EL2257CN 데이터 시트보기 (PDF) - Elantec -> Intersil
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EL2257CN Datasheet PDF : 21 Pages
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EL2257C/EL2357C
125 MHz Single Supply, Clamping Op Amps
Applications Information
Product Description
The EL2257C/EL2357C’s, connected in voltage fol-
lower mode, -3 dB bandwidth is 125 MHz while
maintaining a 275 V/µs slew rate. With an input and out-
put common mode range that includes ground, these
amplifiers were optimized for single supply operation,
but will also accept dual supplies. They operate on a
total supply voltage range as low as +2.7V or up to
+12V. This makes them ideal for +3V applications,
especially portable computers.
While many amplifiers claim to operate on a single sup-
ply, and some can sense ground at their inputs, most fail
to truly drive their outputs to ground. If they do succeed
in driving to ground, the amplifier often saturates, caus-
ing distortion and recovery delays. However, special
circuitry built into the EL2257C/EL2357C allows the
output to follow the input signal to ground without
recovery delays.
Power Supply Bypassing And Printed Circuit
Board Layout
As with any high-frequency device, good printed circuit
board layout is necessary for optimum performance.
Ground plane construction is highly recommended.
Lead lengths should be as short as possible. The power
supply pins must be well bypassed to reduce the risk of
oscillation. The combination of a 4.7 µF tantalum capac-
itor in parallel with a 0.1 µF ceramic capacitor has been
shown to work well when placed at each supply pin. For
single supply operation, where the GND pin is con-
nected to the ground plane, a single 4.7 µF tantalum
capacitor in parallel with a 0.1 µF ceramic capacitor
from the VS+ pin to the GND pin will suffice.
For good AC performance, parasitic capacitance should
be kept to a minimum. Ground plane construction
should be used. Carbon or Metal-Film resistors are
acceptable with the Metal-Film resistors giving slightly
less peaking and bandwidth because of their additional
series inductance. Use of sockets, particularly for the SO
package should be avoided if possible. Sockets add par-
asitic inductance and capacitance which will result in
some additional peaking and overshoot.
Supply Voltage Range and Single-Supply
Operation
The EL2257C/EL2357C have been designed to operate
with supply voltages having a span of greater than 2.7V,
and less than 12V. In practical terms, this means that the
EL2257C/EL2357C will operate on dual supplies rang-
ing from ±1.35V to ±6V. With a single-supply, the
EL2257C/EL2357C will operate from +2.7V to +12V.
Performance has been optimized for a single +5V
supply.
Pins 11 and 4 (14 and 3) are the power supply pins on
the EL2257C (EL2357C). The positive power supply is
connected to pin 11 (14). When used in single supply
mode, pin 4 (3) is connected to ground. When used in
dual supply mode, the negative power supply is con-
nected to pin 4 (3).
As supply voltages continue to decrease, it becomes nec-
essary to provide input and output voltage ranges that
can get as close as possible to the supply voltages. The
EL2257C/EL2357C have an input voltage range that
includes the negative supply and extends to within 1.2V
of the positive supply. So, for example, on a single +5V
supply, the EL2257C/EL2357C have an input range
which spans from 0V to 3.8V.
The output range of the EL2257C/EL2357C is also quite
large. It includes the negative rail, and extends to within
1V of the top supply rail with a 1 kΩ load. On a +5V
supply, the output is therefore capable of swinging from
0V to +4V. On split supplies, the output will swing ±4V.
If the load resistor is tied to the negative rail and split
supplies are used, the output range is extended to the
negative rail.
Choice Of Feedback Resistor, RF
The feedback resistor forms a pole with the input capac-
itance. As this pole becomes larger, phase margin is
reduced. This increases ringing in the time domain and
peaking in the frequency domain. Therefore, RF has
some maximum value which should not be exceeded for
optimum performance. If a large value of RF must be
used, a small capacitor in the few picofarad range in par-
allel with RF can help to reduce this ringing and peaking
at the expense of reducing the bandwidth.
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