AN235

Motor drive topologies

Doc ID 1679 Rev 2

13/23

Figure 11.

Current waveforms for L/R, L/nR and switch mode drive

Figure 11 compares the current rise time for the three drive topologies. The comparison is

done for a 5 V, 1 A motor. The L/R drive uses the motor's rated voltage, the L/nR uses five

times the motor rated voltage and an external resistor equal to four times the winding

resistance and the switch mode drive uses a supply voltage five times the motor rated

voltage with the peak current set equal to the motor rated voltage divided by the winding

resistance (1 A). In each case the final current reaches 1 A but the figure clearly shows the

faster rise time of the higher voltage drives. This faster rise significantly improves the torque

at higher step rates.

Another drive topology that may be used is a bilevel drive, where a higher voltage is applied

during the transition and then the supply voltage is dropped back down to the lower voltage

to maintain the current. The circuit shown in Figure 11 applies a high voltage, without the

additional limit resistor, during the time set by the monostable for each transition. When the

monostable times out, the switch is opened and the voltage is reduced to the lower voltage

that is just enough to maintain the current.

Time

Current

010

20

3040

5060

70

8090 100

0

0.5

1

5Vx

5Vx

Vx

L/R drive

L/5R drive

Chopping drive

Supply voltage

AM16509v1