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REV.-A
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Constant-Current Drive Circuit
The constant-current drive circuit (A phase only) is shown in Figure 2-29, and the various operation
wave forms are shown in Figure 2-30. Figure 2-28 shows current peak detection, chopper off time
controller, current control, and counter electromotive force cancellation circuits for the constant-
current drive circuit. Vref in Figure 2-29 shows the standard voltage which determines the amount
of peak current that flows to resistors R27 and R28. Resistors R21, R22, and condensers C6, C7
determine the chopper OFF time. The operation process for the constant-current controller of the
main circuit is shown below.
Peak Current Detection (to, t1):
(1) MOS FET
Q
1 when excitation input I (A 1,B 1) is ON. Current ION then flows to the wave line channel
to excite the A coil.
(2) The voltage of resistors R27 and R28 rises in proportion to the I ON gain.
(3) f the voltage that runs to resistors R27 and R28 exceeds Vref, CaMP 1 reverses, and the Td voltage
drops to OV.
(4) If Vtd drops and falls below the CaMP 2 threshold voltage, CaMP 2 reverses.
(5) According to the reversal of CaMP 2,
Q
1 gate voltage goes LOW, and CaMP 2 turns
Q
1 OFF.
Chopper OFF Time (t 1, t2) Control:
(6) f
Q
1 goes OFF, a counter electromotive force is generated to the motor coil. For this purpose, the
coil current channel switches from I ON to I OFF.
(7) When I OFF flows, the direction of the current that flows to resistors R27 and R28 changes. The
CaMP 1 feedback voltage VRS (V-) drops from Vref, and CaMP 1 again reverses.
(8) The CaMP 1 output stage is housed in the open collector circuit. In (7), because CaMP 1 output
goes HIGH, the Td voltage Vtd slowly operates according to the time constants of resistor R10 and
condenser C46.
(9) Until the Td voltage reaches the standard voltage (2V) of CaMP 2, the FET gate voltage maintains
the OFF state.
The operation time from 0-2V for the Vtd is suitable for the OFF time TOFF.
Chopper ON Time (t2, t3) Control:
(1 O)When the Td voltage Vtd reaches the standard voltage (2V) of CaMP 2, CaMP 2 reverses, and
Q
1
goes ON.
(11) When
Q
1 goes ON, the current flow switches from I OFF to ION.
(12) For the power voltage Vcc, I ON waits a long time 'according to the constant time of motor coil
A, and then slowly begins to operate.
(13) The electric potential of resistors R27 and R28 rise in proportion to the I ON gain. Until Vrs reaches
Vref,
Q
1 supplies I ON current from the power source to the motor while maintaining the ON state.
The time for VRS to reach Vref is suitable for ON time TON.
Hereafter, steps (6)-(13) repeat, chopper operation is carried out, and the motor current is controlled.
Furthermore, the sum of the chopper ON and OFF times is suitable for the motor drive frequency.
2-29
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