Datasheet

LTC3880/LTC3880-1
26
3880fc
For more information www.linear.com/LTC3880
OPERATION
The I
IN
and I
OUT
overcurrent monitors are performed by
ADC readings and calculations. Thus these values are
based on average currents and can have a time latency of
up to 100ms. The I
OUT
calculation accounts for the sense
resistor and the temperature coefficient of the resistor.
The input current is equal to the sum of output current
times the respective channel duty cycle plus the input
offset current for each channel. If this calculated input
current exceed the IN_OC_WARN_LIMIT the ALERT pin
is pulled low and the IIN_OC_WARN bit is asserted in the
STATUS_INPUT register.
The digital processor within the LTC3880 provides the
ability to ignore the fault, shut down and latch off or shut
down and retry indefinitely (hiccup). The retry interval
is set in MFR_RETRY_DELAY and can be from 120ms
to 83.88 seconds in 1ms increments. The shutdown for
OV/UV and OC can be done immediately or after a user
selectable deglitch time.
Output Overvoltage Fault Response
A programmable overvoltage comparator (OV) guards
against transient overshoots as well as long-term over-
voltages at the output. In such cases, the top MOSFET is
turned off and the bottom MOSFET is turned on until the
overvoltage condition is cleared regardless of the PMBus
VOUT_OV_FAULT_RESPONSE command byte value. This
hardware level fault response delay is typically 2µs from
the overvoltage condition to BG asserted high. Using the
VOUT_OV_FAULT_RESPONSE command, the user can
select any of the following behaviors:
n
OV Pull-Down Only (OV cannot be ignored)
n
Shut Down (Stop Switching) Immediately—Latch Off
n
Shut Down Immediately—Retry Indefinitely at the Time
Interval Specified in MFR_RETRY_DELAY
Either the Latch Off or Retry fault responses can be de
-
glitched in increments of (0-7) • 10µs. See Table 5.
Output Undervoltage Response
The response to an undervoltage comparator output can
be either:
n
Ignore
n
Shut Down Immediately—Latch Off
n
Shut Down Immediately—Retry Indefinitely at the Time
Interval Specified in MFR_RETRY_DELAY
The UV responses can be deglitched. See Table 6.
Peak Output Overcurrent Fault Response
Due to the current mode control algorithm, peak output
current across the inductor is always limited on a cycle by
cycle basis. The value of the peak current limit is specified
in sense voltage in the EC table. The current limit circuit
operates by limiting the I
TH
maximum voltage. If DCR sens-
ing is used, the I
TH
maximum voltage has a temperature
dependency directly proportional to the TC of the DCR
of the inductor. The LTC3880 automatically monitors the
external temperature sensors and modifies the maximum
allowed I
TH
to compensate for this term.
The overcurrent fault processing circuitry can execute the
following behaviors:
n
Current Limit Indefinitely
n
Shut Down Immediately—Latch Off
n
Shut Down Immediately—Retry Indefinitely at the Time
Interval Specified in MFR_RETRY_DELAY
The overcurrent responses can be deglitched in increments
of (0-7) • 16ms. See Table 7
RESPONSES TO TIMING FAULTS
TON_MAX_FAULT_LIMIT is the time allowed for V
OUT
to
rise and settle at start-up. The TON_MAX_FAULT_LIMIT
condition is predicated upon detection of the VOUT_UV_
FAULT_LIMIT as the output is undergoing a SOFT_START
sequence. The TON_MAX_FAULT_LIMIT time is started