NVT2001_NVT2002

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Product data sheet

Rev. 2 — 26 October 2011

8 of 23

NXP Semiconductors

NVT2001; NVT2002

Bidirectional voltage level translator

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:

reduce ringing and reflections.

use.

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.

7.3.1.1

Example maximum frequency

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.