LM1577, LM2577
SNOS658D –JUNE 1999–REVISED APRIL 2013
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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:
•
•
•
•
VIN (min) = Minimum input supply voltage
VOUT = Regulated output voltage
ILOAD(max) = Maximum output load current
Before proceeding any further, determine if the LM1577/LM2577 can provide these values of VOUT and
ILOAD(max) when operating with the minimum value of VIN. The upper limits for VOUT and ILOAD(max) are given by
the following equations.
where
•
•
V
OUT ≤ 60V
V
OUT ≤ 10 × VIN(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 VIN (min) and ILOAD (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 VF = 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)
(6)
IIND,DC, the average inductor current under full load;
B.
Identify Inductor Value:
1. From Figure 36, identify the inductor code for the region indicated by the intersection of E•T and IIND,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 LMIN 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 LMIN
.
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