LP5951_14 Datasheet, PDF (8/26 Pages) Texas Instruments – Micropower, 150mA Low-Dropout CMOS Voltage Regulator

LP5951
SNVS345E – JUNE 2006 – REVISED JANUARY 2011
APPLICATION HINTS
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POWER DISSIPATION AND DEVICE OPERATION
The permissible power dissipation for any package is a measure of the capability of the device to pass heat from
the power source, the junctions of the IC, to the ultimate heat sink, the ambient environment. Thus the power
dissipation is dependent on the ambient temperature and the thermal resistance across the various interfaces
between the die and ambient air.
As stated (see below⁽¹⁾) in the electrical specification section, the allowable power dissipation for the device in a
given package can be calculated using the equation:
P_D = (T_J(MAX) - T_A) / θ_JA
(1)
With a θ_JA = 220°C/W, the device in the SOT-23-5 package returns a value of 454 mW with a maximum junction
temperature of 125°C at T_A of 25°C.
The actual power dissipation across the device can be estimated by the following equation:
P_D ≈ (V_IN - V_OUT) * I_OUT
(2)
This establishes the relationship between the power dissipation allowed due to thermal consideration, the voltage
drop across the device, and the continuous current capability of the device. These two equations should be used
to determine the optimum operating conditions for the device in the application.
EXTERNAL CAPACITORS
As is common with most regulators, the LP5951 requires external capacitors to ensure stable operation. The
LP5951 is specifically designed for portable applications requiring minimum board space and the smallest size
components. These capacitors must be correctly selected for good performance.
INPUT CAPACITOR
An input capacitor is required for stability. It is recommended that a 1.0µF capacitor be connected between the
LP5951 input pin and ground (this capacitance value may be increased without limit).
This capacitor must be located a distance of not more than 1 cm from the input pin and returned to a clean
analogue ground. Any good quality ceramic, tantalum, or film capacitor may be used at the input.
Important: Tantalum capacitors can suffer catastrophic failures 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 (Equivalent Series Resistance) on the input capacitor, but tolerance and
temperature coefficient must be considered when selecting the capacitor to ensure the capacitance will remain
≥0.7µF over the entire operating temperature range.
OUTPUT CAPACITOR
The LP5951 is designed specifically to work with very small ceramic output capacitors. A ceramic capacitor
(dielectric types X7R, Z5U, or Y5V) in the 1.0µF range (up to 47µF) and with ESR between 3 mΩ to 500 mΩ is
suitable in the LP5951 application circuit.
This capacitor must be located a distance of not more than 1cm from the V_OUT pin and returned to a clean
analogue ground.
It is also possible to use tantalum or film capacitors at the device output, V_OUT, but these are not as attractive for
reasons of size and cost (see the section CAPACITOR CHARACTERISTICS).
(1) In applications where high power dissipation and/or poor package thermal resistance is present, the maximum ambient temperature may
have to be derated. Maximum ambient temperature (T_A-MAX) is dependent on the maximum operating junction temperature (T_J-MAX-OP =
125°C), the maximum power dissipation of the device in the application (P_D-MAX), and the junction-to ambient thermal resistance of the
part/package in the application (θ_JA), as given by the following equation: T_A-MAX = T_J-MAX-OP – (θ_JA × P_D-MAX).
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