Theory of Operation Prepared by: Mark Scholten Senior Applications Engineer http://onsemi.com The LM317L wants to see 1.25 V between its VOUT pin and the Vadj pin, and it will do whatever it can to keep that voltage differential between them. So if a resistor is put in series with the output, and the Vadj pin is connected to the load side of the sense resistor, a current source is set up which follows Equation 1: 1.25 IOUT + (Approximately) Rsense But what if a series/parallel combination of LEDs is wanted? The following circuit works fine, assuming that there are not large variations in the forward voltage drop of the LEDs. There could be a problem however if one of the strings opens up for some reason. The LEDs that remain would have 50% more current flowing through them, which could cause them to be driven too hard and fail. So how does one get around this problem? (eq. 1) VIN LM317L VOUT IOUT Rsense VIN LM317L VOUT IOUT Rsense Vadj Rload Vadj LED1 LED4 LED7 LED2 LED5 LED8 LED3 LED6 LED9 Figure 1. Figure 3. In order to set up a constant current source for an LED string, the same circuit can be used by simply substituting Rload with an LED string, as shown below. VIN LM317L VOUT The following circuit takes care of this issue by splitting up the current sense resistor into three resistors, one for each leg of the LED string. The current through any one of the LED strings is set by Equation 2: IOUT Rsense IOUT + LED1 Vadj 1.25 V ) Vsat (Approximately) R1 (eq. 2) LED2 LED3 Figure 2. Semiconductor Components Industries, LLC, 2003 January, 2003 – Rev. 0 1 Publication Order Number: AND8109/D AND8109/D VIN VOUT LM317L Vadj R4 Rsense1 Q1 R5 Rsense2 MPS2222 Q2 R6 MPS2222 Q3 MPS2222 Rsense3 LED1 LED4 LED7 LED2 LED5 LED8 LED3 LED6 LED9 Figure 4. the sense resistor for that leg won’t have any voltage across it, turning ‘off’ the transistor and disconnecting its sense resistor from the Vadj pin. Therefore, the other two LED strings are unaffected by the fault. This same scheme can be expanded to accommodate as many LED strings as needed. When the circuit operates properly and all the LEDs are running, the three sense resistors have about 1.25 V across them, which turns the transistor switches ‘on’. This connects all three sense resistors back to the Vadj pin allowing the proper current to go through each leg. If one string opens up, ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. “Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. SCILLC does not convey any license under its patent rights nor the rights of others. SCILLC products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the SCILLC product could create a situation where personal injury or death may occur. Should Buyer purchase or use SCILLC products for any such unintended or unauthorized application, Buyer shall indemnify and hold SCILLC and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that SCILLC was negligent regarding the design or manufacture of the part. SCILLC is an Equal Opportunity/Affirmative Action Employer. PUBLICATION ORDERING INFORMATION Literature Fulfillment: Literature Distribution Center for ON Semiconductor P.O. Box 5163, Denver, Colorado 80217 USA Phone: 303–675–2175 or 800–344–3860 Toll Free USA/Canada Fax: 303–675–2176 or 800–344–3867 Toll Free USA/Canada Email: [email protected] JAPAN: ON Semiconductor, Japan Customer Focus Center 2–9–1 Kamimeguro, Meguro–ku, Tokyo, Japan 153–0051 Phone: 81–3–5773–3850 Email: [email protected] ON Semiconductor Website: http://onsemi.com For additional information, please contact your local Sales Representative. N. American Technical Support: 800–282–9855 Toll Free USA/Canada http://onsemi.com 2 AND8109/D