8.2.2.3 Capacitor Selection

The LM2776 requires 3 external capacitors for proper operation. TI recommends surface-mount multi-layer ceramic capacitors. These capacitors are small, inexpensive, and have very low ESR (≤ 15 mΩ typical). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors generally are not recommended for use with the LM2776 due to their high ESR, as compared to ceramic capacitors.

For most applications, ceramic capacitors with an X7R or X5R temperature characteristic are preferred for use with the LM2776. These capacitors have tight capacitance tolerance (as good as ±10%) and hold their value over temperature (X7R: ±15% over –55ºC to 125°C; X5R: ±15% over –55°C to 85°C).

Capacitors with a Y5V or Z5U temperature characteristic are generally not recommended for use with the LM2776. These types of capacitors typically have wide capacitance tolerance (80%, …20%) and vary significantly over temperature (Y5V: 22%, –82% over –30°C to 85°C range; Z5U: 22%, –56% over 10°C to 85°C range). Under some conditions, a 1-µF-rated Y5V or Z5U capacitor could have a capacitance as low as 0.1 µF. Such detrimental deviation is likely to cause Y5V and Z5U capacitors to fail to meet the minimum capacitance requirements of the LM2776.

Net capacitance of a ceramic capacitor decreases with increased DC bias. This degradation can result in lower capacitance than expected on the input and/or output, resulting in higher ripple voltages and currents. Using capacitors at DC bias voltages significantly below the capacitor voltage rating usually minimizes DC bias effects. Consult capacitor manufacturers for information on capacitor DC bias characteristics.

Capacitance characteristics can vary quite dramatically with different application conditions, capacitor types, and capacitor manufacturers. It is strongly recommended that the LM2776 circuit be thoroughly evaluated early in the design-in process with the mass-production capacitors of choice. This helps ensure that any such variability in capacitance does not negatively impact circuit performance.

The voltage rating of the output capacitor must be 10 V or more. For example, a 10-V 0603 1-µF is acceptable for use with the LM2776, as long as the capacitance does not fall below a minimum of 0.5 µF in the intended application. All other capacitors must have a voltage rating at or above the maximum input voltage of the application. Select the capacitors such that the capacitance on the input does not fall below 0.7 µF, and the capacitance of the flying capacitor does not fall below 0.2 µF.