LTC3883 Datasheet, PDF (19/112 Pages) Linear Technology – Single Phase Step-Down DC/DC Controller with Digital Power System Management

LTC3883/LTC3883-1
Operation
delay. If the natural decay time of the output is too long,
it is possible to remove the voltage requirement of the
MFR_RETRY_DELAY command by asserting bit 0 of
MFR_CHAN_CONFIG_LTC3883. Alternatively, the control-
ler can be configured so that it remains latched-off follow-
ing a fault and clearing requires user intervention such
as toggling RUN or commanding the part OFF then ON.
Light Load Current Operation
The LTC3883 has three modes of operation including high
efficiency Burst Mode operation, discontinuous conduction
mode or forced continuous conduction mode. Mode
selection is done using the MFR_PWM_MODE_LTC3883
command (discontinuous conduction is always the start-
up mode, forced continuous is the default running mode).
In Burst Mode operation the peak current in the inductor
is set to approximately one-third of the maximum sense
voltage even though the voltage on the I_TH pin indicates a
lower value. If the average inductor current is higher than
the load current, the error amplifier, EA, will decrease the
voltage on the I_TH pin. When the I_TH voltage drops below
approximately 0.5V, the internal Burst Mode operation as-
serts and both external MOSFETS are turned off. In Burst
Mode operation, the load current is supplied by the output
capacitor. As the output voltage decreases, the EA output
begins to rise. When the output voltage drops sufficiently,
Burst Mode operation is deasserted, and the controller
resumes normal operation by turning on the top external
MOSFET on the next PWM cycle.
If a controller is enabled for Burst Mode operation, the
inductor current is not allowed to reverse. The reverse
current comparator, I_REV , turns off the bottom gate external
MOSFET just before the inductor current reaches zero,
preventing it from reversing and going negative. Thus,
the controller can operate in discontinuous operation.
In forced continuous operation, the inductor current is
allowed to reverse at light loads or under large transient
conditions. The peak inductor current is determined solely
by the voltage on the I_TH pin. In this mode, the efficiency
at light loads is lower than in Burst Mode operation.
However, continuous mode exhibits lower output ripple and
less interference with audio circuitry. Forced continuous
conduction mode may result in reverse inductor current,
which can cause the input supply to boost. The VIN_OV_
FAULT_LIMIT can detect this and turn off the offending
channel. However, this fault is based on an ADC read and
can take up to 120ms to detect. If there is a concern about
the input supply boosting, keep the part in discontinuous
conduction or Burst Mode operation.
If the part is set to Burst Mode operation, as the inductor
average current increases, the controller will automati-
cally modify the operation from Burst Mode operation,
to discontinuous mode to continuous mode.
Switching Frequency and Phase
The switching frequency of the LTC3883’s controller can
be established with internal clock references or with an
external time-base. The LTC3883 can be configured for
an external clock input through the programmed value in
NVM, a PMBus command or setting the R_BOTTOM resistor
of the FREQ_CFG pin to 0Ω and the R_TOP to open. The
PMBus command FREQUENCY_SWITCH is set to external
clock. The MFR_PWM_CONFIG_LTC3883 command
determines the relative phasing. The RCONFIG input can
set the relative phasing with respect to the falling edge of
SYNC. The master should be selected to be out of phase
with the slave. The RUN pin must be low before the
FREQUENCY and MFR_PWM_CONFIG_LTC3883 com-
mands can be written to the LTC3883. The relative phas-
ing of all devices in a PolyPhase rail should be optimally
phased. The relative phasing of each rail is 360/n where
n is the number of phases in the rail.
If the LTC3883 is configured as the oscillator output on
SYNC, the switching frequency source can be selected with
either external configuration resistors or through serial
bus programming. The FREQ_CFG configuration resistor
pin can be used to select the FREQUENCY_SWITCH
and MFR_PWM_CONFIG_LTC3883 values as outlined
in Table 14. Otherwise, the FREQUENCY_SWITCH and
MFR_PWM_CONFIG_LTC3883 PMBus commands can be
used to select PWM switching frequency and the PWM
channel phase relationship. The phase and frequency
relationships are completely independent of each other
providing the numerous application options for the user.
If the LTC3883 is configured to drive the SYNC pin using
the programmed FREQUENCY_SWITCH command value,
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