Datasheet

LM1577, LM2577
SNOS658D –JUNE 1999–REVISED APRIL 2013 www.ti.com
STEP-UP REGULATOR DESIGN PROCEDURE
The following design procedure can be used to select the appropriate external components for the circuit in
Figure 32, based on these system requirements.
Given:
• V
IN (min)
= Minimum input supply voltage
• V
OUT
= Regulated output voltage
• I
LOAD(max)
= Maximum output load current
• Before proceeding any further, determine if the LM1577/LM2577 can provide these values of V
OUT
and
I
LOAD(max)
when operating with the minimum value of V
IN
. The upper limits for V
OUT
and I
LOAD(max)
are given by
the following equations.
where
• V
OUT
≤ 60V
• V
OUT
≤ 10 × V
IN(min)
(3)
These limits must be greater than or equal to the values specified in this application.
1. Inductor Selection (L)
A. Voltage Options:
1. For 12V or 15V output
From Figure 34 (for 12V output) or Figure 35 (for 15V output), identify inductor code for region
indicated by V
IN (min)
and I
LOAD (max)
. The shaded region indicates 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
(max)
, the maximum switch duty cycle (0 ≤ D ≤ 0.9):
(4)
where V
F
= 0.5V for Schottky diodes and 0.8V for fast recovery diodes (typically);
E •T, the product of volts × time that charges the inductor:
(5)
I
IND,DC
, the average inductor current under full load;
(6)
B. Identify Inductor Value:
1. From Figure 36, identify the inductor code for the region indicated by the intersection of E•T and I
IND,DC
.
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 < 0.85, go on to step C. If D ≥ 0.85, then calculate the minimum inductance needed to ensure the
switching regulator's stability:
(7)
If L
MIN
is smaller than the inductor value found in step B1, go 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
MIN
.
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