GB2401261A

GB2401261A - Apparatus for extending the standby time of a mobile unit

Info

Publication number: GB2401261A
Application number: GB0319951A
Authority: GB
Inventors: Anthony Paul Banks
Original assignee: NEC Technologies UK Ltd
Current assignee: NEC Technologies UK Ltd
Priority date: 2003-08-26
Filing date: 2003-08-26
Publication date: 2004-11-03
Anticipated expiration: 2023-08-26
Also published as: GB2401261B; GB0319951D0

Abstract

An apparatus for reducing power consumption in a device comprising functional circuitry 520 requiring a first current in a first mode of operation and a second current in a second mode of operation, a capacitor 530 connected in parallel with the functional circuitry thereby defining a parallel circuit; a power supply 500 connected in series with the parallel circuit, switching means 550 between the power supply and the parallel circuit and means to control the switch to connect the parallel circuit to the power supply when the functional circuitry is required to be in its first mode of operation and to disconnect the power supply from the parallel circuit when the functional circuit is required to be in its second mode of operation.

Description

2401 261 - 1 - Apparatus for Extendina the Standby time of a Mobile Unit

The present invention relates to an apparatus for extending the standby time in a mobile communication device.

Mobile communication devices typically have two main modes of operation, an operational mode and a standby mode.

During operational mode a device is connected to a network via a dedicated link between a base station and the device. Data may be transmitted to and from the device for lo example during a voice call.

In standby mode no dedicated link is established to the network but instead the unit is waiting to receive or transmit data. Standby mode includes a paging period and a low power inactive period. During the paging period the unit monitors signals from the network which are transmitted from local base stations. The unit compares the strength of signals from different base stations and determines whether a handover between base stations is required. These signals are also used to synchronise the internal clock with the network clock. Typically, the paging period accounts for around 10 % of the standby time. During the remainder of the standby time, ie around I, the unit operates in the low power inactive mode during which it merely maintains operation of the internal low frequency, 32768 Hz, clock.

Mobile units include a baseband core logic circuit. The baseband core logic circuits are processing cores such as microprocessors and Digital Signal Processors ( DSP).

These circuits are used during both operational and to standby modes. - 2

The circuits require different operating currents depending on the operational mode in order to activate their functions. When in active mode a DSP core may be running at 120MHz, whereas low power standby mode it may be using a 32KHz clock. The power consumption is linearly related to the clock frequency. For example, in operational and paging modes, the baseband circuits may require currents of around 100 mA, however, during the low power inactive standby period baseband circuits may only lo require a current of around 0.5 mA in order to maintain operation of the low frequency clock.

Mobile units are usually powered by a single battery and, typically, a 3. 6 V lithium cell is utilised. In order to increase the power efficiency within the unit, modern ASIC Is devices which include baseband core logic circuits include two power rails which are driven by the cell. The first of these, known as the 'core' supply, powers the internal logic of the core and typically runs at a voltage of 1.5 Volts in order to reduce power consumption. The second so rail is known as the 'IO' (input/output) supply. This powers the external interface drivers of the part and typically runs at a higher voltage of 2.85 Volts. The IO rail only supplies power to the peripheral interface buffers. These interface between the internal logic, running at the core voltage of say 1.5V, and the external voltage levels, typically 2.85V. The core voltage is lower to minimise power consumption, since there is a V squared dependency.

Conventional approaches to providing power to the baseband so circuits employ a regulator as shown in figure 1. The regulator is used to step down the voltage from 3.6 V to 1.5 V and supply the variable currents required during the different operation modes. However, regulators are inherently inefficient with regard to power transfer. The as maximum efficiency of a regulator which supplies 1.5 V l - 3 - from a 3.6 V source is 1.5/3.6 or about 42 %. The remaining 58 % of power is dissipated as heat in the regulator.

Customer demands for pocket sized units with increased functionality and prolonged standby periods require that high power efficiency of units has become an important design consideration. In essence, any power which is wasted is power which could be better used to increase the operating time.

lo Recently developed systems have improved the efficiency of units by employing highly efficient switch mode power supplies (SMPS) to step down the voltage. Such a circuit is shown in figure 2. SMPS are well known. Commercially available SMPS operating within the voltage requirements of mobile handsets can typically provide conversion efficiencies of around 90 % when reducing voltage from 3.6 to around 1.5 V and supplying currents in the range 10 - loO mA. Therefore, switch mode power supplies provide a more efficient means to drive the baseband circuits during So operation and paging periods in comparison with the 42 % efficient regulators.

