Maxim Integrated MAX4888A - Manual (Page 11)

MAX4888A/MAX4889A
5.0Gbps PCI Express Passive Switches
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Applications Information
PCIe Switching
The MAX4888A/MAX4889A primary applications are
aimed at reallocating PCIe lanes (see Figure 5). For
example, in graphics applications, several manufacturers
have found that it is possible to improve performance by
a factor of nearly two by splitting a single 16-lane PCIe
bus into two 8-lane buses. Two of the more prominent
examples are SLI™ (Scaled Link Interface) and
CrossFire™. The MAX4889A permits a computer
motherboard to operate properly with a single 16-lane
graphics card, and can later be updated to dual cards.
The same motherboard can be used with dual cards
where the user sets a jumper or a bit through software
to switch between single- or dual-card operation.
Common mode below 1V operation requirement.
Board Layout
High-speed switches require proper layout and design
procedures for optimum performance. Keep design-
controlled impedance PCB traces as short as possible
or follow impedance layouts per the PCIe specification.
Ensure that power-supply bypass capacitors are
placed as close to the device as possible. Multiple
bypass capacitors are recommended. Connect all
grounds and the exposed pad to large ground planes.
Common mode below 1V operation requirement.
ESD Protection
As with all Maxim devices, ESD-protection structures
are incorporated on all pins to protect against electro-
static discharges encountered during handling and
assembly. The COM_+ and COM_- lines have extra
protection against static electricity. Maxim’s engineers
have developed state-of-the-art structures to protect
these pins against ESD of ±6kV without damage. The
ESD structures withstand ±6kV of ESD in all states: nor-
mal operation, state output mode, and powered down.
Human Body Model
The MAX4889A COM_+ and COM_- pins are character-
ized for ±6kV ESD protection using the Human Body
Model (MIL-STD-883, Method 3015). Figure 6 shows
the Human Body Model and Figure 7 shows the current
waveform it generates when discharged into low
impedance. This model consists of a 100pF capacitor
charged to the ESD voltage of interest, which is then
discharged into the device through a resistor.
MAX4888A
MAX4889A
ONE LANE
DATA
DIRECTION
DATA IS ROUTED
TO EITHER
BOARD A OR B
NOTE: ONLY ONE LANE IS SHOWN FOR CLARITY
A B
Figure 5. The MAX4888A/MAX4889A Used as a Single-Lane
Switch
CrossFire is a trademark of ATI Technologies, Inc.
SLI is a trademark of NVIDIA Corporation.
Chip Information
PROCESS: CMOS
CHARGE-CURRENT-
LIMIT RESISTOR
DISCHARGE
RESISTANCE
STORAGE
CAPACITOR Cs
100pF
R C
1M Ω
R D
1500 Ω
HIGH-
VOLTAGE
DC
SOURCE
DEVICE
UNDER
TEST
Figure 6. Human Body ESD Test Model
I P 100%
90%
36.8%
t RL
TIME
t DL
CURRENT WAVEFORM
PEAK-TO-PEAK RINGING
(NOT DRAWN TO SCALE) Ir
10%
0
0
AMPERES
Figure 7. Human Body Model Current Waveform