Datasheet A3955 (Allegro) - 9

A3955 Full-Bridge PWM Microstepping Motor Driver
reference value, resulting in increased motor performance in microstepping applications. For a given level of ripple current, slow decay affords the lowest PWM frequency, which reduces heating in the motor and driver IC due to a corresponding decrease in hysteretic core losses and switching losses respectively. Slow decay also has the advantage that the PWM load current regulation can follow a more rapidly increasing reference before the PWM frequency drops into the audible range. For these reasons slow-decay mode is typically used as long as good current regulation can be maintained. Under some circumstances slow-decay mode PWM can fail to maintain good current regulation:
A — Slow-Decay
1) The load current will fail to regulate in slow-decay mode due to a suffi ciently negative back-EMF voltage in conjunction with the low voltage drop across the load during slow decay recirculation. The negative back-EMF voltage can cause the load current to actually increase during the slow decay off time. A negative back-EMF voltage condition commonly occurs when driving stepping motors because the phase lead of the rotor typically causes the back-EMF voltage to be negative towards the end of each step (see fi gure 3A). 2) When the desired load current is decreased rapidly, the slow rate of load current decay can prevent the current from following the desired reference value. 3) When the desired load current is set to a very low value, the current-control loop can fail to regulate due to its minimum duty
B — Fast-Decay
cycle, which is a function of the user-selected value of tOFF and the minimum on-time pulse width ton(min) that occurs each time the PWM latch is reset.
Fast Current-Decay Mode.
When VPFD 0.8 V, the device is in fast current-decay mode (both the sink and source drivers are disabled when the load current reaches ITRIP). During the fi xed off-time, the load inductance causes the current to fl ow from ground to the load supply via the motor winding, ground- clamp and fl yback diodes (see fi gure 1). Because the full motor supply voltage is across the load during fast-decay recirculation, the rate of load current decay is rapid, producing a high ripple current for a given fi xed off-time (see fi gure 2). This rapid rate of decay allows good current regulation to be maintained at the cost of decreased average current accuracy or increased driver and motor losses.
C — Mixed-Decay Figure 3 — Sinusoidal Drive Currents
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