How Cycle-by-Cycle Current Limit Benefits Brushless Motor Designs

How Cycle-by-Cycle Current Limit Benefits Brushless Motor Designs

In-rush current has been a critical obstacle to DC motor drive designs for generations, but with the arrival of a cycle-by-cycle current limit IC, design is greatly simplified.

A Cycle-by-Cycle IC Motor Driver

The new SA30-IHZ from the Cirrus Logic family of Apex Precision Power of motor drivers is a fully-integrated switching amplifier that employs cycle-by-cycle current limit which has been developed expressly to drive 3-phase brushless DC motors,. Three independent half bridges, with each comprising a P-FET and a N-FET, provide over 10 A of PEAK output current under digital control. Thermal and short circuit monitoring is provided, which generates fault signals for the microcontroller to take appropriate action.

If one compares a brushless DC motor with the traditional brush DC motor, the former has a number of advantages. These include higher reliability and smaller size. However, there has been, until now, the issue of in-rush current which has persisted

For instance, a 1 A continuous motor might require a driver amplifier that could deliver well over 10 A of PEAK current, in order to provide the initial inrush current that flows during startup. As a matter of fact, a low-inertia brushless motor can exhibit a peak-to-average ratio which is a much as 30:1! Therefore, until now, designers have had no option but to select a driver which could safely handle the large inrush current, or to create some form of current limit function in the drive circuitry design.

Understanding Cycle-by-Cycle Limit
As with many motor drive discussions, the concept of current limit in the context of a pulse width modulated (PWM) circuit is deceptively simple. In real time, current limit can measure the current in each motor phase or each drive leg and turn off the driver transistors when the current reaches some programmable threshold; followed by current decay in the motor for the rest of the PWM cycle; and finally, begin a new PWM cycle and repeat.

Features & benefits:

• Greater Reliability and Smaller size

Application information:

1st Pulse – During the 1st PWM pulse no current limit occurs because the pulse ends before the rising current reaches the 'Current Limit' threshold. At the end of the 1st pulse there is a short decay before the 2nd PWM pulse begins and the current resumes its rise.

2nd Pulse – During the 2nd PWM pulse the current reaches the 'Current Limit' value before the PWM input pulse ends. The PWM output pulse is shut off early in its cycle. Then the motor current decays until the third pulse is applied which once again causes the current to rise.

Subsequent pulses – The behaviour described above continues until the motor rotation begins to generate sufficient back EMF so that the current no longer rises above the 'Current Limit' threshold.

Notice that the rise and fall of the motor current waveforms is dependant on the resistance and inductance of the motor winding and are asynchronous with the PWM frequency.

Product information table: