A circuit arrangement for monitoring an external voltage supply (VBAT1, VBAT2) and for reliable locking of a signal (Z2), which is emitted from a logic circuit (8), at a voltage level (VDD, VSS) of an internal voltage supply, wherein the circuit arrangement has a voltage divider (6, 7), which is connected between a first and a second external supply voltage (VBAT1, VBAT2) and produces a potential level (VLOCK) for a switching signal; a controllable switch (13) which separates the internal voltage supply, which has a first and a second internal supply voltage (VDD, VSS), from the logic circuit (8) in order to deactivate the latter as a function of a locking signal (LOCKP) which is produced from the switching signal; and a high-value resistor (14) by means of which the signal (Z2) which is emitted from the deactivated logic circuit (8) is drawn to the level of one of the two internal supply voltages (VSS).