LTC3452 Synchronous Buck-Boost MAIN/CAMERA White LED Driver U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO High Efficiency: ≥85% Over Entire Li-Ion Battery Range Wide VIN Range: 2.7V to 5.5V Independent MAIN/CAMERA Current Control Up to 425mA Continuous Output Current Internal Soft-Start Open/Shorted LED Protection PWM Brightness Control LED Current Matching Typically <2.5% Constant Frequency 1MHz Operation Low Shutdown Current: 6.5µA Overtemperature Protection Small Thermally Enhanced 20-Lead (4mm × 4mm) QFN Package The LTC®3452 is a synchronous buck-boost DC/DC converter optimized for driving two banks of white LEDs from a single Li-Ion battery input. Five parallel LEDs can be driven at up to 25mA each in the low power LED bank, while two LEDs can be driven at up to 150mA each (or a single LED at 300mA) in the high power LED bank. The regulator operates in either synchronous buck, synchronous boost or buck-boost mode, depending on input voltage and LED maximum forward voltage. Optimum efficiency is achieved by sensing which LED requires the largest forward voltage drop at its programmed current, and regulating the common output rail for lowest dropout. Efficiency of 85% can be achieved over the entire usable range of a Li-Ion battery (2.7V to 4.2V). Cell Phones Digital Cameras PDAs Portable Devices Maximum LED current for each LED display is programmable with a single external resistor. Dual enable pins allow for PWM brightness control in the low power bank and independent on/off control for the high current bank (optimal for LED camera flash). In shutdown, the supply current is only 6.5µA. U APPLICATIO S ■ ■ ■ ■ , LTC and LT are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. A high constant operating frequency of 1MHz allows the use of a small external inductor. The LTC3452 is offered in a low profile (0.75mm) thermally enhanced 20-lead (4mm × 4mm) QFN package. U TYPICAL APPLICATIO 5 × 20mA White LED Display + 200mA Camera Light Driver L 4.7µH + Torch and Flash Mode Efficiency 2.2µF VIN PVIN SW1 SW2 VOUT LEDH1 ENH 4.7µF D1 CAM 95 93 200mA 91 LEDH2 ISETH EFFICIENCY (%) VIN SINGLE Li-Ion CELL 2.7V TO 4.2V D2 6.19k LEDL1 1MHz BUCK/BOOST VC LEDL2 0.1µF LEDL3 ENL LEDL4 LTC3452 LEDL5 ISETL 0mA TO 20mA D3 0mA TO 20mA D4 0mA TO 20mA D5 GND PGND EXPOSED PAD 89 TORCH MODE AT 100mA 87 85 83 FLASH MODE AT 200mA 81 0mA TO 20mA 79 D6 77 0mA TO 20mA 75 2.7 10.2k GND TA = 25°C (V – VLEDx) • ILEDx EFFICIENCY = Σ OUT VIN • IIN MAIN DISPLAY LED BACKLIGHT D1: AOT 2015 D2 TO D6: NICHIA NSCW100 L: COILCRAFT DO3314-472 3452 TA01a 3.1 3.5 3.9 4.3 VIN (V) 4.7 5.1 5.5 3452 TA01b 3452f 1 LTC3452 W W W AXI U U ABSOLUTE RATI GS U U W PACKAGE/ORDER I FOR ATIO (Note 1) VIN, PVIN, SW1, SW2, VOUT Voltage ........... – 0.3V to 6V LEDL1 to LEDL5 Voltage ... – 0.3V to (VOUT + 0.3V) or 6V LEDH1, LEDH2 Voltage ..... – 0.3V to (VOUT + 0.3V) or 6V VC, ENL, ENH, ISETL, ISETH Voltage ............ – 0.3V to (VIN + 0.3V) or 6V LEDL1 to LEDL5 Current ....................................... 50mA LEDH1, LEDH2 Current ....................................... 250mA Operating Temperature Range (Note 2) .. – 40°C to 85°C Junction Temperature (Note 3) ............................ 125°C Storage Temperature Range ................ – 65°C to 125°C VOUT SW2 PGND SW1 PVIN TOP VIEW 20 19 18 17 16 15 VC VIN 1 14 ENH ENL 2 13 ISETH 21 ISETL 3 7 8 9 10 GND LEDH1 6 LEDL5 11 GND LEDL4 12 LEDH2 LEDL2 5 LEDL3 LEDL1 4 UF PACKAGE 20-LEAD (4mm × 4mm) PLASTIC QFN TJMAX = 125°C, θJA = 40°C/W EXPOSED PAD (PIN 21) IS GND, MUST BE SOLDERED TO PCB ORDER PART NUMBER LTC3452EUF UF PART MARKING 3452 Order Options Tape and Reel: Add #TR Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF Lead Free Part Marking: http://www.linear.com/leadfree/ Consult LTC Marketing for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. VIN = PVIN = VOUT = 3.6V unless otherwise specified. (Note 2) PARAMETER CONDITIONS Input DC Supply Current Normal Operation Shutdown UVLO MIN ● Input Supply Voltage (VIN) TYP MAX UNITS 5.5 V 0.6 6.5 3 1 18 5 mA µA µA 2.0 1.87 2.3 V V 0.54 1.2 V 2.7 2.7V ≤ VIN ≤ 5.5V, RISETL = RISETH = 51.1k, ILEDx = 0 (Note 4) 2.7V ≤ VIN ≤ 5.5V, VENL = VENH = 0V VIN < UVLO Threshold VIN Rising VIN Falling ● ENL,H DC Threshold for Normal Operation (VIH) 2.7V ≤ VIN ≤ 5.5V, VENL,H Rising ● ENL,H DC Threshold for Shutdown (ILEDx = 0) (VIL) 2.7V ≤ VIN ≤ 5.5V, VENL,H Falling ● 0.2 ENL,H Input Current (IIH, IIL) 2.7V ≤ VIN ≤ 5.5V ● –1 ENL PWM Frequency 2.7V ≤ VIN ≤ 5.5V (Note 5) ● 10 ISETL,H Servo Voltage RISETL = RISETH = 20k ● 788 780 800 800 812 812 mV mV ● 730 714 768 768 806 806 mA/mA mA/mA 1 6 Undervoltage Lockout Threshold LEDHx Output Current Ratio (ILEDHx/ISETH) 1.6 ILEDHx = 100mA, VLEDHx = 300mV LEDHx Output Current Matching (Max – Min)/[(Max + Min)/2] • 100%, ILEDHx = 100mA, VLEDHx = 300mV, 2.7V ≤ VIN ≤ 5.5V LEDHx Pin Voltage ILEDHx = 100mA 0.52 V 1 µA kHz 250 % mV 3452f 2 LTC3452 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. VIN = PVIN = VOUT = 3.6V unless otherwise specified. (Note 2) PARAMETER CONDITIONS LEDLx Output Current Ratio (ILEDLx/ISETL) (Note 6) ILEDLx|MAX = 20mA, VLEDLx = 300mV PWM Duty Cycle = 6% MIN TYP MAX UNITS ● 1.8 1.75 2 2 2.16 2.21 mA/mA mA/mA ● 3.66 3.56 4 4 4.28 4.38 mA/mA mA/mA ● 7.32 7.12 8 8 8.56 8.76 mA/mA mA/mA ● 14.72 14.32 16 16 17.04 17.44 mA/mA mA/mA ● 29.44 28.64 32 32 33.92 34.56 mA/mA mA/mA ● 58.88 57.92 64 64 67.2 68.16 mA/mA mA/mA ● 117.12 114.56 128 128 134.4 137.6 mA/mA mA/mA ● 234.24 229.12 256 245 268.8 272.64 mA/mA mA/mA 2.5 8 PWM Duty Cycle = 19% PWM Duty Cycle = 31% PWM Duty Cycle = 44% PWM Duty Cycle = 56% PWM Duty Cycle = 69% PWM Duty Cycle = 81% PWM Duty Cycle = 94% LEDLx Output Current Matching (Max – Min)/[(Max + Min)/2] • 100%, ILEDLx = 20mA, VLEDLx = 300mV LEDLx Pin Voltage ILEDLx = 20mA Regulated Maximum VOUT VLEDLx = VLEDHy = 0V PMOS Switch RON Switches A and D at 100mA 210 mΩ NMOS Switch RON Switches B and C at 100mA 205 mΩ Forward Current Limit Switch A Reverse Current Limit Switch D PMOS Switch Leakage Switches A and D NMOS Switch Leakage Switches B and C Oscillator Frequency Soft-Start Time Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LTC3452E is guaranteed to meet specifications from 0°C to 70°C. Specifications over the –40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls. % 130 ● 4.35 1000 4.5 1600 mV 4.75 V 2400 mA 200 –1 –1 0.9 mA µA 1 µA 1 1 1.1 MHz µs 650 Note 3: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formula: TJ = TA + (PD • θJA°C/W). Note 4: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. Note 5: Do not exceed 50kHz PWM frequency in the application. Note 6: This parameter is tested in a setup which forces conditions equivalent to those programmed by the indicated duty cycle. 3452f 3 LTC3452 U W TYPICAL PERFOR A CE CHARACTERISTICS Shutdown Current vs Temperature Undervoltage Lockout Threshold vs Temperature Shutdown Current vs VIN 12 20 2.5 TA = 25°C 10 14 12 10 8 VIN = 5.5V VIN = 4.2V 6 4 2 0 –55 –35 –15 8 6 4 VIN = 3.6V VIN = 2.7V 2.3 UVLO THRESHOLD (V) 16 SHUTDOWN CURRENT (µA) SHUTDOWN CURRENT (µA) 18 0 2.7 3.1 3.5 3.9 4.3 VIN (V) 4.7 1000 ENABLE THRESHOLDS (mV) ENABLE THRESHOLDS (mV) 700 600 VIH 500 VIL 400 300 200 –55 –35 –15 5 25 45 65 85 105 125 TEMPERATURE (°C) 3452 G03 ISETL,H Servo Voltage vs Temperature 812 900 808 800 804 700 600 VIH VIL 