LM3578 datasheet(9/18 Pages) NSC

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Applications Information (Continued)

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.

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.

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.

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:

out = DxVin = Vin x(ton)/(ton +toff).

DS008711-23

FIGURE 10. Duty Cycle Adjustment

DS008711-24

FIGURE 11. Shutdown Occurs when V

L is High

DS008711-30

FIGURE 12. D1 Prevents Output Transistor from

Improperly Turning ON due to D2’s Forward Voltage

DS008711-25

FIGURE 13. Synchronizing Devices

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