TPS2596: Cdvdt for using electrolytic capacitor

Hi Michiaki,

Is the 2F cap a Super Cap ? What is the maximum voltage that you want the cap to ?

For charging super caps you can consider using the current limit functionality (current limit set at  200mA) of the IC instead of dvdt based startup. By using current limit functionality you have the following advantages,

• The high value of Cdvdt can be avoided
• The turn ON delay due to Cdvdt can be avoided

Below is a test that we have performed on TPS259630,

Test Condition:

• 10F super cap at output
• 4V input,
• Current limit set to 230 mA
• OVLO tied to Output vis resistor divider so as to set Cut off at 2.6V,
• T_A = 25C,  
• Load applied after super cap charges (at point A) to discharge Vout and trigger charging again.

Observation: In the initial charging phase, when Vout is less than 1V the current limit value will be less than the set value. This current foldback feature is implemented in the IC to reduce the current limit value in short circuit conditions. Once the Vout reaches 1V, the current limit value is equal to the set value as expected.

Hi Praveen,

Yes, 2F Cap is ELDC (Super Cap).
As for maximum voltage to Super Cap is 3.8V but let me double-check with customer.

Could you kindly advise your proposal exactly ?

Using the current limit functional is open for Cdvdt ?
In the case of your proposal, you show the waveform as attached but I see the rise time for Vout is slow compared with datasheet.

I would much appreciate if you advised me a bit detail.

Thank you and best regards,
Michiaki

Hi Michiaki,

You can leave the dVdt pin floating and set the current limit to 200mA . By doing so, the super cap will be charged at 200 mA current limit. 

In the testing that we have done, we have tied the OVLO pin to Ouput via a resistor divider such that when the output reaches 2.6V, the device turns off (stops charging super cap). The efuse turn ON again when the output drops below a particular threshold based on the OVLO falling threshold. You can set this OVLO rising threshold to 3.8V at ouput so that the Super cap stops charging when its voltage reaches higher than 3.8V.

The rise time can be calculated from the formula below while charging in current limit (constant current),

i = C * (dv/dt)

⇒ dt = C * dv/i

So for constant current and voltage, as Capacitance increases the time to charge the capacitor also increases. The output capacitance in the datasheet must be much smaller than 2F.

Please let me know if you have more questions.

Hi Praveen,

Please let me make sure that the reason of using OVLO
Is it on the increase of the life for super cap by reducing charge count ?

Configured around 10V for OVLO and in the case of not triggerd OVLO, how the TPS2593x would be worked ?

Thank you and best regards,
Michiaki

Hi Michiaki,

Do you have any other questions ? if not, can we close this thread ?

Hi,

Could you let me know the use case of dvdt function ?

In the case of using super cap, customer should use current limit and OVLO function rather than dvdt function.

What case is dvdt function used ?

Thank you and best regards,

Michiaki

Hi Michiaki,

The function of dvdt circuit is to limit the current during startup. This is achieved by allowing a constant current in the FET(to obtain constant dv/dt)  by proper selection of the Cdvdt capacitor. The current limited by dvdt during startup is generally set to less than current limit value. 

So, dvdt is generally used when the steady state current limit is higher and you do not want that amount of current during start up. Having less amount of current during startup helps to maintain the FET temperature to less that the thermal shutdown threshold thereby helping in having a successful startup.

In your case, the steady state current limit which is the current limit for charging the super cap is nothing but the current limit during startup. This is because your input voltage is higher than output voltage and you turn off the eFuse before the output voltage reaches input voltage (as it reaches the cap max voltage value).

Hi Michiaki,

Did the above reply answer your question ? Do let us know if you have any more queries.