Datasheet
AD8104/AD8105
Rev. 0 | Page 31 of 36
operation as well, as there is a common-mode-to-differential
gain conversion that becomes greater at higher frequencies.
During operation of the AD8104/AD8105, transient currents
flow into the VOCM net from the amplifier control loops.
Although the magnitude of these currents are small (10 µA to
20 µA per output), they can contribute to crosstalk if they flow
through significant impedances. Driving VOCM with a low
impedance, low noise source is desirable.
Power Dissipation
Calculation of Power Dissipation
8
4
15 85
MAXIMUM POWER (W)
AMBIENT TEMPERATURE (°C)
T
J
= 150°C
7
6
5
25 35 45 55 65 75
06612-070
Figure 70. Maximum Die Power Dissipation vs. Ambient Temperature
The curve in Figure 70 was calculated from
JA
AMBIENTMAXJUNCTION
MAXD
TT
P
θ
−
=
,
,
(1)
As an example, if the AD8104/AD8105 is enclosed in an envi-
ronment at 45°C (T
A
), the total on-chip dissipation under all
load and supply conditions must not be allowed to exceed 7.0 W.
When calculating on-chip power dissipation, it is necessary to
include the rms current being delivered to the load, multiplied
by the rms voltage drop on the AD8104/AD8105 output
devices. For a sinusoidal output, the on-chip power dissipation
due to the load can be approximated by
(
)
RMSOUTPUT
RMSUTPUT
OPOS
OUTPUTD
IVVP
,
,
,
×−=
For nonsinusoidal output, the power dissipation should be
calculated by integrating the on-chip voltage drop multiplied by
the load current over one period.
The user can subtract the quiescent current for the Class AB
output stage when calculating the loaded power dissipation. For
each output stage driving a load, subtract a quiescent power
according to
(
)
QUIESCENTOUTPUT
NEGPOS
OUTPUTDQ
IVVP
,,
×−=
where
I
OUTPUT, QUIESCENT
= 1.65 mA for each single-ended output pin.
For each disabled output, the quiescent power supply current in
VPOS and VNEG drops by approximately 9 mA.
QNPN
QPNP
V
NEG
V
POS
V
OUTPUT
I
OUTPUT
I
OUTPUT, QUIESCENT
I
OUTPUT, QUIESCENT
06612-071
Figure 71. Simplified Output Stage
Example
For the AD8104/AD8105, in an ambient temperature of 85°C,
with all 16 outputs driving 1 V rms into 100 loads and power
supplies at ±2.5 V, follow these steps:
1. Calculate power dissipation of AD8104/AD8105 using data
sheet quiescent currents. Disregard VDD current, as it is
insignificant.
(
)
(
)
VNEGNEGVPOSPOS
QUIESCENTD
IVIVP
×
+×
=
,
(
)
(
)
W7.1mA340V5.2mA340V5.2
,
=
×
+
×
=
QUIESCENTD
P
2.
Calculate power dissipation from loads. For a differential
output and ground-referenced load, the output power is
symmetrical in each output phase.
(
)
RMSOUTPUTRMSOUTPUT
POS
OUTPUTD
IVVP
,,,
×
−
=
(
)
()
mW15100/V1V1V5.2
,
=
×
−
=
OUTPUTD
P
There are 16 output pairs, or 32 output currents.
W48.0mW1532
,
=
×
=
OUTPUTD
nP
3.
Subtract the quiescent output stage current for number of
loads (32 in this example). The output stage is either
standing, or driving a load, but the current only needs to
be counted once (valid for output voltages > 0.5 V).
(
)
QUIESCENTOUTPUT
NEGPOS
OUTPUTDQ
IVVP
,,
×
−
=
(
)
mW25.8mA65.1V)5.2(V5.2
,
=
×−
−
=
OUTPUTDQ
P
There are 16 output pairs, or 32 output currents.
W26.0mW25.832
,
=
×
=
OUTPUTDQ
nP
4.
Verify that the power dissipation does not exceed the
maximum allowed value.
OUTPUTDQOUTPUTDQUIESCENTD
CHIPOND
nPnPPP
,,,
,
−
+
=
−
W9.1W26.0W48.0W7.1
,
=−
+
=
−CHIPOND
P
From
Figure 70 or Equation 1, this power dissipation is below
the maximum allowed dissipation for all ambient temperatures
up to and including 85°C.