For the first time the input voltage VS D1 will enable the storage capacitor C1 is charged until its voltage slightly lower than the VS. Initially, the transistor Q2 in a closed state, IC1’s reset input is high, the output (OUTPUT) is high, so the current through R1 to C2 charging.
During this time, R4 pull the discharge side (DISCHARGE) turns on transistor Q1, the current in the inductor L1 IL began to slash increases. Q1 is saturated, so the D3 and the LED lights in reverse bias state.
When the C2 voltage more than IC1 pin 6 threshold voltage (THRESHOLD), output (OUTPUT) and the discharge side (DISCHARGE) goes low, Q1 is cutoff. Back electromotive force generated across the L1, instantly elevated to the VA of the LED anode voltage, VA, is greater than the VS, the LED is lit. At this time, the diode D3 is forward biased state, and pull IC1’s input voltage V + until more than VS ~~ 4V.
Then, C2, once the rapid discharge through the transistor D2 and resistor R2, ready for the next cycle. If the proper choice of the resistance of R5 and R6, Q2 turns on the LED reset input at the same time. L1 stored energy is depleted, the LED and transistor D3 re-enter the reverse bias state, the VA is reduced to a lower level. Q2 is turned off immediately, and allow the the IC1 start another cycle, C2 begins again charging through R1. The process is repeated thousands of times per second, so the LED can be lit continuously.
This circuit uses three “tricks” to optimize performance. Transistor D3, since the gain can increase the input voltage of the timer, and even in VS drops below 1V, the circuit can also continue to work properly. In addition, through R4 Q1 enhance the base gate drive.
Second, through Q2 feedback to ensure that the L1 energy is exhausted, new cycle can begin, so to maximize the average LED current.
Q1 is not the timer output side, but driven by a timer drain discharge terminal, the base drive does not depend on the performance of the 555 timer output terminal current source.
Transistor Q1 must be low in saturated type, the driving time for tON: tON = K × R1 × C2,
Where K is a constant determined by the actual use of the 555 timer type.
LED grow lights peak current is approximately equal to the maximum induced current IL (MAX): IL (MAX) = [(VS-VCE (SAT)) / L1] × tON Q1’s saturation voltage is low, for example, is lower than 50mV, then the VCE (SAT) can be neglected, the above equation simplified to: IL (MAX) = (VS/L1) × tON
Therefore, for a specific VS values, select R1, C2 and L1 values can be IL (MAX), the maximum, which does not exceed the peak current rating of the LED to achieve maximum brightness.
Must be the appropriate choice of resistor R5 and R6, to ensure that VA = Q2 is turned off (in the case of incentives for the first time), the LED forward bias (VA> VS) conduction. Q2 itself must be a high current gain of small signal devices.
In order to achieve high efficiency, low voltage operation mode, you must use the CMOS timer, such as Intersil’s ICM7555 or TI’s TLC555. These types of timers are running under a 2.0V low voltage special features. In addition, their internal discharge transistor to pin 7 of voltage down to 100mV or even lower, to ensure that Q1 is completely turned off.
In a test circuit, IC1 TLC555, Q1 = ZTX649, Q2 = BC546, L1 = 100 μH, R5 = 56 kΩ, R6 = 10kΩ, that the circuit in the VS is equal to the low voltage of 1.0V to start. This circuit can Lumileds (www.lumileds.com) white LED to the best brightness.
The rise time of the transistor Q1 (tON) about 20 μs, making about 300 mA VS = 1.5 V when the peak inductor current. But to change the value of C2, R1, C2 of R1, L1 can change the peak inductor current, the ICM7555 also achieve the same performance, the minimum turn-on voltage will be slightly higher than 1.2V.
This circuit is ideal for a single LED application, because even if the input voltage drops below 1.0 V, it can make the LED flood lights to maintain sufficient brightness. Of course, two or more LEDs in series with its brightness will be reduced accordingly.
