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
(
PPRREESS--DDEETT,, FFOONN
)
PRES-DET input detects the PRSNT#2 pin on a PCI
Express connector. When the card is plugged in,
PRES-DET goes low and allows the turn-on of the output after a 4ms debounced time. When the card is
removed, an internal 50kΩ pullup forces PRES-DET
high and the PCI Express 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 PCI Express slot (main
supplies and auxiliary) to turn on regardless of the status of the other logic inputs provided the UVLO thresholds are exceeded on all of the inputs.
Active Current Limits
Active current limits are provided for all three outputs.
Connect a current-sense resistor between 12S+ and
12S- to set the current limit for the 12V output. The current limit is set to 54mV/R
SENSE12
. Connect a
current-sense resistor between 3.3S+ and 3.3S- to
set the current limit for the 3.3V main output to
20mV/R
SENSE3.3.
For the auxiliary output (3.3V
AUXO
)
the current limit is fixed at 470mA.
When the voltage across R
SENSE12
or R
SENSE3.3
reaches the current-limit threshold voltage, the
MAX5954 regulates the gate voltage to maintain the
current-limit threshold voltage across the sense resistor. If the current limit lasts for t
FAULT
, then an overcur-
rent fault occurs. The MAX5954 shuts down both the
12V and 3.3V outputs and asserts the FAULT output.
When the auxiliary output reaches the current limit
(470mA) for longer than t
FAULT
, a fault occurs and the
device shuts down all outputs and asserts FAULT.
Undervoltage-Lockout Threshold
The UVLO thresholds prevent the internal auxiliary
MOSFET and the external main channel MOSFETs (Q1
and Q2 in the Typical Application Circuit) 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 (12G
and 3.3G) 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 supply is monitored at 3.3S+ and has a UVLO threshold of
2.65V. The auxiliary supply is monitored at 3.3VAUXIN
and has a 2.65V UVLO threshold. For the main outputs to
operate, V
3.3VAUXIN
must be above its UVLO threshold.
External MOSFET Gate Driver
(12G and 3.3G)
The gate drive for the external MOSFETs is provided at
12G and 3.3G. 12G is the gate drive for the 12V main
supply and is boosted to 5.3V above V
12VIN
by an
internal charge pump. During turn-on, 12G sources
5µA into the external gate capacitance to control the
turn-on time of the external MOSFET. During turn-off,
12G sinks 150µA from the external gate capacitance to
quickly turn off the external MOSFET. During short-circuit events, an internal 120mA current activates to
rapidly bring the load current into the regulation limits.
3.3G is the gate drive for the 3.3V main supply’s
MOSFET and is driven to 5.5V above the 3.3V main supply. The power for 3.3G is supplied from 12VIN and has
no internal charge pump. During turn-on, 3.3G sources
5µA into the external gate capacitance to control the
turn-on time of the external MOSFET. During turn-off,
3.3G sinks 150µA to quickly turn off the external
MOSFET. During short-circuit events, an internal 150mA
current activates to rapidly turn off the external
MOSFET.
Auxiliary Supply (3.3VAUXIN)
3.3VAUXIN provides power to the auxiliary output as
well as the internal logic and references. The drain of the
internal auxiliary MOSFET connects to 3.3VAUXIN
through internal sense resistor and the source connects
to the auxiliary output 3.3VAUXO. The MOSFET’s typical
on-resistance is 0.3Ω. An internal charge pump boosts
the gate-drive voltage to fully turn on the internal n-channel MOSFET. The auxiliary supply has an internal current
limit set to 470mA.
MAX5954
Single PCI Express, Hot-Plug
Controller
______________________________________________________________________________________ 17