Datasheet

APPLICATION INFORMATION
5kW
OPA379
10mAmax
V
S
V
IN
V
OUT
I
OVERLOAD
NOISE
OPERATING VOLTAGE
CAPACITIVE LOAD AND STABILITY
INPUT COMMON-MODE VOLTAGE RANGE
PROTECTING INPUTS FROM
10 toW
20W
OPA379
V
S
V
IN
V
OUT
R
S
R
L
C
L
OPA379
OPA2379
OPA4379
www.ti.com ................................................................................................................................................... SBOS347D – NOVEMBER 2005 – REVISED MAY 2008
The OPA379 family of operational amplifiers
minimizes power consumption without compromising
bandwidth or noise. Power-supply rejection ratio
(PSRR), common-mode rejection ratio (CMRR), and
open-loop gain (A
OL
) typical values are 100dB or
better.
When designing for ultra-low power, choose system
components carefully. To minimize current
consumption, select large-value resistors. Any
Figure 20. Input Current Protection for Voltages
resistors will react with stray capacitance in the circuit
Exceeding the Supply Voltage
and the input capacitance of the operational amplifier.
These parasitic RC combinations can affect the
stability of the overall system. A feedback capacitor
may be required to assure stability and limit
Although micropower amplifiers frequently have high
overshoot or gain peaking.
wideband noise, the OPA379 series offer excellent
Good layout practice mandates the use of a 0.1 µ F
noise performance. Resistors should be chosen
bypass capacitor placed closely across the supply
carefully because the OPA379 has only 2.8 µ V
PP
of
pins.
0.1Hz to 10Hz noise, and 80nV/ √ Hz of wideband
noise; otherwise, they can become the dominant
source of noise.
OPA379 series op amps are fully specified and tested
from +1.8V to +5.5V ( ± 0.9V to ± 2.75V). Parameters
that will vary with supply voltage are shown in the
Follower configurations with load capacitance in
Typical Characteristics curves.
excess of 30pF can produce extra overshoot (see
typical characteristic Small-Signal Overshoot vs
Capacitive Load, Figure 17 ) and ringing in the output
signal. Increasing the gain enhances the ability of the
The input common-mode voltage range of the
amplifier to drive greater capacitive loads. In
OPA379 family typically extends 100mV beyond each
unity-gain configurations, capacitive load drive can be
supply rail. This rail-to-rail input is achieved using a
improved by inserting a small (10 Ω to 20 Ω ) resistor,
complementary input stage. CMRR is specified from
R
S
, in series with the output, as shown in Figure 21 .
the negative rail to 1V below the positive rail.
This resistor significantly reduces ringing while
Between (V+) – 1V and (V+) + 0.1V, the amplifier
maintaining direct current (dc) performance for purely
operates with higher offset voltage because of the
capacitive loads. However, if there is a resistive load
transition region of the input stage. See the typical
in parallel with the capacitive load, a voltage divider is
characteristic, Offset Voltage vs Common-Mode
created, introducing a dc error at the output and
Voltage vs Temperature (Figure 8 ).
slightly reducing the output swing. The error
introduced is proportional to the ratio R
S
/R
L
, and is
generally negligible.
OVER-VOLTAGE
Normally, input currents are 5pA. However, a large
voltage input (greater than 500mV beyond the supply
rails) can cause excessive current to flow in or out of
the input pins. Therefore, as well as keeping the input
voltage below the maximum rating, it is also important
to limit the input current to less than 10mA. This
limiting is easily accomplished with an input voltage
resistor, as shown in Figure 20 .
Figure 21. Series Resistor in Unity-Gain Buffer
Configuration Improves Capacitive Load Drive
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Product Folder Link(s): OPA379 OPA2379 OPA4379