Datasheet

LM2876

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SNAS088C –AUGUST 1995–REVISED MARCH 2013

SUPPLY BYPASSING

The LM2876 has excellent power supply rejection and does not require a regulated supply. However, to eliminate

possible oscillations all op amps and power op amps should have their supply leads bypassed with low-

inductance capacitors having short leads and located close to the package terminals. Inadequate power supply

bypassing will manifest itself by a low frequency oscillation known as “motorboating” or by high frequency

instabilities. These instabilities can be eliminated through multiple bypassing utilizing a large tantalum or

electrolytic capacitor (10 μF or larger) which is used to absorb low frequency variations and a small ceramic

capacitor (0.1 μF) to prevent any high frequency feedback through the power supply lines.

If adequate bypassing is not provided the current in the supply leads which is a rectified component of the load

current may be fed back into internal circuitry. This signal causes low distortion at high frequencies requiring that

the supplies be bypassed at the package terminals with an electrolytic capacitor of 470 μF or more.

LEAD INDUCTANCE

Power op amps are sensitive to inductance in the output lead, particularly with heavy capacitive loading.

Feedback to the input should be taken directly from the output terminal, minimizing common inductance with the

load.

Lead inductance can also cause voltage surges on the supplies. With long leads to the power supply, energy is

stored in the lead inductance when the output is shorted. This energy can be dumped back into the supply

bypass capacitors when the short is removed. The magnitude of this transient is reduced by increasing the size

of the bypass capacitor near the IC. With at least a 20 μF local bypass, these voltage surges are important only if

the lead length exceeds a couple feet (> 1 μH lead inductance). Twisting together the supply and ground leads

minimizes the effect.

LAYOUT, GROUND LOOPS AND STABILITY

The LM2876 is designed to be stable when operated at a closed-loop gain of 10 or greater, but as with any other

high-current amplifier, the LM2876 can be made to oscillate under certain conditions. These usually involve

printed circuit board layout or output/input coupling.

When designing a layout, it is important to return the load ground, the output compensation ground, and the low

level (feedback and input) grounds to the circuit board common ground point through separate paths. Otherwise,

large currents flowing along a ground conductor will generate voltages on the conductor which can effectively act

as signals at the input, resulting in high frequency oscillation or excessive distortion. It is advisable to keep the

output compensation components and the 0.1 μF supply decoupling capacitors as close as possible to the

LM2876 to reduce the effects of PCB trace resistance and inductance. For the same reason, the ground return

paths should be as short as possible.

In general, with fast, high-current circuitry, all sorts of problems can arise from improper grounding which again

can be avoided by returning all grounds separately to a common point. Without isolating the ground signals and

returning the grounds to a common point, ground loops may occur.

Product Folder Links: LM2876