Datasheet

MCP651/1S/2/3/4/5/9
DS22146C-page 26  2009-2011 Microchip Technology Inc.
4.3.0.3 Power Dissipation
Since the output short circuit current (I
SC
) is specified
at ±100 mA (typical), these op amps are capable of
both delivering and dissipating significant power. Two
common loads, and their impact on the op amp’s power
dissipation, will be discussed.
Figure 4-7 shows a resistive load (R
L
) with a DC output
voltage (V
OUT
). V
L
is R
L
’s ground point, V
SS
is usually
ground (0V) and I
OUT
is the output current. The input
currents are assumed to be negligible.
FIGURE 4-7: Diagram for Resistive Load
Power Calculations.
The DC currents are:
EQUATION 4-1:
The DC op amp power is:
EQUATION 4-2:
The maximum op amp power, for resistive loads at DC,
occurs when V
OUT
is halfway between V
DD
and V
L
or
halfway between V
SS
and V
L
:
EQUATION 4-3:
Figure 4-7 shows a capacitive load (C
L
), which is
driven by a sine wave with DC offset. The capacitive
load causes the op amp to output higher currents at
higher frequencies. Because the output rectifies I
OUT
,
the op amp’s dissipated power increases (even though
the capacitor does not dissipate power).
FIGURE 4-8: Diagram for Capacitive Load
Power Calculations.
The output voltage is assumed to be:
EQUATION 4-4:
The op amp’s currents are:
EQUATION 4-5:
The op amp’s instantaneous power, average power
and peak power are:
EQUATION 4-6:
The power dissipated in a package depends on the
powers dissipated by each op amp in that package:
MCP65X
V
DD
V
OUT
R
L
V
L
I
DD
I
SS
I
OUT
V
SS
I
OUT
V
OUT
V
L
–
R
L
--------------------------=
I
DD
I
Q
max 0 I
OUT
,+ 
I
SS
I–
Q
min 0 I
OUT
,+ 
Where:
I
Q
= Quiescent supply current for one
op amp (mA/amplifier)
V
OUT
= A DC value (V)
P
OA
I
DD
V
DD
V
OUT
–I
SS
V
SS
V
OUT
–+=
max P
OA
I
DD
V
DD
V
SS
–=
max
2
V
DD
V
L
– V
L
V
SS
–
4R
L
------------------------------------------------------------------+
C
L
MCP65X
V
DD
V
OUT
I
DD
I
SS
I
OUT
V
SS
V
OUT
V
DC
V
AC
 tsin+=
Where:
V
DC
= DC offset (V)
V
AC
= Peak output swing (V
PK
)
 = Radian frequency (2 f) (rad/s)
I
OUT
C
L
dV
OUT
dt
-----------------
 V
AC
 C
L
 tcos==
I
DD
I
Q
max 0 I
OUT
,+ 
I
SS
I–
Q
min 0 I
OUT
,+ 
Where:
I
Q
= Quiescent supply current for one
op amp (mA/amplifier)
P
OA
I
DD
V
DD
V
OUT
–I
SS
V
SS
V
OUT
–+=
ave P
OA
 V
DD
V
SS
–I
Q
4V
AC
fC
L

------------------------+


=
max P
OA
 V
DD
V
SS
–I
Q
2V
AC
fC
L
+=