LM2792

LM2792 Current Regulated Switched Capacitor LED Driver with Analog Brightness Control July 2002 LM2792 Current Regulated Switched Capacitor LED Driver with Analog Brightness Control General Description The LM2792 is a CMOS charge-pump voltage doubler and regulator that provides two regulated current sources. They are designed to drive two white (or blue) LEDs with matched currents (within ± 0.3%) to produce balanced light sources for display backlights. The LM2792 accepts an input voltage range from 3.0V to 5.8V and maintain a constant current determined by an external set resistor. The LM2792 delivers up to 34mA of load current to accommodate two high forward voltage (typically white) LEDs. The switching frequency is 900kHz (min.) to keep the conducted noise spectrum away from sensitive frequencies within portable RF devices. The LM2792 offers full off to maximum current control through the BRGT pin. The output current linearly tracks the BRGT pin voltage. The LM2792 is available in active high or low shutdown versions. The shutdown pin reduces the operating current to 1µA (max.). The LM2792 is available in a 10 pin leadless leadframe (LLP) CSP package. Features n n n n n n n n n n n n n Output matching of ± 0.3% (typ.) Drives up to two LED’s 3.0V to 5.8V Input Voltage Up to 34mA output current Soft start limits inrush current Analog brightness control Separate shutdown input Very small solution size and no inductor 1.4mA typical operating current 1µA (max.) shutdown current 900kHz switching frequency (min.) Linear regulation generates predictable noise spectrum LLP-10 package: 3mm X 3mm X 0.8mm Applications n White LED Display Backlights n White LED Keypad Backlights n 1-Cell Li-Ion battery-operated equipment including PDAs, hand-held PCs, cellular phones n Flat Panel Dispalys Basic Application Circuit 20024201 © 2002 National Semiconductor Corporation DS200242 www.national.com LM2792 Connection Diagram 20024203 Top View 10-Lead LLP Ordering Information Order Number LM2792LD-L LM2792LDX-L LM2792LD-H LM2792LDX-H Shutdown Polarity Active Low Active Low Active High Active High NSC Package Drawing LLP-10 LLP-10 LLP-10 LLP-10 Package Marking SRB SRB SPB SPB Supplied As 1000 Units, Tape and Reel 4500 Units, Tape and Reel 1000 Units, Tape and Reel 4500 Units, Tape and Reel www.national.com 2 LM2792 Pin Description Pin 1 2 3 4 5 6 7 8 9 10 Name BRGT POUT C1− C1+ D2 D1 GND VIN SD/SD ISET Charge pump output. Connect this pin to the negative terminal of C1. Connect this pin to the positive terminal of C1. Current source outputs. Connect directly to LED. Current source outputs. Connect directly to LED. Power supply ground input. Power supply voltage input. Shutdown input. Device operation is inhibited when pin is asserted. Current Sense Input. Connect resistor to ground to set constant current through LED. Function Variable voltage input controls output current. Block Diagram 20024202 3 www.national.com LM2792 Absolute Maximum Ratings (Note 1) Human Body Model Machine Model 2KV 200V If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN BRGT, SD Power Dissipation (Note 2) TJMAX (Note 2) θJA (Note 7) Storge Temperature Lead Temp. (Soldering, 5 sec.) ESD Rating −0.3 to 6.0V −0.3 to (VIN +0.2V) 400 mW 150˚C 55˚C/W −65˚C to +150˚C 260˚C Operating Conditions Input Voltage (VIN) BRGT Ambient Temperature (TA) 34mAOperating Junction Temperature 3.0V to 5.8V 0 to 3.0V −30˚C to +85˚C −30˚C to 100˚C Electrical Characteristics Limits in standard typeface are for TJ = 25˚C and limits in boldface type apply over the full Operating Temperature Range. Unless otherwise specified, C1 = CIN = CHOLD = 1 µF, VIN = 3.6V, BRGT pin = 2.75V. Symbol IDX Parameter Available Current at Output Dx Conditions 3.0V ≤ VIN ≤ 5.8V VDx ≤ 3.6V RSET = 1650Ω VBRGT = 3V 3.0V ≤ VIN ≤ 4.4V VDx ≤ 3.6V RSET = 1800Ω VBRGT = 2.75V VIN = 3.6V VDx = 3.0V VDx = 4.0V RSET = 1800Ω VBRGT = 2.75V VD1 = VD2 = 3.6V, BRGT = 2.75V, VIN = 3.6V, RSET = 1800Ω 3.0V ≤ VIN ≤ 4.4V, Active, No Load Current 3.0V ≤ VIN ≤ 5.5V, Shutdown At 85˚C 3.0V ≤ VIN ≤ 5.5V, Note5 3.0V ≤ VIN ≤ 5.5V, Note5 0V ≤ VSD ≤ VIN 0 IDx/25 3.0V ≤ VIN ≤ 4.4V ID1 = ID2 ≤ 16mA IDx = 90% steady state 900 1100 10 1800 0.01 250 3.0 0.8* VIN 0.2* VIN