LM2794BL

LM2794, LM2795 www.ti.com SNVS168L – JANUARY 2002 – REVISED MAY 2013 LM2794 /LM2795 Current Regulated Switched Capacitor LED Supply with Analog and PWM Brightness Control Check for Samples: LM2794, LM2795 FEATURES APPLICATIONS • • • • 1 2 • • • • • • • • • • • Regulated Current Sources with ±0.5% Matching between any Two Outputs High Efficiency 3/2 Boost Function Drives One, Two, Three or four White LEDs 2.7V to 5.5V Input Voltage Up to 80mA Output Current Analog Brightness Control Active-Low or High Shutdown Input ('94/95) Very Small Solution Size and no Inductor 2.3µA (typ.) Shutdown Current 325kHz Switching Frequency (min.) Constant Frequency Generates Predictable Noise Spectrum Thin DSBGA Package: 2.08mm X 2.403mm X 0.600mm High White LED Display Backlights White LED Keypad Backlights 1-Cell Li-Ion Battery-Operated Equipment Including PDAs, Hand-Held PCs, Cellular Phones DESCRIPTION The LM2794/95 is a fractional CMOS charge-pump 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. Basic Application Circuit POUT VIN CIN CHOLD 1PF 1PF C1+ C1 D1 1PF D2 C1C2+ C2 LED1 LM2794/95 1PF D3 LED2 C2- D4 LED3 SD BRGT LED4 ISET GND RSET 1 2 Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet. All trademarks are the property of their respective owners. PRODUCTION DATA information is current as of publication date. Products conform to specifications per the terms of the Texas Instruments standard warranty. Production processing does not necessarily include testing of all parameters. Copyright © 2002–2013, Texas Instruments Incorporated LM2794, LM2795 SNVS168L – JANUARY 2002 – REVISED MAY 2013 www.ti.com DESCRIPTION (CONTINUED) 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 linearly vary the LED current. 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 2.3µA (typ.) The LM2794 is shut down when the SD pin is low, and the LM2795 is shut down when the SD pin is high. The LM2794/95 is available in a DSBGA CSP package. Connection Diagram C7 A7 E7 D6 B6 E5 A5 E3 A3 D2 E1 B2 A1 C1 Figure 1. DSBGA Package Bottom View PIN DESCRIPTION Pin (1) Name Function A1 C1+ 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 (1) C7 ISET B6 BRGT Current Sense Input. Connect 1% resistor to ground to set constant current through LED A7 SD The LM2794 has an active-low shutdown pin (LOW = shutdown, HIGH = operating). The LM2795 has an active-high shutdown pin (HIGH = shutdown, LOW = operating) that has a pull-up to VIN. A5 C2+ Positive terminal of C2 A3 POUT Charge pump output Variable voltage input controls output current Note that the pin numbering scheme for the DSBGA 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 These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam during storage or handling to prevent electrostatic damage to the MOS gates. 2 Submit Documentation Feedback Copyright © 2002–2013, Texas Instruments Incorporated Product Folder Links: LM2794 LM2795 LM2794, LM2795 www.ti.com SNVS168L – JANUARY 2002 – REVISED MAY 2013 Absolute Maximum Ratings (1) (2) VIN −0.5 to 6.2V max SD −0.5 to (VIN+0.3V) w/ 6.2V max −0.5 to (VIN+0.3V) w/ 6.2V max BRGT Continuous Power Dissipation (3) Internally Limited TJMAX (3) 135°C θJA (3) (4) 125°C/W −65°C to +150°C Storge Temperature Lead Temp. (Soldering, 5 sec.) ESD Rating 260°C (5) Human Body Model 2kV Machine Model (1) 200V 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. Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and specifications. Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at TJ=150°C (typ.) and disengages at TJ=140°C (typ.). D1, D2, D3 and D4 may be shorted to GND without damage. POUT may be shorted to GND for 1sec without damage. The value of θJA is based on a two layer evaluation board with a dimension of 2in. x1.5in. 