LM1117IMPX-3.3 National Semiconductor, LM1117IMPX-3.3 Datasheet - Page 6

• Manufacturer Part Number: LM1117IMPX-3.3
• Description: IC,VOLT REGULATOR,FIXED,+3.3V,BIPOLAR,SOT-223,3PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM1117IMPX-3.3

• Rohs Compliant: NO

www.national.com
Errors Caused by Adjust Pin
Current
For all adjustable regulators, there is always a small amount
of current that flows at the adjust pin. Ideally, this current
would be zero. For many parts, this current is very low and is
not specified. For a bipolar part like the LM1117-ADJ, the
adjust pin current is typically 60 µA. For the LM2941 it is
about 5 µA. For CMOS regulators, the adjust pin current is
much less of a concern and is usually 100 nA or less.
Equation 1 can be modified to include the effects of the
unwanted adjust pin current.
This equation applies to both circuits. For both circuits, the
polarity of I
defined in Figure 5. Again be reminded that the location of
R1, R2 and the V
the resulting equation 11 is the same. For both circuits, the
adjust pin current will cause positive errors in V
polarity of the adjust pin current is in question, some data
sheets contain a transistor level “equivalent circuit diagram.”
The actual bias current polarity can usually be determined
from the equivalent circuit diagram by examining the polarity
of the transistor junction at the adjust pin. Depending upon
how much adjust pin current error is tolerable, it is possible
to calculate the largest sensible value for R2 (equation 12
and Table 3).
For most circuits, a small value for R2 is not a problem. For
battery powered circuits, the wasted current flowing through
adj
is positive, however, the direction of flow is as
ref
is different for the two circuits, however,
out
. If the
(12)
(11)
6
feedback resistors R1 and R2 may become an issue. If this
is the case then it will be desirable to select the largest
reasonable value for R2.
(Arrow Indicates Actual Direction of Current Flow)
FIGURE 5. Adjust Pin Currents
Not Ground Referenced
Ground Referenced
20148229
20148230