SMM665B
Preliminary Information
APPLICATIONS INFORMATION (CONTINUED)
A pulse of current, either sourced or sunk for 5µs
every 1.7ms, to the capacitors connected to the
TRIM_CAPX pins adjusts the voltage output on the
TRIMX pins. The voltages on the TRIM_CAPX pins are
buffered and applied to the TRIMX pins. The voltage
adjustments on the TRIMX pins cause a slight ripple of
less than 1mV on the power supply voltages. The
amplitude of this ripple is a function of the TRIM_CAP
capacitor and the trim gain of the converter.
Application Note 37 details the calculation of the
TRIM_CAP capacitor to achieve a desired minimum
ripple.
the PWR_ON/OFF pin to its active state will initiate the
Power-on sequence. To enable software control of
the sequencing feature, the SMM665B offers an I2C
command to initiate Power-on sequencing while the
PWR_ON/OFF pin is in its inactive state.
The SMM665B can be programmed to wait until either
or both VDD and 12VIN inputs are within their
respective voltage threshold limits before Power-on
sequencing is allowed to begin. This ensures that the
converters have their full supply voltage before they
are enabled.
Once Power-on sequencing begins, the SMM665B will
wait a Power-on delay time (tDPON) for any channel in
the first sequence position (0) and then activate the
PUPX outputs for those channels. The Power-on
delay times are individually programmable for each
channel. The SMM665B will then wait until all VMX
inputs of the channels assigned to the first sequence
position (0) are above their programmed UV1
thresholds which is called cascade sequencing. At
this point, the SMM665B will enter the second
sequence position (1) and begin to timeout the Power-
on delay times for the associated channels. This
process continues until all of channels in the sequence
have turned on and are above their UV1 threshold.
The status registers indicates that all sequenced
power supply channels have turned on. Once these
channels are above their UV1 thresholds, the
SMM665B will begin the Active DC Control of the
enabled channels. The Power-on sequencing mode
ends when the Active DC Controlled channels are at
their nominal voltage setting. The “Ready” bit in the
status registers signifies that the voltages are at their
set points.
The programmable sequence termination timer can be
used to protect against a stalled Power-on sequence.
This timer resets itself at the beginning of each
sequence position. All channels in the sequence
position must go above their UV1 threshold before the
sequence termination timer times out (tSTT) or the
sequence will terminate and all PUPX outputs will be
switched to their inactive state. The status registers
contain bits that indicate the sequence has been
terminated and in which sequence position the timer
timed out. This timer has four settings of OFF, 100ms,
200ms and 400ms.
Each channel can be programmed to either enable or
disable the Active DC Control function.
When
disabled or not active, the TRIMX pins on the
SMM665B are high impedance inputs. If disabled and
not used, they can be connected to ground. The
voltages on the TRIMX pins are buffered and applied
to the TRIM_CAPX pins charging the capacitors. This
allows a smooth transition from the converter powering
up to its nominal voltage; to the SMM665B controlling
that voltage, and to the Active DC Control nominal
setting.
The pulse of current can be increased to a 10X pulse
of current until the power supply voltages are at their
nominal settings by selecting the programmable
Speed-Up Convergence option. As the name implies,
this option decreases the time required to bring a
supply voltage from the converter’s nominal output
voltage to the Active DC Control nominal voltage
setting.
POWER-ON CASCADE SEQUENCING
The SMM665B can be programmed to sequence up to
six power supplies in any order. Each of these six
channels (A-F) has an associated open drain PUP
output that, when connected to a converter’s enable
pin, controls the turn-on of the converter.
The
channels are assigned sequence positions to
determine the order of the sequence. Any channel
can also be programmed to not take part in the
sequencing in applications with fewer than six
supplies. The polarity of each of the PUPX outputs is
programmable for use with various types of
converters.
Power-on sequencing can be initiated by the
PWR_ON/OFF pin or via I2C control. The polarity of
the PWR_ON/OFF pin is programmable. If hard wired
in its active state the SMM665B will automatically
initiate the Power-on sequence. Otherwise, toggling
Summit Microelectronics, Inc
2089 1.1 10/20/04
14
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