However, the efficiency of switch mode power supplies is strongly dependent on current below about 10 mA in these voltage ranges. Figure 3 shows the efficiency of a commercially available SMPS as a function of current when reducing voltage from 4.2 V, 3.6 V and 2.7 V to 1.8 V. This figure is taken from Linear Technology. Therefore, during the inactive periods of standby operation in a unit, when 0.5 mA is required, the efficiency of such a so SMMPS is around 30 %, ie lower than a regulator.

Many units now include a switch mode power supply to power the baseband circuits when loo mA is required and thereby convert power at 90 % efficiency, and a regulator, to power the baseband circuit during the low power inactive period at 0.5 A at 42 % efficiency. The circuitry for such a unit is shown in figure 4.

We have appreciated that customer and industry demands on mobile communication devices require that improvements to the power efficiency have become valuable to the industry.

In particular, in known systems, the least efficient period is during low power inactive periods which accounts for around 90 % of the standby time.

lo Embodiments of the present invention use a high value capacitor to drive the baseband circuits during the inactive standby period. The capacitor is placed in parallel with the baseband circuits. A switch mode power supply is used to drive the baseband circuits when high current is required. During this period, the SMPS also charges the capacitor. When the baseband circuits require low current the switch mode supply is disconnected and the capacitor discharges through the baseband circuits and supplies the required low current. The capacitor is highly so efficient and typically only has a small internal leakage current. Therefore, the power efficiency during the low power periods is higher than the 42 % available using the SMPS or regulator. Use of a capacitor with a small leakage current will give a power efficiency in the low power mode which is close to the efficiency of the operational mode.

The invention is defined in its various aspects in the appended claims to which reference should now be made.

An embodiment of the invention is now described in detail so with reference to the accompanying figures in which: Figure 1 is a known circuit incorporating a regulator to supply voltage and current to the baseband circuits.

Figure 2 is a known circuit incorporating a switch mode power supply to drive the baseband circuits.

Figure 3 is a graph showing the conversion efficiency of a known switch mode regulator as a function of the current drawn from it.

Figure 4 is a known circuit incorporating a SMPS and a regulator to drive the baseband circuits.

Figure 5 is a circuit diagram of an embodiment of the present invention.

lo Figure 6 is a flow diagram showing the procedure for activating the present invention.

Figure 7 is a circuit diagram used to generate simulation results.

Figure 8 is a graph showing the voltage across the capacitor as a function of time. The results are generated from the circuit of figure 7.

Figure 5 is a circuit diagram of an embodiment of the present invention. The figure only shows the power sources and circuits involved in the baseband circuitry of the go mobile unit The figure only shows the core supply to the baseband. It does not cover the IO supply to the baseband or the RF supply rails. The circuit is driven by a battery 500. The battery 500 is the main battery of the unit which is generally a 3.6 V lithium cell. However, other batteries may be suitable for use in mobile communication devices, for example nickel metal hydride or nickel cadmium. The battery is connected to a switch mode power supply 510. The switch mode power supply 510 is controlled by a controlling means 540. The controlling 3 o means can turn the SMPS on and off. - 6 -

A switch 550 is provided in the circuit in order to connect or disconnect the battery and the switch mode power supply from the baseband core logic 520. In practice, this switch may be included within the SMPS and the circuit can be disconnected simply by turning off the SMPS.

The circuit includes the baseband core logic circuits of the unit 520 and a capacitor 530. The capacitor is a high value capacitor and will typically have a value of around lo 0.2 F. However, the exact value of the capacitor will depend on the specific requirements of the circuit. The baseband core logic and capacitor are connected in parallel and are both connected to ground. In further embodiments further components and circuitry may be positioned between this parallel circuit and ground. The base band circuitry includes all circuitry required to drive the unit during operation, paging and low power inactive periods.

During periods when the baseband circuit requires 100 mA, so ie during paging and operational modes, the controlling means switches on the SMPS. In practice, the exact current and voltage outputs will be specific to the individual unit. The 1.5 V from the SMPS is put across the baseband core logic and capacitor in parallel. During this period the baseband logic is driven by the high current level, eg mA, and the capacitor is charged. The SMPS output of loo mA is generated by the SMPS at around 90 % power efficiency.

