MIC7211 Datasheet, PDF (5/8 Pages) Micrel Semiconductor – IttyBitty™ Rail-to-Rail Input Comparator Preliminary Information

MIC7211/7221
Application Information
The small outline and low supply current (typically 7µA at 5V)
of the MIC7211/21 are the primary advantages of these
comparators. They have been characterized for 2.2V, 2.7V,
5V, and 10V operation.
Their 2.2V capability is especially useful in low-battery volt-
age situations. Low-voltage operation allows longer battery
life or deeper discharge capability. Even at 2.2V, the output
can drive several logic-gate inputs. At 2.5mA, the output
stage voltage drop is guaranteed to not exceed 0.1V.
Outputs
The MIC7211 has a push-pull output while the MIC7221 has
an open-drain output, otherwise both comparators share a
common design.
The open-drain MIC7221 output can be pulled up to 10V,
even when the supply voltage is as low as 2.2V. Conversely,
the output also can be pulled up to voltages that are lower
than the positive supply. Logic-level translation is readily
facilitated by the ability to pull the open-drain output to
voltages above or below the power supply.
Although specified short-circuit output current specified for
these parts typically exceeds 100mA, their output is not
intended to sink or source anywhere near 100mA. The short-
circuit rating is only presented as additional information
regarding output impedance and may be useful for determin-
ing the voltage drop one may experience when driving a given
load.
Input Bias Current
The low input-bias current (typically 0.5pA) requirement of
the MIC7211/21 provides flexibility in the kinds of circuitry and
devices that can be directly interfaced.
Designs using an amplifier for transducer-to-comparator
impedance transformation may be simplified by using the
MIC7211/21’s low-input-current requirement to eliminate the
amplifier.
Input Signal Levels
Input signals may exceed either supply rail by up to 0.2V
without phase inversion or other adverse effects. The inputs
have internal clamp diodes to the supply pins.
V⁺
R_IN
V_IN
(±100V) ≥100k
0.1µF
V_REF
R1
R_F
V_OUT
Note: R_F and R1 control hysteresis (typically, R_F >> R1).
Figure 1. Driving the Input Beyond the Supply Rails
Larger input swings can be accommodated if the input current
is limited to 1mA or less. Using a 100k input resistor will allow
an input to swing up to 100V beyond either supply rail.
Because of the low input bias current of the device, even
larger input resistors are practical. See Figure 1. The ability
Micrel
to swing the input beyond either rail facilitates some other-
wise difficult circuits, such as a single-supply zero-crossing
detector or a circuit that senses its own supply voltage.
The comparator must be powered if an input is pulled above
the rail, even with current limiting in effect. Figure 2 shows a
hypothetical situation where an input is pulled higher than the
rail when the power supply is off or not present. Figure 2 also
shows external clamp diodes for additional input circuit
protection. Discrete clamp diodes can be arbitrarily more
robust than the internal clamp diodes.
The power supply has been simplified (real power supplies do
not have a series output diode); however, this illustrates a
common characteristic of most positive-voltage power sup-
plies: they are designed to source, but not sink, current. If the
supply is off, or disconnected, there is no limiting voltage for
the clamp diode to reference. The input signal can charge the
the bypass capacitor, and possibly the filter capacitor, up to
the applied input (V_IN). This may be high enough to cause a
thin-oxide rupture in a CMOS integrated circuit.
0V WHEN
SUPPLY
IS OFF
Power
Supply
Output
POSSIBLE
DISCONNECT
R_IN
V_IN
(>>V+)
V⁺⁺
V⁺
0.1µF
R_PU
V_REF
R1
R_F
V_OUT
Note: 1V ≤ V⁺⁺ ≤ 10V
Figure 2. Avoid This Condition
Ideally, the supply for the comparator and the input-produc-
ing circuitry should the same or be switched simultaneously.
Bypass Capacitors
CMOS circuits, especially logic gates with their totem-pole
(push-pull) output stages, generate power supply current
spikes (noise) on the supply and/or ground lines. These
spikes occur because, for a finite time during switching, both
output transistors are partially on allowing “shoot-through
current.” Bypass capacitors reduce this noise.
Adequate bypassing for the MIC7211 comparator is 0.01µF;
in low-noise systems, where this noise may interfere with the
functioning or accuracy of nearby circuitry, 0.1µF is recom-
mended. Because the MIC7221 does not have a totem-pole
output stage, this spiking is not evident; however, switching
a capacitive load can present a similar situation.
Thermal Behavior
The thermal impedance of a SOT-23-5 package is 325˚C/W.
The 5V Electrical Characteristics table shows a maximum
voltage drop of 0.1V for a 5mA output current, making the
output resistance about 20Ω (R = 0.1/0.005 = 20Ω). Attempt-
ing to draw the typical specified output short-circuit current of
150mA (sourcing) can be expected to cause a die tempera-
ture rise of 146˚C. (Operating die temperature for ICs should
generally not exceed 125˚C.) Using a series resistance is the
simplest form of protecting against damage by excessive
output current.
June 1999
5
MIC7211/7221