LP2957A_15 Datasheet, PDF (9/24 Pages) Texas Instruments – LP2957/LP2957A 5V Low-Dropout Regulator for μP Applications

LP2957, LP2957A
www.ti.com
APPLICATION HINTS
SNVS102C – JUNE 1998 – REVISED APRIL 2013
EXTERNAL CAPACITORS
A 2.2 µF (or greater) capacitor is required between the output pin and ground to assure stability (refer to
Figure 30). Without this capacitor, the part may oscillate. Most type of tantalum or aluminum electrolytics will
work here. Film types will work, but are more expensive. Many aluminum electrolytics contain electrolytes which
freeze at −30°C, which requires the use of solid tantalums 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 ESR may increase by a
factor of 20 or 30 as the temperature is reduced from 25°C to −30°C). The value of this capacitor may be
increased without limit. At lower values of output current, less output capacitance is required for stability. The
capacitor can be reduced to 0.68 µF for currents below 10 mA or 0.22 µF for currents below 1 mA.
A 1 µF capacitor should be placed from the 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 input is used. This capacitor may have to be increased if the
regulator is wired for snap-on/snap-off output and the source impedance is high (see Snap-On/Snap-Off
Operation section).
SHUTDOWN INPUT
A logic-level signal will shut off the regulator output when a “LOW” (< 1.2V) is applied to the Shutdown input.
To prevent possible mis-operation, the Shutdown input must be actively terminated. If the input is driven from
open-collector logic, a pull-up resistor (20 kΩ to 100 kΩ recommended) must be connected from the Shutdown
input to the regulator input.
If the Shutdown input is driven from a source that actively pulls high and low (like an op-amp), the pull-up resistor
is not required, but may be used.
If the shutdown function is not to be used, the cost of the pull-up resistor can be saved by tying the Shutdown
input directly to the regulator input.
IMPORTANT: Since the Absolute Maximum Ratings state that the Shutdown input can not go more than 0.3V
below ground, the reverse-battery protection feature which protects the regulator input is sacrificed if the
Shutdown input is tied directly to the regulator input.
If reverse-battery protection is required in an application, the pull-up resistor between the Shutdown input and the
regulator input must be used.
MINIMUM LOAD
It should be noted that a minimum load current is specified in several of the electrical characteristic test
conditions, so the value listed must be used to obtain correlation on these tested limits. The part is parametrically
tested down to 100 µA, but is functional with no load.
DROPOUT VOLTAGE
The dropout voltage of the regulator is defined as the minimum input-to-output voltage differential required for the
output voltage to stay within 100 mV of the output voltage measured with a 1V differential. The dropout voltages
for various values of load current are listed under Electrical Characteristics.
If the regulator is powered from a transformer connected to the AC line, the minimum AC line voltage and
maximum load current must be used to measure the minimum voltage at the input of the regulator. The
minimum input voltage is the lowest voltage level including ripple on the filter capacitor . It is also advisable to
verify operation at minimum operating ambient temperature , since the increasing ESR of the filter capacitor
makes this a worst-case test due to increased ripple amplitude.
Copyright © 1998–2013, Texas Instruments Incorporated
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