LM3578A MWC National Semiconductor, LM3578A MWC Datasheet - Page 10

• Manufacturer Part Number: LM3578A MWC
• Description: Switching Regulator
• Manufacturer: National Semiconductor

www.national.com
Typical Applications
Figure 15 is a 15V to 5V buck regulator with an output cur-
rent, I
20% of I
ciency of 75%, a load regulation of 30 mV (70 mA to 350 mA)
and a line regulation of 10 mV (12
Component values are selected as follows:
R1 = (V
R3 = V/I
R3 = 0.15
where:
V is the current limit sense voltage, 0.11V
I
transistor.
L1 is the inductor and may be found from the inductance cal-
culation chart ( Figure 16 ) as follows:
Given
Note that since the circuit will become discontinuous at 20%
of I
70 mA.
Step 1: Calculate the maximum DC current through the in-
ductor, I
top of the chart and show that I
configuration. Thus, I
sw(max)
o(max)
o
, of 350 mA. The circuit becomes discontinuous at
L(max)
o
is the maximum allowable current thru the output
V
V
I
f
Discontinuous at 20% of I
o(max)
sw(max)
, the load current must not be allowed to fall below
o(max)
OSC
− 1) x R2 where R2 = 10 k
in
o
FIGURE 14. Basic Buck Regulator
= 5V
= 15V
. The necessary equations are indicated at the
= 50 kHz
, has 10 mV of output voltage ripple, an effi-
= 350 mA
L(max)
= 350 mA.
L(max)
o(max)
(Continued)
V
= I
in
.
o(max)
18V).
DS008711-5
for the buck
10
Step 2: Calculate the inductor Volts-sec product, E-T
cording to the equations given from the chart. For the Buck:
E-T
= (15 − 5) (5/15) (1000/50)
= 66V-µs.
with the oscillator frequency, f
Step 3: Using the graph with axis labeled “Discontinuous At
% I
maximum inductor current, I
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
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.
op
OUT
= (V
V
V
V
I
f
R1 = 40 k
R2 = 10 k
FIGURE 15. Buck or Step-Down Regulator
” and “I
o
osc
in
o
ripple
= 350 mA
= 5V
= 15V
= 50 kHz
in
= 10 mV
− V
L(max, DC)
o
) (V
o
/V
in
” find the point where the desired
) (1000/f
op
L(max, DC)
. For this example, E-T
osc
R3 = 0.15
C1 = 1820 pF
C2 = 220 µF
C3 = 20 pF
L1 = 470 µH
D1 = 1N5818
, expressed in kHz.
osc
)
intercepts the desired
DS008711-6
op
op
, ac-
is