Power-Up And Initialization - Analog Devices Linear LTM 4700 User Manual

LTM4700
OPERATION
resolved. See the application Information section or con-
tact the factory for details on efficient in-system EEPROM
programming, including bulk EEPROM Programming,
which the LTM4700 also supports.
The LTM4700 contains dual integrated constant frequency
current mode control buck regulators (channel 0 and
channel 1) whose built-in power MOSFETs are capable of
fast switching speed. The factory NVM-default switching
frequency clocks SYNC at 350kHz, to which the regula-
tors synchronize their switching frequency. The default
phase-interleaving angle between the channels is 180°.
A pin-strapping resistor on FSWPH_CFG configures the
frequency of the SYNC clock (switching frequency) and the
channel phase relationship of the channels to each other
and with respect to the falling edge of the SYNC signal.
(Most possible combinations of switching frequency and
phase-angle assignments are settable by resistor pin pro-
gramming; see Table 3. Configure the LTM4700's NVM to
implement settings not available by resistor-pin strapping.)
When a FSWPH_CFG pin-strap resistor sets the channel
phase relationship of the LTM4700's channels, the SYNC
clock is not driven by the module; instead, SYNC becomes
strictly a high impedance input and channel switching
frequency is then synchronized to SYNC provided by
an externally-generated clock or sibling LTM4700 with
pull-up resistor to V . Switching frequency and phase
DD33
relationship can be altered via the I
when switching action is off, i.e., when the module is not
regulating either output. See the Applications Information
section for details.
Programmable analog feedback loop compensation for
channel 0 and channel 1 is accomplished with a capaci-
tor connection from COMPna to SGND, and a capacitor
from COMPnb to SGND. The COMPnb pin is for the high
frequency gain roll off and is the g
has a programmable range. The COMPna pin has the
programmable resistor range along with a capacitor to
SGND that sets the low frequency pole. See Programmable
Loop Compensation section. The LTM4700 module has
sufficient stability margins and good transient performance
with a wide range of output capacitors, even all-ceramic
MLCCs. Table 13 provides guidance on input and output
capacitors recommended for many common operating
24
2
C interface, but only
amplifier output that
m
For more information
conditions along with the programmable compensation
settings. The Analog Devices LTpowerCAD tool is avail-
able for transient and stability analysis, and experienced
users who prefer to adjust the module's feedback loop
compensation parameters can use this tool.

POWER-UP AND INITIALIZATION

The LTM4700 is designed to provide standalone supply
sequencing and controlled turn-on and turn-off operation.
It operates from a single input supply (4.5V to 16V) while
three on-chip linear regulators generate internal 2.5V,
3.3V and 5.5V. If V does not exceed 6V, V
IN
the INTV and SV pins must be tied together. The con-
CC IN
troller configuration is initialized by an internal threshold
based UVLO where SV
IN
the 5.5V, 3.3V and 2.5V linear regulators must be within
approximately 20% of the regulated values. In addition
to the power supply, a PMBus RESTORE_USER_ALL or
MFR_RESET command can initialize the part too.
The LTM4700 has a build-in 5V buck converter between
SV and EXTV of the controller to improve overall ef-
IN CC
ficiency. The 5V buck converter is controlled by RUNP
pin signal and will not take over the internal regulator
unless SV is higher than 7V. This 5V converter serves
IN
for efficiency improvement purposes and is not required
for LTM4700 operation.
During initialization, the external configuration resistors
are identified and/or contents of the NVM are read into the
controller's commands and the power train is held off. The
RUNn , FAULTn and PGOODn are held low. The LTM4700
will use the contents of Table 1 thru 5 to determine the
resistor defined parameters. See the Resistor Configuration
section for more details. 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
configuration command), the LTM4700 will use only the
contents of NVM to determine the DC/DC characteris-
tics. The ASEL value read at power-up or reset is always
respected unless the pin is open. The ASEL will set the
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and SV
IN
must be approximately 4V and
,
IN
Rev. B