500 796 792 788 300 784 200 2.7 3.1 3.5 3.9 4.3 VIN (V) 4.7 5.1 5.5 VIN = 3.6V RISETL = 10.2k RISETH = 4.99k 800 400 5 25 45 65 85 105 125 TEMPERATURE (°C) 780 –55 –35 –15 5 25 45 65 85 105 125 TEMPERATURE (°C) 3452 G05 3452 G04 3452 G06 Maximum Regulated VOUT vs Temperature ISETL,H Servo Voltage vs VIN 812 1.5 –55 –35 –15 5.5 TA = 25°C VIN = 3.6V 800 5.1 VISETL,H (mV) 1200 900 VIN FALLING Enable Thresholds vs VIN Enable Thresholds vs Temperature 1000 1.9 3452 G02 3452 G01 1100 VIN RISING 1.7 2 5 25 45 65 85 105 125 TEMPERATURE (°C) 2.1 4.60 TA = 25°C 4.58 808 VIN = 3.6V 4.56 804 VOUT (V) VISETL,H (mV) 4.54 800 796 792 4.52 4.50 4.48 4.46 788 4.44 784 780 2.7 4.42 3.1 3.5 3.9 4.3 VIN (V) 4.7 5.1 5.5 3452 G07 4.40 –55 –35 –15 5 25 45 65 85 105 125 TEMPERATURE (°C) 3452 G08 3452f 4 LTC3452 U W TYPICAL PERFOR A CE CHARACTERISTICS PMOS RDS(ON) vs Temperature Oscillator Frequency vs Temperature NMOS RDS(ON) vs Temperature 325 325 300 300 1050 1040 VIN = 2.7V 1030 275 250 225 VIN = 5.5V VIN = 4.2V VIN = 2.7V 250 FREQUENCY (kHz) VIN = 3.6V RDS(ON) (mΩ) RDS(ON) (mΩ) 275 200 VOUT = 3V VIN = 3.6V 225 200 VIN = 5.5V VIN = 4.2V VIN = 5.5V 1020 1010 VIN = 4.2V 1000 980 175 175 150 150 960 125 –55 –35 –15 125 –55 –35 –15 950 –55 –35 –15 5 25 45 65 85 105 125 TEMPERATURE (°C) 970 5 25 45 65 85 105 125 TEMPERATURE (°C) VIN = 3.6V 990 VIN = 2.7V 5 25 45 65 85 105 125 TEMPERATURE (°C) 3452 G11 3452 G10 Output Voltage Ripple (Front Page Application) 3452 G12 Start-Up Transient CH1, VOUT 1V/DIV CH2, ILED 300mA FINAL VALUE CH3, ENH 1V/DIV VIN = 3V VOUT = 3.1V ILED = 100mA 3452 G13 VIN = 3.6V ILEDH = 300mA 3452 G14 3452f 5 LTC3452 U U U PI FU CTIO S VIN (Pin 1): Signal Voltage Input Supply Pin (2.7V ≤ VIN ≤ 5.5V). Recommended bypass capacitor to GND is 2.2µF ceramic or larger. Connect to PVIN (Pin 20). ENL (Pin 2): Enable Input Pin and PWM Brightness Control for Low Power LED Bank. Active high. For constant IMAXL operation, connect the ENL pin to VIN (or any voltage >1.2V). For ENL voltage <0.2V, all low power bank LED current source outputs are Hi-Z (if both ENL and ENH are <0.2V, the part is in shutdown and the input supply current drops to ~6µA). For brightness control between zero current and IMAXL, drive the ENL pin with a PWM waveform of frequency ≥10kHz. The low power LED bank currents will be equal to a percentage of IMAXL as given in Table 1. The ENL pin is high impedance and should not be floated. ISETL (Pin 3): Low Power LED Bank Current Programming Pin. A resistor to ground programs each low power bank current source output maximum to ILEDLx|MAX = 256 • (0.8V/RISETL). Enabled by ENL (Pin 2). PWM brightness control also via ENL. ISETH (Pin 13): High Power LED Bank Current Programming Pin. A resistor to ground programs each high power bank current source output to ILEDHx = 768(0.8V/RISETH). Enabled by ENH (Pin 14). ENH (Pin 14): Enable Input Pin for High Power LED Bank. Active high. The ENH pin is high impedance and should not be floated. VC (Pin 15): Compensation Point for the Internal Error Amplifier Output. Recommended compensation capacitor to GND is 0.1µF ceramic or larger. VOUT (Pin 16): Buck-Boost Output Pin. Recommended bypass capacitor to GND is 4.7µF ceramic. SW2 (Pin 17): Switching Node Pin. Connected to internal power switches C and D. External inductor connects between SW1 and SW2. Recommended value is 4.7µH. PGND (Pin 18): Power Ground Pin. Connect to GND (Pins 9 and 11). SW1 (Pin 19): Switching Node Pin. Connected to internal power switches A and B. External inductor connects between SW1 and SW2. Recommended value is 4.7µH. LEDL1 to LEDL5 (Pins 4 to 8): Individual Low Dropout Current Source Outputs