FEATURE OPTOELECTRONICS LED Driver ICs reduce switching loss and EMI noise
The latest LED driver ICs from Ricoh, with PFC and low EMI emission address industry specific demands which supresses the harmonic current and EMI emission and results in a flicker-free LED operation. Particularly when using the light source for recording photos and videos, the resulting footage quality will be lowered by flickering illumination. Striking effects are areas or lines on the footage with a different exposure. Another advantage of the R1580 is its wide dimming range; it extends the standard range from a minimum setting of 5% to 0.5%. In particular for RGB LED configurations, the extended dimming range ensures a wide control of the colour temperature settings. The R1700 has an array of protection circuits contributing to a safe operation of the application: Overcurrent Protection (OCP) Thermal Shutdown (TSD) BST/VCC Pin Undervoltage Lockout (VCC UVLO)
I
Cs targeted at LED lighting applications, powered directly from the
mains convert primary power source into a voltage suitable to drive LEDs. Developed by Ricoh Electronic Devices Co., based in Japan, the ICs are said to be optimised for non-isolated LED driver circuits and have a wide input voltage range up to 650V to tolerate peaks in the supply voltage caused by switching inductive loads and to allow for enough safety margin. The PFC circuit improves the ratio between active and apparent power, resulting in a lower load current from the source (electricity supplier). The company understands from the
market that there is a demand for LED drivers addressing specific issues. Its customers prefer to design a high efficient PFC circuit with synchronous rectification but are concerned about the EMI noise level caused by two switching MOSFETs.
The advanced R1700 has a Zero Voltage
Switch (ZVS) circuit with Edge Resonance Control, which is able to reduce switching loss and EMI noise significantly by synchronous rectification. Figure 2 highlights the basic operation of the circuit in more detail. (A) shows an ideal waveform of the
voltage (Vds) and current (Id). (B) shows the actual waveform of Vds
and Id. When the MOSFET is turn on and off, switching losses occur due to voltage and current overlap. (C) shows the improved waveform of Vds and Id using ZVS, any overlap between voltage and current is eliminated and switching loss minimised. Typically, faster switching operation generates high frequency noise that can lead to interfering system noise and other EMI issues; however, the R1700 can significantly reduce switching noise and EMI.
ONE STAGE SWITCHING CONVERSION A conventional LED driver circuit usually consists of two switching DC/DC converters whilst Ricoh's solution uses a one stage switching conversion with an advanced buck-boost (inverting) topology. This comes as an advantage as it means skipping one noisy switching circuit, reducing the number of bulky components on the board and helping save cost this way. Secondly, the edge resonance controlled
40 JUNE 2018 | ELECTRONICS
Zero Voltage Switching (ZVS) circuit allows for a high operating frequency and the use of small components. These two features makes the application suitable for integration in regular but also small and thin sized lighting fixtures. Compared to conventional circuits this represents a board-space reduction of up to 70% and a height reduction of up to 50% is achievable.
The R1700 has a LED brightness
dimming range from 100% to 5% by connecting a PWM signal with a frequency from 1 to 10kHz to the DIM input. As an alternative option, one could consider adding the R1580 constant current driver into the circuit. This IC converts a PWM signal into a linear dimming signal, the main advantages of this method is that the LED driver MOSFET is not continuously switching
Figure 1: The advanced R1700
VCC Pin Overvoltage Lockout (VCC OVLO)
Feedback Pin Overvoltage Protection (FB OVP)
CMP Pin Overvoltage Protection (CMB OVP)
Figure 2:
The basic operation of the circuit
Latch-Type Protection (optional by product version) The latch circuit becomes active as soon the VCC OVLO, TSD or CMP OVP protection is triggered over 32 times and will stop the operation of the R1700. The latch version remains in this state until the circuit performs a reboot or when the chip is reset by using the DIM input pin. The non-latch version resumes operation automatically when the FB pin voltage returns to normal.
COMPANY PROFILE Ricoh Electronic Devices Co., based in Japan, is experienced in developing semiconductor products as of 1981 and has its own four and six inch production facilities. The company’s portfolio now consists of advanced solutions for Power Management and Time Keeping such as Voltage Regulators, DCDC Converters, Supervisory, Switches, LED drivers, Li- Ion Battery Protection, single chip PMU and Real Time Clock ICs.
Ricoh Electronic Devices Co.
http://www.e-devices.ricoh.co.jp/en/ T: +81-50-3817-8101
/ ELECTRONICS
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