Dead-time is defined as the time the outputs are blanked off

(to prevent shoot-through currents) after a direction change

input See

Figure 3 It can be seen that the dead-time is two

clock periods Since the dead-time scheme introduces de-

lay into the system feedback control loop which could im-

pact system performance or stability it is important that the

dead-time be kept to a minimum From

Figure 3 it can be

seen that the time between a direction change signal and

the initiation of output blanking can vary up to one clock

period due to asynchronous nature of the clock and the

direction signal

Typical Applications

THREE-PHASE EXAMPLES

Figure 4 is a typical LM621 application This circuitry is for

use with a three-phase motor having 30-degree sensor

phasing as indicated by connection of the 3060 SELECT

the DEAD-TIME ENABLE input pin 3 enables this feature

Typical power switches and a simple implementation of an

overcurrent sensing circuit are also detailed in

Figure 4 This

application example assumes a device turn-off time of about

48 ms maximum as evidenced by the choice of R and C

See Typical Performance Characteristics The choice of RC

should be made such that two periods are at least equal to

the maximum device turn-off time

the LM621 outputs to the power switches) depends on the

input current requirements of the power switching devices

These resistors should be chosen to provide only the

amount of current needed by the device inputs up to 50 mA

(typical) The resistors minimize the dissipation incurred by

the LM621 Although

Figure 4 shows the 5 – 40V supply (pin

18) connected to the motor supply voltage this was done

only to emphasize the ability of the part to provide up to 40V

output swings For the bipolar power switches shown con-

necting pin 18 to a 5V supply would reduce on-chip power

dissipation Driving FET power switches however may re-

quire connecting pin 18 to a higher voltage

Figure 5 is the

three-phase application built with MOSFET power-switching

MOSFET gates

TLH8679 – 9

FIGURE 4 Commutation of Three-Phase Motor (Bipolar Switches)

8