LM1117IMPX-3.3+ National Semiconductor, LM1117IMPX-3.3+ Datasheet

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LM1117IMPX-3.3+

Manufacturer Part Number
LM1117IMPX-3.3+
Description
Manufacturer
National Semiconductor
Datasheet

Specifications of LM1117IMPX-3.3+

Notes
NEW
Date_code
10+
© 2005 National Semiconductor Corporation
Method For Calculating
Output Voltage Tolerances
in Adjustable Regulators
Introduction
When working with voltage regulator circuits, the designer is
often confronted with the need to calculate the tolerance of
the regulated output voltage. For fixed voltage regulators this
problem is easily managed because the required information
is directly supplied on the semiconductor manufacturer’s
datasheet.
The tolerancing of adjustable regulators can be more com-
plicated because of the introduction of an external feedback
resistor network, effects of adjust pin current, and the diffi-
culties associated with combining these terms to obtain an
overall estimate of output voltage tolerance.
Traditional “worst case” analysis methods, although valid,
result in unrealistic and excessively conservative estimates
of the total tolerances leading to unnecessary added circuit
costs.
Reduced voltages for modern microprocessors are further
increasing the demands on available voltage tolerances. As
such, a more detailed understanding of the tolerancing prob-
lem is needed.
The Commonly Encountered
Circuits
The most commonly encountered adjustable regulator cir-
cuits are shown in Figure 1. We will examine these circuits
and the statistical effects on V
Ground Referenced
out
FIGURE 1. Adjustable Regulators: Common Topologies
that are caused by varia-
AN201482
20148201
National Semiconductor
Application Note 1378
Thomas Mathews
July 2005
tions in R1, R2, and V
ground pins (the LM317 and LM1117 for example) have the
same mathematical relationship between V
ever, V
output pin the other. Reversing the positions of R1 and R2
makes the mathematical relationship for both circuits identi-
cal (equation 1). Be aware that many datasheets are not
consistent in the identification of R1 and R2 which further
compounds confusion regarding the correct relationship.
For the circuits in Figure 1, V
ref
is referenced to ground in one case and to the
Not Ground Referenced
adj
. Be aware that regulators without
out
is related to V
out
and V
www.national.com
ref
:
adj
20148202
, how-
(1)

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