Power supply circuit using DC/DC converter

Abstract

A power supply circuit comprises an input for receiving a power supply from a battery and a DC/DC converter for supplying a converted voltage to a load. A regulator is used for controlling the DC/DC converter such that the current drawn from the converter is smoothed. A charge storage device at the output of the DC/DC converter enables delivery of a non-constant current to the load.

Claims

1. A power supply circuit, comprising: an input for receiving a power supply from a battery; a switched capacitor DC/DC converter for supplying a converted voltage to a load; a regulator for controlling the switched capacitor DC/DC converter to reduce a ripple in the current drawn from the switched capacitor DC/DC converter; and a charge storage device at an output of the switched capacitor DC/DC converter for delivering a non-constant current to the load, wherein the regulator comprises a controller configured to control an output resistance of the switched capacitor DC/DC converter by either adjusting a switching frequency or a capacitance of the switched capacitor DC/DC converter.

2. The power supply circuit as claimed in claim 1, wherein the regulator ensures the ripple in the current drawn from the converter is less than 10% of an average current drawn.

3. The power supply circuit as claimed in claim 1, wherein the charge storage device comprises: an output capacitor connected between the output of the switched capacitor DC/DC converter and a reference line.

4. The power supply circuit as claimed in any preceding claim, wherein the regulator comprises a current mirror circuit at the output of the switched capacitor DC/DC converter which is fed by a current source.

5. An electronic device comprising: the battery; the power supply circuit as claimed in claim 1; a first circuit powered by the switched capacitor DC/DC converter; and a second circuit powered directly by the battery.

6. The device as claimed in claim 5, wherein the first circuit comprises a digital integrated circuit and the second circuit comprises an analog circuit.

7. The device as claimed in claim 6 comprising a hearing aid, wherein the digital integrated circuit comprises a transceiver circuit.

8. The device as claimed in claim 7, comprising the hearing aid, wherein the analog circuit comprises a loudspeaker and microphone drive circuit.

9. A power supply method, comprising: receiving a power supply from a battery; using a switched capacitor DC/DC converter to supply a converted voltage to a load; controlling the switched capacitor DC/DC converter to reduce a ripple in a current drawn from the switched capacitor DC/DC converter; and using a charge storage device at the output of the switched capacitor DC/DC converter to deliver a non-constant current to the load; and controlling an output resistance of the switched capacitor DC/DC converter by either adjusting a switching frequency or a capacitance of the switched capacitor DC/DC converter.

10. The method as claimed in claim 9, wherein the controlling further comprises: fixing the current drawn from the switched capacitor DC/DC converter using a current source.

11. The method as claimed in claim 9, further comprising: adjusting the switching frequency to adjust the output resistance of the switched capacitor DC/DC converter.

12. The method as claimed in claim 9, further comprising: adjusting the capacitance to adjust the output resistance of the switched capacitor DC/DC converter.

Description

(1) Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:

(2) FIG. 1 shows an example of DC/DC converter which can be used by a device having the monitoring circuit and method of the invention;

(3) FIG. 2 shows the capacitor configurations for various conversion ratios;

(4) FIG. 3 shows the characteristics of the converter for one particular conversion ratio;

(5) FIG. 4 shows a circuit using six of the modules of FIG. 1;

(6) FIG. 5 shows different ways in which a modular approach with multiple capacitor core sections can be used to generate multiple output voltages and swap up- and down conversion re-using the same circuits;

(7) FIG. 6 shows the circuit diagram of a DC/DC converter connected to a load, and shows the timing diagrams to illustrate performance issues;

(8) FIG. 7 shows how the problems with the circuit of FIG. 6 are worse when the battery ages;

(9) FIG. 8 shows how the addition of capacitances can reduce the problems;

(10) FIG. 9 shows how the battery voltage is used directly to power some circuits;

(11) FIG. 10 shows a first example of converter circuit of the invention; and

(12) FIG. 11 shows a second example of converter circuit of the invention.

(13) The invention relates to a power supply circuit which receives a power supply from a battery and has a DC/DC converter for supplying a converted voltage (i.e. a DC regulated voltage) to a load. A regulator is used for controlling the DC/DC converter such that the current drawn from the converter is smoothed, for example so that it is substantially constant.

(14) This reduces the resulting ripple on the battery output voltage. A charge storage device at the output of the DC/DC converter enables delivery of a non-constant current to the load.

(15) Before describing the invention in detail, the problems associated with the known circuit arrangement will first be discussed.

(16) FIG. 6 shows the equivalent circuit and the various voltage and currents flowing when the DC/DC converter is used to drive an IC 64, which for example can be a 2.4 GHz transceiver circuit.

(17) The circuit comprises the battery 60, the DC-DC converter 62 and the load 64 to be driven by the converter output, for example a digital integrated circuit (IC). The IC requires a particular voltage range. For example, a battery voltage of 1.2V can be converted to a value which must always be higher than 1.8V, for example in the range 2V to 3V.

(18) The IC has a known current demand. The example shows a peak current I.sub.OUT of 13.5 mA for a duration of 0.8 ms, with a period of 6 ms. This current spike can be the burst mode of a transceiver.

