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LMC7660 output ripple calculation

Abdullah Ghamdi
Prodigy 60 points
Other Parts Discussed in Thread: ADS1220, LMC7660, TLV2333, TL7660

Hi

I am using LMC7660 to provide a -2.5 supply rail to ADS1220 and TLV2333. The +2.5V supply rail fed to charge pump is coming from TLV7132 (2.5V LDO).

My concern now is that since I am using a bipolar power supply for ADS1220 (+2.5V, -2.5V), I am worried that  charge pump's output ripple might at some point exceed ADS1220's maximum rated supply for AVSS, which -2.3V. I found the following equation to calculate output ripple from TL7660's datasheet, but I am not sure if it's applicable:



Given the pumping frequency of LMC7660 (10KHz), C2 = 10μF, ESR OF C2 = 2.5, Iout (my maximum load current) = 600μA

, then peak-to-peak ripple =~ 116μV


What do you think? is there anything wrong or missing here?

Regards,

Abdullah

  • 0
    TI__Genius 12630 points

    Hello Abdullah,

    The equation you used to calculate the output ripple is for TL7660 rather than LMC7660. The equation for output ripple for LMC7660 is found on page 6 of the datasheet and gives about 24mV of output ripple from the info you provided. The recommended supple for AVSS for ADS1220 is max = -2.3V and min = -2.75V which leaves about 200mV of room if using -2.5V. I think the rated supply of the ADS1220 will not be violated in any circumstance. The example in the LMC7660 (page 6) uses the same value capacitor as you (10uF) and uses the 200mV accepted ripple just like your design but a much larger load current of 5mA so I think it is safe to say your design will be within the recommended operating conditions.

  • 0
    TI__Genius 12630 points
    Abdullah,

    In my calculation, I used your load current as the supply current by mistake. When I use the maximum supply current for LMC7660 which is 400uA (found in datasheet in EC table on page 2), I get a ripple voltage of 160uV which is far less than the 24mV I had previously calculated. This means the recommended operating conditions will definitely not be violated due to the ripple voltage. I hope this helps!