TPS2490: Timer Capacitor(Ctimer) Value

Hi Sakai,

The max Ctimer value that can be used is determined by the system operating parameters and FET SOA. If the design sheet meets SOA for 1uF, you can proceed with out any concern.

Can you please detail your second question.

BR, Rakesh

Dear, Rakesh-san. 

The customer's system condition are ; 

- Input voltage : 24.5V

- RL : 0.3ohm

- Current limit setting : 20A

- Ctimer : 1uF (160ms) 

1. Under above condition, they confirmed the timer period is 272ms versus calculated value 160ms. 

   They are asking the reason why there is a big difference. 

2. As you mentioned, FET's SOA will be so important. Do you mean 1uF value capacitor & 160ms 

    timer period is not suitable considering existing FET's SOA? 

Best Regards, 

H. Sakai

Hi Sakai-san,

Can you please share

1. Schematic
2. Filled up design calculator
3. Test condition under which above issued observed
4. Test waveforms.. Include
   1. Vin, Vout, GATE, timer voltage
   2. Vout, GATE, timer voltage, input current 

BR, Rakesh

Dear, Rakesh-san. 

Thank you so much for your support. 

I requested the customer to provide the information as you pointed, but have not 

obtained, yet. 

I believe the timer function will be simple to act the charging & discharging with the 

external capacitor. If it was true, no result why the timer period become so long after

the schematic or setting checking. 

At this time, I would like to confirm about the characteristics protection diode. 

According the datasheet, the protection diode will be located GND to TIMER pin. 

For instance, the discrete SBD has the reverse current(IR_leakage) and it has the thermal co efficiency.

I am guessing it influenced the timer period to be longer, especially under higher temp.  

Is it possible to confirm the IR of protection diode &  the thermal co efficiency? 

You don't need to assure that, the design spec is enough. 

It was depended on the results, but possible to make the final answer without your loading for the checking. 

Best Regards, 

H. Sakai

Hi Sakai-san,

The timer sourcing current has variation from 15uA to 34uA with 25uA as typical. This variation accounts for all the process, temperature and internal circuit tolerances.

For 1uF timer cap, the typical timer period is 160ms; max timer period is 266ms and min timer period is 117ms.

Can you share few more details...

1. Part number of 1uF timer cap
2. Is the behavior of longer timer period observed on single device or multiple devices ?
3. what is the timer period when low value of timer cap is used ? for example 100nF timer cap

BR, Rakesh

Dear, Rakesh-san. 

Thank you so much for your reply. 

Shall we talk via our private mail. 

Best Regards, 

H. Sakai