(19) The current spike in the battery output is a multiple of the output current spike as a result of the converter amplification. For example, the fluctuation in the battery current I.sub.BAT on the input side of the converter 62 can be approximately 30 mA as shown, based on a converter ratio of 2. With a battery impedance of 50 this corresponds to a spike in the battery voltage V.sub.BAT of 150 mV.

(20) As the battery ages, the voltage decreases and the impedance increases. For example the voltage can drop to 1.0V and the impedance can increase to 10Ω.

(21) FIG. 7 shows the effect on the battery supply. The current ripple increases to 50 mA based on a conversion ratio of 3, and the corresponding voltage ripple in V.sub.BAT increases to 500 mV. This voltage ripple can make the battery output unsuitable for use in powering other devices and it can result in audible noise when used to power a loudspeaker.

(22) A first way to reduce these ripples is to add an output capacitor 80 between the output V.sub.OUT and ground as shown in FIG. 8.

(23) This reshapes the output voltage V.sub.OUT to have a ripple, as shown as plot 81 in FIG. 8. The output capacitor can have a value of around 10 uF, and it combines with Rout to form a low pass filter that filters the ripple at Vout. The output current I.sub.OUT is thus partly supplied from the capacitor C.sub.OUT. This means R.sub.out can made more high-ohmic by reducing the size of the switching capacitor (which for example may be of the order of 6 nF) and/or f.sub.clk in the DC/DC converter.

(24) The reduced ripple in the battery voltage and current is shown as plots 82 and 84. However, the output capacitor takes up area, and the improvement is limited.

(25) A further improvement can be obtained by adding a capacitor 86 to the battery output. This is an additional capacitor that is placed in parallel with the battery to further provide battery ripple reduction. The output voltage V.sub.OUT is not changed further but the voltage and current ripple for the battery are reduced further as shown by plots 88,90, However, placing a limited size capacitor at that location does not improve the ripple significantly.

(26) FIG. 9 shows resistors in series with the two additional capacitors 80,86. These are parasitic resistances, and they have the effect of increasing the ripple on the output voltage V.sub.OUT as shown by plot 92.

(27) FIG. 9 also shows that the battery voltage output can be used to power analogue circuits, such as a class D amplifier 87 as shown.

(28) The problem remains that it is very difficult to achieve a low ripple on the battery, assuming that large external capacitors are to be avoided, for example because they may be too large with respect to the area inside a hearing aid.

(29) The ripple on the battery voltage depends on the supply drop allowed on V.sub.OUT, the capacitance value that can be used in the application, the battery impedance R.sub.BAT, and the equivalent series resistance (ESR) of the chosen external capacitors. The on time T.sub.ON (0.8 ms in the example given) determines the maximum current the DC/DC converter needs to deliver and the minimum permitted battery voltage.

(30) Simulations show that the ripple on the battery voltage V.sub.BAT is mainly determined by the battery resistance R.sub.BAT and the low-pass filtered output current pulse as seen at the input of the DC/DC converter. As explained above, this filtered current pulse is multiplied by the battery resistance Rbat to derive the voltage ripple at the battery.

(31) The mean current that needs to be delivered determines the DC/DC converter output resistance R.sub.OUT, and therefore a certain value of C.sub.OUT can be found that provides sufficient filtering of the current pulse multiplied with the battery resistance R.sub.BAT. Thus, there is a trade-off between C.sub.OUT and R.sub.BAT.

(32) By way of example, a simulation shows that the described 800 us on-time in 6 ms intervals is possible with use of a 10 uF external capacitor C.sub.OUT. Assuming V.sub.BAT=1.2V, R.sub.BAT=5Ω, this results in <100 mV supply dips at the Zn-Air battery and 600 mV supply dips at V.sub.OUT.

(33) V.sub.OUT should be charged to more than 2.4V as a result of this 600 mV ripple.

(34) For lower peak voltages, the <100 mV ripple can be kept on V.sub.BAT by choosing an increased ratio and larger C.sub.DC/DC, but an increase in R.sub.BAT will cause a larger ripple. As explained above, a larger ripple on the battery causes significant performance reduction.

(35) The invention provides a power supply circuit which is supplied by a battery and which has a DC/DC converter for supplying a DC regulated voltage to a load. A current regulator controls the current drawn from the DC/DC converter to be constant (in an ideal implementation). This means an input current ripple is avoided, and the battery output voltage is stabilised, so that other circuits can be powered directly by the battery. A charge storage device at the output of the DC/DC converter enables a non-constant current to be supplied to the load, based on the demand presented by the load.

(36) FIG. 10 shows a first example of DC/DC converter circuit of the invention in which a current mirror is provided at the output of the DC/DC converter to ensure a constant battery current. This current mirror thus provides the regulation function.

(37) The circuit comprises the battery 60, the optional battery capacitor 86 to help filtering the voltage ripple at V.sub.BAT, and the DC/DC converter 62. The load IC 64 is shown as well as the external capacitor 80 placed at the output of the DC/DC converter 62. The parasitic resistances are also shown, to illustrate that they do not prevent the correct operation of the invention.

