LP3985ITLX-3.2 National Semiconductor, LP3985ITLX-3.2 Datasheet - Page 13

• Manufacturer Part Number: LP3985ITLX-3.2
• Description: Micropower/ 150mA Low-Noise Ultra Low-Dropout CMOS Voltage Regulator
• Manufacturer: National Semiconductor

Typical Performance Characteristics
C
Application Hints
EXTERNAL CAPACITORS
Like any low-dropout regulator, the LP3985 requires external
capacitors for regulator stability. The LP3985 is specifically
designed for portable applications requiring minimum board
space and smallest components. These capacitors must be
correctly selected for good performance.
INPUT CAPACITOR
An input capacitance of ) 1µF is required between the
LP3985 input pin and ground (the amount of the capacitance
may be increased without limit).
This capacitor must be located a distance of not more than
1cm from the input pin and returned to a clean analog
ground. Any good quality ceramic, tantalum, or film capacitor
may be used at the input.
Important: Tantalum capacitors can suffer catastrophic fail-
ures due to surge current when connected to a low-
impedance source of power (like a battery or a very large
capacitor). If a tantalum capacitor is used at the input, it must
be guaranteed by the manufacturer to have a surge current
rating sufficient for the application.
There are no requirements for the ESR on the input capaci-
tor, but tolerance and temperature coefficient must be con-
sidered when selecting the capacitor to ensure the capaci-
tance will be ) 1µF over the entire operating temperature
range.
OUTPUT CAPACITOR
The LP3985 is designed specifically to work with very small
ceramic output capacitors. A ceramic capacitor (temperature
characteristics X7R, X5R, Z5U, or Y5V) in 1 to 22 µF range
with 5mΩ to 500mΩ ESR range is suitable in the LP3985
application circuit.
It may also be possible to use tantalum or film capacitors at
the output, but these are not as attractive for reasons of size
and cost (see next section Capacitor Characteristics).
The output capacitor must meet the requirement for mini-
mum amount of capacitance and also have an ESR (Equiva-
lent Series Resistance) value which is within a stable range
(5 mΩ to 500 mΩ).
BYPASS
= 0.01 µF, V
Output Impedance (V
IN
= V
OUT
+ 0.2V, T
IN
= 4.2V)
A
= 25˚C, Enable pin is tied to V
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Unless otherwise specified, C
13
NO-LOAD STABILITY
The LP3985 will remain stable and in regulation with no
external load. This is specially important in CMOS RAM
keep-alive applications.
CAPACITOR CHARACTERISTICS
The LP3985 is designed to work with ceramic capacitors on
the output to take advantage of the benefits they offer: for
capacitance values in the range of 1µF to 4.7µF range,
ceramic capacitors are the smallest, least expensive and
have the lowest ESR values (which makes them best for
eliminating high frequency noise). The ESR of a typical 1µF
ceramic capacitor is in the range of 20 mΩ to 40 mΩ, which
easily meets the ESR requirement for stability by the
LP3985.
The ceramic capacitor’s capacitance can vary with tempera-
ture. The capacitor type X7R, which operates over a tem-
perature range of -55˚C to +125˚C, will only vary the capaci-
tance to within
() 2.2µF) are manufactured with Z5U or Y5V temperature
characteristics. Their capacitance can drop by more than
50% as the temperature goes from 25˚C to 85˚C. Therefore,
X7R is recommended over Z5U and Y5V in applications
where the ambient temperature will change significantly
above or below 25˚C.
Tantalum capacitors are less desirable than ceramic for use
as output capacitors because they are more expensive when
comparing equivalent capacitance and voltage ratings in the
1µF to 4.7µF range.
Another important consideration is that tantalum capacitors
have higher ESR values than equivalent size ceramics. This
means that while it may be possible to find a tantalum
capacitor with an ESR value within the stable range, it would
have to be larger in capacitance (which means bigger and
more costly ) than a ceramic capacitor with the same ESR
value. It should also be noted that the ESR of a typical
tantalum will increase about 2:1 as the temperature goes
from 25˚C down to −40˚C, so some guard band must be
allowed.
NOISE BYPASS CAPACITOR
Connecting a 0.01µF capacitor between the C
and ground significantly reduces noise on the regulator out-
put. This cap is connected directly to a high impedance node
IN
. (Continued)
Output Impedance (V
±
15%. Most large value ceramic capacitors
IN
= C
OUT
IN
= V
= 1 µF Ceramic,
OUT
+ 0.2V)
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BYPASS
pin