LMH6645 Datasheet, PDF (14/28 Pages) National Semiconductor (TI) – 2.7V, 650μA, 55MHz, Rail-to-Rail Input and Output Amplifiers with Shutdown Option

LMH6645, LMH6646, LMH6647
SNOS970C – JUNE 2001 – REVISED APRIL 2013
APPLICATION NOTES
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CIRCUIT DESCRIPTION
The LMH6645/6646/6647 family is based on 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
similar speed devices (patent pending).
• Common Emitter push-pull output stage capable of 20mA output current (at 0.5V from the supply rails) while
consuming 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 Figure 34, 35, and 36 for plots).
For low distortion applications, it is best to keep the input common mode voltage from transversing 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 Figure 37 and 38. 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 variations will not compromise speed.
This device family is designed to avoid output phase reversal. 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 47, below,
shows the input and output voltage when the input voltage significantly exceeds the supply voltages:
INPUT
OUTPUT
V_S = ±2.5V, R_L = 10k: A_V = +1
2 V/DIV
10.0 µs/DIV
Figure 47. Input/Output Shown with Exceeded Input CMVR
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