MIC7211_09 Datasheet, PDF (5/7 Pages) Micrel Semiconductor – IttyBitty Rail-to-Rail Input Comparator

Micrel, Inc.
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 voltage
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 deter-
mining 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 cur-
rent 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 to swing
the input beyond either rail facilitates some otherwise difficult
MIC7211/7221
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-producing
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). At-
tempting to draw the typical specified output short-circuit
current of 150mA (sourcing) can be expected to cause a
die temperature 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.
October 2009
5
M9999-100909