Datasheet

LTC3880/LTC3880-1
20
3880fc
For more information www.linear.com/LTC3880
OPERATION
multiple outputs are controlled by the same GPIO pin,
the decay time of the faulted output determines the retry
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_LTC3880. Alternatively, the con
-
troller can be configured so that it remains latched-off
following a fault and clearing requires user intervention
such as toggling RUNn or commanding the part OFF
then ON.
LIGHT LOAD CURRENT OPERATION
The LTC3880 has three modes of operation including high
efficiency Burst Mode operation, discontinuous conduc
-
tion mode or forced continuous conduction mode. Mode
selection is done using the MFR_PWM_MODE_LTC3880
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 re
-
verse 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 100ms
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.
S
WITCHING FREQUENCY AND PHASE
The switching frequency of the LTC3880’s controller can
be established with internal clock references or with an
external time-base. The LTC3880 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_LTC3880 command determines
the relative phasing. Using the RCONFIG input, channel 0
and channel 1 have a relative phasing of 0° and 180° with
respect to the falling edge of SYNC. The master should
be selected to be out of phase with the slave. Both RUN
pins must be low or both channels commanded off before
the FREQUENCY and MFR_PWM_CONFIG_LTC3880 com
-
mands can be written to the LTC3880. 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 LTC3880 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_LTC3880 values as outlined
in Table 14. Otherwise, the FREQUENCY_SWITCH and
MFR_PWM_CONFIG_LTC3880 PMBus commands can be
used to select PWM switching frequency and the PWM
channel phase relationship. The phase and frequency