LM2794/LM2795 Current Regulated Switched Capacitor LED Supply with Analog Brightness Control General Description Features The LM2794/95 is a fractional CMOS charge-pump and regulator that provides four regulated current sources. It accepts an input voltage range from 2.7V to 5.5V and maintains a constant current determined by an external sense resistor. The LM2794/5 delivers up to 80mA of load current to accommodate four White LEDs. The switching frequency is 325kHz. (min.) to keep the conducted noise spectrum away from sensitive frequencies within portable RF devices. Maximum operating current is 8.2mA (unloaded) and the maximum shutdown current is only 5µA. If not all output pins are used, leave pin(s) unconnected. Brightness can be controlled by both linear and PWM techniques. A voltage between 0V and 3.0V may be applied to the BRGT pin to vary the current over more than a 5 to 1 ratio. Output current will linearly track the voltage applied to the BRGT pin. Alternatively, a PWM signal can be applied to the SD pin to vary the perceived brightness of the LED. The SD pin reduces the operating current to 5µA (max.) The LM2794 uses an active-low shutdown level, and the LM2795 uses an active-high shutdown level. The LM2794/95 is available in a micro SMD-14 CSP package. n Regulated IOUT with ± 0.5% matching between any two outputs n High efficiency 3/2 boost function n Drives one, two, three or four white LEDs n 2.7V to 5.5V Input Voltage n Up to 80mA output current n Soft start limits inrush current n Analog brightness control n Active-low or high shutdown input (’94/95) n Very small solution size and no inductor n 5µA (max.) shutdown current n 325kHz switching frequency (min.) n Linear regulation generates predictable noise spectrum n micro SMD-14 package: 2.08mm X 2.403mm X 0.845mm 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 Displays Basic Application Circuit 20028503 © 2002 National Semiconductor Corporation DS200285 www.national.com LM2794/LM2795 Current Regulated Switched Capacitor LED Supply with Analog Brightness Control September 2002 LM2794/LM2795 Connection Diagram 20028523 Bottom View Ordering Information Order Number Shutdown Polarity Package Number Package Marking LM2794BL Active Low BLP14EHB I LOG Supplied As 250 Units, Tape and Reel LM2794BLX Active Low BLP14EHB I LOG 3000 Units, Tape and Reel LM2795BL Active High BLP14EHB I LOJ 250 Units, Tape and Reel LM2795BLX Active High BLP14EHB I LOJ 3000 Units, Tape and Reel Order Number Shutdown Polarity Package Number Package Marking Supplied As LM2794TL Active Low TLP14EHA I LOG 250 Units, Tape and Reel LM2794TLX Active Low TLP14EHA I LOG 3000 Units, Tape and Reel LM2795TL Active High TLP14EHA I LOJ 250 Units, Tape and Reel LM2795TLX Active High TLP14EHA I LOJ 3000 Units, Tape and Reel Pin Description Pin(*) Name A1 C1+ Function Positive terminal of C1 B2 C1− Negative terminal of C1 C1 VIN Power supply voltage input D2 GND Power supply ground input E1 C2− Negative terminal of C2 E3,E5,E7,D6 D1−4 Current source outputs. Connect directly to LED C7 ISET B6 BRGT A7 SD Current Sense Input. Connect 1% resistor to ground to set constant current through LED Variable voltage input controls output current Shutdown input. On LM2795, a high level inhibits device operation. Internal pull-up current source allows open drain drive. On LM2794, a low level inhibits device operation A5 C2+ Positive terminal of C2 A3 POUT Charge pump output (*) Note that the pin numbering scheme for the Micro SMD package was revised in April, 2002 to conform to JEDEC standard. Only the pin numbers were revised. No changes to the physical location of the inputs/outputs were made. For reference purpose, the obsolete numbering had C1+ as pin 1, C1- as pin 2, VIN as pin 3, GND as pin 4, C2- as pin 5, D1-D4 as pin 6,7,8 & 9, Iset as pin 10, BRGT as pin 11, SD as pin 12, C2+ as pin 13, Pout as pin 14 www.national.com 2 LM2794/LM2795 Block Diagram 20028502 3 www.national.com LM2794/LM2795 Absolute Maximum Ratings Storge Temperature (Note 1) Lead Temp. (Soldering, 5 sec.) