As circuit designers, we encounter similar challenges, albeit less dramatic. As a young electrical engineering student, we were also taught to separate sources, currents, and voltages when analyzing circuits using techniques such as superposition, Kirchhoff's current laws, and nodal analysis. All the tools we use to simplify dauntingly complex circuits into something easier to understand.

　　The same logic applies to designing motor drive circuits. Sparks fly as the brushes in a brushed motor make and disconnect from the components in the commutator. We've all seen this in our handheld power drills at home. Brushed motors, used in thousands of applications, are noisy beasts. As the brushes wear out over time, a momentary short between power and ground can occur and cause the power supply to crash repeatedly. These very small dips (100s of nanoseconds) can cause the system to malfunction and overload the microprocessor with interrupts. Using a differential probe between the motor supply and the board d GND, the noise is very noticeable and causes undervoltage faults in the system.

　　This problem is solved in devices such as the DRV8835 and DRV8836, DRV8837 and DRV8838 and DRV8839 by separating the motor supply from the logic supply.

　　The DRV8835 provides an integrated motor driver solution for video cameras, consumer products, toys, and other low-voltage or battery-powered motion control applications. The device has two H-bridge drivers and is capable of driving two DC motors or a stepper motor, as well as other devices such as solenoids. Each output driver block consists of N-channel power MOSFETs configured as an H-bridge to drive the motor windings. An internal charge pump generates the required gate drive voltage. Each H-bridge of the DRV8835 is capable of delivering up to 1.5A of output current. It operates over a motor supply voltage range of 0V to 11V, and a device supply voltage range of 2V to 7V. Selectable phase/enable and IN/IN interfaces are compatible with industry standard devices. An internal shutdown function supports over-current protection, short-circuit protection, under-voltage lockout, and over-temperature protection. The DRV8835 is available in a very small 12-pin WSON package with PowerPAD™ (Eco-Friendly: RoHS Compliant and Sb/Br Free).

　　●Characteristics:

　　■Double H-bridge motor driver

　　▲Able to drive two DC motors or one stepper motor

　　▲Low metal oxide semiconductor field effect transistor (MOSFET) on-resistance:

　　◆High side + low side (HS + LS) 305mΩ

　　■Maximum drive current of 1.5A per H-bridge

　　■Two bridges connected in parallel can achieve 3A drive current

　　■ Separate motor and logic supply pins:

　　▲ 0V to 11V motor operating power supply voltage range

　　▲ 2V to 7V logic supply voltage range

　　■ Separate logic and motor supply pins

　　■ Flexible pulse width modulation (PWM) or phase/enable interface

　　■Low-power sleep mode with 95nA maximum supply current

　　■ Very small 2mm x 3mm wafer-scale small outline no-lead (WSON) package

　　These devices are specified to operate as low as 1.8V on the logic supply (VCC), which is typically connected to a regulated microprocessor supply rail. But here's the solution to your problem - the motor supply (VM) is specified as low as 0V! So all those annoying power interruptions you see in Figure 1 lead to zero faults on the logic core.

　　There is another advantage that may be overlooked. The input logic thresholds are scaled by the logic supply (VCC). This allows the device to operate from 1.8V logic, which is more common in battery-operated devices.

　　Complex problems are always easier to solve when the variables are separated. No exception in the challenging world of motors, separate your logic power from your motor power and enjoy your free time instead of spending it on power filtering or interrupt service routines.