# Why does this H-bridge have PWM and Enable pins?

The BTS7960 H-brige has 6 pins (excluding current warning pins).

1. Right Turn PWM
2. Left Turn PWM
3. Right Turn Enable
4. Left Turn Enable
5. VCC
6. GND

I usually connect all the "Enable" pins to HIGH and only use the PWM pins to control the motor speed and direction. That works perfectly, but is that a good practice?

Are the "Enable" pins really unnecessary or am I missing something?

• What does the datasheet say? – Leon Heller Jun 16 '18 at 16:41

If you don't need the enable pins, there is nothing wrong with tying them high. Just because they are unnecessary in your design, doesn't mean they are in some other design.

The purpose of the pin is to allow both transistors in each half-bridge to be turned off. The PWM pin (called IN by the datasheet) is used to select whether the high or low transistor is on, whist the Enable pin (called INH by datasheet) is used to switch of both transistors.

If for example you wanted the motor to freewheel (soft breaking in @vicatcu's answer), you would need to turn off both high and low sides, leaving the motor current to flow through the diodes in the bridge. If you want to stop the motor instantly (hard breaking), you switch both h-bridges to the same transistor (both high, or both low).

• So if enable is high and PWM is low, is it breaking hard or soft? – MatMis Jun 16 '18 at 17:45
• @MatMis enable low = both transistors off = soft. – Tom Carpenter Jun 16 '18 at 17:57
• but if enable is high? – MatMis Jun 16 '18 at 18:00
• @MatMis then you can use the PWM inputs to control whether each half-bridge drives high or drives low. – Tom Carpenter Jun 16 '18 at 18:02
• If for example Left Enable is HIGH and Left PWM is 0%, then is it soft breaking? – MatMis Jun 16 '18 at 18:07

It's the difference between a "soft brake" and a "hard brake". In one case the motor is floating and not under control, in the other case, it's locked in position.

Read the BTS7960B datasheet until you fully understand deadtime and motor stored energy calaculations with inertia load currents driving the motor in controlled accelerating and braking.

Since you are using PWM extermal commutation, you must be aware of cross-conduction when changing directions and to stop slowly with PWM ramp before doing so..

This smart chip board allows flexibility on methods of controlling PWM for optimal control of switching losses in each direction rather than one PWM method for both acceleration and braking.

Some combinations of 4 inputs shunt the motor windings for braking BEMF currents rather than switching between supply and ground which cannot be used in your simple method and thus draws more power. This is why it is not suggested. But it depends on your motor and load energy size relative to Pd dissipation capacity of board and heatsink/fan and estimated inefficiency losses.

It may simplify the topic to think of it from another angle than that of "hard braking" Perhaps you have another axis driving the same mechanical load and therefor have the same considerations in as far as things like inertia. In this case you may wish to have a "PWM bus" which gives the speed control to both axes. You would then use the enable pins to address the different axes.