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN −0.5 to 6.2V max SD −0.5 to (VIN+0.3V) w/ 6.2V max BRGT −0.5 to (VIN+0.3V) w/ 6.2V max Power Dissipation (TA = 25˚C (Note 2) TJMAX (Note 2) −65˚C to +150˚C 260˚C ESD Rating (Note 4) Human Body Model 2KV Machine Model 200V Operating Conditions Input Voltage (VIN) 400 mW 2.7V to 5.5V Ambient Temperature (TA) −30˚C to +85˚C Junction Temperature (TJ) −30˚C to +85˚C 135˚C θJA (Notes 2, 3) 125˚C/W 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 = C2 = CIN = CHOLD = 1 µF, VIN = 3.6V, BRGT pin = 0V; RSET =124Ω ; LM2794:VSD = VIN (LM2795: VSD = 0V). Symbol IDX Parameter Available Current at Output Dx Min Typ 3.0V ≤ VIN ≤ 5.5V VDX ≤ 3.8V BRGT = 50mV Conditions 15 16.8 2.7V ≤ VIN ≤ 3.0V VDX ≤ 3.6V BRGT = 0V 10 VDX ≤ 3.8V BRGT = 200mV 20 Max Units mA mA mA VDX Available Voltage at Output Dx 3.0V ≤ VIN ≤ 5.5V IDX ≤ 15mA BRGT = 50mV 3.8 IDX Line Regulation of Dx Output Current 3.0V ≤ VIN ≤ 5.5V VDX = 3.6V 14.18 15.25 16.78 mA 3.0V ≤ VIN ≤ 4.4V VDX = 3.6V 14.18 15.25 16.32 mA 14.18 15.25 16.32 mA V IDX Load Regulation of Dx Output Current VIN = 3.6V 3.0V ≤ VDX ≤ 3.8V ID-MATCH Current Matching Between Any Two Outputs VIN = 3.6V, VDX = 3.6V 0.5 IQ Quiescent Supply Current 3.0V ≤ VIN ≤ 4.2V, Active, No Load Current RSET = OPEN 5.5 8.2 mA ISD Shutdown Supply Current 3.0V ≤ VIN ≤ 5.5V, Shutdown 2.3 5 µA IPULL-SD Shutdown Pull-Up Current (LM2795) VIN = 3.6V 1.5 µA VCP Input Charge-Pump Mode To Pass Mode Threshold 4.7 V VCPH Input Charge-Pump Mode To Pass Mode Hysteresis (Note 5) 250 mV SD Input Logic High (LM2794) 3.0V ≤ VIN ≤ 5.5V VIH SD Input Logic High (LM2795) VIL SD Input Logic Low (LM2794) 1.0 V 0.8VIN 3.0V ≤ VIN ≤ 5.5V 0.2 SD Input Logic Low (LM2795) ILEAK-SD SD Input Leakage Current RBRGT BRGT Input Resistance www.national.com % V 0.2VIN 0V ≤ VSD ≤ VIN 4 100 nA 240 kΩ (Continued) 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 = C2 = CIN = CHOLD = 1 µF, VIN = 3.6V, BRGT pin = 0V; RSET =124Ω ; LM2794:VSD = VIN (LM2795: VSD = 0V). Symbol Parameter ISET ISET Pin Output Current fSW Switching Frequency (Note 6) Conditions Min Typ Max Units 675 kHz IDX/10 3.0V ≤ VIN ≤ 4.4V 325 515 mA 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, D2, D3 and D4 may be shorted to GND without damage. POUT may be shorted to GND for 1sec without damage. Note 3: The value of θJA is based on a two layer evaluation board with a dimension of 2in. x1.5in. Note 4: 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 5: Voltage at which the device switches from charge-pump mode to pass mode or pass mode to charge-pump mode. For example, during pass mode the device output (Pout) follows the input voltage. Note 6: The output switches operate at one eigth of the oscillator frequency, fOSC = 1/8fSW. 5 www.national.com LM2794/LM2795 Electrical Characteristics LM2794/LM2795 Typical Performance Characteristics Unless otherwise specified, C1 = C2 = CIN = CHOLD = 1µF, VIN = 3.6V, BRGT pin = 0V, RSET = 124Ω. Supply Current with Load ID 1-4 = 15mA Supply Current with No Load 20028515 20028514 Shutdown Supply Current Shutdown Threshold vs VIN 20028505 20028513 IDIODE vs VIN @ RSET = 1KΩ, 3KΩ BRGT =3V IDIODE vs Supply Voltage 20028507 20028512 www.national.com 6 = 3.6V, BRGT pin = 0V, RSET = 124Ω. (Continued) IDIODE