CMC7106 Datasheet, PDF (11/11 Pages) California Micro Devices Corp – CMC7106 Low Power Operational Amplifier, RRIO, with Shutdown, SOT23-6

CALIFORNIA MICRO DEVICES
CMC7106
Applications of the CMC7106
High Side Current Sense
To monitor a load current, it is necessary to insert a current
sense resistor into the power supply feed, as shown in
Figure 1. 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 CMC7106 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 R_sense 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
damage (out gassing or over heating).
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.
Figure 1. High Side Current Sense
Low Current 3.3V Regulator with Disable
Over the past few years, power supply voltages in systems,
particularly in PC’s, have migrated from 5 volts to 3.3 volts
while at the same time, saving power – particularly in portable
systems has become a major issue. On the other hand, there
are “legacy” devices which still mandate the use of a 5 volt V_cc.
Frequently in systems, a finite amount of current is needed for
the 3.3 volt IC’s. The circuit shown below provides a stable 3.3
voltage supply derived from the 5 volts at currents up to 100
mA while drawing only 300 µA’s when enabled. It takes advantage
of the CMC7106 op amp which provides a power down disable
(shutdown) compatible with all 5 volt logic families and an ultra
low current reference, the LT1389. When disabled, the circuit
draws essentially zero current but when enabled exhibits load
regulation of 0.05% from 0 to 100 mA, and line regulation of
0.01% from 4.5 V < Vdd < 5.5 V.
Figure 2. 5V to 3.3V Regulator with Disable
Input Common Mode Range Considerations
The CMC7106 is capable of accommodating input common
mode and output voltages equal to both power supply rails.
Nor will voltages that exceed the supply voltages cause phase
inversion of the output. However, ESD diode clamps are
provided at the inputs that can be damaged if static currents
in excess of ± 5 mA are allowed to flow through them. This
can occur when the magnitude of input voltage exceeds the
rail by more than 0.3 volt. To preclude damage, a current
limiting resistor, R_s, in series with the input is recommended
as illustrated in Figure 1, whose value for R_s is given by:
V_in - (V+ 0.3 V)
R_s > ———————————— (1)
5 mA
For V+ (or V-) equal to 2.7 volts and V_in equal to 10
volts, Rs should be chosen for a value of 1.5 KΩ or
g r e a t e r.
Shut-down Pin Current Limiting Considerations
The Shut-down pin also provides ESD clamp diodes that will
be damaged if the signal exceeds the rail by 0.3 volt either
from a logic gate or other signal source. If possible, the current
should be limited to less than ± 5 mA by inserting a resistor
between the gate or source and the Shut-down Pin whose
value is determined by Equation (1) above.
©1999 California Micro Devices Corp. All rights reserved.
10/99
215 Topaz Street, Milpitas, California 95035 Tel: [phone] Fax: [phone] www.calmicro.com
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