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LM2794/LM2795 Current Regulated Switched Capacitor LED Supply with Analog Brightness Control
PRELIMINARY
January 2002
LM2794/LM2795 Current Regulated Switched Capacitor LED Supply with Analog Brightness Control
General Description
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 5A. 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 5A (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.
Features
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 5A (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
(c) 2002 National Semiconductor Corporation
DS200285
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LM2794/LM2795
Connection Diagram
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Bottom View
Ordering Information
Order Number LM2794BL LM2794BLX LM2795BL LM2795BLX Shutdown Polarity Active Low Active Low Active High Active High Package Number BLP14EHB BLP14EHB BLP14EHB BLP14EHB Package Marking I LOG I LOG I LOJ I LOJ Supplied As 250 Units, Tape and Reel 3000 Units, Tape and Reel 250 Units, Tape and Reel 3000 Units, Tape and Reel
Pin Description
Pin 1 2 3 4 5 6, 7, 8, 9 10 11 12 13 14 Name C1+ C1- VIN GND C2- D1-4 ISET BRGT SD C2+ POUT Positive terminal of C1 Negative terminal of C1 Power supply voltage input Power supply ground input Negative terminal of C2 Current source outputs. Connect directly to LED 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 Positive terminal of C2 Charge pump output Function
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LM2794/LM2795
Block Diagram
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LM2794/LM2795
Absolute Maximum Ratings
(Note 1)
Storge Temperature Lead Temp. (Soldering, 5 sec.) ESD Rating (Note 4) Human Body Model Machine Model
-65C to +150C 260C 2KV 200V
If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN SD BRGT Power Dissipation (TA = 25C (Note 2) TJMAX (Note 2) JA (Notes 2, 3) -0.5 to 6.2V max -0.5 to (VIN+0.3V) w/ 6.2V max -0.5 to (VIN+0.3V) w/ 6.2V max 400 mW 135C 125C/W
Operating Conditions
Input Voltage (VIN) Ambient Temperature (TA) Junction Temperature (TJ) 2.7V to 5.5V -30C to +85C -30C to +125C
Electrical Characteristics
Limits in standard typeface are for TJ = 25C and limits in boldface type apply over the full Operating Temperature Range. Unless otherwise specified, C1 = C2 = CIN = CHOLD = 1 F, VIN = 3.6V, BRGT pin = 0V; RSET =124 ; LM2794:VSD = VIN (LM2795: VSD = 0V). Symbol IDX Parameter Available Current at Output Dx Conditions 3.0V VIN 5.5V VDX 3.8V BRGT = 50mV 2.7V VIN 3.0V VDX 3.6V BRGT = 0V VDX 3.8V BRGT = 200mV VDX Available Voltage at Output Dx 3.0V VIN 5.5V IDX 15mA BRGT = 50mV 3.0V VIN 5.5V VDX = 3.6V 3.0V VIN 4.4V VDX = 3.6V IDX ID-MATCH IQ Load Regulation of Dx Output Current Current Matching Between Any Two Outputs Quiescent Supply Current VIN = 3.6V 3.0V VDX 3.8V VIN = 3.6V, VDX = 3.6V 3.0V VIN 4.2V, Active, No Load Current RSET = OPEN 3.0V VIN 5.5V, Shutdown VIN = 3.6V Min 15 Typ 16.8 Max Units mA
10 mA 20 3.8
mA V
IDX
Line Regulation of Dx Output Current
14.18 14.18 14.18
15.25 15.25 15.25 0.5 5.5
16.78 16.32 16.32
mA mA mA %
8.2
mA
ISD IPULL-SD VCP VCPH VIH VIL ILEAK-SD RBRGT
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Shutdown Supply Current Shutdown Pull-Up Current (LM2795) Input Charge-Pump Mode To Pass Mode Threshold Input Charge-Pump Mode To Pass Mode Hysteresis SD Input Logic High (LM2794) SD Input Logic High (LM2795) SD Input Logic Low (LM2794) SD Input Logic Low (LM2795) SD Input Leakage Current BRGT Input Resistance
2.3 1.5 4.7
5
A A V mV V
(Note 5) 3.0V VIN 5.5V 3.0V VIN 5.5V 0V VSD VIN 1.0 0.8VIN
250
0.2 0.2VIN 100 240
V nA k
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LM2794/LM2795
Electrical Characteristics
(Continued) Limits in standard typeface are for TJ = 25C and limits in boldface type apply over the full Operating Temperature Range. Unless otherwise specified, C1 = C2 = CIN = CHOLD = 1 F, VIN = 3.6V, BRGT pin = 0V; RSET =124 ; LM2794:VSD = VIN (LM2795: VSD = 0V). Symbol Parameter ISET Pin Output Current Switching Frequency (Note 6) 3.0V VIN 4.4V 325 Conditions Min Typ IDX/10 515 675 Max Units mA kHz
ISET fSW
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 JA is based on a two layer evaluation board with a dimension of 2in. x1.5in. Note 4: In the test circuit, all capacitors are 1.0F, 0.3 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.
