THS4271_16 Datasheet, PDF (22/50 Pages) Texas Instruments – LOW NOISE, HIGH SLEW RATE, UNITY GAIN STABLE VOLTAGE FEEDBACK AMPLIFIER

THS4271
THS4275
SLOS397F – JULY 2002 – REVISED OCTOBER 2009
SINGLE-SUPPLY OPERATION
The THS4271 is designed to operate from a single
5-V to 15-V power supply. When operating from a
single power supply, care must be taken to ensure
the input signal and amplifier are biased appropriately
to allow for the maximum output voltage swing. The
circuits shown in Figure 77 demonstrate methods to
configure an amplifier in a manner conducive for
single-supply operation.
+V_S
50 Ω Source
V_I
R_T 49.9 Ω
+
THS4271
_
+V_S
2
R_f
R_g
249 Ω
249 Ω
+V_S
2
50 Ω Source
R_g
V_I
249 Ω
61.9 Ω R_T
+V_S
+V_S
2
2
R_f
249 Ω
V_S
_
THS4271
+
V_O
499 Ω
V_O
499 Ω
Figure 77. DC-Coupled Single-Supply Operation
Saving Power with Power-Down Functionality and
Setting Threshold Levels with the Reference Pin
The THS4275 features a power-down pin (PD) which
lowers the quiescent current from 22 mA down to 700
μA, ideal for reducing system power.
The power-down pin of the amplifier defaults to the
positive supply voltage in the absence of an applied
voltage, putting the amplifier in the power-on mode of
operation. To turn off the amplifier in an effort to
conserve power, the power-down pin can be driven
towards the negative rail. The threshold voltages for
power-on and power-down are relative to the supply
rails and given in the specification tables. Above the
Enable Threshold Voltage, the device is on. Below
the Disable Threshold Voltage, the device is off.
Behavior in between these threshold voltages is not
specified.
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Note that this power-down functionality is just that;
the amplifier consumes less power in power-down
mode. The power-down mode is not intended to
provide a high-impedance output. In other words, the
power-down functionality is not intended to allow use
as a 3-state bus driver. When in power-down mode,
the impedance looking back into the output of the
amplifier is dominated by the feedback and gain
setting resistors, but the output impedance of the
device itself varies depending on the voltage applied
to the outputs.
The time delays associated with turning the device on
and off are specified as the time it takes for the
amplifier to reach 50% of the nominal quiescent
current. The time delays are on the order of
microseconds because the amplifier moves in and out
of the linear mode of operation in these transitions.
Power-Down Reference Pin Operation
In addition to the power-down pin, the THS4275 also
features a reference pin (REF) which allows the user
to control the enable or disable power-down voltage
levels applied to the PD pin. Operation of the
reference pin as it relates to the power-down pin is
described below.
In most split-supply applications, the reference pin is
connected to ground. In some cases, the user may
want to connect it to the negative or positive supply
rail. In either case, the user needs to be aware of the
voltage level thresholds that apply to the power-down
pin. Table 2 and Table 3 show examples and
illustrate the relationship between the reference
voltage and the power-down thresholds.
Table 2. Power-Down Threshold Voltage Levels
(REF ≤ MIDRAIL)
SUPPLY
VOLTAGE
(V)
±5
5
REFERENCE
PIN VOLTAGE
(V)
GND
-2.5
-5
GND
1
2.5
ENABLE
LEVEL
(V)
≥ 1.8
≥ -0.7
≥ -3.2
≥ 1.8
≥ 2.8
≥ 4.3
DISABLE
LEVEL
(V)
≤1
≤ –1.5
≤ -4
≤1
≤2
≤ 3.5
In Table 2, the threshold levels are derived by the
following equations:
REF + 1.8 V for enable
REF + 1 V for disable
Note that in order to maintain these threshold levels,
the reference pin can be any voltage between VS_– or
GND up to Vs/2 (midrail).
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