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. (2) (3) (4) (5) Operating Conditions Input Voltage (VIN) 2.7V to 5.5V Ambient Temperature (TA) −30°C to +85°C Junction Temperature (TJ) −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 Junction Temperature Range (−30°C ≤ TJ ≤ +100°C). 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 % Submit Documentation Feedback Copyright © 2002–2013, Texas Instruments Incorporated Product Folder Links: LM2794 LM2795 3 LM2794, LM2795 SNVS168L – JANUARY 2002 – REVISED MAY 2013 www.ti.com Electrical Characteristics (continued) Limits in standard typeface are for TJ = 25°C and limits in boldface type apply over the full Operating Junction Temperature Range (−30°C ≤ TJ ≤ +100°C). 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 Conditions IPULL-SD Shutdown Pull-Up Current (LM2795) VCP Input Charge-Pump Mode To Pass Mode Threshold VCPH Input Charge-Pump Mode To Pass Mode Hysteresis VIH SD Input Logic High (LM2794) (1) 3.0V ≤ VIN ≤ 5.5V SD Input Logic High (LM2795) VIL SD Input Logic Low (LM2794) Min VIN = 3.6V Typ SD Input Leakage Current RBRGT ISET fSW (1) (2) 4 Units µA 4.7 V 250 mV 1.0 V 0.8VIN 3.0V ≤ VIN ≤ 5.5V 0.2 SD Input Logic Low (LM2795) ILEAK-SD Max 1.5 V 0.2VIN 0V ≤ VSD ≤ VIN 100 nA BRGT Input Resistance 240 kΩ ISET Pin Output Current IDX/10 mA Switching Frequency (2) 3.0V ≤ VIN ≤ 4.4V 325 515 675 kHz 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. The output switches operate at one eigth of the oscillator frequency, fOSC = 1/8fSW. Submit Documentation Feedback Copyright © 2002–2013, Texas Instruments Incorporated Product Folder Links: LM2794 LM2795 LM2794, LM2795 www.ti.com SNVS168L – JANUARY 2002 – REVISED MAY 2013 Typical Performance Characteristics Unless otherwise specified, C1 = C2 = CIN = CHOLD = 1µF, VIN = 3.6V, BRGT pin = 0V, RSET = 124Ω. 20 IDIODE vs VIN IDIODE vs BRGT Figure 2. Figure 3. IDIODE vs VIN BRGT = 3V IDIODE vs RSET Figure 4. Figure 5. IDIODE vs RSET VBRGT = 0V IDIODE vs VDIODE 18 16 IDX (mA) 14 12 10 8 6 4 2 0 100 300 500 700 900 1100 130015001700 1900 2100 RSET(Ÿ) Figure 6. Figure 7. Submit Documentation Feedback Copyright © 2002–2013, Texas Instruments Incorporated Product Folder Links: LM2794 LM2795 5 LM2794, LM2795 SNVS168L – JANUARY 2002 – REVISED MAY 2013 www.ti.com Typical Performance Characteristics (continued) Unless otherwise specified, C1 = C2 = CIN = CHOLD = 1µF, VIN = 3.6V, BRGT pin = 0V, RSET = 124Ω. VSET vs VBRGT RSET = 1KΩ Duty Cycle vs. Led Current (LM2794) IDIODE 1- 4 = 15mA Figure 8. Figure 9. Supply Current vs VIN IDIODE 1-4 = 15mA Supply Current vs VIN IDIODE 1-4 = Open 120 100°C -30°C 80 25°C 60 40 I SUPPLY (mA) 100 20 0 2.7 3.2 3.7 4.2 4.7 5.2 5.7 V IN (V) Figure 10. Figure 11. Shutdown Supply Current vs VIN Shutdown Threshold vs VIN 5 SHUTDOWN SUPPLY CURRENT (µA) 25°C -30°C 4 3 2 1 100°C 0 2.7 3.2 3.7 4.2 4.7 5.2 5.7 V IN (V) Figure 12. 6 Figure 13. Submit Documentation Feedback Copyright © 2002–2013, Texas Instruments Incorporated Product Folder Links: LM2794 LM2795 LM2794, LM2795 www.ti.com SNVS168L – JANUARY 2002 – REVISED MAY 2013 Typical Performance Characteristics (continued) Unless otherwise specified, C1 = C2 = CIN = CHOLD = 1µF, VIN = 3.6V, BRGT pin = 0V, RSET = 124Ω. Start-Up Response @ VIN = 2.7V (LM2794) Start-Up Response @ VIN = 2.7V (LM2795) Figure 14. Figure 15. Start-Up Response @ VIN = 3.6V (LM2794) Start-Up Response @ VIN = 3.6V (LM2795) Figure 16. Figure 17. Start-Up Response @ VIN = 4.2V (LM2794) Start-Up Response @ VIN = 4.2V (LM2795) Figure 18. Figure 19. Submit Documentation Feedback Copyright © 2002–2013, Texas Instruments Incorporated