LMH6612MA Datasheet, PDF (21/32 Pages) Texas Instruments – Single Supply 345 MHz Rail-to-Rail Output Amplifiers

Application Information
The LMH6611 and LMH6612 are based on National
Semiconductor’s proprietary VIP10 dielectrically isolated
bipolar process. This device family architecture features the
following:
• Complimentary bipolar devices with exceptionally high f_t
(∼8 GHz) even under low supply voltage (2.7V) and low
bias current.
• Common emitter push-push output stage. This
architecture allows the output to reach within millivolts of
either supply rail.
• Consistent performance with little variation from any
supply voltage (2.7V - 11V) for the most important
specifications (e.g. BW, SR, I_OUT.)
• Significant power saving compared to competitive devices
on the market with similar performance.
With 3V supplies and a common mode input voltage range
that extends beyond either supply rail, the LMH6611 is well
suited to many low voltage/low power applications. Even with
3V supplies, the −3 dB BW (at A_V = +1) is typically 305 MHz.
The LMH6611 and LMH6612 are designed to avoid output
phase reversal. With input overdrive, 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 shows the
input and output voltage when the input voltage significantly
exceeds the supply voltages.
30033622
FIGURE 1. Input and Output Shown with CMVR Exceeded
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.
SHUTDOWN CAPABILITY AND TURN ON/OFF
BEHAVIOR
The LMH6611 can be shutdown by connecting the
DISABLE pin to a voltage 0.5V below the supply midpoint
which will reduce the supply current to typically 120 µA. The
DISABLE pin is “active low” and can be connected through a
resistor to V⁺ or left floating for normal operation. Shutdown
is guaranteed when the DISABLE pin is 0.5V below the supply
midpoint at any operating supply voltage and temperature.
Typical turn on time is 20 ns and the turn off time is 60 ns.
In the shutdown mode, essentially all internal device biasing
is turned off in order to minimize supply current flow and the
output goes into high impedance mode. During shutdown, the
input stage has an equivalent circuit as shown in Figure 2.
30033639
FIGURE 2. Input Equivalent Circuit During Shutdown
When the LMH6611 is shutdown, there may be current flow
through the internal diodes shown, caused by input potential,
if present. This current may flow through the external feed-
back resistor and result in an apparent output signal. In most
shutdown applications the presence of this output is incon-
sequential. However, if the output is “forced” by another de-
vice, the other device will need to conduct the current
described in order to maintain the output potential.
To keep the output at or near ground during shutdown when
there is no other device to hold the output low, a switch using
a transistor can be used to shunt the output to ground.
SELECTION OF R_F AND EFFECT ON STABILITY AND
PEAKING
The peaking of the LMH6611 depends on the value of the
R_F. From the graph shown in Figure 3, as the R_F value in-
creases, the peaking increases.
For A_V = 2, at R_F = 1 kΩ, the −3 dB bandwidth is 113 MHz
and peaking is about 0.6 dB whereas at R_F = 665Ω, the −3
dB bandwidth is about 110 MHz and peaking is 0 dB. R_F and
the input capacitance form a pole in the amplifier’s response.
If the time constant is too big, it will cause peaking and ringing.
Except for A_V = 1 when R_F should be 0Ω, across all other gain
settings it is recommended that R_F remain between 500Ω and
1 kΩ to ensure optimum performance.
30033692
FIGURE 3. Closed Loop Gain vs. Frequency and R_F = R_G
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