LED driver circuit with a boosted voltage output

Details LED driver circuit with a boosted voltage output LED driver circuit with a boosted voltage output

2001-05-18

2002-11-26

Ton, My-Trang Nu (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

External effect

Light

C327S589000

Reexamination Certificate

active

06486726

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The field of the invention is Light Emitting Diode (LED) driver circuits, and more particularly, LED driver circuits with a boosted voltage output for driving an LED from a voltage power supply having a voltage that is lower than the turn “on” voltage of the LED.
2. Description of the Related Art
LEDs are commonly used as status indicator lights and in numerical and alpha-numerical displays. LEDs are also used in opto-couplers to transmit optical signals between circuits. Such opto-couplers are typically used to provide a communications link between circuits that are electrically isolated from each other.
LEDs are typically driven by an integrated circuit using an LED driver circuit. This is because the output of many integrated circuits can not supply the current needed to turn on an LED.
FIG. 1
shows a conventional LED driver circuit for driving an LED
100
. The LED driver circuit comprises a bipolar transistor
115
, a collector resistor
111
, and a base resistor
120
. The collector resistor
110
, which is used to set the current level supplied to the LED
100
, is connected between a voltage power supply
105
and the anode of the LED
100
. The collector terminal of the transistor
115
is connected to the cathode of the LED
100
, and the emitter terminal of the transistor
115
is connected to ground. One end of the base resistor
120
is connected to the base terminal of the transistor
115
. The other end of the base resistor
120
is used for the input
122
of the LED driver circuit, which may be driven by the output of a logic gate in an integrated circuit (not shown). The base resistor
120
determines the input
122
impedance of the LED driver circuit and, thus, the drive requirement at the input
122
.
Historically, the voltage power supplies used to power integrated circuits have had voltages greater than the turn “on” voltages of many LEDs, which typically range from about 1.6 V to 3.6 V. This has allowed the use of the same voltage power supplies to power both integrated circuits and LEDs using conventional LED driver circuits. However, as the dimensions of integrated circuits have continued to be scaled down, the voltages of many voltage power supplies used to power integrated circuits have been reduced to values approaching the turn “on” voltage of many LEDs. Recently, the voltage used to power many integrated circuits have been migrating from 5.0 V to 3.3 V. Ultimately, the voltage used to power many integrated circuits may be reduced to a value below 1 V. As a result, the voltage power supplies used to power many integrated circuits may eventually be unable to power LEDs using conventional LED driver circuits, thereby creating the need for separate voltage power supplies to power the LEDs.
Even as the voltages used to power integrated circuits remain above the turn “on” voltage of many LEDs, the trend towards reduced voltages to power integrated circuits may lead to a degradation of the current control of conventional LED driver circuits. This can be illustrated by way of an example in which a 3.3 V voltage power supply
105
is used to power the LED driver circuit of FIG.
1
. In this example, the 3.3V voltage power supply
105
has a maximum and a minimum worst case voltage of 3.6V and 3.0 V, respectively, and the LED
100
has a turn “on” voltage of 2.5 V. The resistance of the collector resistor
10
is chosen such that the current supply to the LED
100
is about 10 mA at a nominal voltage power supply
105
voltage of 3.3 V. Assuming that the transistor
115
has a collector-emitter voltage drop of approximately 0.2 V, the current supply level to the LED
100
varies between 15 mA and 5 mA for the maximum and minimum worst case voltages of the voltage power supply
105
, respectively. As a result, the LED driver circuit of
FIG. 1
may exhibit poor current control to an LED
100
when the voltage of the voltage power supply
105
approaches the turn “on” voltage of the LED
100
.
Therefore, there is a need for an LED driver circuit having a boosted voltage output capable of driving an LED from a voltage power supply having a voltage that is lower than the turn “on” voltage of the LED. This would allow an integrated circuit and the LED to be powered by the same voltage power supply when the voltage power supply has a voltage that is lower than the turn “on” voltage of the LED. In addition, the expense of having to provide separate voltage power supplies to power the integrated circuit and the LED may be avoided. There is also a need for an LED circuit driver that exhibits good current supply control to an LED when the voltage of the voltage power supply approaches the turn “on” voltage of the LED.
SUMMARY OF THE INVENTION
Embodiments of the present invention provide LED driver circuits with a boosted voltage output for driving an LED from a voltage power supply having a voltage that is lower than the turn “on” voltage of the LED.
An LED driver circuit, built in accordance with one embodiment of the present invention, comprises an input buffer and a charge pump, both of which are connect to a voltage power supply having a voltage of Vdd. The charge pump boosts the voltage of the voltage power supply at its output in order to drive an LED having a turn “on” voltage that is comparable to or greater than the voltage of the voltage power supply. The input buffer receives a control signal at its input and either enables or disables the charge pump based upon the received control signal.
In another embodiment of a preferred embodiment, the charge pump of the LED driver circuit comprises an oscillator, an inverter, a capacitor, and a diode. The oscillator has an input connected to the output of the input buffer and an output. The inverter has an input connected to the output of the oscillator and an output. The capacitor has a low electrode connected to the output of the inverter and a high electrode connected to the anode of the LED being driven. The anode of the diode is connected to the voltage power supply and the cathode of the diode is connected to the high electrode of the capacitor.
When enabled by the input buffer, the oscillator outputs a pulsing signal that alternately switches the inverter between a high output state and a low output state. When switched to the low output state by the pulsing signal, the inverter pulls the low electrode of the capacitor down to ground. The diode is forward biased and current flows from the voltage power supply to the high electrode of the capacitor through the diode. This current charges up the capacitor, raising the voltage at the high electrode of the capacitor to approximately Vdd−Vt, where Vt is the potential drop across the diode. When switched to the high output state by the pulsing signal, the inverter raises the voltage at the low electrode of the capacitor to Vdd. This voltage rise at the low electrode of the capacitor causes the voltage at the high electrode of the capacitor to rise above Vdd−Vt due to capacitive coupling. The high electrode of the capacitor rises until it reaches the turn “on” voltage of the LED, at which point, the capacitor discharges through the LED, causing the LED to turn “on”.
In another embodiment of a preferred embodiment, the diode of the charge pump is implemented using a diode-connected PFET. In the forward direction, the source of the PFET is connected to the voltage power supply, the drain of the PFET is connected to the high electrode of the capacitor, and the body of the PFET is connected to the drain of the PFET via a large value resistor. This type of body connection weakly forward biases the body-source junction of the diode connected PFET in the forward direction, thereby lowering the magnitude of the threshold voltage V
t
of the PFET. By lowering the magnitude of the threshold voltage V
t
in the forward direction, the potential drop across the diode connected PFET is decreased, thereby increasing the voltage boosting capability of the LED driver circuit.
In another embodiment of a preferred

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