Rainbow Electronics MAX5958 User Manual

Page 18

MAX5957/MAX5958

Triple PCI Express, Hot-Plug Controllers

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Debounced Logic Gate

(INPUT_ and OUTPUT_)

INPUT1, INPUT2, and INPUT3 accept inputs from
mechanical switches. The corresponding outputs are
OUTPUT1, OUTPUT2, and OUTPUT3. OUTPUT_ is
debounced for 4ms. When INPUT_ goes from high to
low, OUTPUT_ goes low immediately and stays low for
at least 4ms. After the debounce time, OUTPUT_ fol-
lows INPUT_. If INPUT_ goes from low to high, OUT-
PUT_ goes high immediately and stays high for at least
4ms. After the debounce time, OUTPUT_ follows
INPUT_. Figure 5 shows the timing diagram describing
the INPUT_/OUTPUT_ debounced feature.

Present-Detect and Forced-On Inputs

(PRES-DET_, FON_)

PRES-DET_ input detects the PRSNT#2 pin on a PCIe
connector. When the card is plugged in, PRES-DET_
goes low and allows the turn-on of the outputs of the
respective slot after a 4ms debounced time. When the
card is removed, an internal 50kΩ pullup resistor forces

PRES-DET_ high and the respective slot is shut down
with no delay. PRES-DET_ works in conjunction with
ON_ and AUXON_ and only enables the device when
ON_ and AUXON_ are high.

A logic-low on FON_ forces the respective slot (main
supplies and auxiliary) to turn on regardless of the sta-
tus of the other logic inputs, provided the UVLO thresh-
olds are exceeded on all the inputs.

Active Current Limits

Active current limits are provided for all three outputs of
the three slots (slot A, slot B, and slot C). Connect a
current-sense resistor between 12S_+ and 12S_- to set
the current limit for the 12V outputs. The current limit is
set to 54mV / R

SENSE12

. Connect a current-sense resis-

tor between 3.3S_+ and 3.3S_- to set the current limit
for the 3.3V main outputs to 20mV / R

SENSE3.3

. For the

auxiliary output (3.3VAUXO_) the current limit is fixed at
450mA in the MAX5957 and 700mA in the case of the
MAX5958.

When the voltage across R

SENSE12

or R

SENSE3.3

reaches the current-limit threshold voltage, the
MAX5957/MAX5958 regulate the gate voltage to main-
tain the current-limit threshold voltage across the sense
resistor. If the current limit lasts for t

FAULT

, then an

overcurrent fault occurs. The MAX5957/MAX5958 shut
down both the 12V and 3.3V outputs and assert the
FAULT_ output of the respective slot.

When the auxiliary output reaches the current limit for
longer than t

FAULT

, a fault occurs and the device shuts

down all outputs and asserts FAULT of the respective slot.

UVLO Threshold

The UVLO thresholds prevent the internal auxiliary
MOSFETs and the external main channel MOSFETs
from turning on if V

12VIN

, V

3.3VIN

, and V

3.3VAUXIN

are

not present. Internal comparators monitor the main
supplies and the auxiliary supply and keep the gate-
drive outputs (12GA, 12GB, 12GC, 3.3GA, 3.3GB, and
3.3GC) low until the supplies rise above their UVLO
threshold. The 12V main supply is monitored at 12VIN
and has a UVLO threshold of 10V. The 3.3V main sup-
ply is monitored at 3.3SA+ and has a UVLO threshold
of 2.65V. The auxiliary supply is monitored at
3.3AUXIN and has a 2.65V UVLO threshold. For either
main channels to operate, V3.3AUXIN must be above
its UVLO threshold.

Figure 5. INPUT_ and OUTPUT_ Debounced Feature

INPUT

DEBOUNCED

OUTPUT

DBC