TPS22920: Min/Max data for tOff

Hey Carolus,

tOFF (and tFALL) is dependent on the load resistance and capacitance of the tps22920.
The below app note goes into detail in section 1.4 (Links to 2.3 and 3.3) on why fall time may be different than expected.

www.ti.com/.../slva883.pdf

To determine the max and min of your customers tOFF and tFALL times, use sections 2.3 and 3.3 to estimate the RC decay of the load capacitance.
where:
R = Rpd || Rload
C = Cload

Best,
Nick

Hey Nick!

The app note above only goes in depth to explain t_fall, not t_off. By the diagram below:

It appears that t_off is a function of the device only, and should not be affected by the load (just a slew down, no RC drop out). My customer is looking to OR in at the node and drive from a separate source, so t_fall would not be relevant here. Do we have any data on t_off? If it helps, the customer is looking to drive Vin = 1.2V with logic at the gate of 2.5V.

Hey Carolus,

This datasheet characterizes tOFF as the enable signal going low, until Vout reaches 50%.
There is not a designated timing parameter for this datasheet to characterize the delay between the enable signal going low and Vout beginning to fall.
The best approximation of this is from Figure 17 and Figure 21.
- From Figure 17: @0.9Vin, the off delay is approximately 15us
- From Figure 21: @3.6Vin, the off delay is approximately 2us

If your customer is looking to OR the output rail, they should not use this device.
1. This device DOES NOT HAVE Reverse current blocking, which means that at any point when Vout>Vin there could be current flowing from the output to the input.
2. This device HAS quick output discharge, which means when the TPS22920 is off, there will be a connection from Vout to Gnd through an internal pull-down resistor. This is not ideal when the output rail is being supplied by the other source because it will constantly be sinking power.

Best,
Nick

According to the literature previously reference to in this thread, the t,off timing is defined as EN low to Vout falling to 90%, not 50%:

Hello Antennahead,

You are correct the Application Note defines tOFF as the enable going low until Vout hits 90% (starts to fall). In section 1.2 of the app note, right below figure 1, a second figure is shown with tOFF going until Vout hits 50%. Different timing methods can be used depending on the part/manufacturer. It is best to verify how each part defines its timing parameters.

The TPS22920, as seen in Figure 41 of the datasheet, uses tOFF as the enable going low until Vout hits 50%.

Best,

Nick

OK, thanks.

Also, please note that we have successfully demostrated that this device can be used as a diode-OR, when driven backwards, so long as the Vin pin is able to get enough bias to properly set up the charge pump. In one application we used a discrete schottky diode in parallel with the substrate diode to obtain a bias of high enough voltage under the circuit conditions.

Nick-

We need to further follow up on this and/or have an internal call about this.

In this particular application we are running the device BACKWARDS, essentially in a power mux'ing solution. In this case, the substrate diode is pointing INTO the load, rather than away from it. Figure 31, page 11, of the data sheet indicates t,off vs Vin, and for Vin = 1.2V, it appears that t,off ~= 8 us.

What would be the "gauranteed by design" upper limit for t,off? Remember, we are driving the device backwards, so the usual "load effect" is not present- we need to know the propagation delay from falling edge of EN to gate bias falling.

If we can provide some evidence of a max value less that about 50 us, we are good to go.

Please contact me if you need more details or otherwise lets set up a call so we can resolve this quickly in the next few days.

thanks, Best, Steve