Datasheet
SLUS318B – APRIL 1999 – REVISED JANUARY 2002
12
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APPLICATION INFORMATION
When configured for two-quadrant operation, (QUAD=0), the UCC3626 modulates only the low-side devices
of the output power stage. The current paths within the output stage during the PWM on- and off-times are
illustrated in Figure 9. During the on interval, both switches are on, and current flows through the load down to
ground. During the off time, the lower switch is shut off, and the motor current circulates through the upper half
bridge via the flyback diode. The motor is assumed to be operating in either quadrant I or III.
If operation is attempted in quadrants II or IV by changing the DIR bit and reversing the torque, switches 1 and
4 are turned off and switches 2 and 3 turned on. Under this condition motor current very quickly decays, reverses
direction and increases until the control threshold is reached. At this point, switch 2 turns off and current once
again circulates in the upper half bridge. However, in this case, the motor’s BEMF is in phase with the current,
(i.e. the motor’s direction of rotation has not yet changed.) Figure 10 illustrates the current paths when operating
in this mode. Under these conditions there is nothing to limit the current other than motor and drive impedance.
These high-circulating currents can result in damage to the power devices in addition to high, uncontrolled
torque.
Figure 9. Two-Quadrant Chopping
IPHASE
+ BEMF –
VM
O
T
IOFF
ION
S1
S3 S5
S2
S4
S6
UDG–01119
Figure 10. Two-Quadrant Reversal
IPHASE
+ BEMF –
VMOT
IOFF
ION
S1
S3 S5
S2
S4
S6
UDG–01120