PBL 3771/1

5

FAST Current Decay

SLOW Current Decay

Motor Current

Time

1

3

2

Figure 5. Output stage with current paths

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

Figure 6. Typical stepper motor application with 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, R

S

, 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, V

R

,

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 V

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

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:

I

[A]

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

for the sensing resistor, R

reference voltage will produce an output

current of approximately 450 mA. To

improve noise immunity on the V

the control range may be increased to

5 volts if R

to 2 ohms.

3

2 1

R

s

7

8

9

16

15

14

Phase

CD

V

Phase

CD

V

1

1

2

2

R1

R2

E

C

E

C

GND

RC

PBL 3771/1

15 kW

3 300 pF

1.0 W

1.0 W

M

M

M

M

A1

B1

A2

B2

V

CC

VV

MM1

MM2

+5 V

4

1

19

22

11

3

20

12

5,

6,

17, 18

2

13

21

10

11

2

2

R S

STEPPER

MOTOR

V

MM

Pin numbers refer

to DIL package.

GND (V

)

1 kW

1 kW

820 pF

820 pF

RS

0.1 mF

0.1 mF

+

10 mF

V

(+5 V)

CC

GND

(V

)

CC

MM