LP3928 Datasheet, PDF (9/11 Pages) National Semiconductor (TI) – High Speed Bi-Directional Level Shifter and Ultra Low-Dropout CMOS Voltage Regulator

Typical Performance Characteristics Unless otherwise specified: C_VBAT = 1 µF, C_VCCA = 1 µF,
C_VCCB = 1 µF, V_BAT = 3.3V, V_CCA = 1.8V, T_A = 25˚C. (Continued)
Power Supply Rejection Ratio (V_BAT=3.46V)
20039108
Application Hints
EXTERNAL CAPACITORS
Like any low-dropout regulator, the LP3928 requires external
capacitors for regulator stability. The LP3928 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 ≅ 1 µF is required between the
LP3928 V_BAT pin and ground (the amount of the capacitance
may be increased without limit).
This capacitor must be located a distance of not more than
1 cm from the V_BAT 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 ≅ 1 µF over the entire operating temperature
range.
FAST ON-TIME
The LP3928 utilizes a speed up circuitry to ramp up the
internal V_REF voltage to its final value to achieve a fast
output turn on time.
CAPACITOR CHARACTERISTICS
The LP3928 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 1 µF to 4.7 µF 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 1 µF
ceramic capacitor is in the range of 20 mΩ to 40 mΩ, which
easily meets the ESR requirement for stability by the
LP3928.
The ceramic capacitor’s capacitance can vary with tempera-
ture.
Most large value ceramic capacitors (≅ 2.2 µF) are manufac-
tured with Z5U or Y5V temperature characteristics, which
results in the capacitance dropping by more than 50% as the
temperature goes from 25˚C to 85˚C.
A better choice for temperature coefficient in ceramic capaci-
tor is X7R, which holds the capacitance within ±15%.
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
1 µF to 4.7 µF 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.
OUTPUT CAPACITOR
The LP3928 is designed specifically to work with very small
ceramic output capacitors, any ceramic capacitor (dielectric
types Z5U, Y5V or X7R) in 1.5 µF to 22 µF range with 5 mΩ
to 500 mΩ ESR range is suitable in the LP3928 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 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.
The output capacitor should be placed as near to the V_CCB
pin as possible.
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