LMH6639_14 Datasheet, PDF (17/26 Pages) National Semiconductor (TI) – LMH6639 190MHz Rail-to-Rail Output Amplifier with Disable

LMH6639
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SNOS989G – JANUARY 2002 – REVISED MARCH 2013
In the circuit of Figure 46, the outputs of IC1 and IC2 are tied together such that their output impedances are
placed in parallel at the input of IC3. The output impedance of the disabled amplifier is high compared both to the
output impedance of the active amplifier and the 330Ω gain setting resistors. The closed loop output resistance
for the LMH6639 is around 0.2Ω. Thus the active state amplifier output impedance dominates the input node to
IC3, while the disabled amplifier is assured of a high level of suppression of unwanted signals which might be
present at the output.
SHUTDOWN OPERATION
With SD pin left floating, the device enters normal operation. However, since the SD pin has high input
impedance, it is best tied to V⁺ for normal operation. This will avoid inadvertent shutdown due to capacitive pick-
up from nearby nodes. LMH6639 will typically go into shutdown when SD pin is more than 1.7V below V⁺,
regardless of operating supplies.
The SD pin can be driven by push-pull or open collector (open drain) output logic. Because the LMH6639's
shutdown is referenced to V+, interfacing to the shutdown logic is rather simple, for both single and dual supply
operation, with either form of logic used. Typical configurations are shown in Figure 48 and Figure 49 below for
push-pull output:
V⁺
PUSH-PULL
OUTPUT
LOGIC GATE
V_S
V^-
+ _V+
- ^{V- SD}
LMH6639
Figure 48. Shutdown Interface (Single Supply)
V⁺
PUSH-PULL
OUTPUT
LOGIC GATE
V⁺
V^-
+ _V+
- ^{V- SD}
LMH6639
V^-
Figure 49. Shutdown Interface (Dual Supplies)
Common voltages for logic gates are +5V or +3V. To ensure proper power on/off with these supplies, the logic
should be able to swing to 3.4V and 1.4V minimum, respectively.
LMH6639’s shutdown pin can also be easily controlled in applications where the analog and digital sections are
operated at different supplies. Figure 50 shows a configuration where a logic output, SD, can turn the LMH6639
on and off, independent of what supplies are used for the analog and the digital sections:
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