Pwm led regulator with sample and hold

Description of equivalent WO 2004060023 A1

PWM LED REGULATOR WITH SAMPLE AND HOLD

The invention relates to regulated LED current sources. More particularly the invention relates to techniques for configuring an LED regulator for improved stability.

LED lighting systems generally employ regulated power sources for supplying power to the LEDs. In the art of LED drivers, it is known to use a pulse-width modulated (PWM) drive current as a power source to the LED. Generally, a regulator circuit includes several sub-circuits with active and passive elements that operate in concert to provide power regulation.

A simple circuit diagram for a typical regulator for driving LED strings is shown in FIG. 1. A Buck-Boost converter is formed of Ql, L, Dl and Cl. A serial LED string is denoted as D5. The OP-AMP1 along with the surrounding resistors, R5, R6, R7, R8 forms a differential amplifier for the sensed current signal form Rl. An analog PID controller is formed by OP-AMP2 along with the surrounding components R9, R10, Rll, R12, C5, C6, and C7. A PWM signal is introduced to the regulator circuit through the modulator COMP1. In steady-state DC operation, the LED string D5 current is regulated by the regulator circuit.

FIG. 2 illustrates the regulator circuit configured to provide the LED string D5 with light output adjustment or dimming functionality. It is known to be beneficial to use a low- frequency PWM current for the LED string D5 by invoking a series switch Q2 as is depicted in FIG. 2. In order to reduce the current peak pulse in the LED string D5 at each turn on event, a simple sample-and-hold 210 sub-circuit consisting of R2, R4, C2 and D2 is provided. As shown in FIG. 2, the sample-and-hold sub-circuit has an output voltage V3 and an input voltage V6. It can be shown that when the diode D2 conducts, the transfer function of the sample-and-hold 210 sub-circuit is:

—=κ(s)=κ₀-!— , (l)

V6 ^w_]+ ω

R2 where K„ = , and (2)

⁰ R2+R4 ^;

R2+R4 ω= . (3)

R2*R3*C2

Inspection of equation (1) reveals that the sample-and-hold introduces a pole, with an associated 90 degree phase delay, into the current regulation loop. The LED regulator phase margin is therefore reduced and the regulator circuit tends to oscillate. It would therefore be desirable to provide an improved LED regulator configuration that addressed these and other limitations.

The present invention is directed to a system and method for improving stability in an LED regulator. In accordance with the invention a method for configuring a regulator circuit having a sample-and-hold circuit is provided. Coupling an input voltage to an input node of the sample-and-hold circuit is provided. Activating the sample-and hold circuit in response to the input voltage and sensing an output voltage at an output node coupled to the sample and hold circuit is also provided. Determining whether the input voltage at the input node is greater than the output voltage at the output node and providing a sample-and-hold function based on the determination are also provided.

In accordance with another aspect of the invention, a regulator circuit having a sample-and-hold circuit with improved stability is provided. A regulation circuit is provided. A sample-and-hold circuit coupled to input and output nodes is also provided. The transfer function of the sample-and-hold circuit is pseudo-all-pass if the input voltage at the input node is greater than an output voltage at the output node and is a substantially constant signal if the input voltage at the input node is less than the output voltage at the output node.

The foregoing and other features and advantages of the invention are apparent from the following detailed description of exemplary embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof.

FIG. 1 illustrates a prior art LED regulating system. - FIG. 2 i]lustøtes_a„μrior.art low-frequejicy_.PWM_ha.sed LEDjrøgulating system.

FIG. 3 is a block diagram of an pseudo-all-pass sample-and-hold circuit in accordance with the present invention.

FIG. 4 is a block diagram illustrating an embodiment of the pseudo-all-pass sample- and hold circuit of FIG. 3 in accordance with the present invention.

