Figure 4 is an additional circuit that can implement overvoltage protection and open LED protection. In fact, many peak current mode controller ICs have dedicated RT pins. The resistor connected to this pin can be used to set the internal current, which is used to charge the oscillator capacitor (which can be internal or external). The ramp voltage on the oscillator capacitor controls the switching frequency so that the switching frequency is proportional to the output current at the RT pin. The smaller the resistance (larger), the larger the current (smaller) and the higher the switching frequency (lower). Based on this principle, the output frequency feedback can be used to adjust the switching frequency.
In the circuit shown in Figure 4, resistors R3 and R4 form a voltage divider. The voltage across R4 minus the voltage drop (Vbe) between the base and emitter of transistor Q2 is the voltage across R5. Therefore, the current flowing through R5 (IR5) is:
This current is obtained from pin RT of the control IC using a matched transistor pair.
Resistor R2 in Figure 4 is used to start the converter. In the startup state, the output voltage is zero, so IR5 is also zero. The converter cannot start because there is no current from the controller's RT pin. Increasing the resistance R2 can get a small part of the current in the startup state, and the size of R2 satisfies:
IR5 >> V (RT) / R2
Where V (RT) is the voltage on the RT pin of the controller. Meeting this condition ensures startup of the converter and minimizes the error introduced by R2. If you choose R3 = R4, you have:
IR5 >> VO / 2R5
It is assumed here that the output voltage is much larger than the base-emitter voltage drop of Q2.
In this way, according to the above formulas, the output LED current can be obtained as:
iLED = KICLi2pk / (2 × 2R5)
In this way, the LED current will no longer be determined by the input or output voltage. The use of resistor R6, transistor Q3 and Zener diode D2 can increase the overvoltage protection function. In the open LED state, when the switch is on, the inductor stores energy, and when the switch is off, this energy is transferred to the output capacitor. Because there is not enough load to discharge the capacitor, the output voltage will gradually increase every cycle. When the voltage rises above the Zener diode's turn-on voltage, the Zener diode branch circuit consisting of D2 and R6 begins to conduct. This also provides a path through the base current of Q3, turning Q3 on. At this time, the resistor R4 is actually shorted. Therefore, the PN junction of the base-emitter of Q2 will be closed, resulting in zero current on R5. This will stop the internal oscillation of the controller until the output voltage drops below the Zener diode voltage, and the above process continues. This burst mode minimizes the average power of the LED in an open circuit state. This overvoltage protection method will force the control IC into a low-frequency, low-power operating mode.
The current in the Zener diode's resistance branch circuit must be able to generate enough voltage on R6 to bias the PN junction between the base and emitter of the transistor.