LM2794EVAL

User's Guide SNVA047B – February 2002 – Revised April 2013 AN-1218 LM2791/2/4/5 Evaluation Board 1 Introduction The LM2791, LM2792, LM2794 and LM2795 are a family of CMOS Current Regulated Switched Capacitor. They are designed to drive white (or blue) LEDs with matched currents to produce balanced light sources for display backlights. The LM2791/2 is offered in a 10 pin (QFN) package. The LM2791/2 is a charge-pump voltage doubler that provides two regulated current sources. The LM2791 delivers up to a total of 36mA through the LEDs with an offset voltage of 200mV at the Iset pin. The LM2792 delivers up to 34mA with zero offset at the Iset to provide fully off to maximum current control. The switching frequency is between 450KHz to 850KHz for the LM2791 and 900KHz to 1800KHz for LM2792. Both devices accept an input voltage range of 3V to 5.8V. The LM2791 and the LM2792 are also available in active high and low shutdown versions. The LM2794/5 is offered in a 14 bump DSGBA package. LM2794/5 is a fractional charge pump that provides four regulated current sources. The devices deliver up to 80mA with an offset voltage of 188mV at the Iset pin. The switching frequency for both device are 325kHz to 675kHz and the input voltage range is 2.7V to 5.5V. Note that if VIN is greater than 4.5V, the device will switch from charge pump mode to pass mode. During pass mode, the device output (POUT) will follow VIN. This is done to prevent POUT from exceeding the package voltage rating if VIN is greater than 4.5V. The LM2794 option offers active low for shutdown while the LM2795 offers active high for shutdown. All trademarks are the property of their respective owners. SNVA047B – February 2002 – Revised April 2013 Submit Documentation Feedback AN-1218 LM2791/2/4/5 Evaluation Board Copyright © 2002–2013, Texas Instruments Incorporated 1 Typical Application Circuit www.ti.com 2 Typical Application Circuit 3 RSET Selection (LM2791/2) RSET is the resistor that sets the current through both LEDs. It is left intentionally for the end users to set the desired current range. Equations below are used for determining RSET value: Eq. 1 for LM2792: RSET = ((0.42 • BRGT) / ILED) •25 Eq. 2 for LM2791: RSET = (((0.42 • BRGT) + 0.200)/ ILED) •25 (1) (2) For example, if 15mA is the desired current per LED and BRGT = 3V, using the above equation 1 yields RSET = 2.1K ohms. Table 1 below shows a typical LED current when BRGT is at 3V(LM2792) maximum and Table 2 shows a typical LED current when BRGT is at 0V(LM2791). If BRGT voltage is other than 3V, Table 5 shows RSET and BRGT combination per LED current. 2 AN-1218 LM2791/2/4/5 Evaluation Board SNVA047B – February 2002 – Revised April 2013 Submit Documentation Feedback Copyright © 2002–2013, Texas Instruments Incorporated RSET Selection (LM2794/5) www.ti.com Table 1. RSET when BRGT = 3V (example given for LM2792) ILED (1) RSET (1) 15mA 2.1kΩ 10mA 3.15kΩ 5mA 6.3kΩ Use 1% resistor for Rset Table 2. RSET when BRGT = 0V (example given for LM2791) ILED (1) 4 RSET (1) 15mA 330Ω 10mA 500Ω 5mA 1KΩ Use 1% resistor for Rset RSET Selection (LM2794/5) Similar to the LM2791/2, RSET is left intentionally for the end users to set the desired current range. Below is the equation for determining RSET value: Eq. 3 for LM2794/5: RSET = ((0.188 + (0.385 • BRGT)) / ILED) •10 (3) For example, if 15mA is the desired current per LED and BRGT = 0V (or ground), using the above equation 3 yields RSET = 124 ohms. Table 3 below shows typical LED current when BRGT is tied to ground and Table 4 shows typical LED current when BRGT is at 3V maximum. If BRGT is used in the application, Table 5 shows RSET and BRGT combination per LED current. Table 3. RSET when BRGT = 0V ILED (1) RSET (1) 15mA 124Ω 10mA 196Ω 5mA 383Ω Rset value is rounded off to the nearest 1% value Table 4. RSET when BRGT = 3V ILED (1) 5 RSET (1) 15mA 909Ω 10mA 1.4KΩ 5mA 2.67Ω Rset value is rounded off to the nearest 1% value BRGT (LM2791/2) A voltage from 0 to 3V may be applied to the BRGT pin to control the brightness of the LEDs by varying the current through the LEDs. Note that some voltage must be provided at BRGT pin or no current will flow through the LEDs for the LM2792. BRGT pin is connected to an internal resistor divider that gives a factor of 0.42 (LM2792). The product of this factor and the voltage at BRGT is fed to the input of an internal amplifier that sets the current mirror resistor RSET. Table 5 shows the relationship between LED current with various R SET and BRGT values. Care must be taken to ensure that the voltage at BRGT does not cause LEDs current to exceed a total of 34mA (LM2792). Note that calculations are based on when both D1 and D2 are in use. SNVA047B – February 2002 – Revised April 2013 Submit Documentation Feedback AN-1218 LM2791/2/4/5 Evaluation Board Copyright © 2002–2013, Texas Instruments Incorporated 3 BRGT (LM2794/5) www.ti.com Table 5. LED Current When Using BRGT Input (Example for LM2792 & both D1 & D2 are in use) Voltage on BRGT (V) RSET 1000Ω 1500Ω 2000Ω 2500Ω Current through LED (mA) 0 0 0 0 0 0.5 5.25 3.5 2.6 2.1 1.0 10.05 7 5.25 4.2 1.5 15.75 10.5 7.88 6.3 2.0 21 14 10.5 8.4 2.5 26.25 17.5 13.1 10.5 3.0 31.5 21 15.75 12.6 (Values Highlighted in Boldface exceed maximum current range of the device if both D1 & D2 are in use) By rearranging equation 1, the following equation can be used to determine ILED: Eq. 4 for LM2792: ILED = ((0.42 • BRGT) / RSET) •25 Eq. 5 for LM2791: ILED = (((0.42) + 0.200) • BRGT) / RSET) •25 6 (4) (5) BRGT (LM2794/5) A voltage from 0 to 3V may be applied to the BRGT pin to control the brightness of the LEDs by varying the current through the LEDs. BRGT pin is connected to an internal resistor divider and summed with an offset voltage from the internal bandgap (188mV). This voltage is fed to the input of an internal amplifier that sets the current mirror resistor RSET. Table 6 below shows the relationship between LED current with various R SET and BRGT values. Care must be taken to ensure that the voltage at BRGT does not cause LEDs current to exceed total of 80mA. By rearranging equation 3, the following equation can be used to determine the ILED: Eq. 6: ILED = ((0.188 + (0.385 • BRGT)) / RSET ) •10 (6) Table 6. LED Current When Using BRGT Input (D1-D4 are active) Voltage on BRGT (V) RSET 124Ω 500Ω 900Ω 1750Ω Current through LED (mA) 0 15.16 3.76 2.09 1.07 0.5 1.0 30.69 7.61 4.23 2.17 46.21 11.46 6.37 3.27 1.5 61.73 15.31 8.51 4.37 2.0 77.26 19.16 10.64 5.47 2.5 92.78 23.01 12.78 6.57 3.0 101.88 26.86 14.92 7.67 NOTE: Values Highlighted in Boldface exceed Maximum curent range of the device if all D1-D4 are in use. Besides adjusting the BRGT pin to control the brightness of the LEDs, SD pin can also be used to control the brightness by applying a PWM signal at the SD pin and varying the duty cycle. A duty cycle of zero will turn off the device and a 50% duty cycle waveform will produce an average current of 7.5mA if the intended LED(s) current is 15mA. 4 AN-1218 LM2791/2/4/5 Evaluation Board SNVA047B – February 2002 – Revised April 2013 Submit Documentation Feedback Copyright © 2002–2013, Texas Instruments Incorporated Shutdown (LM2791/2) www.ti.com 7 Shutdown (LM2791/2) During normal operation, SD pin is connected to VIN (for LM2792LD-H) or connected to ground (for LM2792LD-L). If SD pin is used to control the brightness instead of the BRGT pin, applying a pulse width modulation (PWM) signal in the range of 100Hz to 1KHz is recommended for best result. In the case of the LM2792, connect BRGT to 3V before applying a PWM signal at SD or connect BRGT pin to GND if LM2791 is used), 8 Shutdown (LM2794/5) During normal operation, SD pin is connected to VIN (for LM2794) or connected to ground (for LM2795). If SD pin is used to control the brightness instead of the BRGT pin, a PWM signal in the range of 100Hz to 1KHz is recommended. Table 7. Components List for LM2791/2 9 Component Name Type Value Size Manufacturer U1 LM2791/2 LM2791/2 WSON-10 Texas Instruments CIN X7R C2012X7R1C105K, 16V uF 0805 TDK C1 X7R C2012X7R1C105K, 16V uF 0805 TDK CHOLD X7R C2012X7R1C105K, 16V uF 0805 TDK RSET Through hold Value to be determined by the end users, +/-1% White LEDs Surface Mount Device, White LEDs (TOPLED) LWT67C/LWT673 Schottky Diode Surface Mount Device BAT-54 =4P Osram SOT23-3 Fairchild LM2791/2 Evaluation Board Figure 1. Silksreen SNVA047B – February 2002 – Revised April 2013 Submit Documentation Feedback AN-1218 LM2791/2/4/5 Evaluation Board Copyright © 2002–2013, Texas Instruments Incorporated 5 LM2791/2 Evaluation Board www.ti.com Figure 2. PCB Layout Table 8. Component List for LM2794/5 6 Component Name Type Value Size U1 LM2794/5 LM2794 --- Active Low Shutdown LM2795 --- Active High Shutdown 14 Bump DSBGA CIN X7R C2012X7R1C105K, 16V uF 0805 TDK C1 X7R C2012X7R1C105K, 16V uF 0805 TDK 0805 TDK SOT23-3 Osram CHOLD X7R C2012X7R1C105K, 16V uF RSET Through hold Value to be determined by the end uers, +/-1% Diode1 - 4 Surface Mount Device, White LEDs (TOPLED) LWT67C/LWT673 AN-1218 LM2791/2/4/5 Evaluation Board Manufacturer SNVA047B – February 2002 – Revised April 2013 Submit Documentation Feedback Copyright © 2002–2013, Texas Instruments Incorporated LM2794/5 Evaluation Board www.ti.com 10 LM2794/5 Evaluation Board Figure 3. Silksreen Figure 4. PCB Layout SNVA047B – February 2002 – Revised April 2013 Submit Documentation Feedback AN-1218 LM2791/2/4/5 Evaluation Board Copyright © 2002–2013, Texas Instruments Incorporated 7 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. All semiconductor products (also referred to herein as “components”) are sold subject to TI’s terms and conditions of sale supplied at the time of order acknowledgment. TI warrants performance of its components to the specifications applicable at the time of sale, in accordance with the warranty in TI’s terms and conditions of sale of semiconductor products. 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