PBL3771 데이터 시트보기 (PDF) - Ericsson

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PBL3771

Precision Stepper Motor Driver

Ericsson  

PBL 3771/1

Functional Description

Each channel of the PBL 3771/1

consists of the following sections: an

H-bridge output stage, capable of driving

up to 650 mA continuous motor current

(or 500 mA, both channels driven), a

logic section that controls the output

transistors, an S-R flip-flop, and two

comparators. The oscillator is common

to both channels.

Constant current control is achieved

by switching the current to the windings.

This is done by sensing the (peak)

voltage across a current-sensing

resistor, RS, effectively connected in

series with the motor winding, and

feeding that voltage back to a

comparator. When the motor current

reaches a threshold level, determined by

the voltage at the reference input, VR,

the comparator resets the flip-flop, which

turns off the output transistors. The

current decreases until the clock

oscillator triggers the flip-flop, which

turns on the output transistors again,

and the cycle is repeated.

The current-decay rate during the

turn-off portion of the switching cycle,

can be selected fast or slow by the CD

input.

In slow current-decay mode, only one

of the lower transistors in the H-bridge

(those closest to the negative supply) is

switched on and off, while one of the

upper transistors is held constantly on.

During turn-off, the current recirculates

through the upper transistor (which one

depends on current direction) and the

corresponding free-wheeling diode

connected to VMM, see figure 5.

In fast current decay mode, both the

upper and lower transistors are

switched. During the off-time, the

freewheeling current is opposed by the

supply voltage, causing a rapid dis-

charge of energy in the winding.

Fast current decay may be required in

half- and microstepping applications

when rapid changes of motor current are

necessary. Slow current decay,

however, gives less current ripple, and

should always be selected, if possible, to

mini-mize core losses and switching

noise.

Applications Information

Current control

The output current to the motor winding

is mainly determined by the voltage at

the reference input and the value of the

sensing resistor, R .

S

Chopping frequency, winding

inductance, and supply voltage will affect

the current level, but to much less

extent. Fast current decay setting will

produce somewhat lower (average)

current than slow current decay. The

peak current through the sensing

resistor (and motor winding) can be

expressed as:

IM,peak = 0.18 • (VR / RS) [A]

i.e., with a recommended value of 1 ohm

for the sensing resistor, RS, a 2.5 V

reference voltage will produce an output

current of approximately 450 mA. To

improve noise immunity on the VR input,

the control range may be increased to

5 volts if R is correspondingly changed

S

to 2 ohms.

21

3

Rs

Motor Current

1

2

FAST Current Decay

SLOW Current Decay

3

Time

VCC (+5 V)

0.1 mF

0.1 mF

V MM

+

10 mF

11

3

20

7

V CC

Phase 1

V MM1

V MM2 MA1 4

8 CD1

9 VR1

16 Phase 2

15 CD2

14 V R2

RC GND

PBL 3771/1

C1

E1 C2

MB1 1

19

MA2

MB2 22

E2

12 5, 6, 10 2 13

21

+5 V 15 kW

17, 18

1 kW

1 kW

3 300 pF

GND

(VCC )

820 pF

820 pF

1.0 W R S

1.0 W RS

STEPPER

MOTOR

Pin numbers refer

to DIL package.

GND (VMM)

Figure 5. Output stage with current paths Figure 6. Typical stepper motor application with PBL 3771/1.

during turn -on, turn-off and phase shift.

5

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