lm8272mmx National Semiconductor Corporation, lm8272mmx Datasheet - Page 13

• Manufacturer Part Number: lm8272mmx
• Description: Rrio, High Output Current & Unlimited Cap Load Op Amp In Miniature Package
• Manufacturer: National Semiconductor Corporation

Application Notes
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:
where:
I
V
V
V
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:
S
P
P
P
P
S
O
r
: Supply Current
total
Q
DC
AC
: V
: Total Supply Voltage (V
: Average Output Voltage
= I
+
= I
= See Table 1 below
= P
for sourcing and V
S
O
· V
Q
· (V
S
+ P
r
- V
DC
o
+ P
)
AC
−
for sinking current
+
- V
−
(Continued)
)
Op Amp Quiescent Power
DC Load Power
AC Load Power
Dissipation
13
The table entries are normalized to V
AC load current component of power dissipation, simply
multiply the table entry corresponding to the output wave-
form by the factor V
a 600Ω load, and triangular waveform power dissipation in
the output stage is calculated as:
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 (
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
• Nearly constant Unity gain frequency (f
• No output phase reversal under input overload condition.
TABLE 1. Normalized AC Power Dissipated in the
voltage swing range and close to either rail.
Margin (Phi
tions.
50.7 x 10
Sinusoidal
P
AC
Output Stage for Standard Waveforms
= (46.9 x 10
−3
m
) for all operating supplies and load condi-
S
2
/R
P
−3
L
46.9 x 10
AC
. For example, with
Triangular
) · [24
(W.Ω/V
2
/600] = 45.0mW
−3
2
)
S
2
/R
L
. To figure out the
62.5 x 10
±
u
12V supplies,
) and Phase
Square
www.national.com
>
4.7µF).
−3