GB2383479A

GB2383479A - Driver circuit for light emitting devices

Info

Publication number: GB2383479A
Application number: GB0130634A
Authority: GB
Inventors: David Murdoch
Original assignee: Matsushita Communication Industrial UK Ltd; Matsushita Electric Industrial Co Ltd
Current assignee: Matsushita Communication Industrial UK Ltd; Panasonic Holdings Corp
Priority date: 2001-12-20
Filing date: 2001-12-20
Publication date: 2003-06-25
Anticipated expiration: 2021-12-20
Also published as: GB2383479B; GB0130634D0; CN1440225A

Abstract

A driver circuit for mobile communications devices boosts the battery voltage to provide an operating voltage suitable for light emitting devices. A voltage greater than the supply is maintained by a clamping circuit 2, with the clamping circuit driven by a current switch 1 from a clocking input. The driver circuit uses a single transistor TR1 and can provide for two luminance levels from the light emitter.

Description

Driver circuit for light emitting devices.

This invention relates to driver circuits for light emitting devices and it has particular utility for light emitting diodes in portable electronic terminals.

Light emitting devices such as light emitting diodes (LED) are in common use for illumination of displays in portable electronic terminals, e. g. mobile phones. Light emitting devices are useful also as indicators to signal the occurrence of an event in portable electronic terminals or in accessories therefor.

In order for the LED's to operate and emit light of the desired luminance a forward voltage across each device of around 2 to 4 volts is required. The main power supply for a portable terminal is normally a re-chargeable battery whose output voltage may be less than the forward voltage of the LED. Although voltages will differ for various types of devices and batteries, in general the battery supply voltage needs to be boosted in order to operate the LED's properly.

Despite considerable efforts by manufacturers in recent years to reduce the size and cost of portable terminals, pressures to reduce size and cost remain. The incorporation of additional features with their associated components in portable terminals also encourages displacement of unnecessary components from existing circuits.

It is an object of the present invention to provide a simple and economical but effective driver circuit for light emitting devices.

According to the present invention there is provided apparatus as set out in the attached claims.

An example of the invention is described below with reference to the accompanying figures in which like numerals identify equivalent elements and in which; Figure 1 is a block diagram of a driver circuit according to the invention, Figure 2 is a circuit diagram of an LED driver circuit for a mobile phone, The basic elements of a driver circuit in accordance with the invention are illustrated in the block diagram of figure 1. An alternating input to a current switch 1 operates a clamping circuit 2 and the clamping circuit maintains the potential on a voltage booster 3. The current switch is provided by the transistor TRI in series with a resistor Rl connected across the supply potential. The voltage booster comprises ballast resistor R2, LED and capacitor Cres connected in series across the supply potential.

The clamping circuit is provided by capacitor C and diodes D 1 and D2.

A square wave clocking signal is applied as an input to the LED driver circuit when the light emitting device is to be lighted or when its luminance is to be increased. This would occur for example when a call is received at the terminal or when a key is pressed on the keypad by the user. A suitable clocking signal is normally readily available from a processor within portable communication terminals.

A circuit arrangement suitable for a low current driver is shown in figure 2.

Transistor TR1 in series with resistor Rl is connected between a positive voltage rail +V and earth providing the supply potential for the driver circuit.

The initial condition of capacitor C is with a zero voltage on plate'b'and with a +V voltage on plate'a'. The square wave input drive voltage with

respect to OV is applied to a resistor Rase which limits the current flowing into the base of TR1. When TRI switches on, a fast discharge from plate 'a'of capacitor C to ground causes the voltage at plate'a'to reduce towards zero volts. Resistor Rl ensures that there is an impedance to +V to prevent shorting of +V to ground, and ensures also that ground potential is available to plate'a'of capacitor C.

