Datasheet

The UC3717A’s drive circuit shown in the block diagram

includes the following components.

(1) H-bridge output stage

(2) Phase polarity logic

(3) Voltage divider coupled with current sensing compa-

rators

(4) Two-bit D/A current level select

(5) Monostable generating fixed off-time

(6) Thermal protection

OUTPUT STAGE

The UC3717A’s output stage consists of four Darlington

power transistors and associated recirculating power di-

odes in a full H-bridge configuration as shown in Figure

7. Also presented, is the new added feature of inte-

grated bootstrap pull up, which improves device per-

formance during switched mode operation. While in

switched mode, with a low level phase polarity input, Q2

is on and Q3 is being switched. At the moment Q3 turns

off, winding current begins to decay through the commu-

tating diode pulling the collector of Q3 above the supply

voltage. Meanwhile, Q6 turns on pulling the base of Q2

higher than its previous value. The net effect lowers the

saturation voltage of source transistor Q2 during recircu-

lation, thus improving efficiency by reducing power dissi-

pation.

UC3717A

Figure 6. UC3717A Test Circuit

Figure 7. Simplified Schematic of Output Stage

Note: Dashed lines indicate current decay paths.

FUNCTIONAL DESCRIPTION