NJM3775

Thermal resistance [

°C/W]

PCB copper foil area [cm ]

2

80

70

60

50

40

30

20

510

15

20

30

35

25

PLCC package

DIP package

V

R1

Dis

1

Phase

1

Phase

2

Dis

2

I

MA1

I

MA2

100%

–100%

–100%

100%

Full step mode

Half step mode

Modified half step mode

V

R2

140%

100%

140%

100%

140%

–140%

140%

–140%

22-pin

DIP

24-pin

EMP

28-pin

PLCC

Figure 9. Stepping modes

Figure 8. Typical thermal resistance vs. PC Board

copper area and suggested layout.

Current sense filtering

At turn-on a current spike occurs, due to the recovery of the recirculation diodes and the capacitance of the motor

winding. To prevent this spike from reseting the flip-flops through the current sensing comparators, the clock

oscillator generates a blanking pulse at turn-on. The blanking pulse pulse disables the comparators for a short time.

Thereby any voltage transient across the sensing resistor will be ignored during the blanking time.

Choose the blanking pulse time to be longer than the duration of the switching transients by selecting a proper C

T

value. The time is calculated as:

t

As the C

33 000 pF, a blanking time ranging from 0.5

µs to 7 µs is

possible. Nominal value is 4 700 pF, which gives a blanking time of 1.0

µs.

As the filtering action introduces a small delay, the peak value across the sensing resistor, and hence the peak

motor current, will reach a slightly higher level than what is defined by the reference voltage. The filtering delay also

limits the minimum possible output current. As the output will be on for a short time each cycle, equal to the digital

filtering blanking time plus additional internal delays, an amount of current will flow through the winding. Typically this

current is 1-10 % of the maximum output current set by R

When optimizing low current performance, the filtering may be done by adding an external low pass filter in series

with the comparator C input. In this case the digital blanking time should be as short as possible. The recommended

filter component values are 1 kohm and 820 pF. Lowering the switching frequency also helps reducing the minimum

output current.

To create an absolute zero current, the Dis input should be HIGH.

Switching frequency

The frequency of the clock oscillator is set by the timing components R

filter blanking time, the clock oscillator frequency is adjusted by R

frequency is approximately calculated as:

f

Nominal component values of 12 kohm and 4 700 pF results in a clock frequency of 23.0 kHz. A lower frequency will

result in higher current ripple, but may improve low level linearity. A higher clock frequency reduces current ripple,

but increases the switching losses in the IC and possibly the iron losses in the motor.

Phase inputs

A logic HIGH on a Phase input gives a current flowing from pin M

A logic LOW gives a current flow in the opposite direction. A time delay prevents cross conduction in the H-bridge

when changing the Phase input.