RT8477GS

.... RT8477 High Voltage High Current LED Driver General Description Features Buck, Boost Constant Current Converter The RT8477 is a current mode PWM controller designed High Voltage : V up to 50V, V up to 50V IN OUT to drive an external MOSFET for high current LED 380kHz Fixed Switching Frequency applications with wide input voltage (4.5V to 50V) and Analog or PWM Control Signal for LED Dimming output voltage (up to 50V) ranges. With internal 380kHz Internal Soft-Start to Avoid Inrush Current operating frequency, the size of the external PWM inductor Under-Voltage Lockout and input/output capacitors can be minimized. High Thermal Shutdown efficiency is achieved by a 100mV current sensing control. RoHS Compliant and Halogen Free LED Dimming control can be done by analog. The RT8477 is now available in the SOP-8 package. Applications Desk Lights and Room Lighting Ordering Information Industrial Display Backlight RT8477 Package Type Pin Configurations S : SOP-8 (TOP VIEW) Lead Plating System 8 VCC CREG G : Green (Halogen Free and Pb Free) 2 7 ISP DRV Note : 3 6 ISN GND Richtek products are : 4 5 CTL SENS RoHS compliant and compatible with the current require- SOP-8 ments of IPC/JEDEC J-STD-020. Suitable for use in SnPb or Pb-free soldering processes. Marking Information RT8477GS : Product Number RT8477 YMDNN : Date Code GSYMDNN Simplified Application Circuit D1 R1 V IN C1 C4 RT8477 LEDs ISP VCC L1 Analog Dimming CTL ISN CREG DRV M1 C2 R3 GND SENS C3 R2 Copyright 2013 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. DS8477-00 December 2013 www.richtek.com 1+ - RT8477 Functional Pin Description Pin No. Pin Name Pin Function 1 VCC Supply Voltage Input. For good bypass, a low ESR capacitor is required. 2 ISP Positive Input Current Sense. 3 ISN Negative Input Current Sense. Voltage threshold between ISP and ISN is 100mV. 4 CTL Analog Dimming Control Input. Effective programming range is 0.33V to 2V. Current Sense Input for LED Current. Connect the current sense resistor between 5 SENS external N-MOSFET switch and the ground. 6 GND Ground. 7 DRV External MOSFET Switch Gate Driver Output. 8 CREG Regulator Output. Placed 1F capacitor to stabilize the 5V regulator output. Function Block Diagram OSC S - VCC R DRV + 4.5V R 5V CREG LDO SENS Soft-Start - ISN GM + ISP CTL + - GND Opertation by the control loop. The RT8477 is a current mode PWM controller designed to drive an external MOSFET for high current LED The current through the sense resistor is set by the applications. This device uses a fixed frequency, current- programmed voltage and the sense resistance. The voltage mode control scheme to provide excellent line and load across the sense resistor can be programmed by the regulation. analog or digital signal at the CTL pin with good dimming linearity. The max sense threshold of 100mV can be The control loop has a current sense amplifier which obtained with CTL pin voltage greater than 2V (max senses the voltage between the ISP and ISN pins. dimming point). The sense threshold is intentionally forced A PWM comparator then turns off the external power to zero by an internal comparator when the CTL pin voltage switch when the sensed power switch current exceeds is less than around 0.33V (min dimming point). Because the internal compensated voltage. The power switch will of that, the actual sense threshold right before cut off may not be reset by the oscillator clock in each cycle. If the vary from part to part over process variation. comparator does not turn off the switch in a cycle, the The RT8477 provides protection functions which include power switch will be on for more than a full switching period over-temperature, and switch current limit to prevent until the comparator is tripped. In this manner, the abnormal situations. programmed voltage across the sense resistor is regulated Copyright 2013 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. www.richtek.com DS8477-00 December 2013 2