The micropower power chips available on the market have the following control modes:
PFM, PWM, chargepump, FPWM, PFM/PWM, and pulse-skipPWM, digitalPWM
Common among them are PFM, PWM, chargepump and PFM/PWM.
1. PFM is a technique for realizing regulated output by adjusting the pulse frequency (ie, the operating frequency of the switching tube). Its pulse width is fixed and the internal oscillation frequency is varied, so filtering is more difficult than PWM. However, the PFM is limited by the output power and can only provide a small current. Therefore, PFM mode control can be adopted when the output power requirement is low and the static power consumption is low.
2. The principle of PWM is that in the case of input voltage, internal parameters and external load changes, the control circuit performs closed-loop feedback by the difference between the control signal and the reference signal to adjust the conduction pulse width of the internal switching device of the integrated circuit, so that the output The voltage or current is stabilized by the control signal. The switching frequency of the PWM is generally a constant value, so it is easier to filter. However, due to the influence of the error amplifier, the PWM gain and response speed are limited, especially the loop gain is low, which is difficult to use for LED constant current driving. Although many products currently use this scheme, there is a constant current problem. PWM mode control can be used in applications where the output power is large and the output noise is low.
3, chargepump charge pump solution is to use a discrete capacitor to send power from the input to the output, the entire process does not require the use of any inductor. The main disadvantage of chargepump is that it can only provide a limited voltage output range (the output generally does not exceed 2 times the input voltage), because when multi-stage chargepump is cascaded, its efficiency is significantly reduced. When driving more than one white LED with a chargepump, it must be driven in parallel, so it is only suitable for applications where the input and output voltages are not much different.
4, using DigitalPWM (digital pulse width modulation) through the digital management of independent digital control loop and phase, to achieve DC / DC point-of-load power conversion monitoring, control and management to provide a stable power supply, reduce the traditional power supply module The voltage amplitude causes instability of the system, and DigitalPWM does not require the use of traditional higher amounts of liquid capacitors for storage and filtering. DigitalPWM digital control technology enables the MOSFET to operate at higher frequencies, effectively mitigating the stress on the capacitor. digitalPWM is suitable for high current density, its response speed is fast, but the loop gain is still limited, and the current cost is relatively high. Therefore, its application in LED constant current driving still needs further research.
5. FPWM (Forced Pulse Width Modulation) is a constant current output based control method. Its working principle is that it always operates at a fixed frequency regardless of the output load, and the high side FET is turned on in one clock cycle. The current flows through the inductor, and the inductor current rises to produce a voltage drop across the inductive reactance. This voltage drop is amplified by a current sense amplifier. The voltage from the current sense amplifier is applied to the input of the PWM comparator and compared to the control terminal of the error amplifier. Once the current sense 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. To operate at a light load, in order to maintain a fixed frequency, the inductor current must flow in the opposite direction through the low side FET. The FPWM technology driver chip currently only sees the chips used by MAXIM and National Semiconductor.
As above, PFM and PWM use constant voltage driving to control LED, while FPWM and PFM/PWM are constant current driving mode control technology, which has proved to be more suitable for LED driving .
The company recently introduced the IV0101/IV0102 boost converter chip. Its control mode is PFM/PWM control technology improved on the basis of PFM. It is a combination of PFM and PWM control (not switching between PFM and PWM). It determines the N switch opening time, output voltage and input with input voltage. The voltage difference determines the synchronizing tube turn-on time, rather than adjusting the pulse width as the PWM uses the error amplifier feedback output. In the case of a certain load, the switching frequency depends on the N tube opening time tN and the P tube opening time tP.
Where tP≧KP/(Vout-Vin); tN≦KN/Vin
At light loads, the charge cycle continues at the maximum value tN. When the inductor current is zero and the synchronous rectifier is turned on, the chip operates in discrete mode (DCM). As the load increases, the output quickly drops to the set point due to heavy loads. If the load current increases, the chip operates in continuous mode (CCM), that is, there is always a current flowing through the inductor. As long as the peak value of the inductor current does not reach the maximum, the N-tube turn-on time tN is always maintained at the set point. When the discharge begins and the discharge cycle begins, the switch current will reach its maximum. However, the full load is still not reached because the output is still adjustable after the minimum discharge time has elapsed. When the discharge time reaches the minimum value tP, the full load will be reached. Therefore, the 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. In the PWM control mode, in order to avoid system oscillation fault caused by parasitic inductance, the input capacitor Cin is generally connected. The chip does not have an input capacitor at the power input end, thus eliminating the PCB board capacitance position and reducing the board area. It also avoids the phenomenon of glitch generated by the capacitor during the PWM cycle, which prevents the system from deteriorating. Because it is a combination of PFM and PWM, it has a fast response speed and high loop gain. And PWM high current output characteristics, can be combined with PWM dimming, become an ideal small and medium power LED constant current driver chip.
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