Product Folder Links: LM2794 LM2795 7 LM2794, LM2795 SNVS168L – JANUARY 2002 – REVISED MAY 2013 www.ti.com Typical Performance Characteristics (continued) Unless otherwise specified, C1 = C2 = CIN = CHOLD = 1µF, VIN = 3.6V, BRGT pin = 0V, RSET = 124Ω. 8 Available Additional Current @ POUT IDIODE 1− 4 = 15mA, RSET = 124 Ω Switching Frequency Figure 20. Figure 21. Submit Documentation Feedback Copyright © 2002–2013, Texas Instruments Incorporated Product Folder Links: LM2794 LM2795 LM2794, LM2795 www.ti.com SNVS168L – JANUARY 2002 – REVISED MAY 2013 FUNCTIONAL BLOCK DIAGRAM C1 C2 1 …F 1 …F LM2794/95 VIN 2.7 - 5.5V CIN 1 …F POUT (3/2)x Charge Pump * *(Only on LM2795) CPOUT 10PA 1 …F SD Brightness Control Voltage Reference 330 k: BRGT 130 k: 110 k: GND RSET D1 D2 D3 D4 Submit Documentation Feedback Copyright © 2002–2013, Texas Instruments Incorporated Product Folder Links: LM2794 LM2795 9 LM2794, LM2795 SNVS168L – JANUARY 2002 – REVISED MAY 2013 www.ti.com APPLICATION INFORMATION CIRCUIT DESCRIPTION The LM2794/5 is a 1.5x/1x CMOS charge pump with four matched constant current outputs, each capable of driving up to 20mA through White LEDs. This device operates over the extended Li-Ion battery range from 2.7V to 5.5V. The LM2794/5 has four regulated current sources connected to the device's 1.5x charge pump output (POUT). At input voltages below 4.7V (typ.), the charge-pump provides the needed voltage to drive high forward voltage drop White LEDs. It does this by stepping up the POUT voltage 1.5 times the input voltage. The charge pump operates in Pass Mode, providing a voltage on POUT equal to the input voltage, when the input voltage is at or above 4.7V (typ.). The device can drive up to 80mA through any combination of LEDs connected to the constant current outputs D1-D4. To set the LED drive current, the device uses a resistor connected to the ISET pin to set a reference current. This reference current is then multiplied and mirrored to each constant current output. The LED brightness can then be controlled by analog and/or digital methods. Applying an analog voltage in the range of 0V to 3.0V to the Brightness pin (BRGT) adjusts the dimming profile of the LEDs. The digital technique uses a PWM (Pulse Width Modulation) signal applied to the Shutdown pin (SD). (see ISET AND BRGT PINS). SOFT START Soft start is implemented internally by ramping the reference voltage more slowly than the applied voltage. During soft start, the current through the LED outputs will ramp up in proportion to the rate that the reference voltage is being ramped up. SHUTDOWN MODE The shutdown pin (SD) disables the part and reduces the quiescent current to 2.3µA (typ.). The LM2795 has an active-high shutdown pin (HIGH = shutdown, LOW = operating). An internal pull-up is connected between SD and VIN of the LM2795. This allows the use of open-drain logic control of the LM2795 shutdown, as shown in Figure 22. The LM2795 SD pin can also be driven with a rail-to-rail CMOS logic signal. LM2795 VIN * Shutdown Control SD *Only on LM2795 10PA Figure 22. Open-Drain Logic Shutdown Control The LM2794 has an active-low shutdown pin (LOW = shutdown, HIGH = operating). The LM2794 SD pin can be driven with a low-voltage CMOS logic signal (1.5V logic, 1.8V logic, etc). There is no internal pull-up or pull-down on the SD pin of the LM2794. CAPACITOR SELECTION The LM2794/5 requires 4 external capacitors for proper operation. Surface-mount multi-layer ceramic capacitors are recommended. These capacitors are small, inexpensive and have very low equivalent series resistance (ESR, ≤15mΩ typ.). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors are generally not recommended for use with the LM2794/5 due to their high ESR, as compared to ceramic capacitors. 