NVT2002 PHILIPS [NXP Semiconductors], NVT2002 Datasheet - Page 8

• Manufacturer Part Number: NVT2002
• Description: Bidirectional voltage level translator for open-drain and push-pull applications
• Manufacturer: PHILIPS [NXP Semiconductors]

NXP Semiconductors
NVT2001_NVT2002
Product data sheet
7.3.1.1 Example maximum frequency
7.3.1 Maximum frequency calculation
The maximum frequency is totally dependent upon the specifics of the application and the
device can operate > 33 MHz. Basically, the NVT20xx behaves like a wire with the
additional characteristics of transistor device physics and should be capable of performing
at higher frequencies if used correctly.
Here are some guidelines to follow that will help maximize the performance of the device:
In a 3.3 V to 1.8 V direction level shift, if the 3.3 V side is being driven by a totem pole type
driver no pull-up resistor is needed on the 3.3 V side. The capacitance and line length of
concern is on the 1.8 V side since it is driven through the ON resistance of the NVT20xx.
If the line length on the 1.8 V side is long enough there can be a reflection at the
chip/terminating end of the wire when the transition time is shorter than the time of flight of
the wire because the NVT20xx looks like a high-impedance compared to the wire. If the
wire is not too long and the lumped capacitance is not excessive the signal will only be
slightly degraded by the series resistance added by passing through the NVT20xx. If the
lumped capacitance is large the rise time will deteriorate, the fall time is much less
affected and if the rise time is slowed down too much the duty cycle of the clock will be
degraded and at some point the clock will no longer be useful. So the principle design
consideration is to minimize the wire length and the capacitance on the 1.8 V side for the
clock path. A pull-up resistor on the 1.8 V side can also be used to trade a slower fall time
for a faster rise time and can also reduce the overshoot in some cases.
Question — We need to make the PLL area of a new line card backwards compatible and
need to need to convert one GTL signal to LVTTL, invert it, and convert it back to GTL.
The signal we want to convert is random in nature but will mostly be around 19 MHz with
very long periods of inactivity where either a HIGH or LOW state will be maintained. The
traces are 1 or 2 inches long with trace capacitance of about 2 pF per inch.
Answer — The frequency of the NVT20xx is limited by the capacitance of the part, the
capacitance of the traces and the pull-up resistors used. The limiting case is probably the
LOW-to-HIGH transition in the GTL to LVTTL direction, and there the use of the lowest
acceptable resistor values will minimize the rise time delay. Assuming 50 pF capacitance
and 220  resistance, the RC time constant is 11 ns (50 pF  220 ). With 19 MHz
corresponding to 50 ns period the NVT20xx will support this application.
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Keep trace length to a minimum by placing the NVT20xx close to the processor.
The trace length should have a time of flight less than half of the transition time to
reduce ringing and reflections.
The faster the edge of the signal, the higher the chance for ringing.
The higher the drive strength (up to 15 mA), the higher the frequency the device can
use.
All information provided in this document is subject to legal disclaimers.
Rev. 2 — 26 October 2011
NVT2001; NVT2002
Bidirectional voltage level translator
© NXP B.V. 2011. All rights reserved.
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