LM2578A, LM3578A
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SNVS767E –AUGUST 2000–REVISED APRIL 2013
EMITTER OUTPUT
When the LM2578A 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 Below 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 15). For better efficiency of a
high output current buck regulator, an external PNP transistor should be used as shown in Figure 29.
Figure 25. 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 oscillators 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 capacitive loads and 2.) delivering
up to 500 μA for each LM2578A.
Capacitors C1 thru CN are to be selected for a 20% slower frequency than the synchronization frequency.
Figure 26. Synchronizing Devices
Typical Applications
The LM2578A may be operated in either the continuous or the discontinuous conduction mode. The following
applications (except for the Buck-Boost Regulator) are designed for continuous conduction operation. That is, the
inductor current is not allowed to fall to zero. This mode of operation has higher efficiency and lower EMI
characteristics than the discontinuous mode.
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