GB2332760A - Low voltage stabilised current source - Google Patents

Abstract

A current source circuit produces a temperature stable current IOUT as the sum of a first current from a circuit A and a second current from a circuit B. The two currents have opposite temperature dependence. In circuit A, the base-emitter voltage of a transistor 3 is applied across a known resistor 5. The resulting current is stabilised by feedback through transistors 4,2 and then reflected into transistor 1. Circuit B includes a bandgap circuit 11-16 and output transistor 10. The device may be constructed as an integrated circuit operable at 0.9 volts, and may be used in a portable radio or mobile telephone.

Description

LOW VOLTAGE STABILISED CURRENT SOURCES Field of the Invention The present invention relates to low voltage stabilised current sources and more particularly to a stabilised current source for use with supply voltages as low as 0.9v. The invention has particular application to portable battery powered equipment such as radios or mobile phones.

Background of the Invention In our parallel co-pending application (Motorola reference SCO643EG) we disclose a low voltage crystal oscillator circuit which is particularly adapted to be manufactured in integrated form, i.e. on a so-called chip.

In that parallel application there is also disclosed a circuit for providing the current source to the crystal oscillator circuit. This current supply circuit is a relatively simple configuration but does not provide the optimum degree of current stability when there are variations in temperature and variations in supply line current source.

Integrated circuits often need current sources to be stable despite supply line and temperature variations. A known implementation is to generate a 1.25 volt bandgap reference and this plus a diode voltage drives the bases of transistors having emitter resistors to define the desired currents. The function of the diode is to cancel the temperature coefficient introduced by the 0.75 volt drop of the aforesaid transistors emitter/base junctions.

However a clear problem exists for supply lines of less than 2 volts, where this method is not applicable.

It is also known to take current sources of opposite temperature coefficients and sum them in proportions such as to arrive at a substantially zero temperature coefficient. Known solutions are similar to the above mentioned solution without the temperature compensation diode but with the addition of current derived from a diode voltage. Again for supply lines having bandgap reference of less than 1.25 volts this method will not work.

The present invention is concerned with dealing with this problem and providing a stable current source in situations where the supply voltage is less than the bandgap voltage.

The present invention seeks to provide an improved integrated current control circuit, and in particular providing an alternative circuit to the one disclosed in the aforementioned parallel application.

Summarv of the Invention According to the present invention there is provided a low voltage stabilised current source circuit which includes first and second control circuit mechanisms, the first control circuit mechanism comprising the current generated by applying the voltage of a forward conducting diode across a known resistance and the second current control mechanism being the current generated by applying a known multiple of the said voltage given by KT/Q across a known resistance.

In the expression kT/q, k is Boltzmann's Constant, T is the temperature and q is the charge of an electron.

Thus the essence of the present invention is that a low voltage current source which is stable in respect of temperature and supply line voltage is achieved by using a combination of two current control mechanisms, one with a rising characteristic with temperature and the other with a falling characteristic with temperature.

Brief Description of the Drawings An exemplary embodiment of the invention will now be described with reference to the drawing in which: FIG. 1 is a circuit showing a preferred embodiment of the present invention; and FIG. 2 is a graph showing the operating characteristics of the circuit of FIG. 1.

Detailed Description of a Preferred Embodiment Referring to FIG.1, there is shown a current control circuit which consists essentially of two sub circuits A and B which are outlined in broken lines.

In order to solve the problem of low voltage operation, two parallel current sources of opposite temperature coefficients are provided such that the minimum supply voltage does not exceed a forward biased base-emitter junction voltage plus a collector-emitter saturation voltage. These voltages are in the region of 0.7 volt and 0.1 volt respectively thus allowing operation down to 0.9 volts even at -50 degrees centigrade.

In essence circuit A is designed to generate current which reduces with increase in temperature whereas circuit B is designed to generate current which increases with increased temperature.

The circuits A and B are both designed to produce current which is stable against supply line variations.

The circuits A and B are interconnected in such a way that the respective currents from them are arithmetically added in the correct proportions such that when summed they give a current output which is substantially constant against temperature variation.

The construction of circuits A and B will now be described.

