LP2953QML_16 Datasheet, PDF (14/26 Pages) Texas Instruments – Adjustable Micropower Low-Dropout Voltage Regulators

LP2953QML, LP2953QML-SP
SNVS395C – NOVEMBER 2010 – REVISED APRIL 2013
APPLICATION HINTS
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Ground Pins
For the LP2953 16–Pin Ceramic SOIC, Pins 1, 8, 9, 16 MUST BE SHORTED TOGETHER ON CUSTOMER'S
P.C. BOARD APPLICATION.
Heatsink Requirements
The maximum allowable power dissipation for the LP2953 is limited by the maximum junction temperature
(+150°C) and the two parameters that determine how quickly heat flows away from the die: the ambient
temperature and the junction-to-ambient thermal resistance of the part.
The military parts which are manufactured in ceramic DIP packages contain a KOVAR lead frame (unlike the
industrial parts, which have a copper lead frame). The KOVAR material is necessary to attain the hermetic seal
required in military applications.
The KOVAR lead frame does not conduct heat as well as copper, which means that the PC board copper can
not be used to significantly reduce the overall junction-to-ambient thermal resistance.
The power dissipation calculations are done using a fixed value for θ_(J–A), the junction-to-ambient thermal
resistance, of 134°C/W and can not be changed by adding copper foil patterns to the PC board. This leads to an
important fact: The maximum allowable power dissipation in any application using the LP2953 is dependent only
on the ambient temperature:
(1)
External Capacitors
A 2.2 μF (or greater) capacitor is required between the output pin and ground to assure stability when the output
is set to 5V. Without this capacitor, the part will 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.
Programming the output for voltages below 5V runs the error amplifier at lower gains requiring more output
capacitance for stability. At 3.3V output, a minimum of 4.7 μF is required. For the worst-case condition of 1.23V
output and 250 mA of load current, a 6.8 μF (or larger) capacitor should be used.
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.
Stray capacitance to the Feedback terminal can cause instability. This problem is most likely to appear when
using high value external resistors to set the output voltage. Adding a 100 pF capacitor between the Output and
Feedback pins and increasing the output capacitance to 6.8 μF (or greater) will cure the problem.
Minimum Load
When setting the output voltage using an external resistive divider, a minimum current of 1 μA is recommended
through the resistors to provide a minimum load.
It should be noted that a minimum load current is specified in several of the electrical characteristic test
conditions, so this value must be used to obtain correlation on these tested limits.
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