MIC2591B
Micrel
Pin Name
IREF
Pin Function
A resistor connected between this pin and GND sets the ADC current
measurement gain for the VAUX[A/B] outputs. This resistor must be
23.2kΩ±1%.
3.3V Standby Input Voltage: Required to support PCI Express VAUX
output(s). These inputs are the primary supply for the MIC2591B and must
be applied at all times for the controller to function properly.
Additionally, the SMBus logic and internal registers run off of VSTBY[A/B]
to ensure that the chip is accessible during standby modes. A UVLO circuit
prevents turn-on of this supply until VSTBY[A/B] rises above its UVLO
threshold. Both pins must be connected together at the MIC2591B controller.
3.3VAUX Outputs to PCI Express Card Slots: These outputs connect
the 3.3AUX pin of the PCI Express connectors to VSTBY[A/B] via internal
400mΩ MOSFETs. These outputs are current limited and protected against
short-circuit faults.
Enable Inputs: Rising-edge triggered. Used to enable or disable the MAINA
and MAINB (+3.3V and +12V) outputs. The outputs can be switched on by
these controls only after the V
STBY
input supply is valid and stabe (i.e., t
POR
elapses - See the Electrical Characteristics Table). Taking ON[A/B] low after
a fault resets the +12V and/or +3.3V fault latches for the affected slot. Tie
these pins to GND if using SMI power control. Also, see pin description for
/FAULTA and /FAULTB.
Enable Inputs: Rising-edge triggered. Used to enable or disable the
VAUX[A/B] outputs. The outputs can be switched on by these controls only
after the V
STBY
input supply is valid and stabe (i.e., t
POR
elapses - See the
Electrical Characteristics Table).Taking AUXEN[A/B] low after a fault resets
the respective slot’s Aux Output Fault Latch. Tie these pins to GND if using
SMI power control. Also, see pin description for /FAULTA and /FAULTB.
Overcurrent Timers: Capacitors connected between these
pins and GND set the duration of t
FLT
for each slot. The overcurrent filter
delay (t
FLT
) is the amount of time for which a slot remains in current limit
before its circuit breaker is tripped.
Power-is-Good Outputs: Open-drain, active-low. Asserted when a slot has
been commanded to turn on and has successfully begun delivering power
to its respective +12V, +3.3V, and VAUX outputs. Each pin requires an
external pull-up resistor to V
STBY
.
Pin Description (continued)
Pin Number
33
11
26
VSTBYA
VSTBYB
15
22
VAUXA
VAUXB
44
43
ONA
ONB
45
42
AUXENA
AUXENB
2
35
CFILTERA
CFILTERB
6
31
/PWRGDA
/PWRGDB
1
36
/FAULTA
/FAULTB
Fault Outputs: Open-drain, active-low. Asserted whenever the
circuit breaker trips due to a fault condition (overcurrent, input undervoltage,
overtemperature). Each pin requires an external pull-up resistor to V
STBY
.
Bringing the slot’s ON[A/B] pin low resets /FAULT[A/B] if /FAULT[A/B]
was asserted in response to a fault condition on one of the slot’s MAIN out-
puts (+12V or +3.3V).
/FAULT[A/B] is reset by bringing the slot’s AUXEN[A/B] pin low if
/FAULT[A/B] was asserted in response to a fault condition on the slot’s VAUX
output. If a fault condition occurred on both the MAIN and VAUX outputs of
the same slot, then both ON[A/B] and AUXEN[A/B] must be brought low to
deassert the /FAULT[A/B] output.
9
28
/FORCE_ONA
/FORCE_ONB
Enable Inputs: Active-low, level-sensitive. Asserting a /FORCE_ON[A/B]
input will turn on all three of the respective slot’s outputs (+12V, +3.3V, and
VAUX), while specifically defeating all protections on those supplies. This
explcitly includes all overcurrent and short circuit protections, and on-chip
thermal protection for the VAUX[A/B] supplies. Additionally included are the
UVLO protections for the +3.3V and +12V main supplies. The
/FORCE_ON[A/B] pins do not disable UVLO protection for the VAUX[A/B]
supplies. These input pins are intended for diagnostic purposes only.
Asserting /FORCE_ON[A/B] will cause the respective slot’s /PWRGD[A/B]
and /FAULT[A/B] pins to enter their open-drain state. Note that the SMBus
register set will continue to reflect the actual state of each slot’s supplies.
There is a pair of register bits, accessible via the SMBus, which can be set
to disable (unconditionally deassert) either or both of the /FORCE_ON[A/B]
pins -- See CNTRL[A/B] Register Bit D[2].
March 2005
5
M9999-033105
MICREL [ MICREL SEMICONDUCTOR ]
