Datasheet MIC7211, MIC7221 (Microchip) - 5

Micrel, Inc. MIC7211/7221 Application Information circuits, such as a single-supply zero-crossing detector or a
circuit that senses its own supply voltage. 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. 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. 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. 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 supplies: 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 (VIN). This may be high enough to cause
a thin-oxide rupture in a CMOS integrated circuit. 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. POSSIBLE
DISCONNECT 0V WHEN
SUPPLY
IS OFF Power
Supply
Output 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 determining the voltage drop one may experience when driving
a given load. V++ V+ VIN
(>>V+) VREF 0.1µF
RPU RIN R1 VOUT RF Note: 1V ≤ V++ ≤ 10V Figure 2.  Avoid This Condition 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. Ideally, the supply for the comparator and the input-producing
circuitry should the same or be switched simultaneously. 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. 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. Bypass Capacitors 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. 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 recommended. 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. V+ VIN
(±100V) VREF 0.1µF RIN ≥100k R1 Thermal Behavior
RF 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). Attempting 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. VOUT Note: RF and R1 control hysteresis (typically, RF >> 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 to swing
the input beyond either rail facilitates some otherwise difficult
October 2009 5 M9999-100909