Figure 1

UC1717

UC2717

UC3717

The UC3717 drive circuit shown in the block diagram in-

cludes the following functions:

(1) Phase Logic and H-Bridge Output Stage

(2) Voltage Divider with three Comparators for current control

(3) Two Logic inputs for Digital current level select

(4) Monostable for off time generation

Input Logic: If any of the logic inputs are left open, the

circuit will treat it as a high level input.

Phase Input: The phase input terminal, pin 18, controls

the direction of the current through the motor winding.

The Schmidt-Trigger input coupled with a fixed time de-

lay assures noise immunity and eliminates cross conduc-

tion in the output stage during phase changes. A low

level on the phase input will turn Q2 on and enable Q3

while a high level will turn Q1 on and enable Q4. (See

Figure 7).

Output Stage: The output stage consists of four Dar-

lington transistors and associated diodes connected in

an H-Bridge configuration. The diodes are needed to pro-

vide a current path when the transistors are being

switched. For fast recovery, Schottky diodes are used

FUNCTIONAL DESCRIPTION

across the source transistors. The Schottky diodes allow

the current to circulate through the winding while the sink

transistors are being switched off. The diodes across the

sink transistors in conjunction with the Schottkys provide

the path for the decaying current during phase reversal.

(See Figure 7).

PHASE INPUT

Q1, Q4

Q2, Q3

Low

Off

On

High

On

Off

3