Overview

The TPUL family of monostable multivibrators is designed to be fully compatible with existing legacy monostable multivibrator offerings across multiple logic families from Texas Instruments. These new devices offer improved power consumption and accuracy while maintaining the same functionality, packages, and pinouts as the legacy devices.

Due to the unique output pulse timing non-linearity of each legacy device, which is influenced by process, voltage, temperature, and RC timing component variations, it is not possible to create a new device using a new technology node that perfectly matches any legacy device. Instead, the TPUL pulse generators target timing similarity with the majority of legacy devices. In most cases, replacing the timing resistor allows matching of the legacy device timing.

Device Comparison

The TPUL logic family is based on the latest technology from Texas Instruments, providing improvements to many existing families. The devices were designed to match or exceed performance for existing devices to provide the best transition from legacy devices to new devices in the majority of use cases.

Table 1 provides a quick reference table for replacing legacy devices with new TPUL function devices. Table 1 includes the expected pulse width error for a typical application if the external timing components are not modified, as well as the replacement resistor value to eliminate the error. These values are based on specific conditions: V_CC = 5V, R_ext = 10kΩ, and C_ext = 0.1µF. The following sections provide more detailed explanations for designing the new TPUL devices into an existing system.

Table 2 provides a quick reference for comparing different logic family performance characteristics.

Modifying an Existing Design for Drop-In Replacement

The vast majority of designs only require a resistor value change to use the new TPUL family of logic. Some designs do not require any external component changes. The equation to change only a resistor is shown in Equation 1.

The K variable in Equation 1 comes from the monostable multivibrator pulse width equation, Equation 2. The K factor value can be found in the data sheet for each individual device and is dependent on multiple variables.

Steps to transition from legacy to TPUL:

1. Identify the operational pulse width of the legacy device.
   1. Best practice is to measure in the existing system.
   2. Alternatively, refer to the legacy data sheet to determine the expected pulse width based on the timing component values.
2. Identify the existing timing component values (R_ext, C_ext).
3. Find the new expected pulse width using the original timing components and new TPUL device.
   1. The easiest method is to use the provided Excel-based calculator to get the expected pulse width. Example product folder: TPUL2G123
   2. The data sheet Application and Implementation section provides a method for calculating the expected pulse with without the provided calculator.
4. If the expected pulse width from (3) is within the operational requirements of the system, a direct replacement is recommended. Stop here.
5. Adjust timing components to match the desired pulse width.
   • The easiest method is to use the provided Excel-based calculator to get the required resistor and capacitor values to replace the existing components.
   • The alternate method is to adjust only the resistor using Equation 1.

Using the Provided Excel-Based Calculator

The Excel-based calculator is linked in each product folder under the Design & development section. This calculator provides three input methods.

The first method, shown in Figure 1, takes the supply voltage, timing resistor, and timing capacitor values and tolerances as inputs, and outputs the expected K Factor, nominal output pulse width, total pulse width range, and total error percentage. These values provide the system designer with important limitations of the accuracy of the TPUL device so that the designer can make informed decisions about the timing component selection. This view is particularly useful when timing components have been selected to provide a detailed look at the expected behavior in a system, especially when manufacturing at volume where tolerances play an important role in system requirements.

The second method, shown in Figure 2, takes the supply voltage and desired output pulse width as inputs, and outputs the best options for the timing component values. If the R_ext entry is blank, this means that the calculated value was outside the recommended range for the device. In particular, this view is helpful for selecting a resistor value to tune the timing for a particular application.

The third method, shown in Figure 3, takes the timing resistor and capacitor values as inputs, and outputs the expected K Factor and pulse widths for each common voltage node between 1.5V and 5V. This view is particularly useful in observing variations in pulse width due to supply changes.

These three methods are provided in the same Excel spreadsheet to give maximum flexibility to the system designer. In many cases, starting with method 2 allows a quick selection of the appropriate timing component values. Then, method 1 can be used to input industry standard component values and tolerances to get the best estimate of performance in the final system design.