TLC4501, TLC4501A, TLC4502, TLC4502A

FAMILY OF SELF-CALIBRATING (Self-Cal

)

PRECISION CMOS RAIL-TO-RAIL OUTPUT OPERATIONAL AMPLIFIERS

SLOS221B – MAY 1998 – REVISED APRIL 2001

21

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

POWER-ON RESET

S

R

Q

Q

ENABLE

RC

OSCILLATOR

COUNTER

RCO

CLOCK

CAL

RESET

SAR

RCO

DAC

LPF

VDD

CORE

AMPLIFIER

+

–

Figure 35. Block Diagram During Calibration Mode

The class AB output stage features rail-to-rail voltage swing and incorporates additional switches to put the

output node into a high-impedance mode during the calibration cycle. Small-replica output transistors (matched

to the main output transistors) provide the amplifier output signal for the calibration circuit. The TLC4502 also

features built-in output short-circuit protection. The output current flowing through the main output transistors

is continuously being sensed. If the current through either of these transistors exceeds the preset limit (60 mA

– 70 mA) for more than about 1

µs, the output transistors are shut down to approximately their quiescent

operating point for approximately 5 ms. The device is then returned to normal operation. If the short circuit is

still in place, it is detected in less than 1

µs and the device is shut down for another 5 ms.

The offset cancellation uses a current-mode digital-to-analog converter (DAC), whose full-scale current allows

for an adjustment of approximately

±5 mV to the input offset voltage. The digital code producing the cancellation

current is stored in the successive-approximation register (SAR).

During power up, when the offset cancellation procedure is initiated, an on-chip RC oscillator is activated to

provide the timing of the successive-approximation algorithm. To prevent wide-band noise from interfering with

the calibration procedure, an analog low-pass filter followed by a Schmitt trigger is used in the decision chain

to implement an averaging process. Once the calibration procedure is complete, the RC oscillator is deactivated

to reduce supply current and the associated noise.