MIC2591B Dual-Slot PCI Express Hot-Plug Controller - Micrel - #19

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March 2005 19 M9999-033105 MIC2591B Micrel /FORCE_EN (From Slot CNTRL Reg.) /FORCE_ON 3VOUT_UV 12VOUT_UV MAIN_EN 3VAUX_UV AUX_EN /PWRGD_[A/B] Figure 8. /PWRGD[A/B] Logic Diagram specifi cations. These outputs are fed via the VSTBY[A/B] input pins and controlled by the AUXEN[A/B] input pins or via their respective bits in the Control Registers. 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 (or the Control Registers' MAINA and MAINB bits) inputs should be deasserted or the MIC2591B will assert /FAULT[A/B] and /INT (if interrupts are enabled) output signals, if an undervoltage condition on the MAIN supply inputs is detected. Circuit Breaker Function The MIC2591B 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 (ILIM = 50mV/RSENSE) for a duration greater than tFLT, 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 VSENSE to exceed VTHFAST, 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 tFLT 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 CNTRL[A/B] registers (see Register Bits D[1:0]). When the circuit breaker trips, /FAULT[A/B] will be asserted if the outputs were enabled through the Hot-Plug Interface inputs. At the same time, /INT will be asserted (unless interrupts are masked). Note that /INT is deasserted by writing a Logic 1 back into the respective fault bit position(s) in the STAT[A/B] register or the Common Status Register. The response time (tFLT) of the MIC2591B’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 CFILTER[A/B] is given by: CFILTER[A..B](ìF) .. tFLT[A/B](ms) .. IFILTER(ìA) VFILTER(V) .. 103 where tFLT[A/B] is the desired response time and quantities IFILTER and V FLT[FILTER are specifi ed in the MIC2591B’s “Electrical Characteristics” table. For applications that require a more accurate response time for a given CFILTER[A/B] tolerance, the MIC2591B employs a patent-pending technique that improves re- FILTER[sponse time accuracy by more than a factor of two. A 110kÙ, 1% resistor connected from the MIC2591B’s RFILTER[A&B] pin (Pin 20) to GND can be used. In this case, the value for CFILTER[A/B] for a desired response time (tFLT) is given by: CFILTER[A/B](ìF) .. tFLT(ms) RFILTER[A/B](k..) .. SF where tFLT is the desired response time, RFILTER[A&B] is 110kÙ, and “SF” is the CFILTER[A/B] response time “Scaling Factor” FILTER[in the “Electrical Characteristics” table. Thermal Shutdown The internal VAUX[A/B] MOSFETs are protected against damage not only by current limiting, but by dual-mode overtemperature protection as well. Each slot controller on the MIC2591B is thermally isolated from the other. Should an