Datasheet

LM2578A, LM3578A
SNVS767E –AUGUST 2000–REVISED APRIL 2013 www.ti.com
Step 1: Calculate the maximum DC current through the inductor, I
L(max)
. The necessary equations are indicated
at the top of the chart and show that I
L(max)
= I
o(max)
for the buck configuration. Thus, I
L(max)
= 350 mA.
Step 2: Calculate the inductor Volts-sec product, E-T
op
, according to the equations given from the chart. For the
Buck:
E-T
op
= (V
in
− V
o
) (V
o
/V
in
) (1000/f
osc
)
=(15 − 5) (5/15) (1000/50)
= 66V-μs.
with the oscillator frequency, f
osc
, expressed in kHz.
Step 3: Using the graph with axis labeled “Discontinuous At % I
OUT
” and “I
L(max, DC)
” find the point where the
desired maximum inductor current, I
L(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-T
op
. For this
example, E-T
op
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 = V
o
(V
in
− V
o
)/(ΔI
L
V
in
f
osc
)
BOOST
L = V
in
(V
o
− V
in
)/(ΔI
L
f
osc
V
o
)
INVERT
L = V
in
|V
o
|/[ΔI
L
(V
in
+ |V
o
|)f
osc
]
where ΔI
L
is the current ripple through the inductor. ΔI
L
is usually chosen based on the minimum load current
expected of the circuit. For the buck regulator, since the inductor current I
L
equals the load current I
O
,
ΔI
L
= 2 • I
O(min)
ΔI
L
= 140 mA for this circuit. ΔI
L
can also be interpreted as
ΔI
L
= 2 • (Discontinuity Factor) • I
L
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.
C2 ≥ V
o
(V
in
− V
o
)/(8f
osc
2
V
in
V
ripple
L1)
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 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 ≤ V
in
≤ 18V).
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