vs BRGT IDIODE vs RSET 20028509 20028508 VSET vs VBRGT RSET = 1KΩ IDIODE vs VDIODE 20028506 20028524 Available Additional Current @ POUT ID 1− 4 = 15mA, RSET = 124 ohms Duty Cycle vs. Led Current (LM2794) ID 1- 4 = 15mA 20028532 20028531 7 www.national.com LM2794/LM2795 Typical Performance Characteristics Unless otherwise specified, C1 = C2 = CIN = CHOLD = 1µF, VIN LM2794/LM2795 Typical Performance Characteristics Unless otherwise specified, C1 = C2 = CIN = CHOLD = 1µF, VIN = 3.6V, BRGT pin = 0V, RSET = 124Ω. (Continued) Start-Up Response @ 3.6VIN (LM2794) Switching Frequency 20028518 20028516 Start-Up Response @ 3.6VIN (LM2795) Start-Up Response @ 2.7VIN (LM2794) 20028522 20028517 Start-Up Response @ 4.2VIN (LM2794) Start-Up Response @ 2.7VIN (LM2795) 20028519 www.national.com 20028520 8 = 3.6V, BRGT pin = 0V, RSET = 124Ω. (Continued) Start-Up Response @ 4.2VIN (LM2795) 20028521 9 www.national.com LM2794/LM2795 Typical Performance Characteristics Unless otherwise specified, C1 = C2 = CIN = CHOLD = 1µF, VIN LM2794/LM2795 Circuit Description The LM2794/5 employs a fractional charge-pump technique to step up the output voltage to 1.5 times the input voltage. The charge-pump provides the voltage that is needed by the four matched internal current sources to drive 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 10 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 a total of 80mA 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. Please refer to table 4 in the application information section for a quick reference table on BRGT voltage and RSET selections. Futhermore, the LM2794/5 can be used for constant brightness by grounding the BRGT pin . Functional Block Diagram 20028530 Application Information CAPACITOR SELECTION Low equivalent series resistance (ESR) capacitors such as X5R or X7R are recommended to be used for CIN, C1, C2, 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. SOFT START LM2794 includes a soft start function to reduce the inrush currents and high peak current during power up of the device. This is done to reduce stress on the LM2794/5 and external components. 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 LM2794/5 and reduce the quiescent current to 5µA maximum. During normal operation mode of the LM2794, applying 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 LM2795, applying 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. www.national.com TABLE 1. Low ESR Capacitor Manufactures 10 Manufacturer Contact website TDK (847) 803 6100 www.component.tdk.com MuRata (800) 831 9172 www.murata.com Taiyo Yuden (800) 348 2496 www.t-yuden.com 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 equation: (Continued) LED SELECTION The LM2794/5 are designed to drive LEDs with a forward voltage of about 3.0V to 3.8V. The typical and maximum VF depends highly on the manufacturer and the technology. Table 2 lists two suggested manufactures and example part numbers. Each supplier makes many LEDs that work well with the LM2794/5. 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 brightness 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 LM2794/5. For best fit selection for an application, consult the manufacturer for detailed information. RSET = ((0.188 + (0.385 • BRGT)) / ISET) • 10 TABLE 3. RSET Selections ( when BRGT pin = 3V maximum) Manufacture Contact LWT673/LWT67C Osram NSCW100/NSCW215 Nichia *RSET (+/-1%) 15mA 909Ω 10mA 1.4KΩ 5mA 2.67KΩ * The Rset values are rounded off to the nearest 1% standard resistors BRGT PIN The BRGT pin can be used to smoothly vary the brightness of the White LEDs. In the LM2794/5, voltage on BRGT is connected to an internal resistor divider which gives a factor of 0.385 and summed with an offset voltage (188mV) from the bandgap (See Functional Block Diagram). 