MAX5957 MAXIM [Maxim Integrated Products], MAX5957 Datasheet - Page 20

• Manufacturer Part Number: MAX5957
• Description: Triple PCI Express, Hot-Plug Controllers
• Manufacturer: MAXIM [Maxim Integrated Products]

Triple PCI Express, Hot-Plug Controllers
The gate drive for the external MOSFETs is provided at
12GA, 12GB, 12GC, 3.3GA, 3.3GB, and 3.3GC. 12G_
is the gate drive for the 12V main supply and is boost-
ed to 5.3V above V
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 sink activates to rapidly bring
the load current into the regulation limits.
3.3G_ is the gate drive for the 3.3V main supply’s MOS-
FET 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 150mA to quickly turn off the external
MOSFET. During short-circuit events, an internal
120mA current sink activates to rapidly turn off the
appropriate external MOSFET.
3.3VAUXIN provides power to the auxiliary outputs as
well as the internal logic and references. The drains of
the internal auxiliary MOSFETs connect to 3.3AUXIN
through internal sense resistors and the sources con-
nect to the auxiliary outputs (3.3VAUXO_). Both
MOSFETs have typical on-resistance of 0.2Ω. Each
channel’s internal charge pump boosts the gate-drive
voltage to fully turn on the internal n-channel MOSFETs.
The auxiliary supplies have an internal current limit set
to 450mA (MAX5957), 700mA (MAX5958).
t
fault must remain for the MAX5957/MAX5958 to disable
the main or auxiliary channels of a particular slot.
Program the fault timeout period (t
ing a resistor (R
calculated by the following equation:
20
FAULT
______________________________________________________________________________________
is the time an overcurrent or overtemperature
Applications Information
External MOSFET Gate Drivers
TIM
Auxiliary Supply (3.3VAUXIN)
Setting the Power-On Reset
) from TIM to GND. t
12VIN
by its internal charge pump.
(12G_ and 3.3G_)
FAULT
FAULT
) by connect-
can be
The t
according to the total capacitance load connected to
the 12G_ and 3.3G_ pins. To properly power up the
main supply outputs, the following constraints need to
be taken:
where t
• I
• V
• C
Maximum and minimum values for R
500kΩ, respectively. Leave TIM floating for a default
t
t
outputs of a slot reach their power-good threshold to
when PWRGD_ pulls low. Program the POR timeout
period (t
PORADJ to GND. t
lowing equation:
Maximum and minimum values for R
and 500kΩ, respectively. Leave PORADJ floating for a
default t
order to completely skip the power-on delay time prior
to the PWRGD_ assertion.
Select the external n-channel MOSFET according to the
applications current requirement. Limit the switch
power dissipation by choosing a MOSFET with an
R
full load. High R
there are pulsed loads. High R
an external undervoltage fault at full load. Determine
the MOSFET’s power-rating requirement to accommo-
date a short-circuit condition on the board during start-
up. Table 3 lists the MOSFETs and sense resistor
manufacturers.
FAULT
POR_HL
DS_ON
+ V
CHG
GATE
LOAD
FAULT
3.3VIN
of 10ms.
SU
POR
low enough to have a minimum voltage drop at
is the time from when the gate voltages of all
POR
= 5µA.
= 2 x t
= 4.8V + V
is the total capacitance load at the gate.
t
POR_HL
programmed time duration must be chosen
) by connecting a resistor (R
t
for 3.3G_.
SU
of 150ms. Connect PORADJ to GND in
t
FAULT
≥ (V
FAULT
DS_ON
POR_HL
= (2.5µs / Ω) x R
GATE
= (166ns / Ω) x R
12VIN
Timeout Period (t
and where:
causes larger output ripple if
x C
can be calculated by the fol-
for 12G_ and V
Component Selection
LOAD
DS_ON
) / I
PORADJ
TIM
PORADJ
TIM
CHG
can also trigger
are 500Ω and
PORADJ
GATE
POR_HL
are 500Ω
= 6.8V
) from
)