AN235

Motor drive topologies

Doc ID 1679 Rev 2

11/23

Figure 8.

Peak winding current decreases at higher speed, limited by motor

inductance

The only way to have the current rise more quickly is to have a higher drive voltage or a

lower inductance. One method used to get better performance is to increase the drive

voltage and use an external resistance to limit the current to the original value as shown in

Figure 9. The exponential time constant is L/R so increasing the resistance reduces the rise

time. Since the asymptotic current is set by the resistance, the time constant and current

can be set by selecting the drive voltage and the external resistance. This topology, referred

to as L/nR drive, was commonly used in early printers. Its significant disadvantage is the

very significant dissipation in the external resistance. For the 1 A motor the dissipation in the

external resistance is around 20 W.

Figure 9.

L/nR drive

What is really needed is a low dissipation drive circuit that would give the fast rise times of a

higher drive voltage and limit the current to the desired value without the high dissipation

associated with the external resistance in the L/nR drive configuration. Such a drive can be

implemented using switch mode techniques as show in Figure 10. Here the peak current is

AM16506v1

Rc

Lc

Vs

Ic = Vs/(Rc+Rs)

Rs

t = L/(Rc+Rs)

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