Min 14.5 Typ 16.8 Max Units mA IDX Line Regulation of Dx Output Current Load Regulation of Dx Output Current 13.5 16 17.8 mA VDX mA 16.1 15.4 0.3 % ID-MATCH Current Matching Between Any Two Outputs Quiescent Supply Current Shutdown Supply Current SD Input Logic High SD Input Logic Low SD Input Leakage Current BRGT Input Resistance Brightness Voltage Range ISET Pin Output Current Switching Frequency (Note 4) Startup Time (Note 6) IQ ISD VIH VIL ILEAK-SD RBRGT BRGT ISET fSW tSTART 1.4 0.1 0.3 2.7 1 mA µA V V µA kΩ V mA kHz µs Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. Electrical specifications do not apply when operating the device beyond its rated operating conditions. Note 2: D1 and D2 may be shorted to GND without damage. POUT may be shorted to GND for 1sec without damage. Note 3: In the test circuit, all capacitors are 1.0µF, 0.3Ω maximum ESR capacitors. Capacitors with higher ESR will increase output resistance, reduce output voltage and efficiency. Note 4: The output switches operate at one half of the oscillator frequency, fOSC = 2fSW. Note 5: The internal thresholds of the Shutdown bar are set at about 40% of VIN Note 6: This electrical specification is quarantee by design Note 7: For more information regarding the LLP package, please refer to National Semiconductor Application note AN1187 www.national.com 4 LM2792 Typical Performance Characteristics 3.6V, BRGT pin =2.75V Input Supply Current vs. VIN ( ID1=ID2=16mA) Unless otherwise specified, C1 = CIN = CHOLD = 1uF, VIN= IDIODE vs. VIN 20024225 20024224 Shutdown Threshold vs. VIN IDIODE vs. Temperature 20024219 20024207 IQ(SHUTSOWN) vs. Temperature IDIODEvs. RSET 20024210 20024213 5 www.national.com LM2792 Typical Performance Characteristics Unless otherwise specified, C1 = CIN = CHOLD = 1uF, VIN= 3.6V, BRGT pin =2.75V (Continued) IDIODE vs. BRGT Switch Frequency vs. Temperature 20024216 20024211 VSET vs. VBRGT IDIODE vs. VDIODE 20024226 20024227 www.national.com 6 LM2792 Circuit Description The LM2792 provides two matched current sources for driving high forward voltage drop LEDs from Li-Ion battery sources. The part has on-chip current regulators which are composed of current mirrors with a 25 to 1 ratio. The mirrors control the LED current without using current limiting resistors in the LED current path. The device can drive up to 34mA through the Leds. The LED brightness can be controlled by both analog and/or digital methods. The digital technique uses a PWM (Pulse Width Modulation) signal applied to the shutdown input. The analog technique applies an analog voltage to the brightness (BRGT) pin (see Application Information section). 20024204 FIGURE 1. Application Information SOFT START LM2792 includes a soft start function to reduce the inrush currents and high peak current during power up of the device. Soft start is implemented internally by ramping the bandgap more slowly than the applied voltage. This is done by holding the bandgap in shutdown for a short time. During soft start, the switch resistances limit the inrush current used to charge the flying and hold capacitors. SHUTDOWN MODE A shutdown pin (SD or SD) is available to disable the LM2792 and reduce the quiescent current to 1µA maximum. The LM2792 is available with both senses of shutdown polarity. During normal operation mode of the ’-L’ options, an active high logic signal to the SD pin or tying the SD pin to VIN, will enable the device. Pulling SD low or connecting SD to ground will disable the device. During normal operation mode of the ’-H’ options, an active low logic signal to the SD pin or tying the SD pin to GND, will enable the device. Pulling SD high or connecting SD to VIN will disable the device. CAPACITOR SELECTION Low equivalent series resistance (ESR) capacitors such as X5R or X7R are recommended to be used for CIN, C1, and CHOLD for best performance. Ceramic capacitors with less than or equal to 0.3 ohms ESR value are recommended for this application. Table 1 below lists suggested capacitor suppliers for the typical application circuit. TABLE 1. Low ESR Capacitor Manufactures Manufacturer TDK MuRata Taiyo Yuden Contact (800) 831 9172 (800) 348 2496 website www.murata.com www.t-yuden.com (847) 803 6100 