MIC2341 Datasheet, PDF (21/32 Pages) Micrel Semiconductor – Dual-Slot PCI Express Hot-Plug Controller

Micrel, Inc.
Standby Mode
Standby mode is entered when one or more of the MAIN
supply inputs (12VIN and/or 3VIN) are below its
respective UVLO threshold or OFF. The MIC2341 also
supplies 3.3V auxiliary outputs (VAUX[A/B]), satisfying
PCI Express specifications. These outputs are fed via
the VSTBY[A/B] input pins and controlled by the
AUXEN[A/B] input pins. These outputs are independent
of the MAIN outputs (12VIN[A/B] and 3VIN[A/B]). Should
the MAIN supply inputs move below their respective
UVLO thresholds, VAUX[A/B] will still function as long as
VSTBY[A/B] is present. Prior to standby mode, ONA and
ONB inputs should be de-asserted or the MIC2341 will
assert the /FAULT_MAIN[A/B] and /INT output signals, if
an undervoltage condition on the MAIN supply inputs is
detected.
Circuit Breaker Function
The MIC2341 provides an electronic circuit breaker
function that protects against excessive loads, such as
short circuits, at each supply. When the current from one
or more of a slot’s MAIN outputs exceeds the current
limit threshold (I_LIM = 50mV/R_SENSE) for a duration greater
than t_FLT, the circuit breaker is tripped and both MAIN
supplies (all outputs except VAUX[A/B]) are shut off.
Should the load current cause a MAIN output’s V_SENSE to
exceed V_THFAST, the outputs are immediately shut off with
no delay. Undervoltage conditions on the MAIN supply
inputs also trip the circuit breaker, but only when the
MAIN outputs are enabled (to signal a supply input
brown-out condition).
The VAUX[A/B] outputs have a different circuit-breaker
function. The VAUX[A/B] circuit breakers do not
incorporate a fast-trip detector, instead they regulate the
output current into a fault to avoid exceeding their
operating current limit. The circuit breaker will trip due to
an overcurrent on VAUX[A/B] when the fault timer
expires. This use of the t_FLT timer prevents the circuit
breaker from tripping prematurely due to brief current
transients.
Following a fault condition, the outputs can be turned on
again by toggling the ON[A/B] input high-low-high (if the
fault occurred on one of the MAIN outputs), or similarly
toggling the AUXEN[A/B] input (if the fault occurred on
the AUX outputs), or by cycling both ON[A/B] and
AUXEN[A/B] (if faults occurred on both the MAIN and
AUX outputs). When the circuit breaker trips, the
corresponding /FAULT_MAIN[A/B] or /FAULT_AUX[A/B]
will be asserted. At the same time, /INT will be asserted.
Note that /INT is only de-asserted by applying a high-to-
MIC2341/2341R
low transition on the corresponding slot’s ON[A/B] or
AUXEN[A/B] input.
The response time (t_FLT) of the MIC2341’s primary
overcurrent detector is set by external capacitors at the
CFILTER[A/B] pins to GND. For Slot A, CFILTER[A] is
located at Pin 2; for Slot B, CFILTER[B] is located at Pin
35. For a given response time, the value for C_FILTER[A/B] is
given by:
C_FILTER[A/B](µF)
=
t_FLT[A/B] (ms) × I_FILTER (µA)
V_FILTER (V) × 10³
where t_FLT[A/B] is the desired response time and quantities
I_FILTER and V_FILTER are specified in the MIC2341’s
“Electrical Characteristics” table.
Digital Filter and Auto-Retry Functions
New timers have been incorporated for additional
protection for overcurrent situations. In many
applications, external power MOSFETs used at
12VOUT[A/B] and at 3VOUT[A/B] have been damaged
during initial start-up and overcurrent conditions because
the response time of the primary OC detectors was set
too long (that is, an incorrect value for CFILTER[A/B]
was used). In these products, a digital filter delay
counter is introduced, and is internally set to a typical
delay time of 40 ms. As shown in the typical applications
circuit of the MIC2341, an external capacitor from
CFILTER[A/B] to ground is used to set the response
time of the primary overcurrent detectors to t_FLT. At the
time a large inrush current causes the primary OC
detector to sense an overcurrent condition, a
CFILTER[A/B] charge-up sequence is initiated. At the
same time, the MIC2341’s digital filter delay counter is
also initiated and commences a count-up to 40 ms. The
MIC2341’s internal logic circuits have been designed to
trip the circuit breaker after t_FLT or t_DFLT, whichever delay
is smaller. If the overcurrent condition causes the
electronic circuit breaker to latch off and thereby
asserting a FAULT condition, the MIC2341 remains
latched-off awaiting system intervention (by toggling
ON[A/B] and/or AUXEN[A/B] and/or cycling the input
power supplies). Internal to the MIC2341R only, a
second, or auto-retry, delay counter is initiated and
commences a count-up to approximately 820 ms to
allow the external power MOSFET to cool before
attempting another power-up sequence. Figure 6
illustrates the filter responses during an overcurrent fault.
October 2007
21
M9999-102507-A
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