LM621N NSC [National Semiconductor], LM621N Datasheet - Page 8

• Manufacturer Part Number: LM621N
• Description: LM621 Brushless Motor Commutator
• Manufacturer: NSC [National Semiconductor]

Figure 4 is a typical LM621 application This circuitry is for
Dead-Time Feature
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
use with a three-phase motor having 30-degree sensor
phasing as indicated by connection of the 30 60 SELECT
input pin 8 to a logic ‘‘1’’ (
the DEAD-TIME ENABLE input pin 3 enables this feature
Typical power switches and a simple implementation of an
a
FIGURE 4 Commutation of Three-Phase Motor (Bipolar Switches)
5V) The same connection of
(Continued)
8
overcurrent sensing circuit are also detailed in Figure 4 This
application example assumes a device turn-off time of about
4 8
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 choice of the value for R
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
components Note that since the output V
at least 1 5V V
MOSFET gates
s maximum as evidenced by the choice of R and C
CC2
can be chosen to avoid overdriving the
limit
(the resistors which couple
drop
(sourcing) is
TL H 8679 – 9