LED constant current drive and other control mode comparison

Micropower power supply chips available in 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 by adjusting the pulse frequency (ie, the operating frequency of the switch) to achieve the output voltage regulator technology. Its pulse width is fixed and the internal oscillation frequency is variable, so the filtering is more difficult than PWM. However, the PFM is limited by the output power and can only provide a smaller current. Therefore, the output power requirements are low, low static power consumption can be used PFM mode control.

 

2, PWM principle is in the input voltage, internal parameters and external load changes, the control circuit is controlled by the difference between the signal and the reference signal for closed-loop feedback, adjust the integrated circuit switching device on-pulse width, making the output Voltage or current is controlled by the signal stability. PWM switching frequency is generally a constant value, so relatively easy to filter. However, due to the influence of the error amplifier, the loop gain and the response speed are limited. Especially, the loop gain is low and it is difficult to be used for LED constant current driving, although many products are using this scheme, but the constant current problem is prevalent. In the required output power and low output noise can be used where the PWM control mode.

 

3, chargepump charge pump solution is to use discrete capacitors to the power supply from the input to the output, the whole process does not require any inductor. The main drawback of chargepump is that it can only provide a limited range of voltage output (the output will not exceed 2 times the input voltage), because when the cascade chargepump cascade, its efficiency decreased significantly. Use chargepump to drive more than one white LED, the parallel drive must be used, and therefore only applies to the input and output voltage difference is not the application.

 

4, DigitalPWM (digital pulse width modulation) through the independent digital control loop and phase digital management, to achieve DC / DC load point power conversion monitoring, control and management to provide a stable power supply, reducing the traditional power supply module Of the voltage amplitude of the instability caused by the system, and DigitalPWM does not require the use of traditional higher volume of liquid capacitors for storage and filtering role. DigitalPWM digital control technology, can make the MOSFET tube running at a higher frequency, effectively alleviate the capacitance of the pressure. DigitalPWM for high current density, the response speed is very fast, but the loop gain is still limited, the current cost is relatively high. So its application in LED constant current drive still needs further study.

 

5, FPWM (forced pulse width modulation) is a constant current output based on the control mode, its working principle is no matter how the output load changes always work at a fixed frequency, high side FET in a clock cycle, So that the current flows through the inductor, the inductor current rise through the inductive anti-voltage drop, the voltage drop through the current sense amplifier to amplify the voltage from the current sense amplifier is added to the PWM comparator input, and error amplifier control terminal for comparison, Once the current sense signal reaches this control voltage, the PWM comparator restarts the logic drive circuit that turns off the high side FET switch and the low side FET turns on after a delay. When operating at light loads, the inductor current must flow through the low-side FET in the opposite direction to maintain a fixed frequency. FPWM technology driver chips currently only see MAXIM and NatiONalSemi conductor chips.

 

As above PFM, PWM is a constant voltage drive control LED, and FPWM and PFM / PWM is a constant current drive mode control technology, proved to be more suitable for LED driver.

 

Recently introduced IV0101 / IV0102 boost converter chip. Its control mode is based on PFM improved PFM / PWM control technology, PFM and PWM organic combination of control (not PFM and PWM switching), is to determine the input voltage N switch on time, the output voltage and input Voltage difference to determine the synchronization tube opening time, rather than the PWM error amplifier feedback output pulse width regulation. In a certain load case, the switching frequency depends on the N-tube opening time tN and P-tube opening time tP.

 

Where tP ≧ KP / (Vout-Vin); tN KN / Vin At light load, the charge cycle continues at the maximum value tN. When the inductor current is zero, the synchronous rectifier is turned on, the chip operates in discrete mode (DCM). When the load increases, the output drops quickly to the set point due to high load. If the load current increases, the chip operates in continuous mode (CCM), ie, there is always current flowing through the inductor, so long as the inductor current peak does not reach its maximum, the N-tube turn-on time tN remains at the setpoint. When the discharge begins to discharge cycle, the switch tube current will reach the 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 to the minimum tP, will reach full load. So the control mode is to adjust the N-tube open time tN and P tube open time tP to adjust the switching frequency to ensure constant current output. In the PWM control mode, in order to avoid the parasitic inductance caused by the system shock failure, generally connected to the input capacitor Cin, the chip in the power supply input terminal is not connected to the capacitor, thus eliminating the PCB board capacitor location, reducing the board area, And to avoid the PWM cycle, the surge generated by the capacitor pulse phenomenon, to prevent system performance decline, because it is PFM and PWM organic combination of control, so it has PFM faster response speed and high loop gain And PWM high current output characteristics, can be matched with PWM dimming, becoming the ideal medium and small power LED constant current driver chip.

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