FIG. 5 is a flow diagram of a method for configuring a regulator circuit having a sample-and-hold circuit in accordance with the present invention. In the following description the term "coupled" means either a direct connection between the things that are connected, or a connection through one or more active or passive devices that may or may not be shown, as clarity dictates. FIG. 3 is a block diagram of a pseudo-all-pass sample-and-hold circuit in accordance with the present invention. FIG. 3 shows a pseudo-all-pass sample-and-hold circuit 300. The pseudo-all-pass sample-and-hold circuit 300 is shown having an input node Vin and an output node Vout both referenced to ground. The pseudo-all-pass sample-and-hold circuit 300 is any circuit that provides a sample- and-hold function and has the transfer function:

Vout(s) /Vin(s)= K(s), K(s) is an all pass function when Vin > Vout, and (4) Vout(t) is a nearly constant signal when Vin < Vout. (5)

Therefore, the pseudo-all-pass sample-and-hold configuration provides a sample-and- hold function in a regulator circuit without introducing a pole into the transfer function of the regulator. A regulator is then able to operate in a more stable manner.

In one embodiment, the pseudo-all-pass sample-and-hold circuit 300 is an active sample-and-hold device configured for all pass operation such as an integrated circuit, for example. In another embodiment, the pseudo-all-pass sample-and-hold circuit 300 is a passive circuit containing passive devices such as resistors, capacitors, diodes and the like. A passive embodiment of a pseudo-all-pass sample-and-hold circuit 300 is discussed in detail with reference to FIG 4.

FIG. 4 is a block diagram illustrating an embodiment of the pseudo-all-pass sample- and hold circuit of FIG. 3. FIG. 3, shows an all sample-and-hold circuit 300 comprising a sample and hold circuit 210 as in FIG. 2, a first pass diode D6 and a second pass diode D7.

The first pass diode D6 is shown coupling the sample-and-hold circuit 210 to an output node V3 with a forward bias. The second pass diode D7 is shown coupling an input node V6 with the output node with a forward bias.

. In operation, Jhe pass diode JD7 passes a current whenever the voltage potential at V6 is greater than the potential voltage at V3. The potential voltage applied to V6 is either time- varying, such as a periodic pulse or a DC value. The bias of diodes D6 and D7 prevents current reversal if the potential voltage of V3 is greater than V6, and therefore configures the sample-and-hold circuit.

In the following process description certain steps may be combined, performed simultaneously, or in a different order without departing from the invention.

FIG. 5 is a flow diagram of a method for configuring a regulator circuit having a sample-and-hold circuit in accordance with the present invention. Process 500 begins in step 510. Generally, the sample-and-hold circuit operates to reduce the current peak pulse in an LED string under PWM drive at each turn-on moment.

In step 510 an input voltage is coupled to an input node V6 of a pseudo-all-pass sample-and hold 300. The input voltage is generally the output of a regulator sub-circuit, such as, for example, a differential amplifier that monitors the current through an LED string D5. The input voltage may be a time-varying signal such as a periodic pulse, or a static DC value. The voltage may be coupled to the input node at any time, and may be selectably operated for specific functionality such as a PWM operational mode.

In step 520, the pseudo-all-pass sample-and-hold circuit 300 is activated in response to the voltage coupled in step 510. The pseudo-all-pass sample-and-hold circuit 300 contains components that are activated when a voltage is coupled to the circuit such as a capacitor. In one embodiment, the capacitor charges in response to the voltage signal. Activation of the sample-and-hold 300 occurs immediately with the coupling of the input voltage in step 510. In step 530, output voltage at an output node is sensed. Generally, a first pass diode

D6 and second pass diode D7 are configured around a sample-and-hold to allow sensing of the output voltage. The diodes will reverse bias if the output voltage is greater than the reference input voltage.

In step 540 a determination is made whether the input voltage at the input node is greater than the output voltage at the output node. Generally, the first pass diode D6 and the second pass diode D7 provide a determination of whether the input voltage is greater than the output voltage, since the forward biased diodes will conduct under those conditions. If the input voltage is less than the output voltage, then the diode D7 will not conduct and the output voltage of the sample-and-hold circuit will be an almost constant signal. In step 550, a sample-and-hold function is provided based on the determination of step 540. The sample-and-hold circuit 300 has a transfer characteristic based on the relative voltages determined in step 540. The sample-and-hold function is provided at all times the sample-and-hold circuit is operational.

While the preferred embodiments of the invention have been shown and described, numerous variations and alternative embodiments will occur to those skilled in the art.

Accordingly, it is intended that the invention be limited only in terms of the appended claims.