Current Measurement

The present invention relates to a current measurement circuit for measuring the current flowing in a circuit and particularly the current flowing into a battery during charging.

It is important to be able to measure the current flowing in electric/electronic circuits in many circumstances. For example, it may be desirable to measure the current flowing into a battery on charging or in linear regulator circuits.

There are a variety of battery chargers in use. Smart chargers are chargers that can charge smart batteries. A smart charger is a charger that complies with the Smart Battery Charger Specification Revision 1.1, December 11, 1998. A smart charger is able to accept information from a smart battery and provide the required current and voltage to charge the battery.

A battery charger incorporates a current sensing circuit in order to measure the current flowing into a secondary battery during charging.

A simple way of measuring the current flowing into a battery during charging is to add a resistor to the OV line of the charger output and measure the voltage drop across that resistor as a representation of the current flowing through it.

However, in some applications the charger OV line, the power supply OV line and the battery OV line are each connected to their conductive cases. This is done to minimise the radio frequency emissions from the charging system. Unfortunately, this means that any contact between the conductive case of the battery and the

conductive case of the charger could therefore result in a short circuit across the resistor in the OV line of the charger and this results in an error in the measurement of the charging current .

Therefore, in order to be able to measure the charging current even in the event of a short circuit, the current sensing resistor is positioned in line with the positive charging terminal of the battery and an integrated circuit is connected across the resistor in order to measure the current flowing in the resistor. This is done by separating out the small voltage drop across the resistor. The drawback to measuring the current in this way is that the integrated circuits typically available for use in current measurement do not operate correctly if the battery voltage is less than about 2.5V. Thus it cannot measure the current supplied to a low voltage battery or limit the current that flows when the charger output terminals are shorted together.

The present invention aims to provide a solution to this problem by providing a current measurement circuit that provides the correct output signal for all battery voltage levels including zero.

Accordingly, the present invention provides a current measurement circuit comprising a positive voltage current sense integrated circuit which is connected between a positive voltage rail and a negative voltage rail of at least -2.5V and is connected across a resistor positioned in the positive rail wherein the voltage sense pins of the integrated circuit are connected across the resistor in the reverse sense and the output from the integrated circuit is inverted.

The negative voltage rail is at at least -2.5V. Thus, there is always a difference in voltage between the two rails even when the output terminals (of the battery) are shorted together. When the output terminals are shorted together the positive voltage rail is at zero.

The negative voltage is typically from -2.5V to -10V, preferably from -2.5V to -7.5V, for example, -4, -5 or -6V.

The high side current sense integrated circuit is an integrated circuit that can be used to sense the current flowing in a circuit and can be positioned on the high side rail. For example a LT1787 or INA193.

The output from the integrated circuit may be inverted using an operational amplifier, for example, a LT2055.

The present invention also provides a method of measuring the current flowing in a circuit comprising

a) measuring the voltage drop across a high side resistor using an integrated circuit connected so as to produce a negative signal, and

b) inverting the signal

wherein the integrated circuit is connected between a positive voltage and a negative voltage.

The expression ' a high side resistor' refers to a resistor in the positive rail, that is to say the high voltage rail.

The present invention also provides the use of a high side current sense integrated circuit which is connected between a positive and a negative voltage and is connected across a resistor positioned in the high side rail wherein the voltage sense pins of the integrated circuit are connected across the resistor in the reverse sense and the output from the integrated circuit is inverted to measure the current flowing in a circuit .

The size of the resistor is chosen so as to be suitable for the current sense integrated circuit.

A capacitor may be connected across the integrated circuit in order to filter out any high frequency components in the voltage across the resistor.

The present invention also provides a battery charger comprising a circuit of the present invention. The present invention also provides a linear regulator circuit comprising a current sense circuit of the present invention .

Specific construction of a current measurement circuit embodying the invention will now be described by way of example and with reference to the drawing filed herewith, in which:

Figure 1 is a schematic diagram of a current measurement circuit according to the present invention.

Figure 1 shows a charger 1 connected between a DC power supply 2 and a secondary battery 3. A high voltage rail 4 connects the power supply 2 to the charger 1 and the charger 1 to the battery 3, via the positive voltage terminals of a DC-DC converter 5. A zero voltage rail 6

is also connected from the power supply 2 through the DC- DC converter 5 to the charger 1 and the battery 3. In each of the power supply 2, the charger 1 and the battery 3 the zero voltage rail 6 is connected to the casing of the object as shown at the positions marked 7.

The charger 1 contains a circuit according to the present invention for measuring the current flowing in the high voltage rail 4. The circuit comprises a resistor 21 which is positioned in the high voltage rail 4. An integrated circuit (LT1787) 8 is connected across the resistor 21. A capacitor 25 is connected across the integrated circuit 8 to the filter terminals 11 and 12 in order to filter out any high frequency components in the voltage across the resistor 21. The voltage sensing pins 14 (+ve) and 13 (-ve) are connected across the resistor

21 in the reverse sense which results in the integrated circuit 8 producing a negative signal. The voltage sensing pins 14 and 13 also provide the power supply to the chip and the chip draws some current through these pins with respect to VEE pin 15. The integrated circuit 8 is connected between the high voltage rail 4 and a -5V voltage source, connected to VEE pin 15, which is provided by a power supply unit 9. The size of the resistor 21 and the capacitor 25 is chosen to be suitable for the specific integrated circuit 8.

Output pins 16 (Vbias) and 17 (Vout) from the integrated circuit 8 are connected together and passed through an operational amplifier 10. Connecting the output pins 16 and 17 together has the effect that no offset is added to the output signal. There is a resistor

22 in the signal lead to the amplifier 10, a resistor 23 connecting the output of the amplifier 10 to the signal input, and a resistor 24 connecting the other input of the amplifier 10 to the zero voltage rail 6. By use of

resistors 22, 23 and 24, the operational amplifier 10 amplifies the signal from the integrated circuit 8. In this embodiment the resistors 22 and 23 are the same size and the operational amplifier 10 therefore does not change the resistance of the signal. Resistor 24 is half the resistance of resistors 22 and 23. The signal from the integrated circuit 8 is inverted by the operational amplifier 10 thus producing a positive signal 26 which indicates the current sensed by the circuit.