PBL 3771/1

6

External components

The voltage across the sensing resistor

is fed back to the comparator via a low-

pass filter section, to prevent erroneous

switching due to switching transients.

The recommended filter component

values, 1 kohm and 820 pF, are suitable

for a wide range of motors and

operational conditions.

Since the low-pass filtering action

introduces a small delay of the signal to

the comparator, peak voltage across the

sensing resistor, and hence the peak

motor current, will reach a slightly higher

level than the threshold, V

reference voltage

(V

The time constant of the low-pass filter

may therefore be reduced to minimize

the delay and optimize low-current

performance, especially if a low (12 V)

supply voltage is used. Increasing the

time constant may result in unstable

switching.

The frequency of the clock oscillator is

set by the R-C combination at pin RC.

The recommended values give a

nominal frequency of 26.5 kHz. A lower

occur if the chopping duty cycle exceeds

approximately 50%. To avoid this, it is

necessary to choose a motor with a low

winding resistance. Low winding

resistance means less inductance and

will therefore enable higher stepping

rates, however it also means less torque

capability. A compromise has to be

made.

Choose a motor with the lowest

possible winding resistance that still

gives the required torque and use as

high supply voltage as possible without

exceeding the maximum recommended

40 V. Check that the chopping duty cycle

does not exceed 50% at maximum

current.

Since the PBL 3771/1 produces a

regulated, constant output current it is

not necessary to use a motor that is

rated at the same voltage as the actual

supply voltage. Only rated current needs

to be considered. Typical motors to be

used together with the PBL 3771/1 have

voltage ratings of 5 to 12 V, while the

supply voltage usually ranges from 24 to

40 V.

Figure 7. Microstepping system with PBM 3960 /1 and PBL 3771/1.

D0

D7

A0

A1

WR

CS

RESET

VV

PBM 3960/1

21

2

19

4

20

3

18

5

1

15

6

22

16

17

7

14

To

µP

+2.5V

Sign

CD

DA

1

1

1

Sign

CD

DA

2

2

2

SS

VDD

Ref

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 k

Ω

3 300 pF

1.0

Ω

1.0

Ω

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 k

Ω

1 k

Ω

820 pF

820 pF

R S

0.1

µF

0.1

µF

+

10

µF

V

(+5 V)

CC

GND

(V

)

CC

MM

frequency will result in higher current

ripple and may cause audible noise from

the motor, while increasing the

frequency results in higher switching

losses and possibly increased iron

losses in the motor.

The sensing resistor, R

selected for maximum motor current.

The relationship between peak motor

current, reference voltage and the value

of R

control” above. Be sure not to exceed

the maximum output current which is

650 mA per channel (or 500 mA per

channel, both channels fully on, see

“Recommended Operating Conditions”).

Motor selection

The PBL 3771/1 is designed for bipolar

motors, i.e., motors that have only one

winding per phase. A unipolar motor,

having windings with a center tap, can

also be used, see figure 14.

The chopping principle in the

PBL 3771/1 is based on a constant

frequency and a varying duty cycle. This

scheme imposes certain restrictions on

motor selection. Unstable chopping can