10 Submit Documentation Feedback Copyright © 2002–2013, Texas Instruments Incorporated Product Folder Links: LM2794 LM2795 LM2794, LM2795 www.ti.com SNVS168L – JANUARY 2002 – REVISED MAY 2013 For most applications, ceramic capacitors with X7R or X5R temperature characteristic are preferred for use with the LM2794/5. These capacitors have tight capacitance tolerance (as good as ±10%), hold their value over temperature (X7R: ±15% over −55°C to 125°C; X5R: ±15% over −55°C to 85°C), and typically have little voltage coefficient. Capacitors with Y5V or Z5U temperature characteristic are generally not recommended for use with the LM2794/5. Capacitors with these temperature characteristics typically have wide capacitance tolerance (+80%, −20%), vary significantly over temperature (Y5V: +22%, −82% over −30°C to +85°C range; Z5U: +22%, −56% over +10°C to +85°C range), and have poor voltage coefficients. Under some conditions, a nominal 1µF Y5V or Z5U capacitor could have a capacitance of only 0.1µF. Such detrimental deviation is likely to cause Y5V and Z5U capacitors to fail to meet the minimum capacitance requirements of the LM2794/5. Table 1 lists suggested capacitor suppliers for the typical application circuit. Table 1. Ceramic Capacitor Manufacturers Manufacturer Contact TDK www.component.tdk.com Murata www.murata.com Taiyo Yuden www.t-yuden.com LED SELECTION The LM2794/5 is designed to drive LEDs with a forward voltage of about 3.0V to 4.0V. The typical and maximum diode forward voltage depends highly on the manufacturer and their technology. Table 2 lists two suggested manufacturers. Forward current matching is assured over the LED process variations due to the constant current output of the LM2794/5. Table 2. White LED Selection Manufacturer Contact Osram www.osram-os.com Nichia www.nichia.com ISET AND BRGT PINS An external resistor, RSET, is connected to the ISET pin to set the current to be mirrored in each of the LED outputs. The internal current mirror sets each LED output current with a 10:1 ratio to the current through RSET. The current mirror circuitry matches the current through each LED to within 0.5%. In addition to RSET, a voltage may be applied to the VBRGT pin to vary the LED current. By adjusting current with the Brightness pin (BRGT), the brightness of the LEDs can be smoothly varied. Applying a voltage on BRGT between 0 to 3 volts will linearly vary the LED current. Voltages above 3V do not increase the LED current any further. The voltage on the VBRGT pin is fed into an internal resistor network with a ratio of 0.385. The resulting voltage is then summed with a measured offset voltage of 0.188V, which comes from the reference voltage being fed through a resistor network (See Functional Block Diagram). The brightness control circuitry then uses the summed voltage to control the voltage across RSET. An equation for approximating the LED current is: § VOFFSET + VBRGT * 0. 385 ¨ R SET © I LED = ¨ § 0.188 + V BRGT * 0. 385 ¨ R SET © I LED = ¨ · ¸¸ * MirrorRati o ¹ · 10 Amps ¸¸ * ¹ 1 ILED CURRENT SELECTION PROCEDURES The following procedures illustrate how to set and adjust output current levels. For constant brightness or analog brightness control, go to “Brightness control using BRGT”. Otherwise refer to “Brightness control using PWM”. Brightness Control Using PWM 1. Set the BRGT pin to 0V. Submit Documentation Feedback Copyright © 2002–2013, Texas Instruments Incorporated Product Folder Links: LM2794 LM2795 11 LM2794, LM2795 SNVS168L – JANUARY 2002 – REVISED MAY 2013 www.ti.com 2. Determine the maximum desired ILED current. Use the ILED equation to