Datasheet

LTC3883/LTC3883-1
17
3883fa
For more information www.linear.com/LTC3883
OPERATION
internal 5V linear regulator. The EXTV
CC
pin is driven by
an external regulator to improve efficiency of the circuit
and minimize power on the LTC3883. 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 BG, TG and RUN pins
are held low. The GPIO pin is in high impedance mode.
The LTC3883 will use the contents of Tables 12 to 15 to
determine the resistor defined parameters. See the Re
-
sistor 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_LT C 3883 configuration command), the LTC3883 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 120ms.
SOFT-START
The part must enter the run state prior to soft-start. The
run pin is released by the LTC3883 after the part initializes
and V
IN
is greater than the VIN_ON threshold. If multiple
LTC3883s 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
LTC3883 can be set to turn off (or remain off) if SHARE_
CLK is low (set bit 2 of MFR_CHAN_CONFIG_LTC3883
to a 1). This allows the user to assure synchronization
across numerous LTC ICs even if the RUN pins can not be
connected 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
and pull up to V
DD33
with a 10k resistor. 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 LTC3883
performs a monotonic initial ramp or “soft-start”. Soft-
start is performed by actively regulating the load voltage
while digitally ramping the target voltage from 0V to the
commanded voltage set-point. Once the LTC3883 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 LTC3883 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
transitions to continuous mode or burst, if so programmed.
If TON_MAX_FAULT_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 present.
Setting TON_MAX_FAULT_LIMIT to a value of 0 is not
recommended. This described method of start-up se
-
quencing is time based.
S
EQUENCING
The default mode for sequencing the output on and off is
time based. The output is enabled after waiting TON_DELAY
amount of time following either the RUN pin going high, a