CMC7101A Datasheet, PDF (10/10 Pages) California Micro Devices Corp – CMC7101A Low Power Operational Amplifier, RRIO, SOT23-5

CALIFORNIA MICRO DEVICES
CMC7101A
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3.0 High Side Current Sense
To monitor a load current it is necessary to insert a
current sense resistor in the power supply feed, as shown
in figure 4. The resulting voltage drop is proportional
to the load current, but as both ends of this resistor are
at (or very near to) the supply voltage, it is difficult to
use a conventional Op Amp.
The CMC7101A is ideal for this application as the inputs
can operate at either supply rail (or at ground when
used with a single supply). The output voltage is directly
proportional to current in the load, scaled by the gain
of the amplifier.
To reduce the power loss in the sense resistor and also
keep the supply voltage to the load immune to load
current variations, a small value Rsense is used.
An excellent application for this circuit is in a battery
charger, where the charging current is monitored to
detect end of charge (battery current will rise as the
cells ‘top off’), and charging must be reduced to prevent
The sense resistor can be placed in the negative side
(known as Low Side Sensing), however this is less desirable
due to disruption of the ground path.
3.1 Rail To Rail Output Swing
Making smaller and lighter products requires the reduction
of battery size, lowering of battery voltage, or both. The
lower voltage products have less headroom, for a given
signal swing, and this demands better amplifier
performance. Rail To Rail Outputs help maintain the
existing signal level when products are redesigned for
lower voltage supplies.
3.2 Rail to Rail input Swing
When the amplifier is used as a buffer, the input of the
amplifier follows the input signal and if this is large
compared to the available supply voltage, it is important
that the Op Amp has a wide CMVR (Common Mode
Voltage Range). RRIO Op Amps have a CMVR equal (or
greater than) the supplies and this allows large signal
swings without clipping or distortion.
Figure 4 High Side Current Sense circuit
CMC7101A Physical Dimensions
For a complete mechanical drawing refer to http://www.calmicro.com/prod/data/package/sot-23.htm.
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