TPS63700: negative output voltage out of regulation

Q:

When the IC runs without load, the negtive output voltage is within regulation; but after I attach a nominal load resistor, the negtive output voltage increase to almost 0V.  Is the IC damaged?


A:

No.  There are two possible causes. 

First, the VREF pin has only been characterized for output currents up to 10uA as shown in the electrical charateristics table. Your resistor between VREF and FB pins may too small and is causing VREF to fall out of regulation. 

Second, an inverting buck-boost converter like the VNEG rail produces significant switching noise.  This switching noise can couple back into the IC through the FB and VNEG pins and false trip the current limit circuit of the power switch.  Therefore, the switch turns off too early and causes the converter to be power limited.  The app note at the link http://focus.ti.com/lit/an/slva219/slva219.pdf explains this in more detail.  If you copy the TPS63700 EVM's board layout as documented  in the user's guide at the link http://focus.ti.com/lit/ug/slvu134a/slvu134a.pdf on your board, the switching noise will be minimized and should not power limit the converter.