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Vfb and Vioc when connected to a SEPIC

Category: Datasheet/Specs
Product Number: LT3041

Hello 

I have some questions related to building  LT3041 arrays in Voic configuration...(Assume R1=Vfb resistor, R3 = limit Vmax, and R2 )

1. Is it correct to assume that the value of Vref in any Vioc type scenario (in this case its LT3759 with +1.6V = Vref and -0.8V for negative) represents ultimately the Voic limit?

If Voic=Vfb(R1+R2/R1) then the smallest ratio is when R2 = 0. Which then sets Voic = Vfb.

Please advise 

2. When calculating the proper R3 values (for establishing an upper ceiling for Vmax-in)

What specific value is to be applied to Isink ? What is a good practice?  Could you please explain thank you.

R3 = Vmax - Vfb(R1+R2/R1) / (Vfb/R1+ Isink) 
Basically here it says that if R2 is zero then R3 is basically V(max)-V(fb) divided by the sum of I(sink) and I(R1).
For our initial prototype I had assumed 250uA as max I(sink) current. 
3. if we assume an inverting configuration then Vfb becomes -0.8V (with the sister chip LT3099)
Which ostensibly would reduce Vioc significantly in the negative array  (and reduce power loss etc.)
Is there a way to reduce Vioc in the positive regime?
Hope this makes sense.
Please elaborate. Thank you

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  • Hi,

    1. Yes, if R2 = 0, then VIOC will be VFB. You can't go lower than VFB.
    2. To set the max LDO input voltage, refer to equation (6) in the LT3041 datasheet (pg. 27). The datasheet example uses 2.5uA for Isink. R1 should be chosen so the current through it is 100uA. I don't see 250uA anywhere on the datasheet.
    3. You can't use this part with a negative output regulator. For those applications, you should look at LT3099. You're limited to whatever the VFB is for the upstream converter. There is not a way to use VIOC to get lower than that.
Reply
  • Hi,

    1. Yes, if R2 = 0, then VIOC will be VFB. You can't go lower than VFB.
    2. To set the max LDO input voltage, refer to equation (6) in the LT3041 datasheet (pg. 27). The datasheet example uses 2.5uA for Isink. R1 should be chosen so the current through it is 100uA. I don't see 250uA anywhere on the datasheet.
    3. You can't use this part with a negative output regulator. For those applications, you should look at LT3099. You're limited to whatever the VFB is for the upstream converter. There is not a way to use VIOC to get lower than that.
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