TPS25982: Applicable Cout(Load capacitor after TPS259827) with our specification

Hi Ryo,

1. Can you share a waveform showing VIN,VOUT,IN? Are you seeing a failed startup?

2. Is the load during startup only capacitive or the other downstream circuitry is also drawing current?

3. One solution is to startup device in current limit. Leave CDVDT open. But for that we have to decrease current limit.

Regards

Kunal Goel

Hi Kunal,

1. Sure.

2. There are multiple input capacitors around ICs. Total amount is about 5mF maximum (ceramic caps and aluminum electrolytic caps)

These ICs will draw current from those capacitors.

There are some pull up resistors and diodes, so there might be negligible amount of circuit those draw current.

3. In case of  CDVDT -> open, slew rate goes faster and inrush current will increase. Is this solution really effective?

Regards,

Ryo

Hi Ryo,

1. I see that problem is thermal shutdown because load is large. What is the restriction on charge time? From design calculator I see you have to use a DVDT cap greater than 3.3uF to have startup but charge time will go upto 20s.

2. CDVDT open helps when current limit is lower than inrush current current because of which inrush current exceeds current limit and device will start in current limit . Also when we leave cdvdt open we do not see delay before VOUT starts rising. But it looks it will not help much in your case because we cannot set current limit to very low value because of application requirement .

3. Other solution I can think is you add an external resistor+diode from IN to OUT in parallel to efuse. First you charge output cap upto some voltage say 15V through this R+D. Now you can use a comparator to check output voltage . When outptut voltage reaches 15V you send a signal to our efuse enable pin to turn it high and turn efuse one. Now ouptut cap will charge through our efuse and eFuse will see less power dissipation now because voltage difference that it see at output is now 24-15=9v but earlier it was seeing 24-0=24V. Now you can have successful startup at lesser CDVDT cap value.

Regards

Kunal Goel

Thank you for your reply.

1. It seems that calculator(rev.A) won't work with large charge time like attached picture.

Am I using old calculator tool?

Anyway, too long charge time like  > several seconds is not preferable in our system.

2. If I could change Cdvdt and Ilim dynamically, it might be manageable. But at this moment, we don't have that kind of structure on our PCB. Will it be feasible (for TPS259827) if I had those kind of control function on my PCB?

3. Thank you for your advice but that cannot be a solution for us.

Regards,

Ryo

Hi Ryo,

1. Yes calculator can not auto calculate cap for large charge time. You need to enter cap value manually . 

2. We havn't tried dynamic control during startup especially for DVDT . I think it will not work. ILIM you can change after startup to a different value. But problem with this startup in ILIM approach is that  device will start then hit thermal shutdown then restart then hit thermal shutdown. So it will add to charge time. You have to first test this approach on your system with lowest current limit for this device that is 2A. 

Can you decrease your output cap?

Regards

Kunal Goel

Hi Ryo,

Let me know if I can close the thread or there is further query.

Regards

Kunal Goel

Hi,

We've done another experiment on this problem but our conclusion is that TPS25982 in not suitable for our application.

Specification shown on datasheet(on Feature section) of this eFuse was really nice but this eFuse cannot manage heavy C load with in acceptable time scale.

We will have to re-design our PCB without this eFuse.

Regards,

Ryo