Optical communications – Transmitter
Reexamination Certificate
2001-01-31
2004-06-08
Pascal, Leslie (Department: 2633)
Optical communications
Transmitter
C398S183000, C398S186000, C398S192000, C398S195000, C398S197000, C398S201000, C372S038060, C372S029011, C372S034000, C372S032000, C372S036000, C372S025000, C372S033000, C372S026000
Reexamination Certificate
active
06748180
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to light emitting diodes, and more particularly to a capacitor-regulated high efficiency driver for a light emitting diode.
2. Description of the Related Art
A light emitting diode (LED) driver in a photoelectric transceiver (e.g., an infrared IR transceiver) is critical to the transmitting operation of the transceiver. The LED driver requires the highest operating current of any component in the transceiver. Typically, the peak current of the LED driver can reach 500 mA or more. Since many infrared transceivers are battery operated, the current draw of the LED driver is an important concern.
Therefore, enhancing the efficiency of the LED driver to reduce power consumption would be a considerable advantage, particularly where the IR transceiver is used in telephone or computer network applications. While in data communications applications the IR transceiver is in the receive mode most of the time, in telephone or synchronized network applications the transceiver is typically in the transmit mode for most of its operating time.
FIG. 1A
illustrates a conventional switching LED driver, comprising an external resistor R and an N-type field effect transistor (FET) N. When the gate of the FET is low, the FET is turned off, and no current flows through the LED. When the gate of the FET is high, the FET is turned on, and a current flows between V
dd
and ground to illuminate the LED. The FET has a very low on-resistance, usually less than 0.1 ohm. Thus, the current depends on the forward voltage V
f
of the LED and the resistance of an external resistor R:
I
LED
=(
V
dd
−V
f
)/(
R
on
+R
) (1)
where
I
LED
is the current through the LED,
V
f
is the forward voltage of the LED, and
R
on
is the on-resistance of the FET.
This circuit is relatively simple, and provides the additional advantage that the IR transceiver internal power consumption excludes the power consumption of the resistor R. However, from the standpoint of power conservation there are significant disadvantages to this circuit. For example, the LED driving current depends on the voltage V
dd
, which is not controllable. Further, the power efficiency of the circuit is low because the power consumption of the resistor R constitutes a large portion of the power provided by V
dd
.
A simple calculation shows that the ratio of the power consumption on resistor R to the total power supplied by V
dd
is
P
R
/P
O
=(
R
*(
V
dd
−V
f
))/(
V
dd
*(
R
on
+R
)) (2)
where
P
R
is the power consumption of resistor R, and
P
O
is the total power provided by V
dd
.
Usually R>>R
on
, so equation (2) can be reduced to
P
R
/P
O
=(
V
dd
−V
f
)
V
dd
=1
−V
f
/V
dd
.
Thus, for example, if V
dd
=5 V and V
f
=2 V at 500 mA, this ratio is 3/5 which means more than one half of the total power supplied by V
dd
is dissipated by resistor R.
The dependency of the LED driving current on V
dd
can be seen in the following example. If V
dd
ranges from 4.5 V to 5.5 V, R
on
is 0.1 ohm, and an LED driving current I
LED
of 500 mA is required, the resistance of the resistor R should be 4.9 ohms to generate the LED driving current I
LED
of 500 mA at 4.5 V, which is the low value of V
dd
. At 5.5 V, the high value of V
dd
, the current would increase to 700 mA if V
f
remains constant. Thus, fluctuations in the supply voltage V
dd
directly affect the LED driving current, and the optical intensity of the LED will vary accordingly.
FIG. 1B
shows a conventional pulsed current source (PCS) LED driver which holds the LED current constant, independent of V
dd
. However, in this circuit, the IR transceiver internal power consumption is very high. The ratio of the power consumption P
PCS
of the pulsed current source to the total power provided by the power supply V
dd
is
P
PCS
/P
O
=1
−V
f
/V
dd
(3)
Thus, in this circuit, the IR chip internal power consumption is very high, which causes significant thermal problems.
SUMMARY OF THE INVENTION
In view of the foregoing and other problems, disadvantages, and drawbacks of the conventional methods and structures, an object of the present invention is to provide a driver circuit for a light emitting diode, which overcomes the above problems.
Another object is to provide a high efficiency LED driver circuit which utilizes a capacitor to regulate the LED driving current, which provides high efficiency and low power consumption. The capacitor provides the LED driver current by discharging through the LED during transmission intervals, and the power supply for the device maintains the capacitor voltage. The capacitor charges to a preselected high threshold voltage, which determines the maximum LED optical emission intensity. As the charge depletes during transmit intervals, the capacitor discharges to a preselected low threshold voltage, at which point the capacitor pump controller opens a charge path from the power supply to recharge the capacitor.
The LED driver circuit of the invention accordingly operates at high efficiency with low power consumption. Moreover, the LED driver current can be regulated by changing the low threshold voltage and the high threshold voltage, thereby to control the optical intensity of the LED.
In a first aspect, the present invention provides a driver circuit for a light emitting diode in an optical transmitter which converts an electrical signal into an optical signal. The driver circuit includes a capacitor having one side connected to a higher voltage terminal of a power supply through a charge switch, a second side of the capacitor being connected to a lower voltage terminal of the power supply, the anode of the light emitting diode being connected to a connection point of the charge switch and the capacitor, the cathode of the light emitting diode being connected to a lower voltage terminal of the power supply through a discharge switch, and a controller for closing the charge switch to charge the capacitor when a voltage on the capacitor reaches a preset low voltage threshold and for opening the charge switch when the voltage on the capacitor reaches a preset high voltage threshold. The discharge switch is closed responsive to an intensity of the electrical signal and the capacitor thereby discharges through the light emitting diode to emit an optical signal corresponding to the electrical signal.
In another aspect, an optical transmitter is provided, which includes a light emitting diode, and a driver circuit for activating the light emitting diode responsive to an electrical signal. The driver circuit includes a capacitor having one side connected to a higher voltage terminal of a power supply through a charge switch, a second side of the capacitor being connected to a lower voltage terminal of the power supply, the anode of the light emitting diode being connected to a connection point of the charge switch and the capacitor, the cathode of the light emitting diode being connected to a lower voltage terminal of the power supply through a discharge switch, and a controller for closing the charge switch to charge the capacitor when a voltage on the capacitor reaches a preset low voltage threshold and for opening the charge switch when the voltage on the capacitor reaches a preset high voltage threshold. The discharge switch is closed responsive to an intensity of the electrical signal and the capacitor thereby discharges through the light emitting diode to emit an optical signal corresponding to the electrical signal.
In further exemplary aspects of the invention, the discharge switch preferably includes an N-type FET having a drain connected to a cathode of the light emitting diode, a source connected to the lower voltage terminal of the power supply and a gate connected to the electrical signal.
Preferably, the controller includes a voltage comparator having a positive input connected to an output of the comparator through a resistor and connected to the capacitor through a comparat
Cheung, Esq., Ph.D. Wan Yee
McGinn & Gibb PLLC
Pascal Leslie
Phan Hanh
LandOfFree
Capacitor regulated high efficiency driver for a light... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Capacitor regulated high efficiency driver for a light..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Capacitor regulated high efficiency driver for a light... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3356134