for Low Power LED Bank Current Biasing. Connect each low power LED between VOUT and an individual LEDLx pin. Unused LEDLx outputs should be connected to VOUT. PVIN (Pin 20): Power Voltage Input Supply Pin. Connect to VIN (Pin 1). GND (Pins 9 and 11): Signal Ground Pins. Connect together and to PGND (Pin 18) and Exposed Pad ground (Pin 21). Exposed Pad (Pin 21): Heat Sink Ground. Connect to GND (Pins 9 and 11) and solder to PCB ground for electrical contact and rated thermal performance. LEDH1, LEDH2 (Pins 10, 12): Individual Low Dropout Current Source Outputs for High Power LED Bank Current Biasing. Connect each high power LED between VOUT and an individual LEDHx pin. Unused LEDHx outputs should be connected to VOUT. 3452f 6 LTC3452 W BLOCK DIAGRA VIN 2.7V TO 5.5V VOUT VIN PVIN SW1 20 1 SW2 19 VOUT 17 16 4 UNDERVOLTAGE LOCKOUT UV OVERTEMPERATURE PROTECTION OT BANDGAP REFERENCE SWITCH A SWITCH D SWITCH B SWITCH C GATE DRIVERS AND ANTI-CROSSFORWARD CONDUCTION CURRENT LIMIT 1.23V + 5 LEDL2 LED DETECT REVERSE CURRENT LIMIT 6 + – – 1600mA LEDL1 LED DETECT LEDL3 LED DETECT LOW POWER LED BANK 200mA 7 AB PWM COMPARATOR CD PWM COMPARATOR LOGIC OT UV – 8 – LEDL5 LED DETECT 1MHz OSCILLATOR VC LEDL4 LED DETECT + + 15 MAIN ERROR AMP – VBIAS VFB SAFETY ERROR AMP 1.23V – VOUT 327k + + 123k SOFT-START CLAMP 1.23V 800mV ISETL + LOW POWER LED CURRENT SETTING AMP IMAXL 256 RISETL 8 LEVELS EXPONENTIAL BRIGHTNESS CONTROL – 3 SDL 10 SDL ENL 2 SHUTDOWN CIRCUIT ENH 14 800mV ISETH + LEDH1 LED DETECT SHUTDOWN HIGH POWER LED BANK 12 HIGH POWER LED CURRENT SETTING AMP SDH LEDH2 LED DETECT IMAXH 768 13 RISETH – SDH 9 GND 11 GND 18 PGND 21 EXPOSED PAD 3452 BD 3452f 7 LTC3452 U OPERATIO Buck-Boost DC-DC Converter Buck Mode (VIN > VOUT) The LTC3452 employs an LTC proprietary buck-boost DC/DC converter to generate the output voltage required to drive the LEDs. This architecture permits high-efficiency, low noise operation at input voltages above, below or equal to the output voltage by properly phasing four internal power switches. The error amp output voltage on the VC pin determines the duty cycle of the switches. Since the VC pin is a filtered signal, it provides rejection of frequencies well below the factory trimmed switching frequency of 1MHz. The low RDS(ON), low gate charge synchronous switches provide high frequency pulse width modulation control at high efficiency. Schottky diodes across synchronous rectifier switch B and synchronous rectifier switch D are not required, but if used, do provide a lower voltage drop during the break-before-make time (typically 20ns), which improves peak efficiency by typically 1% to 2% at higher loads. In buck mode, switch D is always on and switch C is always off. Referring to Figure 2, when the control voltage VC is above voltage V1, switch A begins to turn on each cycle. During the off time of switch A, synchronous rectifier switch B turns on for the remainder of the cycle. Switches A and B will alternate conducting similar to a typical synchronous buck regulator. As the control voltage increases, the duty cycle of switch A increases until the maximum duty cycle of the converter in buck mode reaches DCBUCK|max given by: Figure 1 shows a simplified diagram of how the four internal power switches are connected to the inductor, VIN = PVIN, VOUT and GND. Figure 2 shows the regions of operation of the buck-boost as a function of the control voltage VC. The output switches are properly phased so transitions between regions of operation are continuous, filtered and transparent to the user. When VIN approaches VOUT, the buck-boost region is reached where the conduction time of the four switch region