_DELAY) prior
to initiating this output voltage ramp. The rise time of the
voltage ramp can be programmed using the TON_RISE
command to minimize inrush currents associated with the
start-up voltage ramp. The soft-start feature is disabled
by setting the value of TON_RISE to any value less than
0.25ms. The LTC3880 PWM always uses discontinuous
mode during the TON_RISE operation. In discontinuous
mode, the bottom gate is turned off as soon as reverse
current is detected in the inductor. This will allow the
regulator to start up into a pre-biased load. When the
TON_MAX_FAULT_LIMIT is reached, the part transi
-
tions to continuous mode or burst, if so programmed. If
TON_MAX_FAUL
T_LIMIT is set to zero, there is no time
limit and the part transitions to the desired conduction
mode after TON_RISE completes and V
OUT
has exceeded
the VOUT_UV_FAULT_LIMIT and IOUT_OC is not pres-
ent. Setting TON_MAX_FAULT_LIMIT to a value of 0 is
not recommended. This described method of start-up
sequencing is time based.