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A4975

Full-Bridge PWM Microstepping Motor Driver

Allegro MicroSystems 

A4975

Full-Bridge PWM Microstepping Motor Driver

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:

1) The load current will fail to regulate in slow-decay mode

due to a sufficiently 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 figure 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

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 , and the

opposite pair is turned on). During the fixed off-time, the load

inductance causes the current to flow from ground to the load

supply via the motor winding and the opposite pair of transistors

(see figure 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 fixed

off-time (see figure 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.

A — Slow-Decay

B — Fast-Decay

C — Mixed-Decay

Figure 3 — Sinusoidal Drive Currents

Allegro MicroSystems, Inc.

7

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

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