www.component.tdk.com SCHOTTKY DIODE SELECTION A schottky diode (SD1) must be used between VIN and POUT for proper operation. During start-up, the low voltage drop across this diode is used to charge COUT and start the oscillator. It is necessary to protect the device from turning-on its own parasitic diode and potentially latching-up. As a result, it is important to select a schottky diode that will carry at least 200mA or higher current to charge the output capacitor during start-up. A schottky diode like 1N5817 can 7 www.national.com LM2792 Application Information (Continued) be used for most applications or a surface mount diode such as BAT54-series and MA2J704 can be used to reduce the circuit size.Table 2 below lists suggested schottky diode manufactures. TABLE 2. Diode Manufactures Manufacturer ON Semiconductor Phillips Semiconductors Panasonic Semiconductors Contact (800) 344 3860 (800) 234 7381 (408) 945 5622 website www.onsemi.com www.philipssemiconductor.com www.panasonic.com BRGT PIN The BRGT pin can be used to smoothly vary the brightness of the LEDs. In the LM2792, current on BRGT is connected to an internal resistor divider which gives a factor of 0.42 (see Figure 1) . This voltage is fed to the operational amplifier that controls the current through the mirror resistor RSET. The nominal range on BRGT is 0V to 3V. This means some current must be provided on the BRGT pin or no current will flow through the LEDs. The LM2792 can provide an infinite ratio, from fully off (essentially zero current) to the maximum current set by the RSET resistor. Care must be taken to prevent voltages on BRGT that cause LED current to exceed a total of 34mA. Although this will not cause damage to the IC, it will not meet the guaranteed specifications listed in the Electrical Characteristics. Calculation of LED Current When Using BRGT : VIN = 3.6V RSET = 1800Ω ISET = ((VBRGT * (0.42) / RSET )* 25 ISET = ((2.75*(0.42)) / 1800 )*25 = 16mA Note that making VBRGT = 0V results in ISET ∼ = 0mA BRIGHTNESS CONTROL USING PWM Brigthness control can be implemented by pulsing a signal at the SD pin. The recommended signal should be between 100Hz to 1kHz. If the operating PWM frequency is much less than 100Hz, flicker may be seen in the LEDs. Likewise, if frequency is much higher, brightness in the LEDs will not be linear. When a PWM signal is used to drive the SD pin of the LM2792, connect BRGT pin to a maximun of 3V to ensure the widest range. Similarly, the voltage at the BRGT pin can be set higher than 3V without damage to the IC, it will not increase the brigthness of the LED significantly. RSET value is selected using the above I SET equation as if BRGT pin is used. The brigthness is controlled by increasing and decreasing the duty cycle of the PWM. Zero duty cycle will turn off the brigthness and a 50% duty cycle waveform produces an average current of 7.5mA if RSET is set to produce a maximum LED current of 15mA. So the LED current varies linearly with the duty cycle. PARALLEL DX OUTPUTS FOR INCREASED CURRENT DRIVE Outputs D1 and D2 may be connected together to drive a single LED. In such a configuration, two parallel current sources of equal value drive the single LED. RSET and VBRGT should be chosen so that the current through each of the outputs is programmed to 50% of the total desired LED current. For example, if 30mA is the desired drive current for the single LED, RSET and VBRGT should be selected so that the current through each of the outputs is 15mA. Connecting the outputs in parallel does not affect internal operation of the LM2792 and has no impact on the Electrical Characteristics and limits previously presented. The available Dx output current, maximum Dx voltage, and all other specifications provided in the Electrical Characteristics table apply to this parallel output configuration, just as they do to the standard 2-LED application circuit. THERMAL PROTECTION The LM2792 has internal thermal protection circuitry to disable the charge pump if the junction temperature exceeds 150˚C. This feature will protect the device from damage due to excessive power dissipation. The device will recover and operate normally when the junction temperature falls below the