Electric lamp and discharge devices: systems – Current and/or voltage regulation
Reexamination Certificate
2001-01-04
2002-03-19
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
C323S222000, C323S267000, C323S282000, C315S086000
Reexamination Certificate
active
06359392
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to light emitting diode (LED) circuits, and more particularly to driver circuits for driving LEDs.
BACKGROUND OF THE INVENTION
In portable radio communication devices it is desirable to prolong the operating time and battery life. To reduce the current drain from the battery it is desirable to develop circuits that achieve the lowest power consumption possible. Among those circuits, the display draws a disproportionate amount of current from the battery. The LED is widely used for back lighting in devices such as cellular phones due to its simpler driving circuit compared with the electroluminescent (EL) and fluorescent lighting its comparably lower cost and noise. However, the power consumption of LEDs is generally higher than the EL lights when multiple LEDs are used. In addition, the use of white LEDs, which is necessary for backlighting color liquid crystal displays (LCDs), incurs power considerations in that white LEDs have higher threshold voltages, which are often higher than the battery voltages. Thus DC-DC converter is required to boost the battery voltage and the overall power efficiency is reduced.
A radio communication device, such as a cellular phone, is typically powered from a battery, such as a lithium-ion battery, having a normal operating voltage of about 3.6 volts. Ideally, the device circuits are powered directly from the battery, however, some circuits such as light emitting diodes (LEDs) used in displays will not operate at this low voltage or provide deteriorated performance when the battery runs down, and it becomes necessary to add a DC-DC converter to step-up the voltage. However, the inductor type of DC-DC converter may have a typical efficiency of 85%, while the charge pump type of DC-DC converter usually has efficiencies less than 50% when the battery internal resistance is considered.
Referring to
FIG. 1
, a prior art LED inductive boost driver circuit is illustrated as described in U.S. Pat. No. 4,673,865, including an inductive switching power supply
102
to perform a DC-DC conversion. An inductor
104
is connected between a node
106
and a battery
108
. A transistor
110
is connected to node
106
. The anode of a diode
112
is also connected to node
106
and the cathode is connected to a node
114
. A filter capacitor
116
is connected between node
114
and ground. A duty cycle modulator
118
is connected between node
114
and the base of transistor
110
.
In operation, duty cycle modulator
118
periodically switches on and off transistor
110
. When transistor
110
is switched on, current from battery
108
begins to flow through inductor
104
, building up the magnetic field in the inductor as the current increases. When transistor
110
is switched off, the magnetic field collapses and a positive voltage pulse appears at node
106
. Because inductor
104
is in series with battery
108
, the voltage of the pulse at node
106
is greater than the battery voltage.
Thus, the periodic switching of transistor
110
causes a string of pulses to appear at node
106
. These voltage pulses are then rectified and filtered by diode
112
and filter capacitor
116
to produce a multiplied DC voltage at output node
114
. To regulate the output voltage, duty cycle modulator
118
samples the output voltage at DC output node
114
and adjusts the duty cycle of transistor
110
so that the DC output voltage remains substantially constant. A current limiting resistor
124
is coupled in series with the LED
122
along with a transistor
126
to control the activation of LED
122
via a control circuit (not shown). Although an improvement in the art, there is voltage drop across diode
112
, and power consumed in current limiting resistor
124
, which consumes battery power.
Illustrated in
FIG. 2
is another prior art LED driver circuit that consumes less battery energy than the device of FIG.
1
. The driver circuit uses switching power supply
102
, LED
122
and transistor
126
that were previously described in conjunction with FIG.
1
. Also, LED
122
and transistor
126
are mutually interconnected as in FIG.
1
and transistor
126
functions to control the activation of LED
122
as previously described. However, a capacitor
202
is connected between the anode of LED
122
and the pulse output node
106
. A shunt diode
204
is connected to the junction of capacitor
202
and LED
122
.
In operation, during a positive voltage pulse at output node
106
, current flows through LED
122
via coupling capacitor
202
. The capacitor plate
202
a
of capacitor
202
begins to charge negatively. Between voltage pulses, i.e. when transistor
110
conducts and momentarily grounds node
106
, capacitor plate
202
a
goes below ground potential. When the negative potential on capacitor plate
202
a
is sufficient to overcome the small (typically 0.6 Volts) forward voltage drop across diode
204
, the diode conducts, substantially discharging capacitor
202
. Thus, diode
204
provides a means for discharging capacitor
202
during a portion of each period of the voltage waveform at output node
106
.
Unfortunately, the discharge current is lost, lowering efficiency of the driver circuit. Moreover, this device, as well as that of
FIG. 2
, utilizes an inductor type boost converter to provide a high supply voltage, which increases cost and size of the circuit.
What is needed is a high efficiency LED driver circuit that can drive LEDs requiring higher voltage than available battery power. It would also be of benefit to eliminate the inductive type of boost circuits and the losses associated with current limiting resistors and switching circuits. It would also be advantageous to accomplish this in a low cost, simple circuit architecture.
REFERENCES:
patent: 3869641 (1975-03-01), Goldberg
patent: 4673865 (1987-06-01), DeLuca et al.
patent: 4965561 (1990-10-01), Havel
patent: 5575459 (1996-11-01), Anderson
patent: 6285140 (2001-09-01), Ruston
Lee Wilson
Mancini Brian M.
Motorola Inc.
Wong Don
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