GTL2002 查看數據表（PDF） - Philips Electronics
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GTL2002 Datasheet PDF : 24 Pages
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only pull up to 1.5 V when biased with the 3 V supply. If the 1.5 V level is not high enough a high value pull-up
resistor could be used to the 1.8 V supply to make certain that it gets to 1.8 V.
Connecting the device to the right voltages/components
1. Question: In the recommended operating conditions table of the data sheet, the typical DREF is 3.3 V. Should
DREF be equal to or greater than SREF on the reference transistor?
Answer: The SREF and DREF can be equal, however, for best results the DREF should be connected through a 200
kΩ resistor to a power supply level that is at least 1.5 V above SREF. If the SREF is less than 1 V below the DREF,
the maximum pass voltage may be limited to a voltage below SREF by the threshold of the transistor (e.g. DREF =
2.5 V and SREF = 1.8 V). When the SREF and DREF are equal, the threshold of the transistor determines the
maximum pass voltage. Although the threshold variation within a package is small, the part-to-part variation can be
larger. If the part-to-part threshold variation is acceptable, then the gate (GREF) could be tied directly to the power
supply and the reference transistor used as one more pass transistor.
2. Question: You said "You can use SREF to clamp the Sn port at 1.5 V so you can feed in the 3.3 V signal on the Dn
port side and clamp it at 1.5 V without having to use any pull up resistors on the lower voltage side, but going from
1.5 V to the 3.3 V you have to use pull up resistors on the 3.3 V Dn port side". My understanding is that going from
a higher voltage (3.3 V PCI) to a lower voltage (1.5V AGP) doesn't need to use any pull-up resistors unless the logic
used is based in pull-up resistors (like GTL). So in our case (push-pull logic) we don't need pull-ups on the AGP
side. Am I right?
Answer: If it is a uni-directional signal going from the 3.3 V PCI side (push-pull) to the 1.5 V AGP side (which is
push pull logic so it doesn't need pull up resistors but operates in input mode only) then you don't need pull up
resistors on the 3.3 V or 1.5V side except for the DREF pin which sets the reference voltage.
3. Question: We use only 3 bits on the GTL2010. Seven of the bits are No Connect (Dn and Sn is open). Is this OK or
should they be tied to GREF?
Answer: There are several acceptable ways of dealing with unused data paths and treating them as no connects is
probably the easiest. It is recommended that pads be included on the circuit board for the unused pins so that after
soldering the part will be firmly attached. Alternatively, the unused Dn and Sn pins can be connected together and
tied to GND. It is not recommended connecting unused paths to GREF.
4. Question: I use three GTL2000 devices for translating 66 signals from 3.3 V to 1.8 V. Can the 200 kΩ resistor be
shared by all three GTL2000 GREF and DREF pins or do I need three 200 kΩ resistors? What is the recommended
value for the capacitor next to the 200 kΩ resistor?
Answer: It would be best to use 3 different resistors, because different packages may not have identical
characteristics and separate resistors / biasing allows the circuit to compensate for these differences. Sharing one
resistor would not work well. If the characteristics matched perfectly they would be biased with an effective 600 kΩ
resistor and if not perfectly matched one would be biased correctly and the other two would have too low of a gate
voltage.
For the capacitor, we usually recommend a 0.1 µF value. Note that the capacitor stabilizes the gate node but also
slows its power up: with a 200 kΩ resistor, it will take on the order of 100 ms to get to the correct clamp level with a
0.1 µF capacitor. Since the gate node has over 100 pF capacitance the capacitor needs to be in at least the nF range
to do anything. If you do not have any speed constraint at power up, then 0.1 µF would be safe enough.
5. Question: Is it possible to use a GTL2010 to perform bi-directional voltage translation between a 1.8V device and a
3.0V device B using the 3.0V device B supply for the 200 kΩ pull-up at both the GREF and DREF pins of
GTL2010? As I understand from the GTL2010 datasheet, the pull-up supply has to be at least 1.5V more than the
SREF voltage (1.8V in this case)). How should I connect the different pins of the GTL2010?
Answer: Tie the 1.8 V signal to S side. And connect the 3.0 V signal to D side.
The pull-up resistor is dependant on the driving current and signal level.
For a 3.0 V signal and a 15 mA driving current, the pull-up resistor value is 177 Ω (normal) to 195 Ω (max) as
indicated in page 6 of the GTL2010 data sheet.
6. Question: I am experiencing a problem with the GTL2000 when I reference the SREF input to the voltage supply of
the CPU (1.1 V). The problem is sending a signal from the low I/O voltage device to the high I/O device. With input
16
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