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TTL Load Circuits

Load. A device through which an electric current flows and which changes electrical energy into another form. Power consumed by a device or circuit in performing its function. A load my be represented by a resistor and capacitor.

Resistor Load Circuit

The capacitor used in the load circuit is there to represent the capacitance of the load the IC is driving. However the capacitor may also represent the capacitance of the measuring device used to verify how the IC works with the load [as in the capacitance of the scope probe].

Standatd TTL Load Circuit
TTL Load

There are two other types of TTL loads; a standard resistive load, or an active load. The active load using diodes insures the Device under Test [DUT] switches a more realistic load which mimics another IC load. The passive load shown includes a pull-up resistor only because some ICs need to be pulled high to operate [as in an open-collector].

Standatd TTL Load Circuit
TTL Load
............Standatd TTL Load Circuit with switchable components
Switchable Load

Every input to an IC has some capacitance, so the load should include some value of capacitance. Including an additional value to account for the test jig makes the circuit more realistic.

Standatd TTL Load Circuit Equivelent to 4 Diode Load
TTL Load

Many data sheet do not provide an estimate of input capacitance, which would be the capacitive load seen by the IC driving the chip. However in many cases the data sheet does provide a load capacitance to determine switching time in the electrical characteristics portion of the data sheet.

That value or range of values of load capacitance could be used to determine input capacitance [Ci] when using ICs of the same logic family. Then again, in a few cases an input capacitance is provided, which happens to be much lower than the Load Capacitor [CL] used in the timing measurements. For example a Ci of 2.6pF is provided by the 54ACT11 data sheet, while at the same time a value of 50pF CL is used in the timing characteristics.

The difference in capacitor value is the difference between what is expected through design [Ci], and what is expected in a worst case circuit example [CL]. The input capacitance is designed into the IC at the time of fabrication, with some variation. However the load capacitance seen by the driver can not be determined and will vary depending on the integrated circuit used receiving the signal.

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