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Technical Article

Generating Pulse-width Modulation Signals with SmartDACs

Katlynne Jones(Gavin Bakshi co-authored this technical article.)
The technical article, “What is a smart DAC?” explained what a smart digital-to-analog converter (DAC) is andhow it can bring value to many applications. Smart DACs can improve design efficiencies by reducing theburden of software development, and they provide many useful functions that would otherwise require external
components that may have lower performance or similar performance with an increased cost. The integratedfeatures of smart DACs provide high precision at a lower system cost.
In this article, we will discuss how a smart DAC can produce pulse-width modulation (PWM) signals
controlled directly from an analog signal through the feedback pin of the device. The DAC53701 used in thisexample features nonvolatile memory (NVM) that enables the storage of all register configurations after initialprogramming, even after a power cycle.
For long-distance control and fault management in automotive lighting and industrial applications, smart DACs
can be used as PWM generators to provide configurable analog-to-PWM translation, duty-cycle translationand general-purpose input (GPI)-to-PWM conversion at a lower system cost and with higher performance thancompeting solutions. Let’s take a look at each of these in detail, starting with simple PWM generation.
PWM Function Generation
Unlike microcontrollers (MCUs) or timer-based solutions, smart DACs have a continuous waveform generation(CWG) mode that enables simple PWM generation. The function generator has the ability to output triangle
waves, sawtooth waves with rising or falling slopes, and square waves. You can use the configuration registers
to customize the slew rate and the high and low voltage levels of the waves. The function generator can create a
50% duty-cycle square wave with a limited number of adjustable frequencies.
Analog-to-PWM Translation
For applications such as temperature to PWM, smart DACs create an analog-to-PWM output by feeding asawtooth or triangle wave into one of the inputs of the internal output buffer and the threshold voltage into theother input. The DAC53701 feedback pin exposes the feedback path between the output and inverting inputof the internal output buffer, enabling the buffer’s use as a comparator. In the example shown in Figure 1, an
analog input voltage (VFB) created by a resistor divider is applied to the feedback pin. Comparing a triangular
waveform generated by the DAC53701 against VFB will produce a square wave. Using a negative temperature
coefficient resistor in place of one of the resistors in the ladder produces a variable duty cycle.

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Figure 1. Analog-to-PWM Circuit and Simulation(1) and (2) calculate the frequency setting of the input waveform to generate PWM. The registers in theseequations are programmed to the smart DAC.

Figure 2. (1) and (2)2Generating Pulse-width Modulation Signals with Smart DACs
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Margin-high is the high-voltage level of the waveform, and margin-low is the low-voltage level. The duty cycle of
the PWM output correlates with the margin-high, margin-low and VFB applied to the feedback pin, as expressedby (3):Figure 2. (3)GPI-to-PWM Conversion
In GPI-based dimming for LED automotive taillights, smart DACs provide additional digital interfacing byexpanding the feedback resistor divider network, as seen in Figure 4. Adding two resistors to the DAC53701
feedback network shown in Figure 1 creates two new GPI pins. The voltage at the feedback pin changesdepending on the level of GPI1 and GPI2.
As discussed in the previous section, the voltage at the feedback pin, along with the margin-high and margin-low
voltages of the triangular or sawtooth waveform coming from CWG, determine the duty cycle of the PWM output.
GPI0 can provide on and off functionality to the system by powering up and powering down the DAC53701, or
start and stop functionality for CWG.

Figure 2. GPI-to-PWM Circuit and Simulation

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555 Timer Replacement
A smart DAC’s PWM duty cycle is controlled through changes to the triangular- or sawtooth-wave margin-highand margin-low voltages, while the frequency is controlled by setting the slew rate for the DAC. These
programmable settings remove the need for additional timing circuitry such as 555 timers. Replacing 555
timers with smart DACs has multiple advantages. A typical 555 timer is 9 mm by 6 mm and requires external
components to set its operating frequency. Smart DACs come in a 2-mm-by-2-mm quad flat no-lead package
and require fewer external components, and their frequency is not controlled by any external components thatare subject to drift.
PWM Duty-cycle Conversion
When you need to adjust the PWM duty cycle to fit the input ranges of various devices in your system, smart
DACs can translate the duty cycle of an input PWM signal. Adding a resistor-capacitor (RC) filter to the PWM
input signal will translate it to an analog voltage, applicable to the smart DAC’s feedback pin. In most cases,
you will need to invert the PWM input signal because the RC filter will provide a larger analog voltage for larger
duty cycles. A larger analog voltage on the feedback pin will give a smaller duty cycle output wave from theDAC53701.
After inverting the input PWM and adding the RC filter, it is possible to vary the duty cycle of the output by
dividing down the output of the RC filter using a resistor divider, or adjusting the margin-high and margin-low
values of the triangular or sawtooth wave from the DAC53701 CWG according to the Figure 5. The schematicand simulation using the in Figure 5 shows the inversion and filtering of the input PWM waveform, as well as theduty-cycle translation using the DAC53701.

4Generating Pulse-width Modulation Signals with Smart DACs
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Figure 3. PWM Duty-cycle Conversion Circuit and Simulation
Conclusion
Smart DACs are a good fit for most designs or subsystems that need PWM generation, providing accessto internal components as well as storage and programmability. Smart DACs can help take multiple inputs,
converting temperature, resistance or GPI inputs to an accurate and controllable PWM signal.
Additional Resources• Learn more about smart DACs in the technical article, "What is a smart DAC?"• Browse the smart DAC FAQ on our TI E2ETM forums.
• View the DAC53701 data sheet.
• Explore three other devices in the smart DAC family, the DAC53204, DAC63204 and DAC63004.
• Explore TI's portfolio of smart DACs.

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