LTC3882-1
21
Rev A
OPERATION
Finally, each PWM channel can be commanded off by
pulling the associated RUN pin low. Pulling the RUN pin
low can force the channel to perform a controlled turn off
or immediately disable the power stage, depending on the
programming of the ON_OFF_CONFIG command.
Minimum Output Disable Times
When a PMBus OPERATION command is used to turn off
an LTC3882-1 channel, a minimum output disable time of
120ms is imposed regardless of how quickly the channel
is commanded back on. If bit 4 of MFR_CHAN_CONFIG is
clear, a PMBus command to turn the channel off also pulses
the RUN pin low. Once the RUN pin is pulled low internally
or externally, a minimum output disable time (RUN forced
low) of TOFF TOF 136ms is enforced.
If MFR_RESTART_DELAY is greater than this mandatory
minimum, the larger value of MFR_RESTART_DELAY is
used. In either case the LTC3882-1 holds its own RUN
pin low during the entire disable period. These minimum
off times allow a consistent channel restart with coher-
ent monitor ADC values and make the LTC3882-1 highly
compatible with other LTC PMBus digital power system
management products.
Output Short Cycle
An output short cycle condition is created when a mas-
ter channel is commanded back on while waiting for
TOFF_DELAY or TOFF_FALL to expire. Any time this
occurs, the LTC3882-1 asserts the Short Cycle bit in
STATUS_MFR_SPECIFIC. Device response at that point
is governed by bits in MFR_CHAN_CONFIG_LTC3882-1
and SMBALERT_MASK. Refer to the detailed descriptions
of those commands for additional details. Generally, the
LTC3882-1 should be controlled so that short cycle condi-
tions are not created during normal operation.
Light Load Current Operation
The LTC3882-1 has two modes of PWM operation: discon-
tinuous conduction mode (DCM) and forced continuous
conduction mode (CCM). Mode selection is made with
the MFR_PWM_MODE command.
In DCM, the inductor current is not allowed to reverse.
The reverse current comparator I
REV
disables the external
bottom MOSFET (synchronous rectifier) when the induc-
tor current reaches approximately 0A, preventing it from
going substantially negative. The external gate driver or
power block must have short delays to a high impedance
output, relative to the PWM cycle, to support DCM.
Efficiency at light loads in CCM is lower than in DCM.
Continuous conduction mode exhibits less interference
with audio circuitry but may result in reverse inductor
current, for instance at light loads or under large transient
conditions.
Switching Frequency and Phase
There is a high degree of flexibility for setting the PWM
operating frequency of the LTC3882-1. The switching
frequency of the PWM can be established with an in-
ternal oscillator or an external time base. The internal
phase-locked loop (PLL) synchronizes PWM control to
this timing reference with proper phase relation, whether
the clock is provided internally or externally. The device
can also be configured to provide the master clock to
other ICs through PMBus command, EEPROM setting,
or external configuration resistors as outlined in applica-
tion Table 10. For PMbus or EEPROM configuration, the
LTC3882-1 is designated as a clock master by clearing
bit 4 of MFR_CONFIG_ALL_LTC3882-1. As clock master,
the LTC3882-1 will drive its open-drain SYNC pin at the
selected rate with a pulse width of 125ns. An external
pull-up resistor between SYNC and V
DD33
is required in
this case. Only one device connected to SYNC should be
designated to drive the pin. If more than one LTC3882-1
sharing SYNC is programmed as clock master, just one of
the devices is automatically elected to provide the clock.
The others disable their SYNC outputs and indicate this
with bit 10 of MFR_PADS_LTC3882-1.
The LTC3882-1 will automatically accept an external SYNC
input, disabling is own SYNC drive if necessary, as long
as the external clock frequency is greater than 1/2 of the
programmed internal oscillator. Whether configured to drive
SYNC or not, the LTC3882-1 can continue PWM operation
at the selected frequency (FREQUENCY_SWITCH) using
its own internal oscillator, if an external clock signal is
subsequently lost.
The MFR_PWM_CONFIG_LTC3882-1 command can be
used to configure the phase of each channel. Desired phase
