8.2.2.1 Efficiency

Charge-pump efficiency is defined as

8.2.2.2 Power Dissipation

LM2776 power dissipation (P_D) is calculated simply by subtracting output power from input power:

Power dissipation increases with increased input voltage and output current. Internal power dissipation self-heats the device. Dissipating this amount power/heat so the LM2776 does not overheat is a demanding thermal requirement for a small surface-mount package. When soldered to a PCB with layout conducive to power dissipation, the thermal properties of the SOT package enable this power to be dissipated from the LM2776 with little or no derating, even when the circuit is placed in elevated ambient temperatures when the output current is 200 mA or less.

8.2.2.3 Capacitor Selection

The LM2776 requires 3 external capacitors for proper operation. TI recommends urface-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.

8.2.2.4 Output Capacitor and Output Voltage Ripple

The peak-to-peak output voltage ripple is determined by the oscillator frequency, the capacitance and ESR of the output capacitor C_OUT:

In typical applications, a 1-µF low-ESR ceramic output capacitor is recommended. Different output capacitance values can be used to reduce ripple shrink the solution size, and/or cut the cost of the solution. But changing the output capacitor may also require changing the flying capacitor and/or input capacitor to maintain good overall circuit performance.

High ESR in the output capacitor increases output voltage ripple. If a ceramic capacitor is used at the output, this is usually not a concern because the ESR of a ceramic capacitor is typically very low and has only a minimal impact on ripple magnitudes. If a different capacitor type with higher ESR is used (tantalum, for example), the ESR could result in high ripple. To eliminate this effect, the net output ESR can be significantly reduced by placing a low-ESR ceramic capacitor in parallel with the primary output capacitor. The low ESR of the ceramic capacitor is in parallel with the higher ESR, resulting in a low net ESR based on the principles of parallel resistance reduction.

8.2.2.5 Input Capacitor

The input capacitor (C_IN) is a reservoir of charge that aids a quick transfer of charge from the supply to the flying capacitors during the charge phase of operation. The input capacitor helps to keep the input voltage from drooping at the start of the charge phase when the flying capacitors are connected to the input. It also filters noise on the input pin, keeping this noise out of sensitive internal analog circuitry that is biased off the input line.

Much like the relationship between the output capacitance and output voltage ripple, input capacitance has a dominant and first-order effect on input ripple magnitude. Increasing (decreasing) the input capacitance results in a proportional decrease (increase) in input voltage ripple. Input voltage, output current, and flying capacitance also affects input ripple levels to some degree.

In typical applications, a 1-µF low-ESR ceramic capacitor is recommended on the input. When operating near the maximum load of 200 mA, a minimum recommended input capacitance after taking into the DC-bias derating is 2 µF or larger. Different input capacitance values can be used to reduce ripple, shrink the solution size, and/or cut the cost of the solution.

8.2.2.6 Flying Capacitor

The flying capacitor (C1) transfers charge from the input to the output. Flying capacitance can impact both output current capability and ripple magnitudes. If flying capacitance is too small, the LM2776 may not be able to regulate the output voltage when load currents are high. On the other hand, if the flying capacitance is too large, the flying capacitor might overwhelm the input and output capacitors, resulting in increased input and output ripple.

In typical high-current applications, TI recommends 0.47-µF or 1-µF 10 V low-ESR ceramic capacitors for the flying capacitors. Polarized capacitors (tantalum, aluminum electrolytic, etc.) must not be used for the flying capacitor, as they could become reverse-biased during LM2776 operation.