0

1.0

2.0

3.0

4.0

5.0

DC BIAS (V)

100%

80%

60%

40%

20%

0402, 6.3V, X5R

0603, 10V, X5R

LP3997

SNVS272B – MAY 2004 – REVISED MAY 2013

www.ti.com

X5R or X7R is recommended. This capacitor is connected directly to high impedance node in the band gap

reference circuit. Any significant loading on this node will cause a change in the regulated output voltage. For this

reason, DC leakage current from this pin must be kept as low as possible for best output voltage accuracy.

CAPACITOR CHARACTERISTICS

In common with most regulators, the LP3997 requires external capacitors for regulator stability. The LP3997 is

specifically designed for portable applications requiring minimum board space and can use capacitors in the

range 1µF to 4.7µF.These capacitors must be correctly selected for good performance. Ceramic capacitors are

the smallest, least expensive and have the lowest ESR values (which makes them best for eliminating high

frequency noise). The ESR of a typical 1µF ceramic capacitor is in the range of 20 m

Ω to 40 mΩ, which easily

meets the ESR requirement for stability by the LP3997.These capacitors must be correctly selected to ensure

good performance of the LP3997.

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 dependant on the conditions of operation 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. Capacitance value 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 1 shows a typical graph showing a comparison of

capacitor case sizes in a Capacitance versus DC Bias plot. As shown in the graph, as a result of the DC Bias

condition, the capacitance value may drop below the minimum capacitance value given in the recommended

capacitor table (0.7µF 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 (e.g. 0402)

may not be suitable in the actual application.

Figure 1. Capacitance versus DC Bias Plot

The value of ceramic capacitors can vary with temperature. The capacitor type X7R, which operates over a

temperature range of -55°C to +125°C, will only vary the capacitance to within ±15%. The capacitor type X5R

has a similar tolerance over a reduced temperature range of -55°C to +85°C. Most large value ceramic

capacitors, larger than 1µF are manufactured with Z5U or Y5V temperature characteristics. Their capacitance

can drop by more than 50% as the temperature goes from 25°C to 85°C. Therefore X7R is recommended over

Z5U and Y5V in applications where the ambient temperature will change significantly above or below 25°C.

Tantalum capacitors are less desirable than ceramic for use as output capacitors because they are more

expensive when comparing equivalent capacitance and voltage ratings in the 1µF to 4.7µF range.

10

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