TC1026 datasheet(6/18 Pages) MICROCHIP

6 / 18 page

TC1026

DS21725B-page 6

FIGURE 4-1:

COMPARATOR

EXTERNAL HYSTERESIS

CONFIGURATION

4.2

Precision Battery Monitor

Figure 4-2 is a precision battery low/battery dead

monitoring circuit. Typically, the battery low output

warns the user that a battery dead condition is

imminent. Battery dead typically initiates a forced

shutdown to prevent operation at low internal supply

voltages (which can cause unstable system operation).

The circuit of Figure 4-2 uses two TC1026 devices and

only six external resistors. AMP 1 is a simple buffer

while CMPTR1 and CMPTR2 provide precision voltage

detection using VR as a reference. Resistors R2 and

R4 set the detection threshold for BATT LOW while

resistors R1and R3 set the detection threshold for

BATT FAIL. The component values shown assert

BATT LOW at 2.2V (typical) and BATT FAIL at 2.0V

(typical). Total current consumed by this circuit is

typically 28

µA at 3V. Resistors R5 and R6 provide

hysteresis for comparators CMPTR1 and CMPTR2,

respectively.

4.3

Voice Band Receive Filter

The majority of spectral energy for human voices is in

a 2.7kHz frequency band from 300Hz to 3kHz. To

properly recover a voice signal in applications such as

radios, cellular phones and voice pagers, a low-power

bandpass filter that is matched to the human voice

spectrum can be implemented using Microchip’s

CMOS op amps. Figure 4-3 shows a unity-gain multi-

pole Butterworth filter with ripple less than 0.15dB in

the human voice band. The lower 3dB cut-off frequency

is 70Hz (single-order response), while the upper cut-off

frequency is 3.5kHz (fourth-order response).

4.4

Supervisory Audio Tone (SAT)

Filter for Cellular

Supervisory Audio Tones (SAT) provide a reliable

transmission path between cellular subscriber units

and base stations. The SAT tone functions much like

the current/voltage used in land line telephone systems

to indicate that a phone is off the hook. The SAT tone

may be one of three frequencies: 5970, 6000 or

6030Hz. A loss of SAT implies that channel conditions

are impaired, and if SAT is interrupted for more than 5

seconds, a cellular call is terminated.

Figure 4-4 shows a high Q (30) first order SAT

detection bandpass filter using Microchip’s CMOS op

amp architecture. This circuit nulls all frequencies

except the three SAT tones of interest.

–

VDD

VOUT

VSRC

TC1026

Comparator

TC1026

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