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LM2776

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SNVSA56A – MAY 2015 – REVISED FEBRUARY 2016

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8.2.2.2 Power Dissipation

(3)

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

(4)

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.