LP38691, LP38693, LP38691-Q1, LP38693-Q1

SNVS321N – JANUARY 2005 – REVISED MARCH 2015

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Important: To ensure stable operation it is essential that good PCB design practices are employed to minimize

ground impedance and keep input inductance low. If these conditions cannot be met, or if long leads are used to

connect the battery or other power source to the LP38691 or LP38693, then it is recommended that the input

capacitor is increased. 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 equivalent series resistance (ESR) on the input capacitor, but tolerance and

temperature coefficient must be considered when selecting the capacitor to ensure the capacitance will remain

approximately 1 µF over the entire operating temperature range.

9.2.2.4 Output Capacitor

The LP3869x is designed specifically to work with very small ceramic output capacitors. A 1-µF ceramic

capacitor (temperature types Z5U, Y5V or X7R/X5R) with ESR between 5 m

Ω to 500 mΩ, is suitable in the

LP3869x application circuit.

For this device the output capacitor should be connected between the OUT pin and GND pin.

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

reasons of size and cost.

The output capacitor must meet the requirement for the minimum value of capacitance and also have an ESR

value that is within the range 5 m

Ω to 500 mΩ for stability.

9.2.2.5 No-Load Stability

The LP3869x will remain stable and in regulation with no external load. This is an important consideration in

some circuits, for example CMOS RAM keep-alive applications.

9.2.2.6 Capacitor Characteristics

The LP3869x 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 0.47 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 20 m

Ω to 40 mΩ, which easily meets the ESR

requirement for stability for the LP3869x.

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 dependent on the particular case size, with smaller sizes giving poorer

performance figures in general. As an example, Figure 27 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 specification 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 (for example, 0402) may not

be suitable in the actual application.

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