3957 View Datasheet(PDF) - Allegro MicroSystems

Part Name

Description

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3957

FULL-BRIDGE PWM MICROSTEPPING MOTOR DRIVER

Allegro MicroSystems 

3957

FULL-BRIDGE PWM

MICROSTEPPING

MOTOR DRIVER

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 con-

junction 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 the

A — Slow-Decay Mode

B — Fast-Decay Mode

I TRIP

I PEAK

PFD

SLOW (VPFD ≥ 3.5 V)

MIXED (1.2 V ≤ V PFD≤ 2.9 V)

FAST (VPFD ≤ 0.8 V)

t OFF

Dwg. WP-031-2

Figure 2 — Current-Decay Waveforms

C — Mixed-Decay Mode

Figure 3 — Sinusoidal Drive Currents

115 Northeast Cutoff, Box 15036

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

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