LM2577N-12 National Semiconductor, LM2577N-12 Datasheet - Page 14

• Manufacturer Part Number: LM2577N-12
• Description: SIMPLE SWITCHER Step-Up Voltage Regulator
• Manufacturer: National Semiconductor

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Application Hints
where V
ery diodes (typically);
E • T , the product of volts x time that charges the inductor:
I
B.
IND,DC
From Figure 7 (for 12V output) or Figure 8 (for 15V
output), identify inductor code for region indicated by
V
conditions for which the LM1577/LM2577 output switch
would be operating beyond its switch current rating. The
minimum operating voltage for the LM1577/LM2577 is
3.5V.
From here, proceed to step C .
2. For Adjustable version
Preliminary calculations:
The inductor selection is based on the calculation of the
following three parameters:
D
Identify Inductor Value:
1. From Figure 9 , identify the inductor code for the re-
gion indicated by the intersection of E • T and I
This code gives the inductor value in microhenries. The
L or H prefix signifies whether the inductor is rated for a
maximum E • T of 90 V • µs (L) or 250 V • µs (H).
2. If D
late the minimum inductance needed to ensure the
switching regulator’s stability:
IN (min)
(max)
, the average inductor current under full load;
F
, the maximum switch duty cycle (0
= 0.5V for Schottky diodes and 0.8V for fast recov-
<
and I
0.85, go on to step C. If D
LOAD (max)
. The shaded region indicates
(Continued)
0.85, then calcu-
D
0.9):
IND,DC
.
14
If L
on to step C. Otherwise, the inductor value found in step B1
is too low; an appropriate inductor code should be obtained
from the graph as follows:
1. Find the lowest value inductor that is greater than L
2. Find where E • T intersects this inductor value to determine
if it has an L or H prefix. If E • T intersects both the L and H re-
gions, select the inductor with an H prefix.
MIN
FIGURE 7. LM2577-12 Inductor Selection Guide
FIGURE 8. LM2577-15 Inductor Selection Guide
is smaller than the inductor value found in step B1, go
DS011468-28
DS011468-27
MIN
.