Power supply arrangement for battery powered device

1. A method for conversion of a battery voltage in a battery operated system, the method comprising;

providing a voltage reduction to a fixed part of the input voltage using a switched capacitor conversion element, an equivalent model for which comprises an equivalent output resistor R sc, and

providing a variable reduction that ensures that a minimum voltage is maintained,

providing that the fixed part reduction and the variable part reduction are performed in parallel, so that the load current is a sum of the fixed reduction part I sc and the variable reduction part I lin, and

providing a feedback loop from the variable reduction part to the fixed reduction part for adapting the equivalent output resistance R sc to the actual load of the system whereby the battery efficiency is automatically optimized.

2. A method according to claim 1 , wherein the fixed voltage conversion is a ⅔ conversion of the input voltage.

3. A method according to claim 1 , wherein the switched capacitor equivalent output resistance R sc is controlled by setting the clock frequency of the switched capacitor voltage reduction system.

4. A method according to claim 1 , wherein the contribution to the load current from the variable reduction part I lin decreases for an increasing battery voltage V bat , becoming substantially zero at a critical battery voltage V bat,crit so that only the fixed reduction part is active for V bat >V bat,crit .

5. A method according to claim 4 , wherein the feedback loop is adapted to determine the critical battery voltage V bat,crit .

6. A method according to claim 1 , wherein R sc is adapted to provide that the critical battery voltage V bat,crit is substantially equal to a typical battery voltage for the battery type in question.

7. A voltage conversion system for converting battery voltage in a battery operated system, the voltage conversion system comprising a switched capacitor conversion element for providing a voltage reduction to a fixed part of input voltage; means arranged in parallel therewith so that the load current is a sum of the fixed reduction part I sc and the variable reduction part I lin and providing a variable reduction that ensures that a minimum voltage is maintained, wherein an equivalent model for the switched capacitor conversion element comprises an equivalent output resistor R sc ; and a feedback loop from the variable reduction means to the switched capacitor conversion element for adapting the equivalent output resistance R sc to the actual load of the system whereby the battery efficiency is automatically optimized.

8. A voltage converter conversion system according to claim 7 , wherein the switched capacitor conversion element is adapted to perform a ⅔ conversion of the input voltage.

9. A battery operated device comprising a voltage converter conversion system according to claim 7 , wherein the equivalent output resistor R sc of the switched capacitor is balanced in relation to the load current by controlling the switched capacitor system frequency.

10. A hearing aid operating at a voltage lower than the battery voltage comprising a voltage conversion system according to claim 7 .

11. A voltage conversion system according to claim 7 , adapted to provide that the contribution to the load current from the variable reduction part I lin decreases for an increasing battery voltage V bat becoming zero at a critical battery voltage V bat,crit .

12. A voltage conversion system according to claim 7 , wherein the feedback loop comprises a current comparator and a voltage comparator adapted to determine the critical battery voltage V bat,crit .

13. A hearing aid comprising a voltage conversion system according to claim 7 .