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LM239: how to handle negative input with single supply

Genius 11595 points

Replies: 2

Views: 468

Part Number: LM239

Hi,

my customer is using LM239 with single 12V supply. There will be inevitable -6.25V pulse(less than 50ms) at the negative input IN-, IN+ is always 3.4V.

They observed that -6.25V pulse is clamped to -0.6V at IN- pin, and in the datasheet the absolute maximum input at Vin- is -0.3V to 36V.

The question is:

1. what internal circuit clamps IN- to -0.6V?

2. Is it possible that they use an external diode that clamps Vin- above -0.3V? What kind of diode is recommended?

3. If it's not possible to clamp Vin- through external diode. Are there any suggestion to handle the -6.25V pulse at IN-?

If we use dual supply, the output low will be the same level as the negative supply, for example -12V, it's not acceptable for the subsequent circuit.

  • Hi Howard,

    have a look at figure 6 of datasheet. You will see that the collector base juntion of input transistor will be biased when applying a negative input voltage. Use a Schottky diode to clamp the negative input voltage to less than 0.3V. This will at least prevent destruction of chip.

    But take care, the common mode input voltage range of LM239 is only 0V to VCC-1.5V. So, even with the clamp the comparator might not work properly.

    Kai
  • Hello Howard,

    The LM239 is an older design (~45years), before ESD was an issue...

    The LM239 does not have dedicated ESD protection devices. The process is junction isolated, which means there is essentially a reverse biased diode under every internal circuit node to the GND pin. If any pin is brought below the GND pin, these substrate "diodes" will conduct. These diodes are not designed to carry current and can be damaged, and can also create multiple undesired current paths that interfere with "normal" operation. With too much current, these diodes can turn into SCR's and latch. The LM293 family is more susceptible to negative voltages than more modern ESD protected devices.

    As Kai recommended, a Schottky diode should be used to clamp the negative portion to a max of -400mV. "Abnormal" operation ("reversals") could occur if the input is brought 800mV or more below ground, and damage can occur if pushed further.

    Current should be limited to 1mA or less. This can be accomplished by adding a series resistance to the input (this resistance can also be part of any input dividers). A rule of thumb is 1kohm per volt of expected over-voltage. So if you are expecting -6V, then use a minimum of 6k.

    Regards,

    Paul Grohe

    TI Comparators (CMPS) Applications Group

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