LM22671 startup below UVLO threshold or below regulation

Hi Chris,

If you need a 5V output rail, the minimum starting voltage is at least 6.8V for a regulated 5V output (datasheet recommends 8V). However if the input is slowly raised from zero, 4.3V is the minimum starting voltage of the part into no load and this will allow it to startup but NOT at 5Vout. The part will be in drop-out and track the input with a difference of a few tenth's of a volt.

When the Voltage at the input is less than necessary minimum for a 5V output, there is no guarantee of 5 V output regulation due to drop out effect. The part hits maximum duty cycle and then reduced frequency to be able to sustain the output voltage.

You can guarantee startup by increasing the UVLO threshold, or by using an enable supervisor such as a power management IC. But the easiest way is a voltage divider based off the input to set the UVLO.

Thanks,

Anston

The problem that I have is that with a slow startup (the specification requires dealing with an input voltage increasing from 0v at 0.5v per minute) the device does not provide an output even when the voltage exceeds the UVLO threshold and that necessary for good regulation.  If I power down the circuit for say 10 seconds and apply a fast risetime supply it will turn on.  The enable comes from a CAN transceiver, so there may be an external timing issue, but I will permanently enable that to rule that out.

Thanks for reply so far,

Chris

I've investigated the problem, and it's not due to the TI part - it has to do with the voltage thresholds that it and the CAN transceiver which enables the LM22671. The internal UVLO rising threshold is stated in the 'Precision Enable and UVLO' section of the database to be 4.3v (typ) while the falling threshold is 3.9v (typ).  I would like to know what the maximum value of the rising threshold is, and set the external part's enable threshold accordingly, so that the enable signal will only be asserted when the LM22671 has enough voltage to start up.  I don't mind if it's outside regulation, as long as it works correctly when the supply voltage reaches its proper operating level.

Hi Chris,

I now understand your application better. I forwarded your query to our internal Product team and they will get back to me in a few hours. I will communicate when we have exactly what you're looking for.

Thanks,

Anston

Hi Chris,

Taking a little longer than expected. We don't have test data immediately available for the max value of UVLO. Checking a little deeper to see what comes up.

Thanks,

Anston