AN235

Slow versus fast decay

Doc ID 1679 Rev 2

15/23

5

Slow versus fast decay

When implementing current controlled motor drives, the designer has a choice of the

recirculation path the current flows in during the "off" time. Figure 12 shows the two

recirculation options implemented in the L6208, or the L6203. Applying the chopping to only

one side of the bridge allows the current to recirculate around a low voltage loop, in the

upper transistors within the bridge. Since the rate of change of the current is controlled

primarily by the L/R time constant of the motor, the current decays relatively slowly, hence

the designation of slow decay mode. Applying the chopping signal to both sides of the

bridge results in a higher voltage across the coil since the current is recirculating back

through the power supply. The higher voltage across coil forces a faster decay of current,

hence a fast decay mode.

Figure 13.

PWM current decay modes

The selection of the decay mode influences the operation of a drive in several ways. The

most obvious is the magnitude of the ripple current. Drives implemented using the fast

decay mode have, for the same off-time or chopping frequency, a higher ripple current than

drives implemented using a slow decay mode, as shown in Figure 13. This difference in

itself is not significant for most stepper motor drives. Issues with the stability of the current

control loop are discussed elsewhere [1.]. Generally for full-step and half-step drives the

slow decay mode is preferred since it reduces the current ripple and, for many driver ICs, the

dissipation in the device.

V

S

V

S

V

S

ON time

Current builds

In winding

OFF time

Slow decay

Recirculation

OFF time

Fast decay

Recirculation

AM16511v1