Minimum current issues

AN1495

14/25

Doc ID 8700 Rev 3

5

Minimum current issues

When operating a chopping current control that has a minimum duty cycle, the current

cannot be taken below a level that is effectively set by the motor resistance and the

minimum duty cycle. Constant off time controls, like the L6208, have a minimum on time that

is set primarily by the propagation delays from the end of the off time until the comparator

detects a current above the threshold and retriggers the monostable putting the bridge in the

recirculation mode again. This minimum on time and the off time set by the monostable set

a minimum working duty cycle for the circuit. When this duty cycle is applied to the motor, a

current will be established. If a reference corresponding to a current lower than this

minimum is set on the input, the circuit will detect that the motor current is above the

reference. However, since the IC is already operating at its minimum duty cycle, the current

cannot go any lower and thus will not reach the current level desired by the reference level.

The minimum duty cycle in other controllers can sometimes be adjusted. The minimum on

time in the L6506, for example, is set by the width of the sync pulse. By varying the duty

cycle of the oscillator, the minimum duty cycle of the output can also be changed. Since the

sync pulse is also used to mask the switching noise in the system, reducing the minimum

duty cycle is not always possible. [3.]

Figure 12 shows the operating waveforms at the minimum current level.

The traces in Figure 12 are:

●

Ch 1: voltage on output pin

●

Ch 2: Vref

●

Ch 3: Vsense

●

Ch 4: load current (20 mA/div)

Figure 12.

Waveforms at minimum current operation

At the start of each cycle the bridge is turned on and the motor current flows through the

sense resistor to produce the voltage Vsense. However at this operating point the sense

voltage is already greater than the Vref input voltage, as can be seen in Figure 12. The

comparator will detect that Vsense is greater than Vref and cause the circuit switch the

bridge into the recirculation mode and the output is switched off after a delay that is

determined by the response time of the circuit. The output pulse width, and hence the

AM15153v1