AN-1318| Application Note

LM3501 Evaluation Board

National Semiconductor Application Note 1318 Clinton Jensen February 2004

Introduction
The LM3501 is a synchronous step-up DC/DC converter designed for white LED applications. The evaluation board is set up to drive 3 standard VF white LEDs from a single Li-Ion battery. A fourth LED pad (D4) is included for testing of up to 4 low VF white LEDs. The LED current is set using the resistor R2 and the equation ILED = VFB/R2. VFB is determined by the voltage applied to the CNTRL pin. The maximum value of VFB is 0.515V and

occurs with a CNTRL voltage of approximately 2.7V. This condition translates into approximately 20mA LED current. A lower CNTRL voltage translate into a lower VFB voltage and a lower LED current. The feedback voltage can be calculated using the equation VFB = 0.191*CNTRL. A CNTRL voltage of 125mV or greater turns the LEDs on while a CNTRL voltage of 75mV or less turns the LEDs off. Please note that for correct operation the SHDN pin must have a voltage greater than 1.1V applied to it.

Schematic

20097001

Bill of Materials
Designator U1 L CIN COUT D1 D3 D4 R1 Component LM3501TL-ADJ, uSMD 8-lead 22 uH, DT1608C-223 1 uF, 25V Ceramic C3216X7R1E105K 1 uF, 25V Ceramic C3216X7R1E105K White LED, LWT67C 0 Resistor, 1206 Case, CRCW1206 000J 24, 1206 Case, CRCW1206 240J Manufacturer National Semiconductor Coilcraft TDK TDK Osram Vishay Vishay

AN-1318

2004 National Semiconductor Corporation

AN200970

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