9.2.2.2.1.1 Input Capacitor

An input capacitor is required for stability. It is recommended that a 1-µF capacitor be connected between the LDO input pin and ground. (This capacitance value may be increased without limit.)

The input capacitor must be located a distance of not more than 1 cm from the input pin and returned to a clean analog ground. Any good-quality ceramic, tantalum, or film capacitor may be used at the input.

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 remains approximately 1 µF over the entire operating temperature range.

9.2.2.2.1.2 Output Capacitor

The LDOs on the LP3910 are designed specifically to work with very small ceramic output capacitors. A 1-μF ceramic capacitor (temperature types Z5U, Y5V or X7R) with ESR between 5 mΩ to 500 mΩ, are suitable in the application circuit.

Tantalum or film capacitors may also be used at the device output, C_OUT (or V_OUT), 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 mΩ to 500 mΩ for stability.

9.2.2.2.1.3 Capacitor Characteristics

The LDOs are 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 mΩ to 40 mΩ, which easily meets the ESR requirement for stability for the LDOs.

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 must 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 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 76 shows a typical graph comparing different capacitor case sizes.

Figure 76. Typical Variation in Capacitance vs DC Bias

As shown in Figure 76, increasing the DC Bias condition can result in the capacitance value that falls below the minimum value given in the recommended capacitor specifications table. Note that Figure 76 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 (for example, 0402) may not be suitable in the actual application.

Capacitance of a ceramic capacitor can vary with temperature. The capacitor type X7R, which operates over a temperature range of −55°C to +125°C, only varies the capacitance to within ±15%. The capacitor type X5R has a similar tolerance over a reduced temperature range of −55°C to +85°C. Many large value ceramic capacitors, larger than 1 μF are manufactured with Z5U or Y5V temperature characteristics. Their capacitance can drop by more than 50% as the temperature varies from 25°C to 85°C. Therefore X7R is recommended over Z5U and Y5V in applications where the ambient temperature changes 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 0.47-μF to 4.7-μF range.

Another important consideration is that tantalum capacitors have higher ESR values than equivalent size ceramics. This means that while it may be possible to find a tantalum capacitor with an ESR value within the stable range, it would have to be larger in capacitance (which means bigger and more costly) than a ceramic capacitor with the same ESR value. The ESR of a typical tantalum increases about 2:1 as the temperature goes from +25°C down to −40°C, so some guard band must be allowed.

9.2.2.2.1.4 Noise Bypass Capacitors for VREFH Pin

Connecting respectively 100 nF and 1 nF grounded bypass capacitors to the V_REFH pin significantly reduces noise on the LDO outputs. VREFH is a high-impedance node connected to a bandgap reference used for the LDOs. Any significant loading on this node causes a change on the regulated output voltages. For this reason, DC leakage current through these pins must be kept as low as possible for best output voltage accuracy. The types of capacitors best suited for the noise bypass capacitors are ceramic and film capacitors. High-quality ceramic capacitors with either NPI or COG dielectric typically have very low leakage. Polypropylene and polycarbonate film capacitors are available in small surface-mount packages and typically have extremely low leakage current. Residual solder flux is another potential source of leakage, which mandates thorough cleaning of the assembled PCBs.

9.2.2.2.2.1 Input Capacitor Selection for Buck1, Buck2 and Buck-Boost

A ceramic input capacitor of 10 μF, 6.3 V is sufficient for the magnetic DC-DC converters. Place the input capacitor as close as possible to the input of the device. A large value may be used for improved input voltage filtering. The recommended capacitor types are X7R or X5R. Y5V-type capacitors must not be used. DC bias characteristics of ceramic capacitors must be considered when selecting case sizes like 0805 and 0603. The input filter capacitor supplies current to the PFET switch of the DC-DC converter in the first half of each cycle and reduces voltage ripple imposed on the input power source. Low ESR in a ceramic capacitor provides the best noise filtering of the input voltage spikes due to fast current transients. A capacitor with sufficient ripple current rating must be selected. The Input current ripple can be calculated as:

Equation 6.

The worse case is when V_IN = 2 × V_OUT.

9.2.2.2.2.2 Output Capacitor Selection for Buck1, Buck2 and Buck-Boost

A 10-μF, 6.3-V ceramic capacitor must be used on the output of the Buck1 and Buck2 magnetic DC-DC converters. The buck-boost needs a 22-μF capacitor. The output capacitor must be mounted as close as possible to the output of the device. A large value may be used for improved input voltage filtering. The recommended capacitor types are X7R or X5R. Y5V-type capacitors should not be used. DC bias characteristics of ceramic capacitors must be considered when selecting case sizes like 0805 and 0603. DC bias characteristics vary from manufacturer to manufacturer and DC bias curves should be requested from them and analyzed as part of the capacitor selection process.

The output filter capacitor of the magnetic DC-DC converter smooths out current flow from the inductor to the load, helps maintain a steady output voltage during transient load changes and reduces output voltage ripple. These capacitors must be selected with sufficient capacitance and sufficiently low ESD to perform these functions.

The output voltage ripple is caused by the charging and the discharging of the output capacitor and also due to its ESR and can be calculated using Equation 7:

Equation 7.

Voltage peak-to-peak ripple due to ESR can be expressed by Equation 8:

Because the V_PP-C and V_PP-ESR are out of phase, the RMS value can be used to get an approximate value of the peak-to-peak ripple:

Equation 9.

The output voltage ripple is dependent on the inductor current ripple and the ESR of the output capacitor (R_ESR). The R_ESR is frequency dependent as well as temperature dependent. The R_ESR must be calculated with the applicable switching frequency and ambient temperature.