LM621 datasheet(7/12 Pages) NSC

7 / 12 page

Dead-Time Feature

The DEAD-TIME ENABLE input is used to enable this fea-

ture (by connecting a5V to pin 3) The reason for providing

this feature is that the external power switches are usually

totem-pole structures Since these structures switch heavy

currents if either totem-pole device is not completely turned

off when its complementary device turns on heavy ‘‘shoot-

through’’ current spiking will occur This situation occurs

when the motor DIRECTION input changes (when all output

drive polarities reverse) at which time device turn-off delay

can cause the undesired current spiking

Figure 2 shows the logic of the dead-time generator The

dead-time generator includes an RC oscillator to generate a

required clock Pin 4 (CLOCK TIMING) is used to connect

an external RC network to set the frequency of this oscilla-

tor The clock frequency should be adjusted so that two

periods of oscillation just slightly exceed the worst-case

turn-off time of the power switching devices As shown by

the graph in Typical Peformance Characteristics the time of

one clock period (in ms) is approximately (0756 c 10

(R a 1) C where R is in kX and C is in pF the period can

be measured with an oscilloscope at pin 4 The dead-time

generator function monitors the DIRECTION input for

changes synchronizes the direction changes with the inter-

nal clock and inhibits the chip outputs for two clock periods

Flip-flops FF1 through FF3 form a three-bit shift-register

delay line the input of which is the DIRECTION input The

flip-flops are the only elements clocked by the internal clock

generator The shift register outputs must all have the same

state in order to enable gate G1 or G2 one of which must

be enabled to enable the chip outputs As soon as a direc-

tion change input is sensed at the output of FF1 gates G1

and G2 will be disabled thereby disabling the drive to the

power switches for a time equal to two clock periods

FIGURE 2 Dead-Time Generator Logic Diagram

TLH8679 – 7

TLH8679 – 8

FIGURE 3 Dead-Time Generator Waveforms

LM621 Scheda tecnica(PDF) 7 Page - National Semiconductor (TI)