Rainbow Electronics MAX5958 - Manual (Page 21)

MAX5957/MAX5958
Triple PCI Express, Hot-Plug Controllers
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Additional External Gate Capacitance
External capacitance can be added from the gate of
the external MOSFETs to GND to slow down the dV/dt
of the 12V and 3.3V outputs. The maximum gate
capacitance load at 12G_ and 3.3G_ must be consis-
tent with the conditions described in the
Setting the
Power-On Reset section.
Maximum Load Capacitance
Large capacitive loads at the 12V output, the 3.3V out-
put, and the 3.3V auxiliary output can cause a problem
when inserting discharged PCI cards into live back-
planes. A fault occurs if the time needed to charge the
capacitance of the board is greater than the typical
startup time (2 x t
FAULT ). The MAX5957/MAX5958 with-
stand large capacitive loads due to their adjustable
startup times and adjustable current-limit thresholds.
Calculate the maximum load capacitance as follows:
C LOAD < (t SU x I LIM ) / V OUT
V OUT is either the 3.3V output, the 12V output, or the
3.3V auxiliary output for slot A, slot B, or slot C.
Input Transients
The 12V input (12VIN), the 3.3V input (3.3SA+), and the
3.3V auxiliary (3.3AUXIN) must be above their UVLO
thresholds before startup can occur. Input transients
can cause the input voltage to sag below the UVLO
threshold. The MAX5957/MAX5958 reject transients on
the input supplies that are shorter than 4µs (typ).
Because some load fault conditions can cause voltage
transients to propagate to the supply inputs with dura-
tion of greater than 4µs, it is recommended that a small
Schottky diode be placed in series with the 12VIN pin
connection, upstream of the 1µF bypass capacitor. This
provides a hold-up supply that will prevent the 12VIN
input from dropping below V
12UVLO during severe tran-
sients. See the Typical Application Circuit .
Table 3. Component Manufacturers
COMPONENT MANUFACTURER PHONE WEBSITE
Vishay-Date 402-564-3131 www.vishay.com
Sense Resistor
IRC 704-264-8861 www.irctt.com
Fairchild 888-522-5372 www.fairchildsemi.com
International Rectifier 310-322-3331 www.irf.com
Motorola 602-244-3576 www.mot-sps.com/ppd
MOSFETs
Vishay-Siliconix — www.vishay.com