LM2917-N: Tolerance on Internal Zener Diode

Hello Brandon,

Unfortunately, I do not have any additional information about the zener specifications beyond the typical 7.56 V (RDROP = 470 Ω ) value listed in the LM2917 Electrical Characteristics table. That said, there may be some hints in the LM2907/LM2917 datasheet that suggests the zener voltage is close to 7.56 V. Note that the datasheet Section 9.2 Functional Block Diagram indicates that the zener circuit is more than just a diode, but an active zener regulator circuit. So there is more than just the zener at work establishing the voltage that will be seen at the LM2917 pin 9.

Additionally, Figure 4 in the datasheet provides a graph of the zener voltage vs temperature and the resistor that sets the current through the active zener regulator circuit. The vertical zener voltage scale only has a range of 200 mV and that covers wide current and temperature ranges.

There is an LM2907/LM2917 Applications Note, AN-162, that provides additional information about applying this F-to-V converter. It provides some discussion about the zener on Page 2, Active Zener Regulator, and on Page 4, Using Zener Regulated Options (LM2917). Although it doesn't provide a tolerance it does provide another useful graph based on setting a 7.6 V zener voltage. Here is the link to the Application Note:

http://www.ti.com/lit/an/snaa088/snaa088.pdf

Regards, Thomas

Precision Amplifiers Applications Engineering 

The problem is my system is running at 24V, and not 12V, so that single graph does nothing for me. I don't see how TI can sell a part where you give no information on the tolerance of a parameter that is directly used in the output equation. Seems silly.

Hi Brandon,

hmm, as you need a current limiting resistor in series to the zener diode, anyway, the supply voltage is entirely irrelevant. An example:

(12V - 7.56V) / 470R = 9.45mA

->

(24V - 7.56V) / 9.45mA = 1740R

Or am I missing something?

Can you show a schematic of your circuit?

Kai

I don't think you can assume your relationship holds when the Zener plot states 12V, otherwise there would be no need to state the voltage, and they would have referenced it based off the current through the zener. My circuit is just like the reference design.