A3955SLB datasheet(8/16 Pages) ALLEGRO

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3955

FULL-BRIDGE PWM

MICROSTEPPING

MOTOR DRIVER

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

A — Slow-Decay

B — Fast-Decay

C — Mixed-Decay

Figure 3 — Sinusoidal Drive Currents

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). During the fixed off time, the load inductance

causes the current to flow from ground to the load supply

via the motor winding, ground-clamp and flyback diodes

(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

Figure 2 — Current-Decay Waveforms

PFD

I TRIP

Dwg. WP-031-1

I PEAK

OFF

SLOW (V

≥ 3.5 V)

PFD

MIXED (1.1 V

≤ V

≤ 3.1 V)

FAST (V

≤ 0.8 V)

PFD

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