LEGO® introduced the V2 IR receiver because of the serious issue of inrush current resulting from a design change to M Motor. The revised M Motor included a bigger filter capacitor and caused very high inrush current at startup. As a result, it would trip the short-circuit protection in the motor driver chip. LEGO® was forced to introduce the up-rated V2 IR receiver with greater drive current.
We have evaluated 3x different motor driver chips to find one which could reliably tolerate the high inrush current of the revised M Motor. The driver we use is rated to more than 3 A current. In practice, this amount of current is almost never sustained in continuous operation. Most Power Functions motors consume anywhere between 100-600 mA depending on load and applied voltage.
The 3 A rating is only useful for the brief few microseconds where the M Motor capacitor gulps an enormous amount of current until it reaches its full charge. After it reaches its steady state charge, the current drops dramatically to normal current through the motor windings.
The PFx Brick is more than suitable for driving very high motor loads. In summary:
1. It includes dedicated motor driver chips on each channel which are significantly over-rated for the task.
2. It drives the motor channels with a high frequency PWM (pulse width modulation) which results in silent motor operation without the signature "whine" of the IR receiver.
3. We provide an optional "torque compensation" mode which initially drives the motor with a super-low frequency PWM which is randomly modulated. This helps "shake" the motor bearings from their initial static friction and high initial torque.
4. We offer adjustable speed curves for better speed control.
5. You can choose how many speed steps (anywhere from 3 to 100)
6. You can optional invert the polarity of a motor channel (without a separate accessory)
7. You can optionally specify acceleration and deceleration behaviour for each motor channel
The 3 A source current is limited by your power source, which in most cases will be a battery box. However, it is the instantaneous current that is a problem rather than sustained current. The motor driver chip will activate its over-current protection long before the current protection in the battery box. The AA battery box and rechargeable battery box are both capable of delivering bursts of high instantaneous current during the inrush current phase of the M Motor. Therefore, it is still necessary to install the 3 A capable motor driver with a higher threshold for over-current protection.
One final point: Anyone wanting to buy a LEGO® V2 IR reciever, will find that they average $63 USD on Bricklink (as of Apr 1, 2017). Why pay $63 USD for a V2 receiver when a PFx Brick is $65 USD and provides more sophisticated motor control AND gives you an 8 channel lighting Fx controller AND sound Fx AND customization AND USB 2.0, etc...