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TPS65073 -- doesn't power on without a battery

Other Parts Discussed in Thread: TPS65073

In my application (a battery-powered device), I'm worried about the case where the battery is dead and the device won't turn on with "AC" or USB power.  I'm using a TPS65073 with all 3 power input types -- "AC" from a wall-wart, USB, and a battery.  I don't understand why, but apparently the TPS65073 (at least in my implementation) requires voltage at the BAT input before voltage at the "AC" or USB inputs in order to successfully turn on.  Is there a way to get the TPS65073 to turn on without a battery attached?

Also, I wanted the system to turn on automatically if USB power is installed, so the device can communicate to a PC with minimal interaction.

Experimental results

When I have a battery installed (and "AC" and USB open circuited), I can use a push button attached to  /PB_IN to turn on the device.

However, when I do NOT have a battery attached and I try to either power the device with AC or USB power, the push button does NOT turn on the device.  ("AC" power rail uses 20 mA at 4.5 VDC).

If I connect a battery and "AC" power, the push button does turn on the device, but the device does NOT turn on automatically.  (Upon application of "AC" power, "AC" power rail uses 400 mA at 4.5 VDC -- although my charging circuit is set to use 750 mA).

All registers have been left in their default condition.