is typically 150ns. Referring to Figures 1 and 2, the various regions of operation encountered as VC increases will now be described. DCBUCK|max = 100% – DC4SW where DC4SW equals the duty cycle in % of the “four switch” range. DC4SW = (150ns • f) • 100% where f is the operating frequency in Hz. Beyond this point the “four switch” or buck-boost region is reached. Buck-Boost or Four-Switch Mode (VIN ≈ VOUT) Referring to Figure 2, when the control voltage VC is above voltage V2, switch pair AD continue to operate for duty cycle DCBUCK|max, and the switch pair AC begins to phase in. As switch pair AC phases in, switch pair BD phases out accordingly. When the VC voltage reaches the edge of the buck-boost range at voltage V3, switch pair AC completely phases out switch pair BD and the boost region begins at 75% DMAX BOOST PVIN VOUT 20 16 SW1 SW2 19 17 NMOS B A ON, B OFF BOOST REGION PWM CD SWITCHES DMIN BOOST PMOS D PMOS A V4 (≈2.1V) DMAX BUCK V3 (≈1.65V) FOUR SWITCH PWM BUCK/BOOST REGION V2 (≈1.55V) D ON, C OFF PWM AB SWITCHES BUCK REGION NMOS C 3452 F01 Figure 1. Simplified Diagram of Internal Power Switches V1 (≈0.9V) 0% DUTY CYCLE 3452 F02 CONTROL VOLTAGE, VC Figure 2. Switch Control vs Control Voltage, VC 3452f 8 LTC3452 U OPERATIO duty cycle DC4SW. The input voltage VIN where the four switch region begins is given by: VIN = VOUT 1 – (150ns • f) and the input voltage VIN where the four switch region ends is given by: [ ] VIN = VOUT • 1 – (150ns • f) Boost Mode (VIN < VOUT) In boost mode, switch A is always on and switch B is always off. Referring to Figure 2, when the control voltage VC is above voltage V3, switches C and D will alternate conducting similar to a typical synchronous boost regulator. The maximum duty cycle of the converter is limited to 88% typical and is reached when VC is above V4. Forward Current Limit If the current delivered from VIN through PMOS switch A exceeds 1600mA (typical), switch A is shut off immediately. Switches B and D are turned on for the remainder of the cycle in order to safely discharge the forward inductor current at the maximum rate possible. Reverse Current Limit If the current delivered from VOUT backwards through PMOS switch D exceeds 200mA (typical), switch D is shut off immediately. Switches A and C are turned on for the remainder of the cycle in order to safely discharge the reverse inductor current at the maximum rate possible. Undervoltage Lockout To prevent operation of the power switches at high RDS(ON), an undervoltage lockout is incorporated on the LTC3452. When the input supply voltage drops below approximately 1.9V, the four power switches and all control circuitry are turned off except for the undervoltage block, which draws only a few microamperes. Overtemperature Protection If the junction temperature of the LTC3452 exceeds 130°C for any reason, all four switches are shut off immediately. The overtemperature protection circuit has a typical hysteresis of 11°C. Soft-Start The LTC3452 includes an internally fixed soft-start which is active when powering up or coming out of shutdown. The soft-start works by clamping the voltage on the VC node and gradually releasing it such that it requires 650µs to linearly slew from 0.9V to 2.1V. This has the effect of limiting the rate of duty cycle change as VC transitions from the buck region through the buck-boost region into the boost region. Once the soft-start times out, it can only be reset by entering shutdown, or by an undervoltage or overtemperature condition. Main Error Amp The main error amplifier is a transconductance amplifier with source and sink capability. The output of the main error amplifier