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參數(shù)資料
| 型號： | MAX5946LETX+T |
| 廠商： | Maxim Integrated |
| 文件頁數(shù)： | 19/23頁 |
| 文件大?。?/td> | 1102K |
| 描述： | IC CNTRLR PCI EXP DL 36TQFN |
| 產(chǎn)品培訓(xùn)模塊： | Lead (SnPb) Finish for COTS Obsolescence Mitigation Program |
| 標(biāo)準(zhǔn)包裝： | 2,500 |
| 類型： | 熱交換控制器 |
| 應(yīng)用： | 通用型，PCI Express? |
| 內(nèi)部開關(guān)： | 無 |
| 電源電壓： | 3.3V，12V |
| 工作溫度： | -40°C ~ 85°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 36-WFQFN 裸露焊盤 |
| 供應(yīng)商設(shè)備封裝： | 36-TQFN 裸露焊盤（6x6） |
| 包裝： | 帶卷 (TR) |
External MOSFET Gate Drivers
(12GA, 12GB, 3.3GA, and 3.3GB)
The gate drive for the external MOSFETs is provided at
12GA, 12GB, 3.3GA, and 3.3GB. 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 5礎(chǔ) into the external gate capacitance to
control the turn-on time of the external MOSFET. During
turn-off, 12G_ sinks 150礎(chǔ) from the external gate
capacitance to quickly turn off the external MOSFET.
During short-circuit events, an internal 100mA current
activates to rapidly bring the load current into the regu-
lation limits.
3.3G_ is the gate drive for the 3.3V main supplys
MOSFET and is driven to 5.5V above the 3.3V main sup-
ply. The power for 3.3G_ is supplied from 12VIN and
has no internal charge pump. During turn-on, 3.3G_
sources 5礎(chǔ) into the external gate capacitance to con-
trol the turn-on time of the external MOSFET. During
turn-off, 3.3G_ sinks 150礎(chǔ) to quickly turn off the exter-
nal MOSFET. During short-circuit events, an internal
150mA current activates to rapidly turn off the appropri-
ate 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 3.3VAUXIN
through internal sense resistors and the sources con-
nect to the auxiliary outputs (3.3VAUXOA and
3.3VAUXOB). Both MOSFETs have typical on-resistance
of 0.3&. An 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 470mA.
Applications Information
Setting the Fault Timeout Period (t
FAULT
)
t
FAULT
is the time an overcurrent or overtemperature
fault must remain for the MAX5946 to disable the main
or auxiliary channels of a particular slot. Program the
fault timeout period (t
FAULT
) by connecting a resistor
(R
TIM
) from TIM to GND. t
FAULT
can be calculated by
the following equation:
t
FAULT
= 166ns / & 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:
where t
SU
= 2 x t
FAULT
and where
1) I
CHG
= 5礎(chǔ).
2) V
GATE
= 18.4V for 12G_ and V
GATE
= 9.4V for
3.3G_.
3) C
LOAD
is the total capacitance load at the gate.
Maximum and minimum values for R
TIM
are 500& and
500k&, respectively. Leave TIM floating for a default
t
FAULT
of 11ms.
Setting the Power-On-Reset
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 power-on-reset
timeout period (t
POR
) by connecting a resistor (R
PO-
RADJ
) from PORADJ to GND. t
POR_HL
can be calculat-
ed by the following equation:
t
POR_HL
= 2.5祍 / & x R
PORADJ
Maximum and minimum values for R
PORADJ
are 500&
and 500k&, respectively. Leave PORADJ floating for a
default t
POR
of 160ms.
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 MOSFETs power rating requirement to accommo-
date a short-circuit condition on the board during start-
up. Table 3 lists MOSFETs and sense resistor
manufacturers.
Additional External Gate
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.
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 MAX5946 can withstand
t
V
C
I
SU
GATE
LOAD
e
?/DIV>
Dual PCI Express, Hot-Plug
Controller
______________________________________________________________________________________ 19
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