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.

Typical Applications

The LM2578A 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

converted 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 volt-

age by the following equation:

V

Figure 15 is a 15V to 5V buck regulator with an output

current, I

20% of I

ciency of 75%, a load regulation of 30 mV (70 mA to 350 mA)

and a line regulation of 10 mV (12

≤ V

in

≤ 18V).

Component values are selected as follows:

R1=(V

Ω

R3 = V/I

sw(max)

R3 = 0.15

Ω

where:

V is the current limit sense voltage, 0.11V

I

transistor.

L1 is the inductor and may be found from the inductance

calculation chart (Figure 16) as follows:

Given V

V

I

f

Discontinuous at 20% of I

Note that since the circuit will become discontinuous at 20%

of I

70 mA.

00871125

FIGURE 13. Synchronizing Devices

00871105

FIGURE 14. Basic Buck Regulator

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