LM3578AM datasheet(17/30 Pages) TI

17 / 30 page

LM2578A, LM3578A

www.ti.com

SNVS767E – AUGUST 2000 – REVISED APRIL 2013

Component values are selected as outlined for the buck regulator with a discontinuity factor of 10%, with the

addition of R4 and R5:

R4 = 10VBE1Bf/Ip

R5 = (Vin − V − VBE1 − Vsat) Bf/(IL(max, DC) + IR4)

where:

VBE1 is the VBE of transistor Q1.

Vsat is the saturation voltage of the LM2578A output transistor.

V is the current limit sense voltage.

Bf is the forced current gain of transistor Q1 (Bf = 30 for Figure 30).

IR4 = VBE1/R4

Ip = IL(max, DC) + 0.5ΔIL

Vin = 15V

R4 = 200

Vo = 5V

R5 = 330

Vripple = 50 mV

C1 = 1820 pF

Io = 1.5A

C2 = 330

μF

fosc = 50 kHz

C3 = 20 pF

R1 = 40 k

L1 = 220

μH

R2 = 10 k

D1 = 1N5819

R3 = 0.05

Q1 = D45

Figure 30. Buck Converter with Boosted Output Current

BOOST REGULATOR

The boost regulator converts a low input voltage into a higher output voltage. The basic configuration is shown in

Figure 31. Energy is stored in the inductor while the transistor is on and then transferred with the input voltage to

the output capacitor for filtering when the transistor is off. Thus,

Vo = Vin + Vin(ton/toff).

Figure 31. Basic Boost Regulator

The circuit of Figure 32 converts a 5V supply into a 15V supply with 150 mA of output current, a load regulation

of 14 mV (30 mA to 140 mA), and a line regulation of 35 mV (4.5V

≤ Vin ≤ 8.5V).

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