During periods when the unit is operating in inactive so standby mode, eg when only the low frequency clock is active, the switch mode power supply is switched off. This produces a break in the circuit between the battery and parallel circuit. Consequently, the capacitor discharges through the baseband circuits whilst the SMPS is switched off.

The value of the capacitor is selected depending on the specific requirements of the system. The value is chosen by taking into account the resistance of the baseband core logic circuit, the current required by the baseband circuits during low power inactive periods and the relevant time frames during which the capacitor is charged and discharged.

It is important that the time constants for the circuit are calculated correctly. Specifically, the charge placed on the capacitor during charging and maximum voltage across the capacitor, ie while the SMPS is on, should be at least equal to the discharge during the periods when the capacitor drives the circuit in order that the voltage across the capacitor is not reduced over time. The change in current from the capacitor during discharge periods must be calculated when selecting component values in order that the baseband circuits are provided with so sufficient current throughout low power periods. The time constants of the circuit are calculated from t = RC, where t is the time taken for voltage on the capacitor to drop to 37 of its original value and is given in seconds, R is the resistance of the circuit in Ohms and C is the capacitance in Farads. The current is proportional to the voltage across the capacitor during discharge.

Typically, the interval between pages in mobile communication devices is around 2 seconds. During this interval the capacitor is discharged through the baseband So circuit. During the paging sequence, typically 0. 2 seconds, the baseband and capacitor will be connected to the main rail at around 1.5 V and the capacitor will be recharged. The capacitor will also be recharged during operation of the unit, for example during a voice call. - 8 -

Embodiments of the present invention must also take into account the stabilization time for the SMPS. Typically, a SMPS will have an associated time delay between being activated and providing a steady output. This delay must be taken into account when activating the circuit for paging to ensure that baseband circuit is provided with sufficient power when it needs to commence paging operations.

Many mobile units already include a high value capacitor lo which is used to drive the real time clock at times when the unit is powered down. These capacitors are charged while the unit is active and discharged through the real time clock circuits when the unit is switched off.

Embodiments of the present invention may use these existing capacitors to power the baseband circuits during low current inactive periods in order that no additional components are required in the unit.

Figure 6 is a flow diagram showing the steps of operation of embodiments of the present invention. At 600, the capacitor is charged. This may be performed before installation or during operation or production. At 610, the unit determines whether or not the baseband circuitry requires high current. Typically this is determined in dependence on whether the unit is in operating or paging mode or in low power inactive mode. If the circuit requires high current, ie around 100 mA, the switch mode power supply is activated at 630 and the capacitor is charged. If the baseband circuits do not require high current at 620 but only require low current, then the lo switch mode power supply is deactivated at 620 and the capacitor discharges through the baseband circuit. When high current is required, the SMPS is reactivated.

Figure 7 shows a circuit which simulates an embodiment of the present invention. The circuit includes an ac power 9 - supply 700 and diode 710 to simulate the switching the power supply on and off. Furthermore the ac power supply 700 is pulsed in order that the intervals between operation are longer than the periods of operation. The simulation used a 0.5 second on time and a 4.5 seconds off time. This is representative of a 90 % standby duty cycle.

The circuit includes a capacitor C3 630 which is placed in series with a 1 ohm resistor 720. The 1 ohm resistor R2 represents the internal resistance of the high value lo capacitor 630. These are placed in parallel with a 2000 ohm resistor R3 which simulates the current taken by the baseband core logic. When the diode allows current into the circuit the capacitor is charged and, when no current is delivered the capacitor, discharges through the resistor 740. At a 1.5 V supply rail, this circuit gives a current of 0.75 mA for low power standby operation. This is a typical value for third generation chip sets.

In the example of figure 7 the discharge time constant is 2000 x 0.2 = 400 seconds. The charge time constant is 1 x 0.2 = 0.2 seconds.

Figure 8 shows the simulated results from the circuit of figure 7 and displays the calculated voltage across the capacitor as a function of time. The graph produces a sawtooth shape during which the capacitor is charged and discharged as the power supply is switched on and off.

