WORST

BETTER

BEST

1W min.

W

VCC

Unused Input

Input

Output

Output

Input

Unused Input

13

SN74LVC1G38

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SCES538G – JANUARY 2004 – REVISED FEBRUARY 2020

Product Folder Links: SN74LVC1G38

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Copyright © 2004–2020, Texas Instruments Incorporated

10 Power Supply Recommendations

The power supply can be any voltage between the MIN and MAX supply voltage rating located in the

Recommended Operating Conditions table.

The VCC pin should have a good bypass capacitor to prevent power disturbance. It is ok to parallel multiple

bypass caps to reject different frequencies of noise. 0.1-µF and 1-µF capacitors are commonly used in parallel.

The bypass capacitor should be installed as close to the power pin as possible for best results.

11 Layout

11.1 Layout Guidelines

When using multiple bit logic devices, inputs should not float. In many cases, functions or parts of functions of

digital logic devices are unused. Some examples are when only two inputs of a triple-input AND gate are used,

or when only 3 of the 4-buffer gates are used. Such input pins should not be left unconnected because the

undefined voltages at the outside connections result in undefined operational states.

Specified in Figure 8 are rules that must be observed under all circumstances. All unused inputs of digital logic

devices must be connected to a high or low bias to prevent them from floating. The logic level that should be

applied to any particular unused input depends on the function of the device. Generally they are tied to GND or

VCC, whichever makes more sense or is more convenient.

Even low data rate digital signals can have high frequency signal components due to fast edge rates. When a

printed-circuit board (PCB) trace turns a corner at a 90° angle, a reflection can occur. A reflection occurs

primarily because of the change of width of the trace. At the apex of the turn, the trace width increases to 1.414

times the width. This increase upsets the transmission-line characteristics, especially the distributed capacitance

and self–inductance of the trace which results in the reflection. Not all PCB traces can be straight and therefore

some traces must turn corners. Figure 9 shows progressively better techniques of rounding corners. Only the last

example (BEST) maintains constant trace width and minimizes reflections.

11.2 Layout Example

Figure 8. Proper multi-gate input termination diagram

Figure 9. Trace Example