03-01-2013, 03:03 PM
LP2950/LP2951 Series of Adjustable Micropower Voltage Regulators
1Micropower Voltage.pdf (Size: 651.85 KB / Downloads: 17)
General Description
The LP2950 and LP2951 are micropower voltage regulators
with very low quiescent current (75μA typ.) and very low
dropout voltage (typ. 40mV at light loads and 380mV at
100mA). They are ideally suited for use in battery-powered
systems. Furthermore, the quiescent current of the LP2950/
LP2951 increases only slightly in dropout, prolonging battery
life.
The LP2950-5.0 is available in the surface-mount D-Pak
package, and in the popular 3-pin TO-92 package for pincompatibility
with older 5V regulators. The 8-lead LP2951 is
available in plastic, ceramic dual-in-line, LLP, or metal can
packages and offers additional system functions.
One such feature is an error flag output which warns of a low
output voltage, often due to falling batteries on the input. It
may be used for a power-on reset. A second feature is the
logic-compatible shutdown input which enables the regulator
to be switched on and off. Also, the part may be pin-strapped
for a 5V, 3V, or 3.3V output (depending on the version), or
programmed from 1.24V to 29V with an external pair of resistors.
Careful design of the LP2950/LP2951 has minimized all contributions
to the error budget. This includes a tight initial
tolerance (.5% typ.), extremely good load and line regulation
(.05% typ.) and a very low output voltage temperature coefficient,
making the part useful as a low-power voltage reference.
Features
■ 5V, 3V, and 3.3V versions available
■ High accuracy output voltage
■ Guaranteed 100mA output current
■ Extremely low quiescent current
■ Low dropout voltage
■ Extremely tight load and line regulation
■ Very low temperature coefficient
■ Use as Regulator or Reference
■ Needs minimum capacitance for stability
■ Current and Thermal Limiting
■ Stable with low-ESR output capacitors (10mΩ to 6Ω)
Application Hints
OUTPUT CAPACITOR REQUIREMENTS
A 1.0 μF (or greater) capacitor is required between the output
and ground for stability at output voltages of 5V or higher. At
lower output voltages, more capacitance is required (2. 2μF
or more is recommended for 3.0V and 3.3V versions). Without
this capacitor the part will oscillate. Most types of tantalum or
aluminum electrolytic work fine here; even film types work but
are not recommended for reasons of cost. Many aluminum
electrolytics have electrolytes that freeze at about −30°C, so
solid tantalums are recommended for operation below −25°
C. The important parameters of the capacitor are an ESR of
about 5Ω or less and a resonant frequency above 500 kHz.
The value of this capacitor may be increased without limit.
INPUT CAPACITOR REQUIREMENTS
A minimum 1 μF tantalum, ceramic or aluminum electrolytic
capacitor should be placed from the LP2950/LP2951 input pin
to ground if there is more than 10 inches of wire between the
input and the AC filter capacitor or if a battery is used as the
input.
ERROR DETECTION COMPARATOR OUTPUT
The comparator produces a logic low output whenever the
LP2951 output falls out of regulation by more than approximately
5%. This figure is the comparator's built-in offset of
about 60mV divided by the 1.235 reference voltage. (Refer to
the block diagram in the front of the datasheet.) This trip level
remains “5% below normal” regardless of the programmed
output voltage of the 2951. For example, the error flag trip
level is typically 4.75V for a 5V output or 11.4V for a 12V output.
The out of regulation condition may be due either to low
input voltage, current limiting, or thermal limiting.
Figure 3 below gives a timing diagram depicting the ERROR
signal and the regulated output voltage as the LP2951
input is ramped up and down. For 5V versions, the ERROR
signal becomes valid (low) at about 1.3V input. It goes high
at about 5V input (the input voltage at which VOUT = 4.75V).
Since the LP2951's dropout voltage is load-dependent (see
curve in typical performance characteristics), the input voltage
trip point (about 5V) will vary with the load current. The
output voltage trip point (approx. 4.75V) does not vary with
load.
REDUCING OUTPUT NOISE
In reference applications it may be advantageous to reduce
the AC noise present at the output. One method is to reduce
the regulator bandwidth by increasing the size of the output
capacitor. This is the only way noise can be reduced on the
3 lead LP2950 but is relatively inefficient, as increasing the
capacitor from 1 μF to 220 μF only decreases the noise from
430 μV(RMS) to 160 μV(RMS) for a 100 kHz bandwidth at 5V
output.
LLP MOUNTING
The SDC08A (No Pullback) 8-Lead LLP package requires
specific mounting techniques which are detailed in National
Semiconductor Application Note # 1187. Referring to the section
PCB Design Recommendations in AN-1187 (Page 5),
it should be noted that the pad style which should be used
with the LLP package is the NSMD (non-solder mask defined)
type. Additionally, it is recommended the PCB terminal pads
to be 0.2 mm longer than the package pads to create a solder
fillet to improve reliability and inspection.
The thermal dissipation of the LLP package is directly related
to the printed circuit board construction and the amount of
additional copper area connected to the DAP.