Datasheet

P
D
=
(T
JMAX
- T
A
)
T
JA
LP5900
www.ti.com SNVS358O –JULY 2005–REVISED MAY 2013
APPLICATION HINTS
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 in Operating Ratings, the allowable power dissipation for the device
in a given package can be calculated using the equation:
(1)
The actual power dissipation across the device can be represented by the following equation:
P
D
= (V
IN
– V
OUT
) x 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
Like any low-dropout regulator, the LP5900 requires external capacitors for regulator stability. The LP5900 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 capacitor is required for stability. The input capacitor should be at least equal to or greater than the
output capacitor. It is recommended that a 0.47 µF capacitor be connected between the LP5900 input pin and
ground.
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: To ensure stable operation it is essential that good PCB practices are employed to minimize ground
impedance and keep input inductance low. If these conditions cannot be met, or if long leads are to be used to
connect the battery or other power source to the LP5900, then it is recommended to increase the input capacitor
to at least 2.2 µF. Also, 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 ensured 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
0.47 μF ±30% over the entire operating temperature range.
OUTPUT CAPACITOR
The LP5900 is designed specifically to work with very small ceramic output capacitors. A ceramic capacitor
(dielectric types X5R or X7R) in the 0.47 μF to 10 μF range, and with ESR between 5 mΩ to 500 mΩ, is suitable
in the LP5900 application circuit. For this device the output capacitor should be connected between the V
OUT
pin
and a good ground connection and should be mounted within 1 cm of the device.
It may also be 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 CAPACITOR CHARACTERISTICS section below).
The output capacitor must meet the requirement for the minimum value of capacitance and have an ESR value
that is within the range 5 mΩ to 500 mΩ for stability.
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