The LMR51610 has an EN pin to turn the output on and off. The typical threshold voltage of the EN pin specify in LMR51610 data sheet high about 1.27 V and low about 1 V. However, in the inverting buck-boost configuration, EN referenced point is change from device GND to –VOUT voltage. Therefore, the threshold for the EN pin is change to rise is 1.27 V + –VOUT, and the threshold for the EN pin to be considered low is 1 V + -VOUT. For example, if –VOUT = –12 V, EN will be considered high for voltages above –12 V-1.27 V=-10.73V and low for voltages below –12 V+1 V=-11 V. So, in the negative application it is require a voltage of at least 1 V + –VOUT to shutdown the device.

Figure 3-2 EN External Control and EN Control by Vin