Overview
LED lighting offers longer service life, lower energy consumption, and reduced maintenance requirements, making it especially suitable for street lighting and other outdoor infrastructure applications. As purchase prices for LED luminaires continue to decline, the total cost of ownership for LED street lights has been significantly reduced. In such applications, long-term reliability, thermal management, and stable current regulation are critical, placing higher demands on both the driver IC and the street light LED PCB.
Key Features and Operation
The U6117D supports quasi-resonant buck-type LED constant-current and constant-voltage output modes. Selecting the buck quasi-resonant mode is done by shorting the SEL pin to GND. In quasi-resonant buck operation the chip maintains a fixed CS peak and uses valley turn-on to achieve high-precision constant-current regulation while improving system efficiency. The formula for average output current is as follows:
In constant-voltage mode the U6117D uses a multi-mode control strategy combining amplitude modulation (AM) and frequency modulation (FM) to improve efficiency and reliability. In constant-current mode the chip operates in frequency modulation control and integrates line-voltage and load-voltage constant-current compensation. The U6117D is designed to operate without audible noise while maintaining strong dynamic performance. Integrated line-loss compensation enables high-performance constant-voltage output.
Primary-Side Constant-Current Control
The U6117D derives high-precision constant-current control from the timing relationship between the FB pin voltage and the CS pin voltage. In constant-voltage mode, when system output power increases and approaches the constant-current control point, the primary-side inductor current reaches its maximum. The primary inductor current, transformer turns ratio, secondary demagnetization time (Tdem), and switching period (Tsw) determine the average secondary output current. If leakage inductance is neglected, the formula for average secondary output current can be found in the device documentation.
When output current reaches the primary-side constant-current reference, the chip switches to frequency-modulation operation. Regardless of whether the output voltage is below the constant-voltage reference, the device will continue to operate as long as VDD remains above the shutdown threshold. Internally, in constant-current mode the U6117D strictly controls the ratio of demagnetization time Tdem to switching period Tsw to 1/2.
Recommended Output Power
- Flyback primary-side feedback applications
- U6117D: 230VAC: 15W; 85-265VAC: 12W
- Buck step-down applications
- U6117D: 175-265VAC: 25W (Iout < 250mA); 85-265VAC: 18W (Iout < 250mA)
Package Options
The U6117D is available in a DIP-8 package. The related U6117S is offered in an SOP-8 package. For the U6117S, recommended powers are 7.5W or 5.5W in flyback primary-side feedback applications and 18W or 10W in buck step-down applications.
