June 1999

5

MIC7211/7221

MIC7211/7221

Micrel

Application Information

The small outline and low supply current (typically 7

µA 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.

(

±100V)

R1

≥100k

0.1µF

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 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

thin-oxide rupture in a CMOS integrated circuit.

(>>V+)

R1

0.1µF

Note: 1V

POSSIBLE

DISCONNECT

0V WHEN

SUPPLY

IS OFF

Power

Supply

Output

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 0.01

µF;

in low-noise systems, where this noise may interfere with the

functioning or accuracy of nearby circuitry, 0.1

µF 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 20

Ω (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.