Applying the CWG Output Modes

The Complementary Waveform Generator (CWG) provides six different output modes, each of which is well-suited to support particular applications. The following page outlines what each of these modes does, and accompanying applications or suggested uses.

Half-Bridge Mode


At its very simplest, the Half-Bridge mode is useful for turning two things on and off at exactly opposite times. This mode creates two signals which are true and inverted signals based upon the input signal used. This mode can be used to control a half-bridge circuit using two transistors to control power through a load. The configurable dead-band delay prevents both gates to be open simultaneously thus allowing the current to flow directly from power to ground. This is called shoot-through current and can damage both the load and the transistors in series. This mode of operation can also be useful in driving stepper motors, brushed DC motors, or power supplies if your specific applications need only a true and inverted signal.

Forward Full-Bridge Mode


Forward Full-Bridge mode uses all four of the CWG output pins to drive transistor bridges. The example application shows how the full-bridge mode can control the direction of current flow through a load using a transistor bridge. Signals CWGB and CWGD are driven LOW, CWGA is driven HIGH, and CWGC replicates the input signal. This mode is very useful in combination with the reverse full-bridge mode to control brushed DC motors by controlling the direction of rotation. The input signal controls the power allowed across the load, thus controlling the speed of the motor.

Reverse Full Bridge Mode


This mode can be used with the forward full-bridge mode above to control a brushed DC motor. The reverse mode can reverse the current flow through a transistor bridge in order to reverse the direction a brushed DC motor spins.

Push-Pull Mode


Push-Pull mode creates two output signals which are alternating copies of the signal input to the CWG. Many transformer based power supplies use some kind of a push-pull circuit to drive the transformer to create power. This mode allows you to save board space and extra part costs by digitally creating these alternating signals. The figure above shows two pins using transistors to control the current flow through an inductive coil which is contained within a transformer. Some version of this set-up can be used in Half and Full bridge power supplies, DC to AC inverters, and Class-D output drives. This mode also has motor control applications in synchronous drives and induction motor drives.

Steering Mode


Steering Mode allows you to drive the input signal to up to four output pins. Those familiar with Microchip may notice that this can also be done with the Peripheral Pin Select. While this is true, the process is faster if controlled by the CWG. The steering mode is useful when the same signal needs to be sent to different parts of a design at different times. The figure above shows a high-level illustration of the steering mode driving a brushless DC motor. This mode can also increase the output current by using multiple pins with the same signal which can prevent the need for external drivers when driving high-speed FETs.

Asynchronous Steering Mode

This mode is similar to steering mode but operates slightly differently. Asynchronous (not sync) steering mode begins replicating the input signal on the pin as soon as the instruction is passed to begin steering mode, and will stop sending the signal the exact moment the instruction to stop steering on that pin is passed. This mode should be used if the signal needs to stop as close to instantaneously as possible. In contrast, the synchronous mode starts and ends based on the input signal. For example, when the steering command is set, the CWG will output the next time the input signal goes HIGH and will continue to output until the steering command has been stopped and the input signal has gone HIGH one last time.

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