LTC6257 Linear Technology, LTC6257 Datasheet - Page 14

• Manufacturer Part Number: LTC6257
• Description: 65uA Power Efficient Rail-to-Rail I/O Op Amps
• Manufacturer: Linear Technology

LTC6255/LTC6256/LTC6257
ApplicAtions inForMAtion
the negative supply to 0.2V below the positive supply, the
low input bias current allows the amplifiers to be used in
applications with high resistance sources.
Ground Sensing and Rail to Rail Output
The LTC6255 family has excellent output drive capability,
delivering over 10mA of output drive current. The output
stage is a rail-to-rail topology that is capable of swinging to
within 30mV of either rail. If output swing to the negative
rail is required, an external pull down resistor to a negative
supply can be added. For 5V/0V op amp supplies, a pull
down resistor of 2.1k to –2V will allow a ‘true zero’ output
swing. In this case, the output can swing all the way to
the bottom rail while maintaining 80dB of open loop gain.
Since the inputs can go 100mV beyond either rail, the op
amp can easily perform ‘true ground’ sensing.
The maximum output current is a function of total supply
voltage. As the supply voltage to the amplifier increases,
the output current capability also increases. Attention must
be paid to keep the junction temperature of the IC below
150°C when the output is in continuous short-circuit. The
output of the amplifier has reverse-biased diodes con-
nected to each supply. The output should not be forced
more than 0.5V beyond either supply, otherwise current
will flow through these diodes.
Input Protection and Output Overdrive
To prevent breakdown of the input transistors, the input
stages are protected against a large differential input
voltage by two pairs of back-to-back diodes, D5 to D8. If
the differential input voltage exceeds 1.4V, the current in
these diodes must be limited to less than 10mA. These
amplifiers are not intended for open loop applications such
as comparators. When the output stage is overdriven,
internal limiting circuitry is activated to improve overdrive
recovery. In some applications, this circuitry may draw as
much as 1mA supply current.
ESD
The LTC6255 family has reverse-biased ESD protection
diodes on all inputs and output as shown in Figure 1.

Supply Voltage Ramping
Fast ramping of the supply voltage can cause a current
glitch in the internal ESD protection circuits. Depending on
the supply inductance, this could result in a supply voltage
transient that exceeds the maximum rating. A supply volt-
age ramp time of greater than 1ms is recommended.
Feedback Components
Care must be taken to ensure that the pole formed by the
feedback resistors and the parasitic capacitance at the
inverting input does not degrade stability. For example, in
a gain of +2 configuration with gain and feedback resistors
of 10k, a poorly designed circuit board layout with parasitic
capacitance of 5pF (part +PC board) at the amplifier’s
inverting input will cause the amplifier to oscillate due to
a pole formed at 3.2MHz. An additional capacitor of 5pF
across the feedback resistor as shown in Figure 2 will
eliminate any ringing or oscillation.
Shutdown
The single and dual versions have SHDN pins that can
shut down the amplifier to less than 7µA supply current.
The SHDN pin voltage needs to be within 0.6V of V
the amplifier to shut down. During shutdown, the output
will be in high output resistance state, which is suitable
for multiplexer applications. When left floating, the SHDN
pin is internally pulled up to the positive supply and the
amplifier remains enabled.
10k
C
PAR
Figure 2.
V
IN
–
+
LTC6255
5pF
10k
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625567 F02
V
OUT
625567f
–
for