MIC2590B_08 Datasheet, PDF (12/23 Pages) Micrel Semiconductor – Dual-Slot PCI Hot Plug Controller

Micrel, Inc.
equation above computes the minimum value to
guarantee the device does not enter into current limit. The
slew rate dv/dt is computed by:
dv/dt at load = ^ISLEW
C_SLEW × 10 ⁶
By appropriate selection of the value of C_SLEW, it can be
ensured that the magnitude of the inrush current never
exceeds the current limit for a given load capacitance.
Since 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 = 25µA
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) (12V_IN, 12MV_IN, 5V_IN and
3V_IN) drops below its respective UVLO threshold. The
MIC2590B supplies two 3.3V auxiliary outputs, V_AUX[A/B],
satisfying PCI 2.2 specifications. These outputs are fed
via the V_STBY 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, V_AUX[A/B] still function as long as V_STBY 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 0.2Afor –12V) for a duration greater than t_FLT, the
circuit breaker is tripped and all MAIN supplies (all outputs
except V_AUX[A/B]) are shut off. Should the load current
exceed I_THFAST (+12V and –12V), or cause a MAIN
September 2008
MIC2590B
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).
The V_AUX[A/B] outputs have their own separate circuit
breaker functions. V_AUX [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 V_AUX [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 condition can alternatively be cleared under SMI
control of the ENABLE bits in the C_NTRL[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 C_FILTER[A/B] pins. The equation below can be used to
determine the capacitor value for a given duration of t_FLT:
C_FIL
≅
2.0μ.
×
⎜⎜⎝⎛
t _FLT
1second
⎟⎟⎠⎞
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 V_AUX) will be shut off, and the slot’s /FAULT
output will be asserted. The other slot’s operation will
remain unaffected. However, should the MIC2590B’s
overall die temperature exceed 160°C, both slots (all
outputs, including V_AUXA and V_AUXB) 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.
A/D Converter
The MIC2590B has a 20-channel, 8-bit A/D converter
capable of monitoring the output voltage and current of
each supply. This information is available via the System
Management Interface. The information is particularly
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M9999-091808