LM3578AM datasheet(12/18 Pages) NSC

12 / 18 page

Typical Applications (Continued)

For a full line of standard inductor values, contact Pulse En-

gineering (San Diego, Calif.) regarding their PE526XX se-

ries, or A. I. E. Magnetics (Nashville, Tenn.).

A more precise inductance value may be calculated for the

Buck, Boost and Inverting Regulators as follows:

BUCK

L = V

o (Vin −Vo)/(∆IL Vin fosc)

BOOST

L = V

in (Vo −Vin)/(∆IL fosc Vo)

INVERT

L = V

in |Vo|/[∆IL(Vin +|Vo|)fosc]

where

∆I

L is the current ripple through the inductor. ∆IL is

usually chosen based on the minimum load current expected

of the circuit. For the buck regulator, since the inductor cur-

rent I

L equals the load current IO,

∆I

L = 2 • IO(min)

∆I

L = 140 mA for this circuit. ∆IL can also be interpreted as

∆I

L = 2 • (Discontinuity Factor) • IL

where the Discontinuity Factor is the ratio of the minimum

load current to the maximum load current. For this example,

the Discontinuity Factor is 0.2.

The remainder of the components of

Figure 15 are chosen

as follows:

C1 is the timing capacitor found in

Figure 1.

≥ V

o (Vin −Vo)/(8fosc

inVrippleL1)

where V

ripple is the peak-to-peak output voltage ripple.

C3 is necessary for continuous operation and is generally in

the 10 pF to 30 pF range.

D1 should be a Schottky type diode, such as the 1N5818 or

1N5819.

BUCK WITH BOOSTED OUTPUT CURRENT

For applications requiring a large output current, an external

transistor may be used as shown in

Figure 17. This circuit

steps a 15V supply down to 5V with 1.5A of output current.

The output ripple is 50 mV, with an efficiency of 80%, a load

regulation of 40 mV (150 mA to 1.5A), and a line regulation

of 20 mV (12V

≤ V

in ≤ 18V).

Component values are selected as outlined for the buck

regulator with a discontinuity factor of 10%, with the addition

of R4 and R5:

R4 = 10V

BE1Bf/Ip

R5 = (V

in −V−VBE1 −Vsat)Bf/(IL(max, DC) +IR4)

where:

BE1 is the VBE of transistor Q1.

sat is the saturation voltage of the LM1578A output transis-

tor.

V is the current limit sense voltage.

f is the forced current gain of transistor Q1 (Bf = 30 for Fig-

ure 17 ).

R4 = VBE1/R4

p = IL(max, DC) + 0.5∆IL

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).

DS008711-8

Vin = 15V

R4 = 200

Ω

Vo = 5V

R5 = 330

Ω

Vripple = 50 mV

C1 = 1820 pF

Io = 1.5A

C2 = 330 µF

fosc = 50 kHz

C3 = 20 pF

R1 = 40 k

Ω

L1 = 220 µH

R2 = 10 k

Ω

D1 = 1N5819

R3 = 0.05

Ω

Q1 = D45

FIGURE 17. Buck Converter with Boosted Output Current

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