maximum operating junction temperature of 100˚C. It is 8 LED SELECTION The LM2792 is designed to drive LEDs with a forward voltage of about 3.0V to 4.0V or higher. The typical and maximum VF depends highly on the manufacturer and their technology. Table 3 lists two suggested manufactures and example part numbers. Each supplier makes many LEDs that work well with the LM2792. The LEDs suggested below are in a surface mount package and TOPLED or SIDEVIEW configuration with a maximum forward current of 20mA. These diodes also come in SIDELED or SIDEVIEW configuration and various chromaticity groups. For applications that demand color and brigthness matching, care must be taken to select LEDs from the same chromaticity group. Forward current matching is assured over the LED process variations due to the constant current output of the LM2792. For best fit selection for an application, consult the manufacturer for detailed information. TABLE 3. White LED Selection: Component LWT673/LWT67C Manufacture Contact Osram www.osram-os.com www.nichia.com NSCW100/NSCW215 Nichia ISETPIN An external resistor, RSET, sets the mirror current that is required to provide a constant current through the LEDs. The current through RSET and the LED is set by the internal current mirror circuitry with a ratio of 25:1 The currents through each LED are matched within 0.3%. RSET should be chosen not to exceed the maximum current delivery capability of the device. Table 3 shows a list of RSET values when maximun BRGT=3V is applied. For other BRGT voltages, RSET can be calculated using this formula: RSET = ((0.42*BRGT) / ISET)*25 TABLE 4. RSETSelections ( when BRGT pin = 3V maximum) ILED per LED 15mA 10mA 5mA RSET 2.1KΩ 3.15KΩ 6.3KΩ www.national.com LM2792 Application Information (Continued) important to have good thermal conduction with a proper layout to reduce thermal resistance. POWER EFFICIENCY An ideal power efficiency for a voltage doubler switched capacitor converter is given as the output voltage of the doubler over twice the input voltage as follows: Efficiency = (VDIODE* IDIODE) / ( VIN * IDIODE* Gain) = VDIODE / 2VIN In the case of the LM2792, a more accurate efficiency calculation can be applied as the given formula below. Efficiency = ((VD1* ID1) + (VD2* ID2)) / (ISUPPLY* VIN) It is clear that the efficiency will depend on the supply voltage in the above equation. As such, the lower the supply voltage, the higher the efficiency. POWER DISSIPATION The maximum allowable power dissipation that this package is capable of handling can be determined as follows: PDMax = (TJMax - TA) / θJA where TJMax is the maximum junction temperature, TA is the ambient temperature, and θJA is the junction-to-ambient thermal resistance of the specified package. The actual power dissipation of the device can be calculated using this equation: PDissipation = (2VIN -VDIODE)*ILOAD As an example, if VIN in the target application is 4.2V, VDIODE = 3.0V and worse case current consumption is 32mA (17mA for each diode). PDissipation = ((2*4.2) -3.0)*0.032 = 173mW Power dissipation must be less than that allowed by the package. Please refer to the Absolute Maximum Rating of the LM2792. PCB LAYOUT CONSIDERATIONS The LLP is a leadframe based Chip Scale Package (CSP) with very good thermal properties. This package has an exposed DAP (die attach pad) at the center of the package measuring 2.0mm x 1.2mm. The main advantage of this exposed DAP is to offer lower thermal resistance when it is soldered to the thermal land on the PCB. For PCB layout, National highly recommends a 1:1 ratio between the package and the PCB thermal land. To further enhance thermal conductivity, the PCB thermal land may include vias to a ground plane. For more detailed instructions on mounting LLP packages, please refer to National Semiconductor Application Note AN-1187. 9 www.national.com LM2792 Current Regulated Switched Capacitor LED Driver with Analog Brightness Control Physical Dimensions inches (millimeters) unless otherwise noted LLP-10 Pin Package (LDA) For Ordering, Refer to Ordering Information Table NS Package Number LDA10A LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 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