calculate RSET by setting BRGT to 0V or use Table 3 to select a value for RSET when BRGT equals 0V. 3. Brightness control can be implemented by pulsing a signal at the SD pin. LED brightness is proportional to the duty cycle (D) of the PWM signal. For linear brightness control over the full duty cycle adjustment range, the PWM frequency (f) should be limited to accommodate the turn-on time (TON = 100µs) of the device. D × (1/f) > TON fMAX = DMIN ÷ TON If the PWM frequency is much less than 100Hz, flicker may be seen in the LEDs. For the LM2794, zero duty cycle will turn off the LEDs and a 50% duty cycle will result in an average ILED being half of the programmed LED current. For example, if RSET is set to program 15mA, a 50% duty cycle will result in an average ILED of 7.5mA. For the LM2795 however, 100% duty cycle will turn off the LEDs and a 50% duty cycle will result in an average ILED being half the programmed LED current. Brightness Control Using BRGT 1. Choose the maximum ILED desired and determine the max voltage to be applied to the BRGT pin. For constant brightness, set BRGT to a fixed voltage between 0V to 3V. 2. Use Table 3 to determine the value of RSET required or use the ILED equation above to calculate RSET. 3. Use Table 4 as a reference for the dimming profile of the LEDs, when BRGT ranges from 0V to 3V. Table 3. RSET Values LED Current BRGT 5mA 10mA 15mA 20mA 0.0V 374Ω 187Ω 124Ω 93.1Ω 0.5V 768Ω 383Ω 255Ω 191Ω 1.0V 1.15KΩ 576Ω 383Ω 287Ω 1.5V 1.54KΩ 768Ω 511Ω 383Ω 2.0V 1.91KΩ 953Ω 624Ω 475Ω 2.5V 2.32KΩ 1.15KΩ 768Ω 576Ω 3.0V 2.67KΩ 1.33KΩ 909Ω 665Ω Table 4. LED Current RSET Values BRGT 2.67KΩ 1.33KΩ 909Ω 665Ω 0.0V 0.7mA 1.4mA 2.1mA 2.8mA 0.5V 1.4mA 2.9mA 4.2mA 5.7mA 1.0V 2.1mA 4.3mA 6.3mA 8.6mA 1.5V 2.9mA 5.8mA 8.4mA 11.5mA 2.0V 3.6mA 7.2mA 10.5mA 14.4mA 2.5V 4.3mA 8.7mA 12.7mA 17.3mA 3.0V 5.0mA 10.1mA 14.8mA 20.2mA CHARGE PUMP OUTPUT (POUT) The LM2794/5 charge pump is an unregulated switched capacitor converter with a gain of 1.5. The voltage at the output of the pump (the POUT pin) is nominally 1.5 × VIN. This rail can be used to deliver additional current to other circuitry. Figure 23 shows how to connect additional LEDs to POUT. A ballast resistor sets the current through each LED, and LED current matching is dependent on the LED forward voltage matching. Because of this, LEDs driven by POUT are recommended for functions where brightness matching is not critical, such as keypad backlighting. Since POUT is unregulated, driving LEDs directly off POUT is usually practical only with a fixed input voltage. If the input voltage is not fixed (Li-Ion battery, for example), using a linear regulator between the POUT pin and the LEDs is recommended. Texas Instruments LP3985-4.5V low-dropout linear regulator is a good choice for such an application. 12 Submit Documentation Feedback Copyright © 2002–2013, Texas Instruments Incorporated Product Folder Links: LM2794 LM2795 LM2794, LM2795 www.ti.com SNVS168L – JANUARY 2002 – REVISED MAY 2013 The voltage at POUT is dependent on the input voltage supplied to the LM2794/5, the total LM2794/5 output current, and the output resistance (ROUT) of the LM2794/5 charge pump. Output resistance is a model of the switching losses of the charge pump. Resistances of the internal charge pump switches (MOS transistors) are a primary component of the LM2794/5 output resistance. Typical LM2794/5 output resistance is 3.0Ω. For worstcase design calculations, using an output resistance of 3.5Ω is recommended. (Worst-case recommendation accounts for parameter shifts from part-to-part variation and applies over the full operating temperature range). C1 1 …F 3.0V VIN C1- C2 1 …F C1+ C2- C2+ (3/2) VIN = approx. 