LM2578A, LM3578A

www.ti.com

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.

Copyright © 2000–2013, Texas Instruments Incorporated

Submit Documentation Feedback

13

Product Folder Links: LM2578A LM3578A