参数资料
型号: AD8175
厂商: Analog Devices, Inc.
英文描述: Video Crosspoint Switch
中文描述: 视频交叉点开关
文件页数: 33/40页
文件大小: 533K
代理商: AD8175
AD8175
Power Dissipation
Calculation of Power Dissipation
10
Rev. 0 | Page 33 of 40
0
AMBIENT TEMPERATURE (°C)
M
3
4
5
6
7
8
9
15
25
35
45
55
65
75
85
T
J
= 150°C
Figure 51. Maximum Die Power Dissipation vs. Ambient Temperature
The curve in Figure 51 was calculated from
JA
AMBIENT
T
MAX
θ
JUNCTION
T
MAX
D
P
=
,
,
(1)
As an example, if the AD8175 is enclosed in an environment 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 power dissipated in the output devices due to
current flowing in the loads. For a sinusoidal output about
ground and symmetrical split supplies, the on-chip power
dissipation due the load can be approximated by
(
UTPUT
O
POS
OUTPUT
D
V
V
P
,
For nonsinusoidal output, the power dissipation should be
calculated by integrating the on-chip voltage drop across the
output devices 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
V
V
P
,
(
)
RMS
OUTPUT
RMS
I
,
,
×
=
(2)
QUIESCENT
OUTPUT
NEG
POS
OUTPUT
DQ
I
,
)
×
=
(3)
where
I
OUTPUT, QUIESCENT
= 1.65 mA for each single-ended output
pin for the AD8175.
For each disabled RGB output channel, the quiescent power
supply current in VPOS and VNEG drops by approximately
34 mA.
QNPN
QPNP
V
NEG
V
POS
V
OUTPUT
I
OUTPUT
I
O, QUIESCENT
I
O, QUIESCENT
0
Figure 52. Simplified Output Stage
Example
For the AD8175, with an ambient temperature of 85°C, all nine
RGB output channels driving 1 V
rms
into 100 Ω loads, and
power supplies at ±2.5 V, follow these steps:
1.
Calculate power dissipation of AD8175 using data sheet
quiescent currents. Neglecting V
DD
current, as it is
insignificant.
(
VPOS
POS
QUIESCENT
D
I
V
P
,
(
600
V
5
,
QUIESCENT
D
P
) (
+
)
VNEG
NEG
I
V
×
×
=
(4)
) (
+
)
W
3
mA
600
V
5
mA
=
×
×
=
2.
Calculate power dissipation from loads. For a differential
output and ground-referenced load, the output power is
symmetrical in each output phase.
(
OUTPUT
POS
OUTPUT
D
V
V
P
,
(
)
(
1
V
1
V
5
,
OUTPUT
D
P
)
RMS
OUTPUT
)
Ω
=
RMS
I
,
,
×
=
(5)
mW
15
100
/
V
×
=
There are 27 output pairs, or 54 output currents.
mW
15
54
,
OUTPUT
D
nP
W
81
.
=
×
=
3.
Subtract quiescent output stage current for number of
loads (54 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).
(
)
NEG
POS
OUTPUT
DQ
V
V
P
,
(
)
V)
5
(
V
5
,
OUTPUT
DQ
P
QUIESCENT
OUTPUT
I
,
×
=
(6)
mW
25
.
mA
65
.
=
×
=
There are 27 output pairs, or 54 output currents.
25
.
54
,
OUTPUT
DQ
nP
W
45
.
mW
=
×
=
4.
Verify that the power dissipation does not exceed the
maximum allowed value.
P
P
,
,
+
=
CHIP
ON
D
P
OUTPUT
DQ
OUTPUT
D
QUIESCENT
D
CHIP
ON
D
nP
nP
,
,
+
=
(7)
W
36
.
W
45
.
W
81
.
W
3
,
=
From Figure 51 or Equation 1, this power dissipation is below
the maximum allowed dissipation for all ambient temperatures
up to and including 85°C.
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