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TLC2274 misbehavior

Other Parts Discussed in Thread: TLC2274, OPA4171, LM2901

Dear all


Now one of my customer is using our TLC2274, and find two misbehavior for the TLC2274.

  1. TLC2274 cannot supply enough load. Here below is the basic test circuit, when the input is 3.3V(DC), and the output drives a 1kohm load, the output cannot reach 3.3V, it is only 2.55V. But the according to the datasheet, TLC2274 can supply more than 5mA output current. Can you give some explanation.
  2. They are using TLC2274 as voltage follower, the output of TLC2274 is connected to C2000’s ADC, and also connected to a comparator (LM2901), which is used to achieve over current protection. They do basic test as below, but the rise edge of TLC2274’s output is too slow. Following is the testing waveform. Yellow is input signal, the signal amplitude is 0~3.3V, and the frequency is 1kHz. When they test OPA4171 in their board, the waveform is like below. It is quite different from TLC2274, and the rise time of OPA4171 is much shorter than TLC2274 although the slew rate of TLC2274(3.6V/us) is much faster than OPA4171 (1.5V/us). Can any of you help me on this? Thanks a lot.



  • Hi George,

    I think you are expecting more current from the TLC2274 output than it can necessarily provide when in the sourcing current mode. The highest current level listed in the Electrical Characteristics table when sourcing current is 1 mA. That is the level they would have been tested in production. Shown below is data sheet Figure 14 shows the output current sourcing capability of the TLC2274. This is a typical curve and it shows that the TLC2274 output voltage falls off fast once the current exceeds about 2 mA. If the devices you have are to the low end in the drive capability distribution, then indeed the output will not be able to swing higher. 

    The curve shwoing the TLC2274 driving the RC (560 ohm, 20 nF) exhibits two characteristics. Initially, in the first portion of the output waveform the output can be seen slewing quickly; up to a little over 1 V. However, after that the output begins to current limit and charges the capacitor at a constant rate.  It is a matter of dV/ dt = i/ C, during that time. Hence, the long linear region with a constant slope.

    The OPA4171 has higher output current drive capability than the TLC2274. And that is evident from the output curve observed when it is driving the same RC network. It does not show any evidence of the current limiting observed with the TLC2274. The output voltage curve shows the RC time constant response that we would expect when it is driven by a low impedance voltage source.

    The TLC2274 cannot provide enough output current to drive the RC network any faster. A different operational amplifier having higher output current capability should be used.

    Regards, Thomas

    PA - Linear Applications Engineering