GS2988 Datasheet, PDF (11/17 Pages) Gennum Corporation – Dual-Slew-Rate, Cable Driver with 3Gb/s Capability

The RSET resistor is part of an internal DC feedback loop in the GS2988. The resistor
should be placed as close as possible to the R_SET pin, and connected directly to the VCC
plane (traces/wires may cause instability). In addition, the PCB capacitance should be
minimized at this node by removing the PCB groundplane beneath the RSET resistor and
the RSET pin.
NOTE: Care should be taken when considering layout of the RSET resistor. Please refer
to Section 5.1 for more details.
2.0
1.75
1.5
1.25
1.0
0.75
0.5
0.25
0.250
0.500
0.750
1.0
1.25
1.5
RSET (kΩ)
Figure 4-1: V_OUT vs. RSET
In order to determine the best starting value for Rset, the following formula should be
used:
Rset = 8*(Rtrm/VoutppSE)
Where VoutppSE is in Volts, and both resistances are in Ω.
Rtrm is the value of the termination resistors, which should be equal to the
characteristic impedance of the cable, and is typically 75Ω.
The cable must be short (≤1m), and terminated at both ends for the formula to be valid.
Example: For a 75Ω cable, Rtrm = 75Ω (at both ends), VoutppSE = 800mV
Rset = 8*(75/0.8) = 750Ω
GS2988 Dual-Slew-Rate, Cable Driver with 3Gb/s
Capability
Data Sheet
52131 - 4
December 2010
11 of 17