TPS22965: question regarding soft start calculation

Hi Eyal,

1. The table data is gained from testing. The formula is only a approximation tool to help choose a starting CT capacitance. I would recommend using the table and then fine tuning the capacitor as your application requires.

2. The table and the values are for VBIAS=5V. We do not have a table for VBIAS at other conditions or a formula for other conditions. VBIAS is directly proportional to the rise time and with lower BIAS, you will see slower rise times. If a CT value is chosen using the table, a lower VBIAS, will probably result in a rise time that is slower. I would recommend starting with the table once again and the fine tuning the value as needed.

3. This recommendation is mainly for weaker power supplies that cannot provide the inrush current. This is not a strict recommendation. The use of QOD will also provide a less resistive path to GND during turn off so this recommendation can be ignored but I still do recommend you to add some input capacitance.

Thanks,

Shreyas

Hi.

thanks for your answer.

i want to verify some additional issues regarding the switch:

1. we use at other project at ELBIT the TPS22965QWDSGRQ, i want to verify - is it with QOD and without wettable flanks?

2. i see that the QOD internal resistance is 225ohm. that with the 250uF CL cause a long discharge time.

do you have a similar load switch, same size or less, with lower resistance?

3. if not - if i connect reverse external diode from Vout to Vin, can i ignore the recommendation of Cin = 10xCL?

4. at my design, Vbias is activated 1.5msec before Vin. is it OK? (didnt find it at the data sheet)

thanks

eyal

Hi Eyal,

1. I believe this package has wettable flanks. A QOD, non wettable flanks option would be TPS22965TDSGRQ1. The "W" devices use wettable flanks.

2. For most our our devices with QOD we will have an internal resistance of around 180Ohm to 300Ohm. This is in order to avoid a high current to flow through the discharge path and heat up the device. One option may be to use a secondary active low switch that connects a fast pull down resistance on the output.

3. By the description of your application, I think the recommendation need not be followed. I do not think a diode like this would work unless it is placed in series at the output of the switch. This is because the major concern is the body diode of the FET. This would prevent any reverse current flow from occurring. In my opinion, the reverse current path is limited. I also believe some input capacitance can help here but it doesn't have to be the 10:1 ratio.

4. VBIAS before VIN is an acceptable power sequence for the device.

Thanks,

Shreyas

Hi Shreyas.

i'm not sure i understand.

1. regarding you 3rd answer on the external diode - usualy at VRMs that contains FET with internal reverse diode, the internal diode is "week", and if CL > Cin, we add an additional external diode - from Vout to Vin, to protect reverse current from Vout to Vin at poerdown.

Q1: why do you reccomend to place it  in series at the output of the switch, and not from Vout to Vin?

2. as mentioned, i have CL=250uF, and Cin = 50uF (at VRMoutput, driving the TPS22965).

Q2: so i understand from your reccomendation that i can avoid placing the external diode?

3. regarding switch P/N. i understand that you have 2 families at the TPS22965:

a. TPS22965 - industrial. with QOD option only

b. TPS22965x - Q1 - automotive. with option for QOD and wettable flanks.

Q3: - are both families electronaics identical (FFF)?

Q4: i need the exact P/N for:

1. TPS22965  with QOD option

2. TPS22965x - Q1 with QOD and wettable flanks

thanks again

eyal

Shreyas Dmello said:

in series at the output of the switch