Circuit A comprises transistors 1 to 4 and 7 and resistors 5 and 6 and the circuit represented by those components produces a current which depends on the Vb of the transistor 3 and the resistor 5. The DC negative feedback from the collector of transistor 3 via transistors 4 and 2, causes the current in transistor 3 to stabilise at a value equal to the current provided by resistor 6 and transistor 7. Transistor 2 will provide the exact current required by resistor 5 to reach the Veb of transistor 3. Transistor 1 copies this current as its base and emitter are in common with those of transistor 2. For AC stability a further resistor 8 and the capacitor 9 are provided. Neglecting the temperature coefficient of the resistor 5, the collector current of the first transistor will be given by: I = Veb/R3 = 0.75/55k = 15uA dI/dT = 1/R3 x -1.5mV/degC = -0.027 uA/degree Circuit B comprises transistors 10 to 14 and the resistors 15 and 16. The current of this circuit is defined by the bandgap voltage expression KT/Q, the area ratio of transistor 12 with respect to transistor 13, the area ratio of transistor 11 with respect to transistor 14 and the resistors 15 and 16.

The emitter areas of transistor 12 and transistor 14 is arranged to be ten times those of transistor 13 and transistor 11 respectively. In the configuration shown with the current mirror of transistors 11 and 14 returning the current from the current mirror of transistors 12 and 13 back to transistor 12 there are only two possible transistor current states: "all off' or each at a similar current. The "all off' state is prevented by the resistors 6 and 17, which facilitate the starting up of the circuit. Neglecting the base currents of transistors 10, 11 and 14, this common current value is defined by the following expressions: I = 1/R x kT/q x ln(AT12/AT13) =7.7us dI/dT = 1/R x k/q x ln(AT12/AT13) = 0.027uA/degree where k = Boltzmann's constant q = electron charge T = Temperature degrees Kelvin kT/q = Thermal voltage = 25mV at 290deg Kelvin (17degC) R=R1=R2=7500 ln(AT12/AT13) = natural log of (transistor 12 area divided by transistor 13 area = ln10 = 2.3 The temperature coefficient of kT/q is +86uV/degC.

It is reasonable to replace all the bipolar transistors except 12, 13 and 3 by corresponding N or P channel FETs by shorting resistor 16 and making transistors 11 and 14 of a substantially equal size. The temperature coefficient of the resistors 15 and 16 may be compensated for.

In this embodiment the area ratios of the transistors 12:13 is in the ratio 10:1 and the area ratios of the transistors 11:14 is 1:10. It will be appreciated that these ratios are preferred, but exemplary only, and other ratios are possible.

With these ratios there will be a 60 mV across resistor 15, which has a value of 7.5K and also across resistor 16 which also has a value 7.5K, thus giving an output current at 17 of 8uk The resistors 5,6 and 17 have the values 55K, 100K and 300K respectively.

With the values of the components already indicated the sum of the collector currents of transistors 1 and 10 will be close to 21uk By changing the ratios of the two currents generated by the two circuits A and B, the resultant current sum can be made substantially independent of temperature even though the resistors in the circuit have temperature coefficients.

To summarise, the output current of the circuit A is proportional to the Veb of transistor 3 divided by resistor 5 whereas the current output of the circuit B is defined by the bandgap voltage expression KT/Q, the area ratio of transistor 12 to transistor 13 and the area ratio of the transistor 11 to the transistor 14 and also to the value of the resistors 15 and 16.

Figure 2 is a graph of temperature in deg. C against output current in 17, the current output characteristic of circuit A being indicated at 20 and that of circuit B at 21, the combined effects of the two circuits giving the current output indicated at 22. The circuit of the present invention is adapted to be manufactured as an integrated circuit and it is particularly useful in cellular telephones although there are other possible applications.

Claims (7)

1. Claims 1. A low voltage stabilised current source, comprising: a first control means formed by a current generated by applying a voltage of a forward conducting diode across a known resistance; and, a second control means formed by a "bandgap" type reference voltage source.
2. 2. The low voltage stabilised current source of claim 1, wherein the first control means is arranged to operate with a minimum supply line voltage of a base-emitter junction voltage plus a transistor collector-emitter saturation voltage.
3. 3. The low voltage stabilised current source of claim 1 or claim 2 wherein the second control means is defined by the forward biased baseemitter junction voltage of a transistor.
4. 4. The low voltage stabilised current source of any preceding claim wherein the value of the current source is defined by the Thermal voltage (kT/q).
5. 5. The low voltage stabilised current source of any preceding claim when manufactured as an integrated circuit.
6. 6. A cellular telephone incorporating the low voltage stabilised current source of any previous claim.
7. 7. A circuit substantially as hereinbefore described with reference to and as shown in the accompanying drawings.