Trouble with 555 timer

Hi

I am currently using LCM555 timer for generating variable frequency based on relative Humidity (RH) of surrounding. The LCM555 is configured as astable multivibrator and centered at 20KHz. RA = 24K, RB = 200K and C is capacitive sensor having value of 160pF at 30C, 0%RH. The C is given by formula

C = C0*[1 + HC0*RH]

C0 = 160pF at 30C, 30%
HC0 = 3420ppm/%C
RH = % relative humidity

temperature derating factor is
dC= - 0.0019*(T-30) pF
T = temperature in C

When I sweep the RH from 0% to somewhere 45% my 555 timer stops working. I have to replace the LCM555 timer and keep RH below 45%. I have similar circuit with RA = 40.2K, RB = 360K and C is 105pF and centered at frequency 18KHz and it works very fine till 98%.

When I talk to National about this problem, I received a feedback saying I can not use this chip beyond 5.6KHz (max), according to datasheet. I have queries based on my experiment

1. How can upper limit being 5.6KHz is possible. We have many application where 555 used goes much above 100KHz.
2. What causes failure of LCM555 and why it works in other configuration? I have verified the current requirement of discharge pin and trigger pin. What am I missing?
3. What is the solution to my problem.

Could anyone through light on this?

Girish

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What happens if you change the ratio between Ra & Rb (increasing Ra and decreasing Rb)?

With your circuit you will be measuring humidity by measuring the frequency, correct? Wouldn't it be easier to use one 555 running at a constant frequency to trigger a second one as a monostable The averaged output voltage (pulse width) would then indicate humidity.

Thanks for input

The operating frequency is between 10KHz to 100kHz, recommended is 20KHz. So I chose the values closest to 52% duty cycle. I checked with RA = 40.2K and RB = 360K and it failed at 70% RH. To keep accurate RH calculation I chose these resistors in the range of 0.05% so I have upper limit on the resistors.

My worries are, 555 chip is physically damaged at these values so reducing the Resistor will worsen the problem ( I feel so)

Adding another 555 timer will increase board size and will add up to the cost. Also I will have to redesign the complete system but I am not sure is it worth to take these efforts.