LM621 National Semiconductor, LM621 Datasheet - Page 7

• Manufacturer Part Number: LM621
• Description: LM621 Brushless Motor Commutator
• Manufacturer: National Semiconductor

Figure 2 shows the logic of the dead-time generator The
Dead-Time Feature
The DEAD-TIME ENABLE input is used to enable this fea-
ture (by connecting
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
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
a
5V to pin 3) The reason for providing
FIGURE 2 Dead-Time Generator Logic Diagram
FIGURE 3 Dead-Time Generator Waveforms
7
the graph in Typical Peformance Characteristics the time of
one clock period (in
(R
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
a
1) C where R is in k
s) is approximately (0 756
and C is in pF the period can
TL H 8679– 7
TL H 8679– 8
c
10
b
3
)