Rainbow Electronics MAX5958 - Manual (Page 20)

MAX5957/MAX5958
Triple PCI Express, Hot-Plug Controllers
20 ______________________________________________________________________________________
External MOSFET Gate Drivers
(12G_ and 3.3G_)
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
12VIN by its 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 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.
Auxiliary Supply (3.3VAUXIN)
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).
Applications Information
Setting the Power-On Reset
t FAULT is the time an overcurrent or overtemperature
fault must remain for the MAX5957/MAX5958 to disable
the main or auxiliary channels of a particular slot.
Program the fault timeout period (t
FAULT
) by connect-
ing a resistor (R
TIM
) from TIM to GND. t
FAULT
can be
calculated by the following equation:
t FAULT = (166ns / Ω) x R TIM
The t FAULT programmed time duration must be chosen
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:
t
SU ≥ (V GATE
x C
LOAD
) / I
CHG
where t SU = 2 x t FAULT and where:
• I
CHG
= 5µA.
• V GATE = 4.8V + V 12VIN for 12G_ and V GATE = 6.8V
+ V 3.3VIN for 3.3G_.
• C LOAD is the total capacitance load at the gate.
Maximum and minimum values for R TIM are 500Ω and
500kΩ, respectively. Leave TIM floating for a default
t
FAULT of 10ms.
Timeout Period (t POR_HL )
t POR_HL is the time from when the gate voltages of all
outputs of a slot reach their power-good threshold to
when PWRGD_ pulls low. Program the POR timeout
period (t
POR
) by connecting a resistor (R
PORADJ
) from
PORADJ to GND. t POR_HL can be calculated by the fol-
lowing equation:
t
POR_HL
= (2.5µs / Ω) x R
PORADJ
Maximum and minimum values for R PORADJ are 500Ω
and 500kΩ, respectively. Leave PORADJ floating for a
default t
POR of 150ms. Connect PORADJ to GND in
order to completely skip the power-on delay time prior
to the PWRGD_ assertion.
Component Selection
Select the external n-channel MOSFET according to the
applications current requirement. Limit the switch
power dissipation by choosing a MOSFET with an
R
DS_ON
low enough to have a minimum voltage drop at
full load. High R DS_ON causes larger output ripple if
there are pulsed loads. High R DS_ON can also trigger
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.