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LM2794/LM2795
Typical Performance Characteristics
VIN = 3.6V, BRGT pin = 0V, RSET = 124. Supply Current with Load ID 1-4 = 15mA
Unless otherwise specified, C1 = C2 = CIN = CHOLD = 1F,
Supply Current with No Load
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Shutdown Supply Current
Shutdown Threshold vs VIN
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IDIODE vs Supply Voltage
IDIODE vs VIN @ RSET = 1K, 3K BRGT =3V
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20028507
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LM2794/LM2795
Typical Performance Characteristics
VIN = 3.6V, BRGT pin = 0V, RSET = 124. (Continued) IDIODE vs BRGT
Unless otherwise specified, C1 = C2 = CIN = CHOLD = 1F,
IDIODE vs RSET
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VSET vs VBRGT RSET = 1K
IDIODE vs VDIODE
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Available Additional Current @ POUT ID 1- 4 = 15mA, RSET = 124 ohms
Duty Cycle vs. Led Current (LM2794) ID 1- 4 = 15mA
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LM2794/LM2795
Typical Performance Characteristics
VIN = 3.6V, BRGT pin = 0V, RSET = 124. (Continued) Switching Frequency
Unless otherwise specified, C1 = C2 = CIN = CHOLD = 1F,
Start-Up Response @ 3.6VIN (LM2794)
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Start-Up Response @ 3.6VIN (LM2795)
Start-Up Response @ 2.7VIN (LM2794)
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Start-Up Response @ 4.2VIN (LM2794)
Start-Up Response @ 2.7VIN (LM2795)
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LM2794/LM2795
Typical Performance Characteristics
VIN = 3.6V, BRGT pin = 0V, RSET = 124. (Continued) Start-Up Response @ 4.2VIN (LM2795)
Unless otherwise specified, C1 = C2 = CIN = CHOLD = 1F,
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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
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Application Information
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 5A 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. 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. TABLE 1. Low ESR Capacitor Manufactures Manufacturer TDK MuRata Taiyo Yuden Contact (947) 803 6100 (800) 831 9172 (800) 348 2496 website www.component.tdk.com www.murata.com www.t-yuden.com
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LM2794/LM2795
Application Information
LED selection
(Continued)
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. TABLE 2. White LED Selection: Component LWT673/LWT67C Manufacture Contact Osram www.osram-os.com www.nichia.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: RSET = ((0.188 + (0.385 * BRGT)) / ISET) * 10 TABLE 3. RSET Selections ( when BRGT pin = 3V maximum) ILED per LED 15mA 10mA 5mA *RSET (+/-1%) 909 1.4K 2.67K
* The Rset values are rounded off to the nearest 1% standard resistors
NSCW100/NSCW215 Nichia
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
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. 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.
TABLE 4. LED Current When Using BRGT Input (Values Highlighted in Boldface exceeded maximum current range of the device) RSET () VBRGT (V) 0.0 0.5 1 1.5 2 2.5 3 124 ILED (mA) 15.16 30.69 46.21 61.73 77.26 92.78 108.31 500 ILED (mA) 3.76 7.61 11.46 15.31 19.16 23.01 26.86 900 ILED (mA) 2.09 4.23 6.37 8.51 10.64 12.78 14.92 1750 ILED (mA) 1.07 2.17 3.27 4.37 5.47 6.57 7.67
Calculation of LED Current When Using BRGT : VIN = 3.6V RSET = 1000, 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
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. 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 power addi11 www.national.com
LM2794/LM2795
Application Information
(Continued)
tional 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. Thermal Protection The LM2794/5 has internal thermal protection circuitry to disable the charge pump if the junction temperature exceeds 150C. 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 125C. 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. An approximation of the efficiency for the LM2794/95 is given as: 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. Power Dissipation The maximum allowable power dissipation that this package is capable of handling can be determined as follows: PDMax = (TJMax - TA) / JA
where TJMax is the maximum junction temperature, TA is the ambient temperature, and JA 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 = (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.
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LM2794/LM2795 Current Regulated Switched Capacitor LED Supply with Analog Brightness Control
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
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