LM3578AM datasheet(15/20 Pages) NSC

15 / 20 page

Typical Applications (Continued)

BOOST REGULATOR

The boost regulator converts a low input voltage into a

higher output voltage. The basic configuration is shown in

Figure 18. Energy is stored in the inductor while the transis-

tor is on and then transferred with the input voltage to the

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

o =Vin +Vin(ton/toff).

The circuit of Figure 19 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

≤ V

≤ 8.5V).

R1=(V

o − 1) R2 where R2 = 10 k

Ω.

R3 = V/(I

L(max, DC) + 0.5

∆I

where:

∆I

L = 2(ILOAD(min))(Vo/Vin)

∆I

L is 200 mA in this example.

R4, C3 and C4 are necessary for continuous operation and

are typically 220 k

Ω, 20 pF, and 0.0022 µF respectively.

C1 is the timing capacitor found in Figure 1.

≥ I

o (Vo −Vin)/(fosc Vo Vripple).

00871108

in = 15V

R4 = 200

Ω

o =5V

R5=330

Ω

ripple = 50 mV

C1 = 1820 pF

o = 1.5A

C2 = 330 µF

osc =50kHz

C3=20pF

R1=40k

Ω

L1 = 220 µH

R2=10k

Ω

D1 = 1N5819

R3 = 0.05

Ω

Q1 = D45

FIGURE 17. Buck Converter with Boosted Output Current

00871109

FIGURE 18. Basic Boost Regulator

00871111

in =5V

R4=200 k

Ω

o = 15V

C1 = 1820 pF

ripple =10mV

C2=470 µF

o = 140 mA

C3 = 20 pF

osc = 50 kHz

C4 = 0.0022 µF

R1 = 140 k

Ω

L1 = 330 µH

R2=10k

Ω

D1 = 1N5818

R3 = 0.15

Ω

FIGURE 19. Boost or Step-Up Regulator

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