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. TABLE 2. White LED Selection: Component ILED per LED www.osram-os.com www.nichia.com Care must be taken to prevent voltages on BRGT that cause LED current to exceed a total of 80 mA. Although this will not cause damage to the IC, it will not meet the guaranteed specifications listed in the Electrical Characteristics. Table 4 shows the current through each LED for the LM2794/5 with various BRGT and RSET values using ILED equation below. ISET PIN 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 10:1 The currents through each LED are matched within 0.5%. RSET should be TABLE 4. LED Current When Using BRGT Input (Values Highlighted in Boldface exceeded maximum current range of the device) RSET (Ω) 124Ω 500Ω 900Ω 1750Ω VBRGT (V) ILED (mA) ILED (mA) ILED (mA) ILED (mA) 0.0 15.16 3.76 2.09 1.07 0.5 30.69 7.61 4.23 2.17 1 46.21 11.46 6.37 3.27 1.5 61.73 15.31 8.51 4.37 2 77.26 19.16 10.64 5.47 2.5 92.78 23.01 12.78 6.57 3 108.31 26.86 14.92 7.67 Calculation of LED Current When Using BRGT : VIN = 3.6V LED 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. RSET = 1000Ω, BRGT = 3V ILED = ((VOFFSET + (0.385 • BRGT))/1000 ) • 10 ILED = ((0.188 + (0.385 • 3)) / 1000 ) • 10 = 13.4mA BRIGHTNESS CONTROL USING PWM Brightness control can be implemented by pulsing a signal at the SD pin. The recommended frequency is between 100Hz to 1kHz. If the 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 LM2794/5, connect BRGT pin to GND. The RSET value is then selected using the above I SET equation when BRGT = 0V. The brightness is controlled by increasing and decreasing the duty cycle of the PWM signal. Zero duty cycle will turn off the POUT The charge-pump output voltage (POUT) on the LM2794/5 can be used to deliver additional current to other circuitry if desired. The available current from POUT depends on the total LED current consumed and the diode forward voltage. The graph on page 7 (typical performance) shows the additional available output current from POUT when all four diodes are consuming a total current of 60mA. The graph shows that the available additional current from POUT will vary with diode forward voltage. Moreover, if the total diode current is reduced below 60mA, then more current will be available from POUT. It is imperative not to exceed the maximum power dissipation of the device when POUT is used to 11 www.national.com LM2794/LM2795 Application Information LM2794/LM2795 Application Information An approximation of the efficiency for the LM2794/95 is given as: (Continued) power additional circuitry in an application. Note that the POUT voltage is not regulated, and will thus be equal to 1.5 times the input voltage. It is possible to regulate the output voltage of the LM2794/5 by using a Linear Dropout Regulator (LDO) such as the LP3985-series LDOs. Efficiency = (VD(AVG) • 4ID(AVG))/ 3/2VIN • 4ID(AVG) = VD(AVG) / (3/2 VIN) where VD(AVG) is the average diode and ID(AVG) 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. PARALLEL Dx OUTPUTS FOR INCREASED CURRENT DRIVE 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. Outputs D1 -D4 may be connected together in any combination to drive higher currents through fewer LEDs. For example, outputs D1 and D2 may be connected together to drive a single LED. Similarly, D3 and D4 may be connected together to drive a second LED. With this configuration, two parallel current sources of equal value provide current to each LED, so RSET and VBRGT should be chosen