LP5951

www.ti.com

SNVS345G – JUNE 2006 – REVISED DECEMBER 2014

The actual power dissipation across the device can be estimated by the following equation:

PD ≈ (VIN – VOUT) × IOUT

(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.

8.2.2.2 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.

8.2.2.3 Input Capacitor

An input capacitor is required for stability. It is recommended that a 1-µF capacitor be connected between the

LP5951 IN 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 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

≥

0.7 µF over the entire operating temperature range.

8.2.2.4 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-µF range (up to 47 µF) and with ESR between 3 m

Ω to 500 mΩ is

suitable in the LP5951 application circuit.

This capacitor must be located a distance of not more than 1 cm from the OUT pin and returned to a clean

analogue ground.

It is also possible to use tantalum or film capacitors at the device output, VOUT, but these are not as attractive for

reasons of size and cost (see Capacitor Characteristics).

8.2.2.5 Capacitor Characteristics

The LP5951 is designed to work with ceramic capacitors on the output to take advantage of the benefits they

offer. For capacitance values in the range of 1 µF to 4.7 µF, ceramic capacitors are the smallest, least expensive

and have the lowest ESR values, thus making them best for eliminating high frequency noise. The ESR of a

typical 1-µF ceramic capacitor is in the range of 3 m

Ω to 40 mΩ, which easily meets the ESR requirement for

stability for the LP5951.

For both input and output capacitors, careful interpretation of the capacitor specification is required to ensure

correct device operation. The capacitor value can change greatly, depending on the operating conditions and

capacitor type.

In particular, the output capacitor selection should take account of all the capacitor parameters, to ensure that the

specification is met within the application. The capacitance can vary with DC bias conditions as well as

temperature and frequency of operation. Capacitor values will also show some decrease over time due to aging.

The capacitor parameters are also dependant on the particular case size, with smaller sizes giving poorer

performance figures in general. As an example, Figure 11 shows a typical graph comparing different capacitor

case sizes in a Capacitance vs. DC Bias plot. As shown in the graph, increasing the DC Bias condition can result

in the capacitance value falling below the minimum value given in the recommended capacitor specifications

table (0.7 µF in this case). Note that the graph shows the capacitance out of spec for the 0402 case size

capacitor at higher bias voltages. It is therefore recommended that the capacitor manufacturers’ specifications for

the nominal value capacitor are consulted for all conditions, as some capacitor sizes (such as 0402) may not be

suitable in the actual application.

Copyright © 2006–2014, Texas Instruments Incorporated

Submit Documentation Feedback

11

Product Folder Links: LP5951