LM27961TLEV

Application Report SNVA105A – June 2005 – Revised April 2013 AN-1362 LM27961- Dual-Display White LED Driver with 3/2x Switched Capacitor Boost ..................................................................................................................................................... 1 Schematic This application note describes how to operate the LM27961 Evaluation Module. C1 1PF VIN 2.7V to 5.5V C1+ ENB + - - C2 1PF C1- VIN ENA + C2+ C2- ENA ENB POUT 3/2× Charge Pump (1× when VIN > 4.7V) CIN LEDA_ON CPOUT LM27961 1PF* 1PF* D1A D2A D3A ISETA D1B D2B D4A D3B Capacitors: TDK C1608X5R1A105K, or equivalent ISETB GND RSETB RSETA LEDB_ON 2 Bill of Materials Table 1. Bill of Materials Component Symbol Value Package [U.S. (Metric)] Dimensions (mm) Temperature Characteristic Manufactrurer Part # LM27961 -- YZR-18 DSBGA 2.1 x 2.4 x 0.6 -- TI LM27961 Cin, Cout, C1, C2 1µF, 10V 0603 (1608) 1.6 x 0.8 x 0.8 X5R TDK C1608X5R1A105K Dxx White LEDs -- 1.5 x 2.3 x 1.4 -- OSRAM LWM67C-T1U1-3C5D Rset 8.3kΩ 0603 (1608) 1.6 x 0.8 x 0.8 -- Vishay-Dale CRCW06048251F Rset' none -- -- -- -- -- All trademarks are the property of their respective owners. SNVA105A – June 2005 – Revised April 2013 Submit Documentation Feedback AN-1362 LM27961- Dual-Display White LED Driver with 3/2x Switched Capacitor Boost Copyright © 2005–2013, Texas Instruments Incorporated 1 LM27961 Evaluation Board Layout 3 www.ti.com LM27961 Evaluation Board Layout Figure 1. Top Layer Figure 2. Bottom Layer (top view, unmirrored) 4 Board Operation 4.1 Basic Connections To operate the LM27961 evaluation board, connect a supply voltage (2.7V-5.5V) between board connectors VIN and GND. Default Jumper Connections: • ENA: Connects the “+” post to the middle post of the ENA header strip. This connects VIN to the ENA pin of the LM27961, enabling the part and the D1A-D4A outputs. • ENB: Connects the “-” post to the middle post of the ENB header strip. This connects GND to the ENB pin of the LM27961, disabling D1B-D3B outputs. 2 AN-1362 LM27961- Dual-Display White LED Driver with 3/2x Switched Capacitor Boost Copyright © 2005–2013, Texas Instruments Incorporated SNVA105A – June 2005 – Revised April 2013 Submit Documentation Feedback Board Operation www.ti.com • • ENB: Connects the “-” post to the middle post of the ENB header strip. This connects GND to the ENB pin of the LM2796, disabling D1B-D3B outputs. LEDx_ON: Jumper connects the two posts of the LEDx_ON header strips. LEDA_ON connects the anodes of all 4 DxA LEDs to POUT, establishing the LED current path. LEDB_ON connects the cathodes of all 3 DxB LEDs to GND, establishing the LED current path. When these connections are all made correctly, the main bank of LEDs will be ON (D1A-D4A). The secondary bank of 3 LEDs will be OFF. 4.2 RSET Setting LET Currents The resistance of the RSET resistor sets the DC output currents of the LM27961 according to the following equation: IDxx = 100 X (1.25V / RSET) (typical) (1) The default RSET on the evaluation board is 8.3kΩ, resulting in a typical DC output current of 15mA. Component RSET’ is an optional leaded (axial) resistor replacement for the surface mount RSET. 4.3 EN, ENA, AND ENB Headers: LED Activation and PWM Brightness Adjustment The header strips ENA and ENB can be used to enable/disable the LM27961 and/or the output currents. The connections to the ENx pins provided by these posts can also be used to connect pulse-width modulated (PWM) signals to the LM27961 in order to adjust the average brightness of the LEDs. On each of these header strips, the post labeled “+” is connected to VIN. The post labeled “-“ is connected to GND. The middle post connects to ENA and ENB, respectively. Jumpers can be used to connect each ENx pin to either VIN or GND. Connecting ENA or ENB to VIN enables the charge pump and other internal circuitry of the LM27961. Connecting both ENA and ENB to GND places the part in Shutdown mode. Connecting ENA to VIN enables the D1A-D4A LEDs. Connecting ENA to GND disables these LEDs. Similarly, connecting ENB to VIN enables the D1B-D3B LEDs, and connecting ENB to GND disables the DxB LEDs. Connecting a pulse signal to the ENA and/or ENB pins can be used to adjust the brightness of each bank of LEDs. The duty cycle of the pulse signal determines the net brightness, as perceived by the human eye. For example, with a duty cycle of 50%, the LEDs will only be ON for 50% of the time, and the perceived brightness will be approximately half of what the brightness is when the output current flows continuously through the LEDs. Recommended frequency range for PWM signals: 100Hz to 1kHz. 4.4 Using the LEDS on Headers to Measure Output Currents By removing the LEDx_ON jumpers, LM27961 output currents can easily be measured. Removing the jumpers disconnects the anodes (DxA) and cathodes (DxB) of all LEDs from POUT (DxA) or GND (DxB), breaking the LED current paths. By placing a current meter between the two header pins the sum total of all LED currents can be measured. With the LEDx_ON jumpers removed, the current of an individual output can be measured by placing a current meter between a DxA header and POUT or DxB and GND. With such a connection, the voltage on pin Dxx will be almost 0V because the series resistance of the current meter is likely to be quite small. Since the regulated output currents of the LM27961 are almost completely independent of Dxx pin voltage (provided VDxx is not too high for regulation to be achieved), this measurement will still be quite accurate. For an even more precise measurement, however, a resistor or LED can be placed in series with the current meter so that the voltage at pin Dxx more closely resembles the expected forward voltage of the LED in the normal application configuration. SNVA105A – June 2005 – Revised April 2013 Submit Documentation Feedback AN-1362 LM27961- Dual-Display White LED Driver with 3/2x Switched Capacitor Boost Copyright © 2005–2013, Texas Instruments Incorporated 3 IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest issue. Buyers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. 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