Applications Information
(Continued)
DS008711-30
DS008711-23
FIGURE 10. Duty Cycle Adjustment
REMOTE SHUTDOWN
The LM1578A may be remotely shutdown by sinking a
greater current from the non-inverting input than from the in-
verting input. This may be accomplished by selecting resistor
R3 to be approximately one-half the value of R1 and R2 in
parallel.
FIGURE 12. D1 Prevents Output Transistor from
Improperly Turning ON due to D2’s Forward Voltage
SYNCHRONIZING DEVICES
When several devices are to be operated at once, their oscil-
lators may be synchronized by the application of an external
signal. This drive signal should be a pulse waveform with a
minimum pulse width of 2 µs. and an amplitude from 1.5V to
2.0V. The signal source must be capable of 1.) driving ca-
pacitive loads and 2.) delivering up to 500 µA for each
LM1578A.
Capacitors C1 thru CN are to be selected for a 20% slower
frequency than the synchronization frequency.
DS008711-24
DS008711-25
FIGURE 11. Shutdown Occurs when V
L
is High
EMITTER OUTPUT
When the LM1578A output transistor is in the OFF state, if
the Emitter output swings below the ground pin voltage, the
output transistor will turn ON because its base is clamped
near ground. The
Collector Current with Emitter Output Be-
low Ground
curve shows the amount of Collector current
drawn in this mode, vs temperature and Emitter voltage.
When the Collector-Emitter voltage is high, this current will
cause high power dissipation in the output transistor and
should be avoided.
This situation can occur in the high-current high-voltage
buck application if the Emitter output is used and the catch
diode’s forward voltage drop is greater than 0.6V. A
fast-recovery diode can be added in series with the Emitter
output to counter the forward voltage drop of the catch diode
(see
Figure 2).
For better efficiency of a high output current
buck regulator, an external PNP transistor should be used as
shown in
Figure 16.
FIGURE 13. Synchronizing Devices
Typical Applications
The LM1578A may be operated in either the continuous or
the discontinuous conduction mode. The following applica-
tions (except for the Buck-Boost Regulator) are designed for
continuous conduction operation. That is, the inductor cur-
rent is not allowed to fall to zero. This mode of operation has
higher efficiency and lower EMI characteristics than the dis-
continuous mode.
BUCK REGULATOR
The buck configuration is used to step an input voltage down
to a lower level. Transistor Q1 in
Figure 14
chops the input
DC voltage into a squarewave. This squarewave is then con-
verted back into a DC voltage of lower magnitude by the low
pass filter consisting of L1 and C1. The duty cycle, D, of the
squarewave relates the output voltage to the input voltage by
the following equation:
V
out
= D x V
in
= V
in
x (t
on
)/(t
on
+ t
off
).
9
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