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參數(shù)資料
| 型號： | LMH6678 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Low Power 2-Channel Central-Office xDSL Driver |
| 中文描述： | 低功耗雙通道中環(huán)辦事處的xDSL驅(qū)動 |
| 文件頁數(shù)： | 12/17頁 |
| 文件大?。?/td> | 1792K |
| 代理商： | LMH6678 |
Application Notes
(Continued)
CHARGE PUMPS
Figure 3
is a simplified schematic of the internal charge
pumps. Each pump consists of a transfer capacitor and a
reservoir capacitor and switches. The states of switches are
driven by an internal 2.75 MHz clock oscillator. The transfer
capacitor of the high charge pump, C1, is connected across
DV
and DGND during one phase of the clock and be-
tween V
and AV
during the opposite phase. This
causes its reservoir capacitor, C
, to charge up to DV
(3.3Volts) potential less a small drop due to finite switch
resistance. V
HIGH
therefore is pumped to nearly V
CC
+ V
DD
potential or approximately +15V.
Similarly, the transfer capacitor of the low charge pump, C
3
,
is connected across DV
DD
and DGND during one phase of
the clock and between AGND and V
LOW
during the opposite
phase. This causes its reservoir capacitor, C
4
, to charge up
to V
DD
potential less a small drop due to finite switch resis-
tance. V
therefore is pumped to nearly V
DD
potential or
approximately 3V.
The charge pumps outputs provide both dc bias currents and
the base current of the output transistors. These base cur-
rents are small compared to the dc bias currents. Typical and
maximum quiescent V
supply currents are given in the
electrical characteristics. Thus, for the charge pump capaci-
tors C
-C
, the suggested values are 0.022 μF 20% X7R
type. With theses values, the ripple on V
and V
will
be approximately 40 mV
. This results in a small spurious
output on the line of typically 120 dBm/Hz at 2.75 MHz.
Spurs produced at harmonics of the clock frequency are at
least 20 dB lower and further attenuated by the transformer.
This is shown in the typical performance characteristics sec-
tion. Ripple and spurious outputs can be further attenuated
by increasing the size of the reservoir capacitors C
2
and C
4
.
MULTI-TONE POWER RATIO AND NOISE
The Multi-Tone Power Ratio of the LMH6678 is shown in the
typical performance characteristics section. MTPR is the
best representation of non-linearity for ADSL modems. The
measurement is accomplished with all ADSL bins transmit-
ting full power except one. The delta between the peak
amplitude of the transmitting carriers and energy left in the
single bin defines the maximum available SNR for that bin.
The test circuit is described in
Figure 4
. Here R
2
, C
3
, R
4
and
C
4
were added for increase gain.
20084042
FIGURE 2. Internal Connections of Integrated Charge
Pumps
20084043
FIGURE 3. Charge Pump Functional Schematic
L
www.national.com
12
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| LMH6678LQX | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Low Power 2-Channel Central-Office xDSL Driver |
| LMH6682 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:190MHz Single Supply, Dual and Triple Operational Amplifiers |
| LMH6682MA | 制造商:Texas Instruments 功能描述:OP Amp Dual Volt Fdbk ±6V/12V 8-Pin SOIC N Rail 制造商:Texas Instruments 功能描述:AMP, VIDEO DUAL, SMD, SOIC8, 6682 |
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