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LMH6645MFX Datasheet(PDF) 14 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part # LMH6645MFX
Description  2.7V, 650關A, 55MHz, Rail-to-Rail Input and Output Amplifiers with Shutdown Option
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Manufacturer  NSC [National Semiconductor (TI)]
Direct Link  http://www.national.com
Logo NSC - National Semiconductor (TI)

LMH6645MFX Datasheet(HTML) 14 Page - National Semiconductor (TI)

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Application Notes
Circuit Description:
The LMH6645/6646/6647 family is based on National Semi-
conductor’s proprietary VIP10 dielectrically isolated bipolar
process.
This device family architecture features the following:
Complimentary bipolar devices with exceptionally high f
t
(
∼8GHz) even under low supply voltage (2.7V) and low
Collector bias current.
Rail-to-Rail input which allows the input common mode
voltage to go beyond either rail by about 0.5V typically.
A class A-B “turn-around” stage with improved noise,
offset, and reduced power dissipation compared to simi-
lar speed devices (patent pending).
Common Emitter push-pull output stage capable of 20mA
output current (at 0.5V from the supply rails) while con-
suming only
∼700µA of total supply current per channel.
This architecture allows output to reach within milli-volts
of either supply rail at light loads.
Consistent
performance
from
any
supply
voltage
(2.7V-10V) with little variation with supply voltage for the
most important specifications (e.g. BW, SR, I
OUT, etc.)
Application Hints:
The total input common mode voltage range, which extends
from below V
to beyond V+, is covered by both a PNP and
a NPN stage. The NPN stage is switched on whenever the
input is less than 1.2V from V
+ and the PNP stage covers the
rest of the range. In terms of the input voltage, there is an
overlapping region where both stages are processing the
input signal. This region is about 0.5V from beginning to the
end. As far as the device application is concerned, this
transition is a transparent operation. However, keep in mind
that the input bias current value and direction will depend on
which input stage is operating (see typical performance
characteristics for plots). For low distortion applications, it is
best to keep the input common mode voltage from transvers-
ing this transition point. Low gain settling applications, which
generally encounter larger peak-to-peak input voltages,
could be configured as inverting stages to eliminate common
mode voltage fluctuations.
In terms of the output, when the output swing approaches
either supply rail, the output transistor will enter a
Quasi-saturated state. A subtle effect of this operational
region is that there is an increase in supply current in this
state (up to 1mA). The onset of Quasi-saturation region is a
function of output loading (current) and varies from 100mV at
no load to about 1V when output is delivering 20mA, as
measured from supplies. Both input common mode voltage
and output voltage level effect the supply current (see typical
performance characteristics for plot).
With 2.7V supplies and a common mode input voltage range
that
extends
beyond
either
supply
rail,
the
LMH6645/6646/6647 family is well suited to many low
voltage/low power applications. Even with 2.7V supplies, the
−3dB BW (@ A
V = +1) is typically 55MHz with a tested limit
of 45MHz. Production testing guarantees that process varia-
tions will not compromise speed.
This device family is designed to avoid output phase rever-
sal. With input over-drive, the output is kept near the supply
rail (or as close to it as mandated by the closed loop gain
setting and the input voltage).
Figure 1, below, shows the
input and output voltage when the input voltage significantly
exceeds the supply voltages:
As can be seen, the output does not exhibit any phase
reversal as some op amps do. However, if the input voltage
range is exceeded by more than a diode drop beyond either
rail, the internal ESD protection diodes will start to conduct.
The current flow in these ESD diodes should be externally
limited.
LMH6647
Micro-power Shutdown
The LMH6647 can be shutdown to save power and reduce
its supply current to less than 50µA guaranteed, by applying
a voltage to the SD pin. The SD pin is “active high” and
needs to be tied to Vfor normal operation. This input is low
current (<20µA, 4pF equivalent capacitance) and a resistor
to V
(
≤20kΩ) will result in normal operation. Shutdown is
guaranteed when SD pin is 0.4V or less from V
+ at any
operating supply voltage and temperature.
In the shutdown mode, essentially all internal device biasing
is turned off in order to minimize supply current flow and the
output goes into Hi-Z (high impedance) mode. Complete
device Turn-on and Turn-off times vary considerably relative
to the output loading conditions, output voltage, and input
impedance, but is generally limited to less than 1µs (see
tables for actual data).
20020233
FIGURE 1. Input/Output Shown with Exceeded Input
CMVR
www.national.com
14


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