Typical Applications
(Continued)
Step 2:
Calculate the inductor Volts-sec product, E-T
op
, ac-
cording 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.
DS008711-5
FIGURE 14. Basic Buck Regulator
Figure 15
is a 15V to 5V buck regulator with an output cur-
rent, I
o
, of 350 mA. The circuit becomes discontinuous at
20% of I
o(max)
, has 10 mV of output voltage ripple, an effi-
ciency of 75%, a load regulation of 30 mV (70 mA to 350 mA)
and a line regulation of 10 mV (12
≤
V
in
≤
18V).
Component values are selected as follows:
R1 = (V
o
− 1) x R2 where R2 = 10 kΩ
R3 = V/I
sw(max)
R3 = 0.15Ω
where:
V is the current limit sense voltage, 0.11V
I
sw(max)
is the maximum allowable current thru the output
transistor.
L1 is the inductor and may be found from the inductance cal-
culation chart (Figure
16)
as follows:
Given V
in
= 15V
V
o
= 5V
I
o(max)
= 350 mA
f
OSC
= 50 kHz
Discontinuous at 20% of I
o(max)
.
Note that since the circuit will become discontinuous at 20%
of I
o(max)
, the load current must not be allowed to fall below
70 mA.
Step 1:
Calculate the maximum DC current through the in-
ductor, 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.
DS008711-6
V
in
= 15V
V
o
= 5V
V
ripple
= 10 mV
I
o
= 350 mA
f
osc
= 50 kHz
R1 = 40 kΩ
R2 = 10 kΩ
R3 = 0.15Ω
C1 = 1820 pF
C2 = 220 µF
C3 = 20 pF
L1 = 470 µH
D1 = 1N5818
FIGURE 15. Buck or Step-Down Regulator
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 accom-
plished 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.
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