4.5V Keypad LEDs POUT CIN 1 …F CPOUT 1 …F LM2794/95 DK1 DK2 DKX 5mA BRGT RSET D1 D2 D3 GND D4 *Optional Independent Shutdown 15mA each = 60 mA Total 124: Figure 23. Keypad LEDs Connected to POUT Output resistance results in droop in the POUT voltage proportional to the amount of current delivered by the pump. The POUT voltage is an important factor in determining the total output current capability of an application. Taking total output current to be the sum of all DX output currents plus the current delivered through the POUT pin, the voltage at POUT can be predicted with the following equations: ITOTAL = ID1 + ID2 + ID3 + ID4 + IPOUT VPOUT = 1.5 × VIN − ITOTAL × ROUT (1) (2) LED HEADROOM VOLTAGE (VHR) Four current sources are connected internally between POUT and D1-D4. The voltage across each current source, (VPOUT − VDX), is referred to as headroom voltage (VHR). The current sources require a sufficient amount of headroom voltage to be present across them in order to regulate properly. Minimum required headroom voltage is proportional to the current flowing through the current source, as dictated by the equation: VHR-MIN = kHR × IDX (3) The parameter kHR, typically 20mV/mA in the LM2794/5, is a proportionality constant that represents the ONresistance of the internal current mirror transistors. For worst-case design calculations, using a kHR of 25mV/mA is recommended. (Worst-case recommendation accounts for parameter shifts from part-to-part variation and applies over the full operating temperature range). Figure 24 shows how output current of the LM2794/5 varies with respect to headroom voltage. Submit Documentation Feedback Copyright © 2002–2013, Texas Instruments Incorporated Product Folder Links: LM2794 LM2795 13 LM2794, LM2795 SNVS168L – JANUARY 2002 – REVISED MAY 2013 www.ti.com 18 16 14 R SET = 124Ω I DX (mA) 12 10 8 R SET = 475Ω 6 4 R SET = 2.67kΩ 2 0 0.05 0.20 0.35 0.50 0.65 0.80 VHR (V) Figure 24. ILED vs VHR 4 LEDs, VIN = 3.0V On the flat part of the graph, the currents regulate properly as there is sufficient headroom voltage for regulation. On the sloping part of the graph the headroom voltage is too small, the current sources are squeezed, and their current drive capability is limited. Changes in headroom voltage from one output to the next, possible with LED forward voltage mismatch, will result in different output currents and LED brightness mismatch. Thus, operating the LM2794/5 with insufficient headroom voltage across the current sources should be avoided. OUTPUT CURRENT CAPABILITY The primary constraint on the total current capability is the headroom voltage requirement of the internal current sources. Combining the VPOUT and VHR equations from the previous two sections yields the basic inequality for determining the validity of an LM2794/5 LED-drive application: VPOUT = 1.5 × VIN − ITOTAL × ROUT VHR-MIN = kHR × IDX VPOUT − VDX ≥ VHR-MIN 1.5 × VIN − ITOTAL × ROUT − VDX ≥ (kHR × IDX) (4) (5) (6) (7) Rearranging this inequality shows the estimated total output current capability of an application: ITOTAL ≤ [(1.5 × VIN-MIN) − VDX-MAX − (kHR × IDX)] ÷ ROUT (8) Examining the equation above, the primary limiting factors on total output current capability are input and LED forward voltage. A low input voltage combined with a high LED voltage may result in insufficient headroom voltage across the current sources, causing them to fall out of regulation. When the current sources are not regulated, LED currents will be below desired levels and brightness matching will be highly dependent on LED forward voltage matching. Typical LM2794/5 output resistance is 3.0Ω. For worst-case design calculations, using an output