LM3578AM datasheet(16/30 Pages) TI

16 / 30 page

LM2578A, LM3578A

SNVS767E – AUGUST 2000 – REVISED APRIL 2013

www.ti.com

Step 1: Calculate the maximum DC current through the inductor, IL(max). The necessary equations are indicated

at the top of the chart and show that IL(max) = Io(max) for the buck configuration. Thus, IL(max) = 350 mA.

Step 2: Calculate the inductor Volts-sec product, E-Top, according to the equations given from the chart. For the

Buck:

E-Top = (Vin − Vo) (Vo/Vin) (1000/fosc)

=(15

− 5) (5/15) (1000/50)

= 66V-

μs.

with the oscillator frequency, fosc, expressed in kHz.

Step 3: Using the graph with axis labeled “Discontinuous At % IOUT” and “IL(max, DC)” find the point where the

desired maximum inductor current, IL(max, DC) intercepts the desired discontinuity percentage.

In this example, the point of interest is where the 0.35A line intersects with the 20% line. This is nearly the

midpoint of the horizontal axis.

Step 4: This last step is merely the translation of the point found in Step 3 to the graph directly below it. This is

accomplished by moving straight down the page to the point which intercepts the desired E-Top. For this

example, E-Top is 66V-μs and the desired inductor value is 470 μH. Since this example was for 20%

discontinuity, the bottom chart could have been used directly, as noted in step 3 of the chart instructions.

For a full line of standard inductor values, contact Pulse Engineering (San Diego, Calif.) regarding their PE526XX

series, 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 = Vo (Vin − Vo)/(ΔIL Vin fosc)

BOOST

L = Vin (Vo − Vin)/(ΔIL fosc Vo)

INVERT

L = Vin |Vo|/[ΔIL(Vin + |Vo|)fosc]

where

ΔIL is the current ripple through the inductor. ΔIL is usually chosen based on the minimum load current

expected of the circuit. For the buck regulator, since the inductor current IL equals the load current IO,

ΔIL = 2 • IO(min)

ΔIL = 140 mA for this circuit. ΔIL can also be interpreted as

ΔIL = 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 28 are chosen as follows:

C1 is the timing capacitor found in Figure 14.

≥ Vo (Vin − Vo)/(8fosc

inVrippleL1)

where Vripple 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 30. 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

≤ Vin ≤ 18V).

Submit Documentation Feedback

Product Folder Links: LM2578A LM3578A

LM3578AM датащи(PDF) 16 Page - Texas Instruments

LM3578AM датащи(HTML) 16 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

датащи скачать

ссылки URL