drives a capacitor to GND at the VC pin. This capacitor sets the dominant pole for the regulation loop. (See the Applications Information section for selecting the capacitor value.) The error amp gets its feedback signal from a proprietary circuit which monitors all 7 LED current sources to determine which LED to close the regulation loop on. Safety Error Amp The safety error amplifier is a transconductance amplifier with sink only capability. In normal operation, it has no effect on the loop regulation. However, if any of the LED pins open-circuits, the output voltage will keep rising, and safety error amp will eventually take over control of the regulation loop to prevent VOUT runaway. The VOUT threshold at which this occurs is approximately 4.5V. 3452f 9 LTC3452 U OPERATIO LED Current Setting Amps The maximum forward current per LED for all LEDs in a given bank is programmed by a single external resistor to ground at the corresponding ISETL,H pin according to the following formulas: ⎛ 0.8 ⎞ ⎛ 0.8 ⎞ IMAXL = 256⎜ ⎟ ⎟ , IMAXH = 768⎜ ⎝ RISETH ⎠ ⎝ RISETL ⎠ For operation at currents below IMAXL in the low power bank, refer to the Exponential Brightness Control section and also to external circuit options given in the Applications Section. For operation at currents below IMAXH in the high power bank, refer only to the external circuit options given in the Applications Section. implemented results in “smoother” brightness and dimming control as perceived by the human eye, which is logarithmic in nature. Table 1. Low Power Bank Brightness Control ENL DUTY CYCLE (% LOGIC HIGH) LEDLx CURRENT 0% (Logic Low) 0 (Shutdown) 0% < Duty Cycle < 12.5% 1/128 • IMAXL 12.5% < Duty Cycle < 25% 1/64 • IMAXL 25% < Duty Cycle < 37.5% 1/32 • IMAXL 37.5% < Duty Cycle < 50% 1/16 • IMAXL 50% < Duty Cycle < 62.5% 1/8 • IMAXL 62.5% < Duty Cycle < 75% 1/4 • IMAXL 75% < Duty Cycle < 87.5% 1/2 • IMAXL 87.5% < Duty Cycle ≤ 100% IMAXL Shutdown Circuit LED Current Sources The shutdown circuit monitors the voltages at the ENL,H pins. Logic high on either/both inputs enables the part and logic low on both puts the part in shutdown. Since the ENL pin doubles as a PWM input for LED brightness control, an output filter in the shutdown circuit is employed to prevent the part from toggling in and out of shutdown for normal PWMing of the ENL input when ENH is low. If ENH is low, the LTC3452 is enabled immediately after a rising edge at the ENL pin, but waits 200µs (typical) after a falling edge to enter shutdown. Consequently, a minimum PWM frequency is required for smooth brightness control at currents below IMAXL. The recommended PWM frequency is 10kHz to 50kHz. Each LED pin is driven by a current source specifically designed for low dropout. The LTC3452 employs a proprietary architecture that determines which of the seven LEDs requires the largest forward voltage drop at its programmed current, and then generates a feedback voltage based on this one for closing the buck-boost regulation loop. This results in the lowest output voltage required for regulating all of the LEDs and thus the highest LED power efficiency. The voltage present at the LED pin of the “controlling LED” will be typically 130mV at 20mA (low power bank) or 250mA at 100mA (high power bank) of current. LED Detect Circuit Exponential Brightness Control (Low Power LED Bank Only) The LTC3452 implements an exponential brightness control function for the low power LED bank only in which the LEDLx current is a function of the PWM duty cycle at the ENL pin. The LED current will be equal to a fraction of IMAXL as given in Table 1. As the duty cycle (that the PWM waveform is logic