resistance of 3.5Ω is recommended. LM2794/5 has a typical kHR constant of 20mV/mA. For worst-case design calculations, use kHR = 25mV/mA. (Worst-case recommendations account for parameter shifts from part-to-part variation and apply over the full operating temperature range). ROUT and kHR increase slightly with temperature, but losses are typically offset by the negative temperature coefficient properties of LED forward voltages. Power dissipation and internal self-heating may also limit output current capability but is discussed in a later section. PARALLEL Dx OUTPUTS FOR INCREASED CURRENT DRIVE Outputs D1 through D4 may be connected together in any combination to drive higher currents through fewer LEDs. For example in Figure 25, outputs D1 and D2 are connected together to drive one LED while D3 and D4 are connected together to drive a second LED. 14 Submit Documentation Feedback Copyright © 2002–2013, Texas Instruments Incorporated Product Folder Links: LM2794 LM2795 LM2794, LM2795 www.ti.com SNVS168L – JANUARY 2002 – REVISED MAY 2013 C2 1 …F C1 1 …F 3.0V VIN C1- C1+ C2- C2+ POUT CPOUT 1 …F CIN 1 …F LM2794/95 BRGT RSET D1 GND D4 D2 D3 15mA 124: 30mA Figure 25. Two Parallel Connected LEDs With this configuration, two parallel current sources of equal value provide current to each LED. RSET and VBRGT should therefore be chosen so that the current through each output is programmed to 50% of the desired current through the parallel connected LEDs. For example, if 30mA is the desired drive current for 2 parallel connected 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, such as in Figure 26. C2 1 …F C1 1 …F 3.0V VIN C1- C1+ C2- C2+ POUT CPOUT 1 …F CIN 1 …F LM2794/95 BRGT RSET D1 D2 D3 GND D4 15mA 124: 60mA Figure 26. One Parallel Connected LED 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 diode output current, maximum diode 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. THERMAL PROTECTION When the junction temperature exceeds 150°C (typ.), the LM2794/5 internal thermal protection circuitry disables the part. This feature protects the device from damage due to excessive power dissipation. The device will recover and operate normally when the junction temperature falls below 140°C (typ.). It is important to have good thermal conduction with a proper layout to reduce thermal resistance. Submit Documentation Feedback Copyright © 2002–2013, Texas Instruments Incorporated Product Folder Links: LM2794 LM2795 15 LM2794, LM2795 SNVS168L – JANUARY 2002 – REVISED MAY 2013 www.ti.com POWER EFFICIENCY Figure 27 shows the efficiency of the LM2794/5. The change in efficiency shown by the graph comes from the transition from Pass Mode to a gain of 1.5. Efficiency (E) of the LM2794/5 is defined here as the ratio of the power consumed by LEDs (PLED) to the power drawn from the input source (PIN). In the equations below, IQ is the quiescent current of the LM2794/5, ILED is the current flowing through one LED, VLED is the forward voltage at that LED current, and N is the number of LEDs connected to the regulated current outputs. In the input power calculation, the 1.5 represents the switched capacitor gain configuration of the LM2794/5. PLED = N × VLED × ILED PIN = VIN × IIN PIN = VIN × (1.5 × N × ILED + IQ) E = (PLED ÷ PIN) (9) (10) (11) (12) Efficiency, as defined here, is in part dependent on LED voltage. Variation in LED voltage does not affect power consumed by the circuit and typically does not relate to the brightness of the LED. For an advanced analysis, it is recommended that power consumed by the circuit (VIN x IIN) be evaluated rather than power efficiency. Figure 28 shows the power consumption of the LM2794/5 Typical Application