MIC2593_08 Datasheet, PDF (12/26 Pages) Micrel Semiconductor – Dual-Slot PCI Hot Plug Controller

Micrel, Inc.
MIC2593
Power-Up Cycle
When a slot is off, the 5VGATE and 3VGATE pins are
held low with an internal pull-down current source. When
a slot’s MAIN outputs are enabled by applying a rising-
edge signal at the ON[A/B] control input and all input
voltages are above their respective undervoltage lockout
thresholds, all four main supplies will then execute a
controlled turn on. The 5VGATE and 3VGATE pins are
each connected to a constant current source of 25µA,
nominal. Both the 5V and 3.3V outputs act as source
followers, where:
[ ] V_SOURCE = V_GATE − V_TH(ON)
until the associated output is equal to its input. The
voltages on the gates of the external MOSFETs for the 5V
and 3.3V MAIN supplies will continue to rise to
approximately 11.5V, ensuring minimum R_DS(ON) of the
MOSFET. Note that a delay exists between the ON
command to a slot and the appearance of voltage at the
slot’s 3.3V or 5V MAIN output. This delay is the time
required to charge the 3V or 5V GATE output up to the
threshold voltage of the external MOSFET (typically about
4V). For the 5V and 3.3V MAIN supplies, the source
(output) side of the external MOSFET will reach the drain
(input) voltage in a time given by:
( ) _{tDELAY =}
C_GATE × V_DRAIN
I_GATE(SOURCE)
Table 1 provides a reference list of the expected GATE
output slew rate for the 3.3V and 5V supplies using
several (decade-scale) standard capacitors.
C_GATE
0.001µF
0.01µF
0.1µF
1µF
I_SLEW = 25µA
dv/dt (load)
25000V/s
2500V/s
250V/s
25V/s
Table 1. 3.3V/5V Output Slew Rate Selection
For the +12V and –12V supplies, the output slew rate is
controlled by capacitors connected to the 12VSLEWA and
12VSLEWB pins. To determine the minimum value of the
slew rate capacitor, (C_SLEW), and to ensure the device
does not enter into current limit during start-up, the
following equation is used:
C_SLEW (min)
=
I_SLEW
I_{LIM[12V/12MV]}
× C_LOAD
where C_LOAD is the load capacitance connected to the
+12V and –12V outputs, I_{LIM[12V/12MV]} are the current limit
slow-trip thresholds and I_SLEW is the slew rate charge
current found in the “Electrical Characteristics” table. The
slew rate dv/dt is computed by:
September 2008
dv/dt (at load) = ^ISLEW
C_SLEW × 10 ⁶
By appropriately selecting the value of C_SLEW, the
magnitude of the inrush current will not exceed 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 ±12V output slew rate for
various values of C_SLEW.
C_GATE
0.001µF
0.01µF
0.1µF
1µF
I_SLEW = 22µA
dv/dt (load)
22V/ms
2.2V/ms
0.22V/ms
0.022V/ms
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 (12V_IN, 12MV_IN, 5V_IN and/or
3V_IN) drops below its respective UVLO threshold. The
MIC2593 supplies two 3.3V auxiliary outputs, VAUX[A/B],
satisfying PCI 2.x specifications. These outputs are fed
via the VSTBY[A/B] 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] will still function as
long as VSTBY[A/B] 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 MIC2593 will assert /FAULT and also
/INT if interrupts are enabled, when the MIC2593 detects
an undervoltage condition on a supply input.
Circuit Breaker Functions
The MIC2593 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 the
overcurrent timer, 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
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