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A step-up converter converts a battery voltage to an intermediate voltage. The intermediate voltage is greater than or equal to the battery voltage. A step-down converter receives the intermediate voltage supplied by the step-up converter and controls an output voltage which is less than or equal to

A step-up converter converts a battery voltage to an intermediate voltage. The intermediate voltage is greater than or equal to the battery voltage. A step-down converter receives the intermediate voltage supplied by the step-up converter and controls an output voltage which is less than or equal to the intermediate voltage. The step-up converter controls the intermediate voltage in an open loop depending on the battery voltage, at least if the battery voltage is lower than a first predetermined value, and at least if at the same time the load on the output of the step-up converter exceeds a minimum load.

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1. A DC/DC voltage converter for a motor vehicle having a battery providing a battery voltage, the DC/DC voltage converter comprising: a step-up converter for converting the battery voltage to an intermediate voltage, with the intermediate voltage being greater than or equal to the battery voltage,

1. A DC/DC voltage converter for a motor vehicle having a battery providing a battery voltage, the DC/DC voltage converter comprising: a step-up converter for converting the battery voltage to an intermediate voltage, with the intermediate voltage being greater than or equal to the battery voltage, said step-up converter configured for open-loop control, at least in one operating mode, of the intermediate voltage in dependence on the battery voltage at a value which is not equal to the battery voltage; anda step-down converter for receiving the intermediate voltage output from said step-up converter and for regulating of an output voltage being less than or equal to the intermediate voltage;said step-up converter including a switch and a control circuit configured to open and close said switch to step-up said battery voltage when the battery voltage is less than a first predetermined value, and to permanently open said switch when the battery voltage is greater than the first predetermined value. 2. The DC/DC voltage converter according to claim 1, wherein: said step-up converter has an output outputting the intermediate voltage; andin the at least one operating mode, in which said step-up converter is configured for open-loop control of the intermediate voltage in dependence on the battery voltage, the battery voltage is less than the first predetermined value, and a load which is connected to said output of said step-up converter exceeds a minimum load. 3. The DC/DC voltage converter according to claim 2, wherein said control circuit is configured to control said switch with a pulse-width-modulated signal when the battery voltage is less than the first predetermined value. 4. The DC/DC voltage converter according to claim 3, wherein a duty ratio of the pulse-width-modulated signal increases as the battery voltage rises, at least when the battery voltage is less than the first predetermined value. 5. The DC/DC voltage converter according to claim 4, wherein said control circuit is set such that the duty ratio which is less than 100%, is not exceeded at a minimum battery voltage. 6. The DC/DC voltage converter according to claim 1, wherein said step-up converter has an input; andfurther comprising an electromagnetic interference filter disposed between one connection of the battery and said input of said step-up converter. 7. The DC/DC voltage converter according to claim 3, wherein said control circuit has a subtractor for subtraction of the battery voltage from a reference voltage, and a comparator for setting a duty ratio such that the duty ratio is proportional to a signal which is output from said subtractor. 8. The DC/DC voltage converter according to claim 7, wherein said comparator is configured to compare an output signal from said subtractor with the reference voltage which has a triangular profile. 9. The DC/DC voltage converter according to claim 1, wherein the intermediate voltage is set such that it is substantially constant in a battery voltage range which extends from a predetermined minimum value to a nominal battery voltage. 10. The DC/DC voltage converter according to claim 2, wherein said step-up converter has a diode for switching said step-up converter off in a situation when the intermediate voltage exceeds the first predetermined value. 11. An assembly, comprising: a battery outputting a battery voltage; anda DC/DC voltage converter connected to said battery, the DC/DC voltage converter having: a step-up converter for converting the battery voltage to an intermediate voltage, with the intermediate voltage being greater than or equal to the battery voltage, said step-up converter configured for open-loop control, at least in one operating mode, of the intermediate voltage in dependence on the battery voltage at a value which is not equal to the battery voltage; anda step-down converter for receiving the intermediate voltage output from said step-up converter and for regulating of an output voltage being less than or equal to the intermediate voltage;said step-up converter including a switch and a control circuit configured to open and close said switch to step-up said battery voltage when the battery voltage is less than a first predetermined value, and to permanently open said switch when the battery voltage is greater than the first predetermined value. 12. An operating method, which comprises the steps of: providing an assembly including a battery outputting a battery voltage and a DC/DC voltage converter connected to the battery, the DC/DC voltage converter having a step-up converter for converting the battery voltage to an intermediate voltage, with the intermediate voltage being greater than or equal to the battery voltage, the step-up converter configured for open-loop control, at least in one operating mode, of the intermediate voltage in dependence on the battery voltage at a value which is not equal to the battery voltage, and a step-down converter for receiving the intermediate voltage output from the step-up converter and for regulating of an output voltage being less than or equal to the intermediate voltage, said step-up converter including a switch and a control circuit configured to open and close said switch to step-up said battery voltage when the battery voltage is less than a first predetermined value, and to permanently open said switch when the battery voltage is greater than the first predetermined value; andusing the assembly in a motor vehicle. 13. A method of operating a DC/DC voltage converter including a step-up converter for converting a battery voltage to an intermediate voltage, with the intermediate voltage being greater than or equal to the battery voltage, the step-up converter configured for open-loop control, at least in one operating mode, of the intermediate voltage in dependence on the battery voltage at a value which is not equal to the battery voltage and a step-down converter for receiving the intermediate voltage output from the step-up converter and for regulating of an output voltage being less than or equal to the intermediate voltage, said step-up converter including a switch and a control circuit configured to open and close said switch to step-up said battery voltage when the battery voltage is less than a first predetermined value, and to permanently open said switch when the battery voltage is greater than the first predetermined value, which comprises the steps of: controlling the step-up converter in the open loop, such that the step-up converter provides the intermediate voltage in dependence on the battery voltage; andregulating an output voltage by means of voltage regulation by the step-down converter.