Circuit. Figure 27. Efficiency vs VIN 4 LEDs, VLED = 3.6V, ILED = 15mA 450 430 410 PIN (mW) 390 370 350 330 310 290 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 V IN (V) Figure 28. PIN vs VIN 4 LEDs, 2.5 ≤ VDX ≤ 3.9V, IDX = 15mA 16 Submit Documentation Feedback Copyright © 2002–2013, Texas Instruments Incorporated Product Folder Links: LM2794 LM2795 LM2794, LM2795 www.ti.com SNVS168L – JANUARY 2002 – REVISED MAY 2013 POWER DISSIPATION The power dissipation (PDISSIPATION) and junction temperature (TJ) can be approximated with the equations below. PIN is the power generated by the 1.5x charge pump, PLED is the power consumed by the LEDs, PPOUT is the power provided through the POUT pin, TAis the ambient temperature, and θJA is the junction-to-ambient thermal resistance for the DSBGA package. VIN is the input voltage to the LM2794/5, VDX is the LED forward voltage, IDX is the programmed LED current, and IPOUT is the current drawn through POUT. PDISSIPATION = PIN - PLED − PPOUT = [1.5×VIN×(4IDX + IPOUT)] − (VDX×4IDX) − (1.5×VIN×IPOUT) TJ = TA + (PDISSIPATION × θJA) (13) (14) (15) The junction temperature rating takes precedence over the ambient temperature rating. The LM2794/5 may be operated outside the ambient temperature rating, so long as the junction temperature of the device does not exceed the maximum operating rating of 100°C. The maximum ambient temperature rating must be derated in applications where high power dissipation and/or poor thermal resistance causes the junction temperature to exceed 100°C. DSBGA MOUNTING The LM2794/5 is a 14-bump DSBGA with a bump size of 300 micron diameter. The DSBGA package requires specific mounting techniques detailed in Application Note (AN -1112 SNVA009). 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 facilitate placement of the DSBGA device. DSBGA is a wafer level chip size package, which means the dimensions of the package are equal to the die size. As such, the DSBGA package lacks the plastic encapsulation characteristics of the larger devices and is sensitive to direct exposure to light sources such as infrared, halogen, and sun light. The wavelengths of these light sources may cause unpredictable operation. Submit Documentation Feedback Copyright © 2002–2013, Texas Instruments Incorporated Product Folder Links: LM2794 LM2795 17 LM2794, LM2795 SNVS168L – JANUARY 2002 – REVISED MAY 2013 www.ti.com REVISION HISTORY Changes from Revision K (May 2013) to Revision L • 18 Page Changed layout of National Data Sheet to TI format .......................................................................................................... 17 Submit Documentation Feedback Copyright © 2002–2013, Texas Instruments Incorporated Product Folder Links: LM2794 LM2795 PACKAGE OPTION ADDENDUM www.ti.com 10-Dec-2020 PACKAGING INFORMATION Orderable Device Status (1) Package Type Package Pins Package Drawing Qty Eco Plan (2) Lead finish/ Ball material MSL Peak Temp Op Temp (°C) Device Marking (3) (4/5) (6) LM2794TL/NOPB ACTIVE DSBGA YPA 14 250 RoHS & Green SNAGCU Level-1-260C-UNLIM -30 to 85 LOG (1) The marketing status values are defined as follows: ACTIVE: Product device recommended for new designs. LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect. NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design. PREVIEW: Device has been announced but is not in production. Samples may or may not be available. OBSOLETE: TI has discontinued the production of the device. (2) RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may reference these types of products as "Pb-Free". RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption. Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of