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參數資料
| 型號： | LMH6738MQX |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 運算放大器 |
| 英文描述： | Very Wideband, Low Distortion Triple Op Amp |
| 中文描述： | TRIPLE OP-AMP, 4500 uV OFFSET-MAX, PDSO16 |
| 封裝： | SSOP-16 |
| 文件頁數： | 10/12頁 |
| 文件大小： | 432K |
| 代理商： | LMH6738MQX |
Application Section
(Continued)
One possible remedy for this effect is to slightly increase the
value of the feedback (and gain set) resistor. This will tend to
offset the high frequency gain peaking while leaving other
parameters relatively unchanged. If the device has a capaci-
tive load as well as inverting input capacitance using a series
output resistor as described in the section on “Driving Ca-
pacitive Loads” will help.
LAYOUT CONSIDERATIONS
Whenever questions about layout arise, use the evaluation
board as a guide. The LMH730275 is the evaluation board
supplied with samples of the LMH6738.
To reduce parasitic capacitances ground and power planes
should be removed near the input and output pins. Compo-
nents in the feedback loop should be placed as close to the
device as possible. For long signal paths controlled imped-
ance lines should be used, along with impedance matching
elements at both ends.
Bypass capacitors should be placed as close to the device
as possible. Bypass capacitors from each rail to ground are
applied in pairs. The larger electrolytic bypass capacitors
can be located farther from the device, the smaller ceramic
capacitors should be placed as close to the device as pos-
sible. The LMH6738 has multiple power and ground pins for
enhanced supply bypassing. Every pin should ideally have a
separate bypass capacitor. Sharing bypass capacitors may
slightly degrade second order harmonic performance, espe-
cially if the supply traces are thin and /or long. In
Figure 1
and
Figure 2
C
SS
is optional, but is recommended for best
second harmonic distortion.Another option to using C
SS
is to
use pairs of .01 μF and .1 μF ceramic capacitors for each
supply bypass.
VIDEO PERFORMANCE
The LMH6738 has been designed to provide excellent per-
formance with production quality video signals in a wide
variety of formats such as HDTV and High Resolution VGA.
NTSC and PAL performance is nearly flawless. Best perfor-
mance will be obtained with back terminated loads. The back
termination reduces reflections from the transmission line
and effectively masks transmission line and other parasitic
capacitances from the amplifier output stage.
Figure 4
shows a typical configuration for driving a 75
Cable. The
amplifier is configured for a gain of two to make up for the 6
dB of loss in R
OUT
.
POWER DISSIPATION
The LMH6738 is optimized for maximum speed and perfor-
mance in the small form factor of the standard SSOP-16
package. To achieve its high level of performance, the
LMH6738 consumes an appreciable amount of quiescent
current which cannot be neglected when considering the
total package power dissipation limit. The quiescent current
contributes to about 40 C rise in junction temperature when
no additional heat sink is used (V
=
±
5V, all 3 channels on).
Therefore, it is easy to see the need for proper precautions
to be taken in order to make sure the junction temperature’s
absolute maximum rating of 150C is not violated.
To ensure maximum output drive and highest performance,
thermal shutdown is not provided. Therefore, it is of utmost
importance to make sure that the T
is never exceeded
due to the overall power dissipation (all 3 channels).
With the LMH6738 used in a back-terminated 75
RGB
analog video system (with 2 V
PP
output voltage), the total
power dissipation is around 435 mW of which 340 mW is due
to the quiescent device dissipation (output black level at 0V).
With no additional heat sink used, that puts the junction
temperature to about 140 C when operated at 85C ambi-
ent.
To reduce the junction temperature many options are avail-
able. Forced air cooling is the easiest option. An external
add-on heat-sink can be added to the SSOP-16 package, or
alternatively, additional board metal (copper) area can be
utilized as heat-sink.
An effective way to reduce the junction temperature for the
SSOP-16 package (and other plastic packages) is to use the
copper board area to conduct heat. With no enhancement
the major heat flow path in this package is from the die
through the metal lead frame (inside the package) and onto
the surrounding copper through the interconnecting leads.
Since high frequency performance requires limited metal
near the device pins the best way to use board copper to
remove heat is through the bottom of the package. A gap
filler with high thermal conductivity can be used to conduct
heat from the bottom of the package to copper on the circuit
board. Vias to a ground or power plane on the back side of
the circuit board will provide additional heat dissipation. A
combination of front side copper and vias to the back side
can be combined as well.
Follow these steps to determine the Maximum power dissi-
pation for the LMH6738:
1.
Calculate the quiescent (no-load) power: P
AMP
= I
CC
*
(V
S
) V
S
= V
+
-V
2.
Calculate the RMS power dissipated in the output stage:
P
(rms) = rms ((V
- V
)*I
) where V
and I
are the voltage and current across the external load and
V
S
is the total supply current
3.
Calculate the total RMS power: P
T
= P
AMP
+P
D
The maximum power that the LMH6738, package can dissi-
pate at a given temperature can be derived with the following
equation (See
Figure 7
):
P
= (150
o
– T
AMB
)/
θ
, where T
= Ambient tempera-
ture (C) and
θ
= Thermal resistance, from junction to
ambient, for a given package (C/W). For the SSOP package
θ
JA
is 120C/W.
20097502
FIGURE 7. Maximum Power Dissipation
L
www.national.com
10
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相關代理商/技術參數 |
參數描述 |
|---|---|
| LMH6738MQX/NOPB | 功能描述:運算放大器 - 運放 RoHS:否 制造商:STMicroelectronics 通道數量:4 共模抑制比（最小值）:63 dB 輸入補償電壓:1 mV 輸入偏流（最大值）:10 pA 工作電源電壓:2.7 V to 5.5 V 安裝風格:SMD/SMT 封裝 / 箱體:QFN-16 轉換速度:0.89 V/us 關閉:No 輸出電流:55 mA 最大工作溫度:+ 125 C 封裝:Reel |
| LMH6739 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Very Wideband, Low Distortion Triple Video Buffer |
| LMH6739_07 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Very Wideband, Low Distortion Triple Video Buffer |
| LMH6739MQ | 功能描述:緩沖器和線路驅動器 RoHS:否 制造商:Micrel 輸入線路數量:1 輸出線路數量:2 極性:Non-Inverting 電源電壓-最大:+/- 5.5 V 電源電壓-最小:+/- 2.37 V 最大工作溫度:+ 85 C 安裝風格:SMD/SMT 封裝 / 箱體:MSOP-8 封裝:Reel |
| LMH6739MQ/NOPB | 功能描述:緩沖器和線路驅動器 RoHS:否 制造商:Micrel 輸入線路數量:1 輸出線路數量:2 極性:Non-Inverting 電源電壓-最大:+/- 5.5 V 電源電壓-最小:+/- 2.37 V 最大工作溫度:+ 85 C 安裝風格:SMD/SMT 封裝 / 箱體:MSOP-8 封裝:Reel |
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