Method of using 555 timer to form a white LED voltage regulator

After first inputting the voltage VS _, D1 will charge the storage capacitor C1 until its voltage is slightly lower than _VS. Initially, transistor Q2 is in the off state, the reset input of IC1 is high level, and the output terminal (OUTPUT) is high level to allow current to charge C2 through R1.

_{During this period, R4 pulls the discharge terminal (DISCHARGE) to turn on the transistor Q1, and the current IL} in the inductor L1 begins to increase in a ramp. Since Q1 is saturated, both D3 and the LED are reverse biased.

When the voltage of C2 exceeds the limit voltage (THRESHOLD) of pin 6 in IC1, both the output terminal (OUTPUT) and the discharge terminal (DISCHARGE) become low level, and Q1 is cut off. The generated back electromotive force crosses L1, causing the anode voltage of the LED to instantly rise to VA _. VA _is greater than _VS , so the LED is lit. At this time, diode D3 is in a forward biased state and pulls the input voltage V+ of IC1 until it is 2~4V higher than _VS.

Then, C2 is immediately discharged through transistor D2 and resistor R2, ready for the next cycle. If the resistance values ​​of R5 and R6 are chosen appropriately, Q2 will be turned on at the same time as the LED reset input. When the energy stored in L1 is exhausted, the LED and transistor D3 enter the reverse biased state again, and V _A decreases to a lower level. Q2 immediately shuts down, allowing IC1 to start another cycle and C2 begins charging via R1 again. This process is repeated thousands of times per second, so the LED is constantly lit.

This circuit utilizes three "tricks" to optimize performance. First, the self-gain of transistor D3 can increase the input voltage of the timer, so that the circuit can continue to operate normally even when VS drops below 1V _. Additionally, it provides enhanced base drive to Q1 via R4.

Second, feedback via Q2 ensures that when L1's energy is exhausted, a new cycle can begin, thereby maximizing the average LED current.

Third, Q1 is driven not by the timer output, but by the discharge terminal of the timer drain, so the base drive does not depend on the current source performance of the 555 timer output terminal.

Transistor Q1 must be a low saturation type, and its driving time is t _ON : t _ON = K×R1×C2

Where K is a constant, determined by the type of 555 timer actually used.

The peak current of the LED is approximately equal to the maximum induced current I _L(MAX) , here: I _L(MAX) = [(V _S - V _CE(SAT) )/L1] × t _ON If the saturation voltage of Q1 is low, such as low is less than 50mV, then V _CE(SAT) can be ignored, and the above formula is simplified to: I _L(MAX) =( V _S /L1) ×t _ON

Therefore, for a specific V _{S value, the maximum value of IL (MAX)} can be obtained by selecting the values ​​of R1, C2 and L1 , so that the LED reaches maximum brightness without exceeding the peak current rating.

Resistors R5 and R6 must be chosen appropriately to ensure that Q2 is turned off when V _A =V _{S (on first excitation) and turned on when the LED is forward biased (V A} >V _S ). Q2 itself must be a high current gain small signal device.

In order to achieve high efficiency and low voltage operating mode, a CMOS timer must be used, such as Intersil's ICM7555 or TI's TLC555. These types of timers have the special function of operating at a low voltage of 2.0V. Additionally, their internal discharge transistor is capable of reducing the voltage at pin 7 to 100mV or less, ensuring that Q1 is completely turned off.

In a test circuit, IC1 uses TLC555, Q1 = ZTX649, Q2 = BC546, L1 = 100 μH, R5 = 56 kΩ, R6 = 10kΩ. The circuit can start at a low voltage of V _S equal to 1.0V. This circuit enables Lumileds (www.lumileds.com) white LEDs to achieve the best brightness.

_{The rise time (t ON} ) of transistor Q1 is approximately 20 μs, resulting in a peak sense current of approximately 300 mA at V _S =1.5 V. But changing the values ​​of C2, R1 or C2, R1, L1 can change the peak sense current, and the same performance can be achieved using ICM7555, although its minimum on-voltage will be slightly higher than 1.2V.

This circuit is ideal for single LED applications because it allows the LED to maintain sufficient brightness even if the input voltage is below 1.0 V. Of course, two or more LEDs can be connected in series, although their brightness will be reduced accordingly.