When selecting a motor driver IC for a particular application, one key consideration is how much maximum current can be driven through the device. The thermal performance of the device and your PCB often limit the current your motor driver is able to safely handle. That’s why calculating the total motor driver power dissipation is important when designing a motor application.
To calculate the approximate power dissipation in a motor driver, all sources of power dissipation inside a motor driver IC must be considered, such as the power dissipation in the FET ON-resistance.
- Rds(on) Dissipation
The biggest source of power dissipation inside a motor driver IC is the power dissipation in the FET ON-resistance or Rds(on). For example, the power dissipation in an H-bridge (using a high side FET and a low side FET) is:
where P(Rds) is the power dissipated in the output FETs,
HS is the resistance of the high-side FET,
LS is the resistance of the low side FET,
Io is the RMS output current being applied to the motor.
Please note that Rds(on) increases with temperature, as the device heats, the power dissipation increases.
- Switching Losses
When an output transitions from high to low or low to high, the output devices traverse a linear region where they are dissipating significantly more power than when fully turned on. This power dissipation is referred to as switching loss.
In approximate calculation, the switching loss of each output:
where Psw is the total switching loss (in watts) for one output,
P(RISE) is the power dissipated during the rising edge,
P(FALL) is dissipated during the falling edge
To expand the above equation.
where Vm is the supply voltage (in volts),
Iout is is the output current (in amps),
tR is the rise time (in seconds),
tF is the fall time (in seconds),
fsw is the switching frequency (in Hz)
In most case the value of tR and tF can find in the driver datasheet.
Operating Supply Current Dissipation
In real applications, some current is consumed by the motor driver IC. This power dissipation can be calculated as:
where Vm is motor driver’s operating voltage, Im is motor driver operating current
Other Power Dissipation
In some motor applications, you may want to use LDO regulators to provide an external power loads. This power dissipation is calculated as