Functional description
2
Functional description
The circuit is intended to drive both windings of a bipolar stepper motor.
The peak current control is generated through switch mode regulation.There is a choice of
three different current levels with the two logic inputs I
01
- I
11
for winding 1 and I
02
- I
12
for
winding 2.
The current can also be switched off completely.
2.1
Input logic (I
0
and I
1
)
The current level in the motor winding is selected by these inputs. (See
If any of
the logic inputs is left open, the circuit treats it as a high level input.
Table 6.
I
0
H
L
H
L
Current levels
I
1
H
H
L
L
No current
Low current 1/3 I
O
max
Medium current 2/3 I
O
max
Maximum current I
O
max
Current level
2.2
Phase
This input determines the direction of current flow in the windings, depending on the motor
connections. The signal is fed through a Schmidt trigger for noise immunity, and through a
time delay in order to guarantee that no short-circuit occurs in the output stage during
phase-shift. A high level on the Phase input causes the motor current flow from out A
through the winding to out B.
2.3
Current sensor
This part contains a current sensing resistor (R
S
), a low pass filter (R
C
, C
C
) and three
comparators. Only one comparator is active at a time. It is activated by the input logic
according to the current level chosen with signals I
o
and I
1
. The motor current flows through
the sensing resistor R
S
. When the current has increased so that the voltage across R
S
becomes higher than the reference voltage on the other comparator input, the comparator
goes high, triggering the pulse generator.
The max peak current I
max
can be defined by:
Equation 1
V
ref
I
max
= -------------
10R
s
Doc ID 4970 Rev 4
7/12
STMICROELECTRONICS [ STMICROELECTRONICS ]
