Analysis and comparison of different control modes of LED constant current drive

The micro-power supply chips available on the market have the following control modes: PFM, PWM, cha rge pump, FPWM, PFM/PWM, pulse-skipPWM, digitalPWM. Among them, the most common ones are PFM, PWM, cha rge pump and PFM/PWM.

1. PFM is a technology that achieves voltage regulation output by adjusting the pulse frequency (i.e. the operating frequency of the switch tube). Its pulse width is fixed and the internal oscillation frequency is variable, so filtering is more difficult than PWM. However, PFM is limited by the output power and can only provide a small current. Therefore, PFM control can be used in situations where the output power requirement is low and the static power consumption is low.

2. The principle of PWM is that when the input voltage, internal parameters and external load change, the control circuit performs closed-loop feedback through the difference between the controlled signal and the reference signal, adjusts the conduction pulse width of the internal switch device of the integrated circuit, and stabilizes the output voltage or current and other controlled signals. The switching frequency of PWM is generally a constant value, so it is easier to filter. However, due to the influence of the error amplifier, the loop gain and response speed of PWM are limited, especially the low loop gain, which makes it difficult to use it for constant current driving of LEDs. Although many products currently use this solution, there is a common constant current problem. PWM control can be used in situations where a large output power and low output noise are required.

3. The charge pump solution uses discrete capacitors to send power from the input to the output, and no inductor is required in the entire process. The main disadvantage of the charge pump is that it can only provide a limited voltage output range (the output generally does not exceed 2 times the input voltage) because when multiple stages of the charge pump are cascaded, its efficiency drops significantly. When using a charge pump to drive more than one white light LED, a parallel drive method must be used, so it is only suitable for applications where the input and output voltages are not much different.

4. DigitalPWM (digital pulse width modulation) is used to monitor, control and manage the DC/DC load point power conversion through digital management of independent digital control loops and phases to provide a stable power supply and reduce the instability of the system caused by the voltage fluctuation of the traditional power supply module. In addition, DigitalPWM does not need to use traditional high-volume liquid capacitors for wave storage and filtering. DigitalPWM digital control technology can make the MOSFET tube run at a higher frequency, effectively alleviating the pressure on the capacitor. DigitalPWM is suitable for large current density and has a fast response speed, but the loop gain is still limited and the current cost is relatively high. Therefore, its application in LED constant current drive still needs further research.

5. FPWM (forced pulse width modulation) is a control method based on constant current output. Its working principle is that no matter how the output load changes, it always works at a fixed frequency. The high-side FET is turned on in a clock cycle, allowing current to flow through the inductor. The inductor current rises and generates a voltage drop through the inductor. This voltage drop is amplified by the current sensing amplifier. The voltage from the current sensing amplifier is added to the input of the PWM comparator and compared with the control end of the error amplifier. Once the current sensing signal reaches this control voltage, the PWM comparator will restart the logic drive circuit that turns off the high-side FET switch, and the low-side FET will turn on after a delay. When working under light load, in order to maintain a fixed frequency, the inductor current must flow through the low-side FET in the opposite direction. FPWM technology driver chips are currently only used by MAXIM and National Semiconductor chips.

As mentioned above, PFM and PWM use constant voltage drive mode to control LEDs, while FPWM and PFM/PWM are constant current drive mode control technologies, which have been proven to be more suitable for LED driving.

The IV0101/IV0102 boost converter chip recently launched by our company. Its control mode is an improved PFM/PWM control technology based on PFM. It is a control mode that organically combines PFM and PWM (not the switching between PFM and PWM). The input voltage determines the N switch tube opening time, and the output voltage and input voltage difference determines the synchronous tube opening time, instead of using the error amplifier feedback output to adjust the pulse width like PWM. Under certain load conditions, the switching frequency depends on the N tube opening time tN and the P tube opening time tP.
Among them, tP≧KP/(Vout-Vin); tN≦KN/Vin

. Under light load, the charging cycle continues at the maximum value tN. When the inductor current is zero and the synchronous rectifier is turned on, the chip works in discrete mode (DCM). When the load increases, the output quickly drops to the set point due to the large load. If the load current increases, the chip works in continuous mode (CCM), that is, there is always current flowing through the inductor. As long as the inductor current peak does not reach the maximum, the N tube opening time tN always remains at the set point. When charging ends and the discharge cycle begins, the switch tube current will reach the maximum. However, the full load is not reached yet, because after the minimum discharge time is over, the output is still adjustable. When the discharge time reaches the minimum value tP, the full load will be reached. Therefore, this control mode is to adjust the switching frequency by continuously adjusting the N-tube opening time tN and the P-tube opening time tP to ensure constant current output. Under the PWM control mode, in order to avoid system oscillation failure caused by parasitic inductance, the input capacitor Cin is generally connected. This chip does not connect the input capacitor at the power supply access end, thus eliminating the PCB board capacitor position, reducing the board area, and avoiding the surge pulse phenomenon generated by the capacitor during the PWM cycle, preventing the system performance from declining. Because it is a control method that organically combines PFM and PWM, it has a faster response speed of PFM and a very high loop gain and PWM large current output characteristics, which can be matched with PWM dimming to become an ideal small and medium power LED constant current driver chip.