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OPERATION
bottom 4LSBs and the MSBs are set by NVM. See the
Applications Information section for more details.
After the part has initialized, an additional comparator
monitors V
. The VIN_ON threshold must be exceeded
IN
before the output power sequencing can begin. After V
is initially applied, the part will typically require 70ms to
initialize and begin the TON_DELAY timer. The readback
of voltages and currents may require an additional 0ms
to 90ms.
SOFT-START
The method of start-up sequencing described below is
time-based. The part must enter the run state prior to
soft-start. The run pins are released by the LTM4700
after the part is initialized and SV
VIN_ON threshold. If multiple LTM4700s are used in an
application, they all hold their respective run pins low until
all devices are initialized and SV
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
been initialized after SV
IN
set to turn-off (or remain off) if SHARE_CLK is low (set
bit 2 of MFR_CHAN_CONFIG to 1). This allows the user
to assure synchronization across numerous ADI devices
even if the RUN pins cannot be connected together due
to board constraints. In general, if the user cares about
synchronization between chips it is best not only to connect
all the respective RUN pins together but also to connect
all the respective SHARE_CLK pins together and pulled up
with a 10k resistor. This assures all chips begin
to V
DD33
sequencing at the same time and use the same time base.
After the RUN pins release and prior to entering a constant
output voltage regulation state, the LTM4700 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 com-
manded voltage set-point. Once the LTM4700 is com-
manded 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 us-
ing the TON_RISE command to minimize inrush currents
is greater than the
IN
exceeds the VIN_ON
IN
is applied. The LTM4700 can be
For more information
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 LTM4700 PWM always
uses discontinuous mode during the TON_RISE operation.
In discontinuous mode, the bottom MOSFET is turned
IN
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, 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
the VOUT_UV_FAULT_LIMIT and IOUT_OC is not present.
However, setting TON_MAX_FAULT_LIMIT to a value of 0
is not recommended.
TIME-BASED SEQUENCING
The default mode for sequencing the outputs on and
off is time-based. Each output is enabled after waiting
TON_DELAY amount of time following either a RUN pin
going high, a PMBus command to turn on or the V
above a preprogrammed voltage. Off sequencing is handled
in a similar way. To assure proper sequencing, make sure
all ICs connect the SHARE_CLK pin together and RUN pins
together. If the RUN pins cannot be connected together
for some reasons, set bit 2 of MFR_CHAN_ CONFIG to 1.
This bit requires the SHARE_CLK pin to be clocking before
the power supply output can start. When the RUN pin is
pulled low, the LTM4700 will hold the pin low for the MFR_
RESTART_DELAY. The minimum MFR_RESTART_ DELAY
is TOFF_DELAY + TOFF_FALL + 136ms. This delay assures
proper sequencing of all rails. The LTM4700 calculates
this delay internally and will not process a shorter delay.
However, a longer commanded MFR_RESTART_DELAY
can be used by the part. The maximum allowed value is
65.52 seconds.
VOLTAGE-BASED SEQUENCING
The sequence can also be voltage-based. As shown
in Figure 4, The PGOOD
threshold is exceeded for each output. It is possible to
feed the PGOOD pin from one LTM4700 into the RUN pin
of the next LTM4700 in the sequence, especially across
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LTM4700
has exceeded
OUT
IN
pin is asserted when the UV
n
25
rising
Rev. B
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