CMV1020
CALIFORNIA MICRO DEVICES
ground in a single rail application or to the opposite
Applications Information
supply voltage in split rail applications. Since device
only draws 60µA supply current (100µA maximum), its
contribution to the junction temperature, TJ, is negli-
gible. As an example, let us analyze a situation in
which the CMV1020 is operated from a 5 volt supply
and ground, the output is “programmed” to positive
saturation, and the output pin is indefinitely shorted to
ground. In general:
1. Input Common Mode Range and Output
Voltage Considerations
The CMV1020 is capable of accommodating an input
common mode voltage equal to one volt below the
positive rail and all the way to the negative rail. It is
also capable of output voltages equal to both power
supply rails. Voltages that exceed the supply voltages
will not 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 ±5mA are
allowed to flow in them. This can occur when the
magnitude of input voltage exceeds the rail by more
than 0.3 volt. To preclude damage, an applications
resistor, RS, in series with the input is recommended
as illustrated in Figure 1 whose value for Rs is given
by:
PDISS = (V+ – VOUT)*IOUT + IS*V+
Where: PDISS = Power dissipated by the chip
V+ = Supply voltage
VOUT = The output voltage
IS = Supply Current
The contribution to power dissipation due to supply
current is 200µW and is indeed negligible as stated
above.
The primary contribution to power dissipation occurs in
the output stage. V+ – VOUT would equal 5V – 0V =
5V, and power dissipation would be equal to 35mW.
VIN – (V+ + 0.3V)
RS > —————————
5 mA
TJ = TA + θJA* PDISS
For V+ (or V–) equal to 2.2 volts and VIN equal to 10
volts, RS should be chosen for a value of 2.5KΩ or
greater.
Where: TA = The ambient temperature
θJA = The thermal impedance of the package
junction to ambient
The SOT23 exhibits a θJA equal to 325°C/W. Thus for
our example the junction rise would be about 11.4
which is clearly not a destructive situation even under
an ambient temperature of 85°C.
3. Input Impedance Considerations
The CMV1020 exhibits an input impedance typically in
excess of 1 Tera Ω (1 X 10 12 ohms) making it very
appropriate for applications involving high source
impedance such as photodiodes and high output
impedance transducers or long time constant integra-
tors. High source impedances usually dictate large
feedback resistors. But, the output capacitance of the
source in parallel with the input capacitance of the
CMV1020 (which is typically 3pF) create a parasitic
pole with the feedback resistor which erodes the
phase margin of the amplifier. The usual fix is to
bypass, RF, as shown in Figure 2 with a small capaci-
tor to cancel the input pole. The usual formula for
calculating CF always results in a value larger than that
is required:
Figure 1.
2. Output Current and Power Dissipation
Considerations
The CMV1020 is capable of sinking and sourcing
output currents in excess of 7mA at voltages very
nearly equal to the rails. As such, it does not have any
internal short circuit protection (which would in any
event detract from its rail to rail capability). Although
the power dissipation and junction temperature rise are
small, a short analysis is worth investigating.
1
1
————— ≥ —————
2 Π RS CS 2 Π RF CF
Since the parasitic capacitance can change between
the breadboard and the production printed circuit
board, we favor the use of a "gimmick", a technique
Obviously, the worst case from a power dissipation
point of view is when the output is shorted to either
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Tel: (408) 263-3214
Fax: (408) 263-7846
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