so that the current through each of the outputs is programmed to 50% of the desired LED current. For example, if 30mA is the desired drive current for the LEDs, RSET and VBRGT should be selected so that the current through each of the outputs is 15mA. Other combinations of parallel outputs may be implemented in similar fashions (connecting all four outputs to drive a single LED, for example). Connecting outputs in parallel does not affect internal operation of the LM2794/95 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 parallel output configurations, just as they do to the standard 4-LED application circuit. The actual power dissipation of the device can be calculated using this equation: PDissipation = (3/2 VIN -VDIODE) • ILOAD As an example, if VIN in the target application is 4.2V, VDIODE = 3.0V and worse case current consumption is 60mA (15mA for each diode). PDissipation = ((1.5 • 4.2) - 3.0) • 0.06 = 198mW Power dissipation must be less than that allowed by the package. Please refer to the Absolute Maximum Rating of the LM2794/5. MICRO SMD MOUNTING The LM2794/5 is a 14-bump micro SMD with a bump size of 300 micron. The micro SMD package requires specific mounting techniques which are detailed in National Semiconductor Application Note (AN -1112). NSMD (non-solder mask defined) layout pattern is recommended over the SMD (solder mask defined) since the NSMD requires larger solder mask openings over the pad size as opposed to the SMD. This reduces stress on the PCB and prevents possible cracking at the solder joint. For best results during assembly, alignment ordinals on the PC board should be used to faciliate placement of the micro SMD device. Micro SMD is a wafer level chip size package which means the dimensions of the package is equal to the die size. As such, the micro SMD package are lacks the plastic encapsulation characteristic of the larger devices and ; it is sensitive to direct exposure to sun light and light sources such as infrared light and halogen light. These wavelenghts may cause unpreditabled operation. THERMAL PROTECTION The LM2794/5 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 125˚C. It is important to have good thermal conduction with a proper layout to reduce thermal resistance. POWER EFFICIENCY The efficiency of the LM2794/5 is calculated by dividing the output power by the input power. This is shown in the following equation: Efficiency = ( VD1 • ID1 + VD2 • ID2 + VD2 • ID3 + VD4 • ID4) / (VIN • ISUPPLY) Where VDX is the corresponding diode voltage and IDX is the corresponding diode currrent. www.national.com 12 LM2794/LM2795 Physical Dimensions inches (millimeters) unless otherwise noted For Ordering, Refer to Ordering Information Table NS Package Number BLP14 The dimensions for X1, X2, X3 are given as: X1 = 2.08mm ± 0.03mm X2 = 2.403mm ± 0.03mm X3 = 0.845mm ± 0.01mm 13 www.national.com LM2794/LM2795 Current Regulated Switched Capacitor LED Supply with Analog Brightness Control Physical Dimensions inches (millimeters) unless otherwise noted (Continued) For Ordering, Refer to Ordering Information Table NS Package Number TLP14 The dimensions for X1, X2, X3 are given as: X1 = 2.08mm ± 0.03mm X2 = 2.403mm ± 0.03mm X3 = 0.600mm ± 0.075mm 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. National Semiconductor Corporation Americas Email: [email protected] www.national.com National Semiconductor Europe Fax: +49 (0) 180-530 85 86 Email: [email protected] Deutsch Tel: +49 (0) 69 9508 6208 English Tel: +44 (0) 870 24 0 2171 Français Tel: +33 (0) 1 41 91 8790 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. 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