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Comparison of LED Constant Current Drive with Other Control Modes

2019/10/09

There are several control modes available in the market for micropower chips:

PFM, PWM, chargepump, FPWM, PFM/PWM and pulse-skipPWM, digital PWM

Among them, PFM, PWM, chargepump and PFM/PWM are common.

1. PFM is a technology to realize constant voltage output by adjusting pulse frequency (i.e. switching frequency). Its pulse width is fixed and its internal oscillation frequency is variable, so it is more difficult to filter than PWM. However, PFM is limited by the output power and can only provide a small current. Therefore, the PFM mode can be used in the case of low output power requirement and low static power consumption.

2. The principle of PWM is that when the input voltage, the internal parameters and the external load change, the control circuit adjusts the on-off pulse width of the internal switching device of the integrated circuit by closed-loop feedback through the difference between the control signal and the reference signal, so that the output voltage or current is stabilized by the control signal. The switching frequency of PWM is usually constant, so it is easy to filter. However, due to the influence of error amplifier, the loop gain and response speed of PWM are limited, especially the low loop gain, which makes it difficult to be used in constant current drive of LED. Although many products use this scheme at present, there is a common problem of constant current. The PWM mode can be used in the case of high output power and low output noise.

3. The charge pump charge pump solution is to use discrete capacitors to send power from the input to the output. No inductance is needed in the whole process. The main disadvantage of chargepump is that it can only provide a limited range of voltage output (output usually does not exceed 2 times the input voltage), because when multistage chargepump is cascaded, its efficiency decreases obviously. When using charge pump to drive more than one white LED, parallel drive must be adopted, so it is only suitable for applications with little difference between input and output voltage.

4. Digital PWM (Digital Pulse Width Modulation) is used to monitor, control and manage the DC/DC load point power conversion through the digital management of independent digital control loops and phases, so as to provide stable power supply, reduce the voltage amplitude of traditional power supply modules and cause system instability. Moreover, DigitalPWM does not need to use traditional high-volume liquid capacitors as wave storage and filtering functions. This is the case. DigitalPWM digital control technology can make MOSFET operate at higher frequencies, effectively alleviate the pressure on capacitors. Digital PWM is suitable for high current density and its response speed is very fast, but the loop gain is still limited and the cost is relatively high at present. Therefore, its application in constant current drive of LED still needs further study.

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 opens in a clock cycle, so that the current flows through the inductance. The inductance current rises and produces a voltage drop through the inductance. This voltage drop is amplified by the influenza induction amplifier and comes from the current induction amplifier. Voltage is added to the input of the PWM comparator and compared with the control end of the error amplifier. Once the current induction signal reaches the control voltage, the PWM comparator will restart the logic drive circuit that closes the high-side FET switch, and the low-side FET will open after a delay period of time. When working under light load, in order to maintain a fixed frequency, the inductance current must flow in the opposite direction through the low side of FET. At present, only MAXIM and NatiONal Semi conductor chips are used in FPWM technology driver chips.

For example, PFM and PWM control LED by constant voltage drive mode, while FPWM and PFM/PWM are constant current drive mode control technology. Practice has proved that they are more suitable for LED drive.

Recently launched iv0101 / iv0102 boost converter chip. Its control mode is an improved PFM/PWM control technology based on PFM. It is an organic combination of PFM and PWM (not the switch between PFM and PWM). It is based on input voltage to determine the opening time of N switch, output voltage and input voltage difference to determine the opening time of synchronous tube, unlike PWM, which uses error amplifier feedback output to adjust pulse width. Under a certain load, the switching frequency depends on the N-tube opening time tN and the P-tube opening time tP.

 

Among them, tP KP/(Vout-Vin) and tN KN/Vin continue to charge at the maximum tN under light load. When the inductance current is zero and the synchronous rectifier tube is turned on, the chip works in the discrete mode (DCM). When the load increases, the output quickly drops to the set point due to the heavy load. If the load current increases, the chip works in continuous mode (CCM), that is, the total current flows through the inductance. As long as the peak inductance current does not reach the maximum, the N-tube opening time TN remains at the set point. When the discharge cycle begins at the end of charging, the switching current will reach its maximum. However, the full load has not yet been achieved, because the output is still adjustable after the minimum discharge time is over. When the discharge time reaches the minimum tP, it will reach the full load. So this control mode adjusts the switching frequency by continuously adjusting the N-tube opening time tN and P-tube opening time tP to ensure constant current output. In the PWM control mode, in order to avoid the system oscillation fault caused by parasitic inductance, the input capacitor Cin is usually connected. This chip does not connect the input capacitor at the power supply access terminal, so it saves the capacitance position of PCB board, reduces the area of the board, and avoids the sudden wave pulse phenomenon caused by the capacitor during the PWM cycle, which prevents the system efficiency from declining, because it is the controller of the organic combination of PFM and PWM. Therefore, it has fast response speed of PFM, high loop gain and high current output characteristics of PWM. It can cooperate with PWM dimming and become an ideal constant current driver chip for small and medium power LED.

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