Datasheet

MCP621/1S/2/3/4/5/9
DS22188C-page 26 © 2009-2011 Microchip Technology Inc.
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:
EQUATION 4-7:
The maximum ambient-to-junction temperature rise
(ΔT
JA
) and junction temperature (T
J
) can be calculated
using the maximum expected package power (P
PKG
),
ambient temperature (T
A
) and the package thermal
resistance (θ
JA
) found in Table 1-4:
EQUATION 4-8:
The worst-case power derating for the op amps in a
particular package can be easily calculated:
EQUATION 4-9:
Several techniques are available to reduce ΔT
JA
for a
given package:
• Reduce θ
JA
- Use another package
- Improve the PCB layout (ground plane, etc.)
- Add heat sinks and air flow
• Reduce max (P
PKG
)
- Increase R
L
- Decrease C
L
- Limit I
OUT
using R
ISO
(see Figure 4-9)
- Decrease V
DD
C
L
V
DD
V
OUT
I
DD
I
SS
I
OUT
V
SS
MCP62X
V
OUT
V
DC
V
AC
ω t()sin+=
Where:
V
DC
= DC offset (V)
V
AC
= Peak output swing (V
PK
)
ω = Radian frequency (2π f) (rad/s)
I
OUT
C
L
dV
OUT
dt
-----------------
⋅ V
AC
ω C
L
ω 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
π
------------------------+
⎝⎠
⎛⎞
=
max P
OA
() V
DD
V
SS
–()I
Q
2V
AC
fC
L
+()=
P
PKG
P
OA
k1=
n
∑
=
Where:
n = Number of op amps in package (1 or 2)
Δ T
JA
P
PKG
θ
JA
=
T
J
T
A
Δ T
JA
+=
P
PKG
T
Jmax
T
A
–
θ
JA
-------------------------- ≤
Where:
T
Jmax
= Absolute maximum junction
temperature (°C)
T
A
= Ambient temperature (°C)