(38) The current mirror circuit 110 comprises a first branch which is fed by a reference current source 112. The current mirror multiplies the reference current by a factor of n. This multiplied current is provided as the output current I.sub.OUT.

(39) The current mirror 110 drives the output capacitor 80 with a constant current equal to the average current consumed by the load 64 (which in the example above is an integrated circuit, for example an RF transceiver IC). The average current consumption of the load is thus used in the design of the circuit components. Assuming a 13.5 mA current consumption during 800 us activation time in a 6 ms interval (as shown above), the average current is 13.5 mA*800 us/6 ms=1.8 mA. This current will charge the output capacitor Cout with a constant current whereas the IC discharges Cout in a pulsed way. Thus, the capacitor Cout functions as a charge reservoir, leading to a triangular ripple at Vout. This triangular ripple on V.sub.OUT is shown in FIG. 10. The voltage remains above the minimum threshold at all times, which in this example is given as 1.8V.

(40) The output capacitor may be as small as 10 uF to enable implementation of the invention. However, a larger capacitance, for example 47 uF, makes the ripple on the output voltage Vout smaller, and increases power efficiency.

(41) The IC 64 may not be sensitive to voltage ripple, for example if it contains an on-board voltage regulator. The current mirror implementation is used to set the output current to the average current consumed by the IC 64. A high multiplication factor of n (e.g. n=100) is used, so that the overall current consumption resulting from the use of the current mirror is not increased significantly.

(42) The current mirror current can have a static current setting (which is possible when the average current consumption of the IC is known exactly) or it can operate in a dynamic manner. When dynamically controlled, the output voltage Vout can be monitored, and it can be verified whether or not the minimum value stays above the minimum allowed voltage for the IC.

(43) If the IC consumes more average current, then the voltage Vout will drop over time leading to a functional error. By detecting this, the current coming from the current mirror can be increased, by changing the current of the current source.

(44) The effect of this current mirror circuit can be considered to be a modulation of the output resistance Rout. The modulation is the mathematical result from the fact that the output voltage Vout has a triangular shape and the output current Iout is constant, leading to Rout=Vout/Iout=[triangular shape]/[constant shape]=triangular shape].

(45) The output capacitor 80 is needed to prevent a large ripple at the output of the DC/DC converter. The DC/DC converter basically multiplies the input voltage by a certain constant ratio. Suppose the battery voltage is equal to 1.2V and the ratio is 3×, then the output of the DC/DC will generate 3*1.2V=3.6V each time there is a DC/DC switching cycle.

(46) Although the average charge transfer is regulated by gating the clock, there will be a large ripple at this node. For example, 3.6V can be higher than the maximum voltage allowed by the process limit (which is for example 2.8V).

(47) To reduce this voltage ripple, an external capacitance can be added to the node V.sub.DC/DC OUT. However, this requires an extra pin.

(48) FIG. 11 shows a second example of DC/DC converter circuit of the invention in which the output resistance of the DC/DC converter is modulated using frequency control to provide a constant battery current. This avoids the need for an additional external capacitor mentioned above.

(49) FIG. 11 shows how the output resistance of the DC/DC converter can be varied over time as plot 120 in order to result in the constant current output. As shown in FIG. 3 also, the resistance is inversely proportional to the capacitance and the clock frequency. Thus, by dividing the time interval into segments, for example 20 to 500 segments per conversion cycle, the resistance can be controlled in each segment so that the resistance follows the profile 120.

(50) This approach is possible when it is known in advance when the output load is active and its current profile is known. This information is used to modulate the DC/DC converter output resistance R.sub.OLT in such a way that the battery current is constant, by changing either the capacitance inside the capacitive DC/DC converter or its frequency over time.

(51) The circuit of FIG. 11 corresponds to that of FIG. 10 but the DC/DC converter output resistance is modulated as shown instead of providing a current mirror output. Thus, the circuit corresponds to that of FIG. 9 but with modulation of the output resistance. Thus, the current regulator is implemented as a controller for varying the capacitance and/or clocking frequency of the converter.

(52) The circuit functions by varying Rout in such a way that the output voltage Vout has the same triangular shape as in the above case with a current mirror.

(53) By dividing the 6 ms interval into k bins, each will be equal to Tbin=6 ms/k (for k=128, this gives we have Tbin=6 ms/128=46.875 us). In each Tbin a desired constant value for Rout is obtained:
Rout[k]=m/(fclk[k]*Cswitching[k])

(54) If the value of k is large enough, the same triangular shape can be emulated as in the above case with a current mirror:
Iout=Vout/Rout=[triangular shape]/[triangular shape]=[constant shape]

(55) Of course in practice there are limits to increasing k and there will be some quantization effects remaining. However, overall the ripple on Vbat can be reduced to a large extent.

(56) When requiring changes in capacitance at the different bins, the modular approach explained above can be used to vary the switching capacitance. Thus, the number of converter cores which are switched into circuit can be controlled during the time period. The converter cores can for example have a binary weighted capacitance series so that a large number of different capacitances can be implemented by selection of the converter cores which are operative.

(57) The operation of the circuit has been verified by simulations.

(58) Various modifications will be apparent to those skilled in the art.