Hello I am using a MAX6954 to drive 5 separate single digit 14 segment displays. I have read the datasheet and see their statement about RSET recommended values:
The suggested values of RSET (56kΩ) and CSET
(22pF) set the oscillator at 4MHz, which makes the blink
frequency 0.5Hz or 1Hz.
That reccomendation however must be just a minimum as that would make the mA - 40. My LED displays however are only rated for Intensity of 25,000 ucd @ 10mA & Operating DC Voltage Per Segment = 1.8V-2.1V
So based on the other RSET section that talks about the calculation:
Selecting External Components RSET and CSET
to Set Oscillator Frequency and Peak Segment
Current
The RC oscillator uses an external resistor, RSET, and
an external capacitor, CSET, to set the frequency, fOSC.
The allowed range of fOSC is 1MHz to 8MHz. RSET also
sets the peak segment current. The recommended values of RSET and CSET set the oscillator to 4MHz, which
makes the blink frequencies selectable between 0.5Hz
and 1Hz. The recommended value of RSET also sets the
peak current to 40mA, which makes the segment current
adjustable from 2.5mA to 37.5mA in 2.5mA steps.
ISEG = KL / RSET mA
fOSC = KF / (RSET x CSET) MHz
where:
KL = 2240
KF = 5376
RSET = external resistor in kΩ
CSET = external capacitor in pF
CSTRAY = stray capacitance from OSC pin to GND in
pF, typically 2pF
The recommended value of RSET is 56kΩ and the recommended value of CSET is 22pF.
I have setup a calculation to find the desired RSET. However when I do the calculation I feel like something is wrong based on the value I am getting.
Based on the above if I want my CSET to stay at 22uF like it is in their recommended values I would do:
fOSC=KF/(RSETxCSET)
or
4=5376/(RSETx22)
simplified
RSET=672/11
or
RSET=61.09090909
But I thought that was supposed to be in ohms not in kohms based on the calculator. So that would be way to low...
Maybe its just been to long of a day or I'm doing something completely wrong.. Thank In Advance for the help.
You are correct. However that still does not match what they state it should be. If I put in THIER numbers into the formula it comes out to 61kOhms but they state that it should be 56kOhms with those values. So I'm doing 4=5376/(RSETx22) because they state that their value of RSET 56 would equal 4. But if I do that same formula it comes out to 4.36 not 4. So are they just rounding. Where anywhere in that calculation does my value for mA or forward voltage come into play. I just dont get how I'm supposed to calculate it accurately with those formulas even if I cant get their numbers they recommend to match whats in the datasheet.
I could understand them using the next closest HIGHER value but not lower. Wouldnt going lower risk damaging the chip, or is the rage of tolerance just that large on these types of chips?
With that said I still don't see how I get to what my RSET value should be for my displays. That formula does not take into account my display forward voltage or the desired mA.
That formula only appears to get me the value for what the CHIP wants not what is best for my LEDs to prevent burnout.
No problem. I do think you are worrying too much about that detail. You should do the calculations and then pick a resistor value that gives the same or less current than your calculations result.
aarg:
No problem. I do think you are worrying too much about that detail. You should do the calculations and then pick a resistor value that gives the same or less current than your calculations result.
Ok,
I am clearly missing something. I know you state I was right with my math above but IDK why its right. I want to actually understand how to calculate it not just get lucky because I can copy whats in the datasheet with minor modifications.
So, what is the purpose of the 1st formula in the RSET section. ISEG = KL / RSET mA
How does this work unless I know the ISEG. Is the ISEG just the value of my R = (SupplyV-DropV)/I because I thought that was for a single LED. So would I multiply the result by the number of segments I have (14+1 for DP) for a single display? Or is it for all digits all segments?
Once I have that value I could then get RSET I guess but then is the only purpose of the 2nd formula for me to figure out the CSET value?
I just feel like the formulas are laid out for calculating the Chips values not the LED values.
Thanks for explaining like I'm 5 because I have tried for the last 4 hours to not ask for help again and keep digging but keep feeling even further from the right answer.
