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CMV7106Y Datasheet PDF : 9 Pages
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CALIFORNIA MICRO DEVICES
CMV7106
Furthermore,
T
J
=
T
A
+ θJA
(3)
Where T = The Ambient Temperature
A
θJA = The thermal impedance of the package
For a short circuit on the output with V+ equal to 3 volts
and an output current of 50mA, the power dissipation per
equation (2) would be 150mW. Assuming a worst case
ambient of 85°C, the junction temperature would be
134°C well below the maximum junction rating of 150°C.
2.2 Input Considerations
The CMV7106 exhibits an input impedance which is
typically in excess of 1 TeraΩ making it an excellent
choice for applications requiring high source impedance
such as buffering photo diodes, high impedance trans-
ducers, or long time constant integraters. A high source
impedance usually dictates a large feedback resistor, Rf,
but the parallel combination of Rf and Rs in parallel with
the input capacitance of the amplifier (typically 3 pF)
creates a parasitic pole which can erode the phase
margin of the amplifier. The recommended fix is to
bypass Rf with a small capacitor thus canceling the pole
at the inverting input. The formula for calculating invari-
ably results in a value larger than optimum,
1
2 ΠRs
>
1
2 ΠRf
(4)
Since the parasitic capacitance is bound to change
between the breadboard phase and the production
printed circuit board, we recommend the use of a
“gimmick” which is made by twisting two lengths (about a
foot) of insulated wire (such as AWG 24) that are bared
at both ends. The gimmick is soldered across Rf . With the
circuit in operation, Cf is adjusted by clipping short
lengths until the compensation is nominal. Then simply
measure the capacitance of the gimmick on an imped-
ance bridge.
RS
VIN
GND
V+
Cf
R1
6– 4
CIN
1 CMV7106
3
5+
A1
2
GND
GND
VOUT
Figure 1. Input Capacitance Compensation
2.3 Capacitive Load Considerations
The CMV7106 is capable of driving capacitive loads in
excess of 100pF without oscillation. However, significant
peaking will result. The easiest way to minimize this
problem is to use an isolation resistor as shown
in Figure 2.
V+
6– 4
1 CMV7106
5+
A1 3
2
VIN
GND
GND
VOUT
R1
1KΩ
CL
100pF
GND
Figure 2. Isolating a Capacitive Load
2.4 Power Supply Decoupling
The CMV7106 is not prone to oscillation without the use
of power supply decoupling capacitors, however to
minimize hum and noise pick-up, it is recommended that
the rails be bypassed with 0.01µF capacitors.
Typical Application
3.0 Low Power and Supply Voltage Single Rail
Oscillator
Operational amplifiers have been used for years to
generate frequency stable oscillators finding applications
in toys and hand held games. But the circuit using the
CMV7106 shown in Figure 3 provides unique benefits in
a child’s sneaker where Light Emitting Diodes are
caused to flash any time the child’s foot touches the
ground. The Shutdown feature of the CMV7106 is used
in conjunction with normally closed switch SW1 and pull-
up resistor, R5, which disables A1 until the child’s foot
comes in contact with the ground opening the switch
enabling the amplifier. After a turn-on transient of a few
hundred milliseconds, the circuit will oscillate until SW1
closes again. For (R1 + R2) ¸ R1 = 0.473, the period, T,
of the oscillator is given by:
T = 2 Rf C1
(5)
Where: Rf is the feedback resistor, C1 is the
capacitor
©2000 California Micro Devices Corp. All rights reserved.
8
215 Topaz Street, Milpitas, California 95035
Tel: (408) 263-3214
Fax: (408) 263-7846
www.calmicro.com 9/27/2000
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