9.2.2.1.1 Inductor Selection for SW1 and SW2

TI recommends a nominal inductor value of 2.2 µH. It is important to ensure the inductor core does not saturate during any foreseeable operational situation.

Care should be taken when reviewing the different saturation current ratings that are specified by different manufacturers. Saturation current ratings are typically specified at 25°C, so ratings at maximum ambient temperature of the application should be requested from the manufacturer.

There are two methods to choose the inductor saturation current rating:

Recommended method:

The best way to ensure the inductor does not saturate is to choose an inductor that has saturation current rating greater than the maximum LM26480 current limit of 2.4 A. In this case the device prevents inductor saturation.

Alternate method:

If the recommended approach cannot be used, care must be taken to ensure that the saturation current is greater than the peak inductor current:

Equation 10.

where

• I_SAT: Inductor saturation current at operating temperature
• I_LPEAK: Peak inductor current during worst case conditions
• I_OUTMAX: Maximum average inductor current
• I_RIPPLE: Peak-to-peak inductor current
• V_OUT: Output voltage
• V_IN: Input voltage
• L: Inductor value in Henries at I_OUTMAX
• F: Switching frequency, Hertz
• D: Estimated duty factor
• EFF: Estimated power supply efficiency

I_SAT may not be exceeded during any operation, including transients, start-up, high temperature, worst-case conditions, etc.

9.2.2.1.2 Suggested Inductors and Their Suppliers

Table 7. Suggested Capacitors and Their Suppliers

9.2.2.4.1 Input Capacitor

An input capacitor is required for stability. TI recommends that a 1-μF capacitor be connected between the LDO input 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 analog ground. Any good quality ceramic, tantalum, or film capacitor may be used at the input.

9.2.2.4.2 Output Capacitor

The LDOs on the LM26480 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. It is also possible to use tantalum or film capacitors 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.

9.2.2.4.3 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 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.

Figure 27. Capacitance vs DC Bias

As shown in Figure 27, 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 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.

The ceramic capacitor’s capacitance 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. 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 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 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. It should also be noted that the ESR of a typical tantalum will increase about 2:1 as the temperature goes from 25°C down to −40°C, so some guard band must be allowed.