LM8272 datasheet(13/14 Pages) NSC

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Application Notes (Continued)

OUTPUT SHORT CIRCUIT CURRENT AND

DISSIPATION ISSUES:

The LM8272 output stage is designed for maximum output

current capability. Even though momentary output shorts to

ground and either supply can be tolerated at all operating

voltages, longer lasting short conditions can cause the junc-

tion temperature to rise beyond the absolute maximum rat-

ing of the device, especially at higher supply voltage condi-

tions. Below supply voltage of 6V, output short circuit

condition can be tolerated indefinitely.

With the Op Amp tied to a load, the device power dissipation

consists of the quiescent power due to the supply current

flow into the device, in addition to power dissipation due to

the load current. The load portion of the power itself could

include an average value (due to a DC load current) and an

AC component. DC load current would flow if there is an

output voltage offset, or the output AC average current is

non-zero, or if the Op Amp operates in a single supply

application where the output is maintained somewhere in the

range of linear operation. Therefore:

total =PQ +PDC +PAC

Q =IS ·VS

Op Amp Quiescent Power

Dissipation

DC =IO ·(Vr -Vo)

DC Load Power

AC = See Table 1 below

AC Load Power

where:

S: Supply Current

S: Total Supply Voltage (V

+ -V−)

O: Average Output Voltage

r:V

+ for sourcing and V− for sinking current

Table 1 below shows the maximum AC component of the

load power dissipated by the Op Amp for standard Sinusoi-

dal, Triangular, and Square Waveforms:

TABLE 1. Normalized AC Power Dissipated in the

Output Stage for Standard Waveforms

AC (W.

Ω/V2)

Sinusoidal

Triangular

Square

50.7 x 10

−3

46.9 x 10

−3

62.5 x 10

−3

The table entries are normalized to V

2/R

L. To figure out the

AC load current component of power dissipation, simply

multiply the table entry corresponding to the output wave-

form by the factor V

2/R

L. For example, with ±12V supplies,

a 600

Ω load, and triangular waveform power dissipation in

the output stage is calculated as:

AC = (46.9 x 10

−3) · [242/600] = 45.0mW

OTHER APPLICATION HINTS:

The use of supply decoupling is mandatory in most applica-

tions. As with most relatively high speed/high output current

Op Amps, best results are achieved when each supply line is

decoupled with two capacitors; a small value ceramic ca-

pacitor (

∼0.01µF) placed very close to the supply lead in

addition to a large value Tantalum or Aluminum (> 4.7µF).

The large capacitor can be shared by more than one device

if necessary. The small ceramic capacitor maintains low

supply impedance at high frequencies while the large ca-

pacitor will act as the charge “bucket” for fast load current

spikes at the Op Amp output. The combination of these

capacitors will provide supply decoupling and will help keep

the Op Amp oscillation free under any load.

LM8272 ADVANTAGES:

Compared to other Rail-to-Rail Input/Output devices, the

LM8272 offers several advantages such as:

•

Improved cross over distortion

•

Nearly constant supply current throughout the output

voltage swing range and close to either rail.

•

Nearly constant Unity gain frequency (f

u) and Phase

Margin (Phi

m) for all operating supplies and load condi-

tions.

•

No output phase reversal under input overload condition.

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