As the transistor TR1 turns on fully, capacitor C attempts to maintain the potential difference between its plates'a'and'b'causing the potential on plate'b'to fall towards-V. As plate'b'falls to-V diode D2 conducts and draws charge from Crest The positive plate of Cres is connected to ground and remains at ground potential driving the potential on the negative plate of Cres towards-V. The residual voltage on the negative plate of Cres will be a negative value slightly less than-V due to the forward voltages of Dl and D2 (Voi and VD2). The potential intermediate the LED and capacitor Crues reaches a value a, [-V + (VDi +VD2) < a < [VDl +VD2], and this value is maintained by the clamping circuit as follows.

When the potential on plate'b'of capacitor C reaches approximately zero volts the diode D2 stops conducting. A zero potential difference, or approximately so, then exists across plates'a'and'b'of capacitor C.

A reversal of the clocking pulse input then turns off transistor TR I. This causes plate'a'of capacitor C to charge up through Rl towards the potential +V. As the voltage of plate'a'rises, a corresponding rise of voltage in plate b maintains the zero potential across the two plates of capacitor C.

When the voltage of plate'b'reaches the turn-on voltage of diode Dl, Dl conducts and clamps plate'b'to ground. This ensures that at the end of the cycle capacitor C will have potential'V'minus the turn on voltage of D1 across both plates. During this part of the cycle, diode D2 prevents a flow of current from charging Cres and thereby reducing the negative voltage output.

A further reversal of the input clocking voltage causes transistor TR1 to turn on again so that the cycle described above is repeated. Where a sufficiently large magnitude of clocking input is provided, transistor TR1 may be replaced by a MOSFET (metal oxide silicon field effect transistor) with lower'on'resistance to improve the efficiency of the circuit.

Preferably the diodes Dl and D2 will be selected for a low forward voltage characteristic. The voltages across the diodes subtract from the level of the negative voltage intermediate the LED and Cres. The values of C and Rl and the input switching frequency should be selected to ensure that plate'a'

of capacitor C becomes fully charged during TR1 off state and fully discharged during TR1 on state.

The circuit can be used also to switch the LED between two illumination states by selecting an LED with a forward voltage slightly lower than +V.

The application of +V is then sufficient to illuminate the LED when the driver circuit is not operating. Application of the clocking input to TR1 will cause an increase in the voltage across the LED thereby increasing current through the device and consequently increasing the brightness.

By this means the LED may be switched between two states of illumination. Removal of the voltage +V e. g. by means of a switch (not shown) will extinguish the light emission completely. The negative voltage intermediate the LED and Cresmay be useful external to the driver circuit e. g. as a bias level.

As will be readily apparent to one skilled in the art, with alterations to take account of polarity the driver may be used with a negative supply (-V, 0) and will produce a positive voltage to boost the LED.

Claims

Claims 1. A driver for light emitting devices comprising an alternating input to a current switch, a clamping circuit driven by said current switch and a voltage booster maintained by said clamping circuit.
2. A driver for light emitting devices as in claim 1 in which the voltage booster comprises a ballast resistor, light emitting device and capacitor connected in series across a supply potential V with the potential intermediate said light emitting device and said capacitor maintained at a value a, [-V + (Voi +VD2) < a < [VDi +VD2], by said clamping circuit.
3. A driver for light emitting devices as in claims 1 and 2 in which said current switch comprises a resistor connected in series with a transistor across said supply potential.
4. A driver for light emitting devices as in claims 1 and 2 in which said current switch comprises a resistor connected in series with a MOSFET across supply potential.
5. A driver for light emitting devices as in any preceding claim in which voltage booster and current switch are connected in parallel across said supply potential.

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6. A driver for light emitting devices as in any preceding claim in which the clamping circuit consists of a capacitor connected between a point intermediate the resistor and transistor of the current switch and a common point, a first diode connected at cathode to said common point and at anode to the negative supply and a second diode connected at cathode intermediate the light emitting device and capacitor of the voltage booster and at anode to said common point.
7. A driver for light emitting devices as in any preceding claim in which the forward voltage of the light emitting device is less than the supply potential for the driver circuit.
8. A circuit for driving light emitting devices substantially as hereinbefore described with relation to or as illustrated in figure 2.