From A to B. the power line is activated and the capacitor is charged. At B the power line is switched off and the capacitor discharges through the resistor until C. At C, the power line is reactivated. The 'droop' in voltage JO across the capacitor (between D and E) with time is due to the simulation assuming that the capacitor is fully charged at 1.5 V. The voltage then settles at a state of equilibrium after the droop which is dependent upon the on/off duty cycle, and the component values chosen for simulation. In practice the charge put on the capacitor - 10 during charging should compensate the discharge during discharging periods.

Embodiments of the present invention avoid using the SMPS to supply low currents at low efficiency during low power inactive standby periods. Instead, low currents are supplied by discharging the capacitor. The capacitor is charged at 90 % efficiency by the switch mode power supply and during discharge the stored energy is discharged through the baseband circuits at effectively loo % lo efficiency. Therefore, the present invention removes the low efficiency stage during low current periods.

An example of the increased efficiency of the circuit during standby periods is now provided. This example only takes account of the baseband core logic circuits and does not take into account consumption by some of the other circuitry within the handset, for example the radio module. The example includes the following assumptions: a) When in standby mode the Baseband is active for 3% of the time and inactive for 97% of the time.

b) The core current consumption in the active mode is lOOmA c) The core current consumption in the inactive mode is lmA d) The efficiency of the switch mode power supply is 90% in the active mode e) The efficiency of the switch mode power supply is 50% in the inactive mode f) The core voltage is 1.5V - 11 g) The battery voltage is 3.6V The average battery current consumption for the original PSU configuration is given by I = ((percentage of time in active mode x active current / efficiency at active current) + (percentage of time in inactive mode x inactive current / efficiency at inactive current)) x ( Core voltage / battery voltage) = ((0.03 x 100 / 0.90) + (0.97 x l.0 / 0.50)) x ( l.5 / 3.6) = (3.33 + 1.94) x ( 1.5 / 3.6) 2.19 mA With the revised PSU configuration, we make the assumption that the effective efficiency in the inactive mode is now raised to 90% ( neglecting circuit losses).

This includes the assumption that the power is delivered to the capacitor at 90 % and that the capacitor stores charge and discharges at 100 % efficiency.

Then the new average current is then given by = ( (0.03 x 100 / 0.90) + ( 0.97 x l.0 / 0.90) ) x ( 1.5 / 3.6) = (3.33 + 1.07) x ( 3.5 / 3.6) = 1.83 mA The above example shows that, using realistic assumptions, employing a capacitor to drive the baseband circuits during periods of low current requirements will reduce the - 12 average baseband core logic supply current by 16%. This will lead to a 16 % increase in the standby time, assuming that no other factors affect this value.

It will be obvious to those skilled in the art that embodiments of the present invention may be used to power circuits other than those specifically described in the

description. In particular, the invention has been

described in terms of powering the baseband circuits however, the invention could be used to power other low lo current applications. - 13

Claims

Claims 1. An apparatus for reducing power consumption in a device

comprising: functional circuitry requiring a first current in a first mode of operation and a second current in a second mode of operation; a capacitor connected in parallel with the functional circuitry thereby defining a parallel circuit; a power supply connected in series with the parallel lo circuit; switching means between the power supply and the parallel circuit; and, means to control the switch to connect the parallel circuit to the power supply when the functional circuitry is required to be in its first mode of operation and to disconnect the power supply from the parallel circuit when the functional circuit is required to be in its second mode of operation.
2. An apparatus for reducing power consumption in a so device according to claim 1 wherein the device is a mobile communication device.
3. An apparatus for reducing power consumption in a device according to claim 1 or 2 wherein the first mode of operation is a paging mode.
4. An apparatus for reducing power consumption in a device according to claim 1, 2 or 3 wherein the second mode of operation is a standby mode.
5. An apparatus for reducing power consumption in a device according to claim 1, 2, 3 or 4 further comprising so a means varying the voltage and current supplied to the - 14 parallel circuit between the power supply and the parallel circuit.
6. An apparatus for reducing power consumption in a device according to claim 1, 2, 3, 4 or 5 wherein the functional circuitry comprises a plurality of circuits.
7. An apparatus for reducing power consumption in a device according to claim 6 further comprising a means for controlling which circuits are activated in dependence on the operational mode of the device.

lo
8. An apparatus for reducing power consumption in a device according to any preceding claim wherein the first current is higher than the second current.
9. An apparatus for reducing power consumption in a device substantially as herein described with reference to the figures.