MIC2590B Datasheet, PDF (12/24 Pages) Micrel Semiconductor – Dual-Slot PCI Hot Plug Controller

MIC2590B
one capacitor fixes the slew rate for both +12V and –12V, the
capacitor value should be chosen to provide the slower slew
rate of the two. Table 2 depicts the output slew rate for various
values of C_SLEW.
C_GATE
0.001µF
0.01µF
0.1µF
1µF
I_SLEW = 22µA
dv/dt (load)
22000V/s
2200V/s
220V/s
22V/s
Table 2. ±12V Output Slew Rate Selection
Power Down Cycle
When a slot is turned off, internal switches are connected to
each of the outputs to discharge the PCI board's bypass
capacitors to ground.
Standby Mode
Standby mode is entered when any one (or more) enabled
MAIN supply input(s) (12VIN, 12MVIN, 5VIN and 3VIN) drops
below its respective UVLO threshold. The MIC2590B sup-
plies two 3.3V auxiliary outputs, VAUX[A/B], satisfying PCI
2.2 specifications. These outputs are fed via the VSTBY
input, and controlled by the AUXEN[A/B] inputs or via their
SMI bus Control Registers. These outputs are independent of
the MAIN outputs: should one or more of the MAIN supply
inputs move below its UVLO thresholds, VAUX[A/B] still
function as long as VSTBY is present. Prior to entering
standby mode, ONA and ONB (or the MAINA and MAINB bits
in the Control Registers) inputs should be de-asserted. If this
is not done, the MIC2590B will assert /FAULT, and also /INT
if interrupts are enabled, when the MIC2590B detects an
undervoltage condition on a supply input.
Circuit Breaker Functions
The MIC2590B 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
(50mV/R_SENSE for 3.3V and 5V, 1.0A for +12V, and/or 0.2A
for –12V) for a duration greater than t_FLT, the circuit breaker
is tripped and all MAIN supplies (all outputs except
VAUX[A/B]) are shut off. Should the load current exceed
I_THFAST (+12V and –12V), or cause a MAIN output’s V_SENSE
to exceed V_THFAST (+3.3V and +5V), the outputs are 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).
Micrel
The VAUX[A/B] outputs have their own separate circuit
breaker functions. VAUX[A/B] do not incorporate a fast-trip
threshold, but instead regulate the output current into a fault
to avoid exceeding their operating current limit. The circuit
breaker will trip due to overcurrents 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 via the ON inputs (if the fault occurred on one of the
MAIN outputs), via the AUXEN inputs (if the fault occurred on
the AUX outputs), or by cycling both ON and AUXEN (if faults
occurred on both the MAIN and AUX outputs). A fault condi-
tion can alternatively be cleared under SMI control of the
ENABLE bits in the CNTRL[A/B] registers. When the circuit
breaker trips, /FAULT[A/B] will be asserted if the outputs were
enabled through the hot plug Interface (non-SMI mode)
inputs. At the same time, /INT will be asserted (unless
interrupts are masked). Note that /INT is de-asserted by
writing a logic 1 back into the respective fault bit position(s) in
the STAT[A/B] register or the Common Status Register.
t_FLT is set by external capacitors, C_FIL[A/B], connected to the
CFILTER[A/B] pins. The equation below can be used to
determine the capacitor value for a given duration of t_FLT:
C_FIL
≅
2.0µF
×


1
t_FLT
second


Thermal Shutdown
The internal +12V, –12V and VAUX MOSFETs are protected
against damage not only by current limiting, but by dual-mode
over-temperature protection as well. Each slot controller on
the MIC2590B is thermally isolated from the other. Should an
overcurrent condition raise the junction temperature of one
slot’s controller and internal pass elements to 140°C, all of the
outputs for that slot (including VAUX) will be shut off, and the
slot’s /FAULT output will be asserted. The other slot’s opera-
tion will remain unaffected. However, should the MIC2590B’s
overall die temperature exceed 160°C, both slots (all outputs,
including VAUXA and VAUXB) will be shut off, whether or not
a current-limit condition exists. A 160°C overtemperature
condition additionally sets the overtemperature bit (OT_INT)
in the Common Status Register.
MIC2590B
12
August 2002