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CD74HC125-Q1

SCLS579B – APRIL 2004 – REVISED APRIL 2020

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8 Detailed Description

8.1 Overview

This device contains four independent buffers with 3-state outputs. Each gate performs the Boolean function Y =

A in positive logic.

8.2 Functional Block Diagram

8.3 Feature Description

8.3.1 Balanced CMOS 3-State Outputs

A balanced output allows the device to sink and source similar currents. The drive capability of this device may

create fast edges into light loads so routing and load conditions should be considered to prevent ringing.

Additionally, the outputs of this device are capable of driving larger currents than the device can sustain without

being damaged. It is important for the output power of the device to be limited to avoid damage due to over-

current. The electrical and thermal limits defined in the Absolute Maximum Ratings must be followed at all times.

The CD74HC125-Q1 can drive a load with a total capacitance less than or equal to 50 pF connected to a high-

impedance CMOS input while still meeting all of the datasheet specifications. Larger capacitive loads can be

applied, however it is not recommended to exceed 70 pF. If larger capacitive loads are required, it is

recommended to add a series resistor between the output and the capacitor to limit output current to the values

given in the Absolute Maximum Ratings.

3-State outputs can be placed into a high-impedance state. In this state, the output will neither source nor sink

current, and leakage current is defined by the IOZ specification in the Electrical Characteristics. A pull-up or pull-

down resistor can be used to ensure that the output remains HIGH or LOW, respectively, during the high-

impedance state.

8.3.2 Standard CMOS Inputs

Standard CMOS inputs are high impedance and are typically modeled as a resistor from the input to ground in

parallel with the input capacitance given in the Electrical Characteristics. The worst case resistance is calculated

with the maximum input voltage, given in the Absolute Maximum Ratings, and the maximum input leakage

current, given in the Electrical Characteristics, using ohm's law (R = V ÷ I).

Signals applied to the inputs need to have fast edge rates, as defined by Δt/Δv in the Recommended Operating

Conditions to avoid excessive current consumption and oscillations. If a slow or noisy input signal is required, a

device with a Schmitt-trigger input should be used to condition the input signal prior to the standard CMOS input.