high) increases linearly, the LED current will increase exponentially from 1/128th IMAXL to 128/128ths IMAXL in seven binary steps. The function If fewer than five LED outputs in the low power bank and/ or fewer than two LED outputs in the high power bank are required, unused outputs should be connected to VOUT. Each LED pin has an internal LED detect circuit that disables the output current source to save power if an output is not needed. A small current is employed to detect the presence of an LED at startup. This current is typically 10µA for the low power bank and 30µA for the high power bank. 3452f 10 LTC3452 U W U U APPLICATIO S I FOR ATIO COMPONENT SELECTION Input Capacitor Selection Inductor Selection Since the VIN pin is the supply voltage for the IC it is recommended to place at least a 2.2µF, low ESR bypass capacitor to ground. See Table 3 for a list of component suppliers. The high frequency operation of the LTC3452 allows the use of small surface mount inductors. The inductor current ripple is typically set to 20% to 40% of the maximum average inductor current. For a given ripple the inductance term in boost mode is: L> VIN(MIN)2 • ( VOUT – VIN(MIN) ) • 100% f • IOUT(MAX ) • %Ripple • VOUT 2 Table 3. Capacitor Vendor Information SUPPLIER WEB SITE AVX www.avxcorp.com Sanyo www.sanyovideo.com Taiyo Yuden www.t-yuden.com TDK www.component.tdk.com and in buck mode is: L> ( ) VOUT • VIN(MAX ) – VOUT • 100% f • IOUT(MAX ) • %Ripple • VIN(MAX ) where: f = operating frequency, Hz %Ripple = allowable inductor current ripple, % VIN(MIN) = minimum input voltage, V VIN(MAX) = maximum input voltage, V VOUT = output voltage, V IOUT(MAX) = maximum output load current For high efficiency, choose an inductor with a high frequency core material, such as ferrite, to reduce core loses. The inductor should have low ESR (equivalent series resistance) to reduce the I2R losses, and must be able to handle the peak inductor current without saturating. Molded chokes or chip inductors usually do not have enough core to support peak inductor currents >1A. To minimize radiated noise, use a toroid, pot core or shielded bobbin inductor. For the white LED application, a 4.7µH inductor value is recommended. See Table 2 for a list of component suppliers. Table 2. Inductor Vendor Information SUPPLIER WEB SITE Coilcraft www.coilcraft.com Cooper/Coiltronics www.cooperet.com Murata www.murata.com Sumida www.japanlink.com/sumida Vishay-Dale www.vishay.com Output Capacitor Selection The bulk value of the capacitor is set to reduce the ripple due to charge into the capacitor each cycle. The steady state ripple due to charge is given by: %Ripple _ Boost = %Ripple _ Buck = ( ) IOUT(MAX ) • VOUT – VIN(MIN) • 100 COUT • VOUT 2 • f % ( VIN(MAX) – VOUT ) • 100 % 8 • VIN(MAX ) • f 2 • L • COUT where COUT = output filter capacitor, F The output capacitance is usually many times larger in order to handle the transient response of the converter. For a rule of thumb, the ratio of the operating frequency to the unity-gain bandwidth of the converter is the amount the output capacitance will have to increase from the above calculations in order to maintain the desired transient response. The other component of ripple is due to the ESR (equivalent series resistance) of the output capacitor. Low ESR capacitors should be used to minimize output voltage ripple. For surface mount applications, Taiyo Yuden, TDK, AVX ceramic capacitors, AVX TPS series tantalum capacitors or Sanyo POSCAP are recommended. For the white LED application, a 4.7µF capacitor value is recommended. See Table 3 for a list of component suppliers. 3452f 11 LTC3452 U W U U APPLICATIO S I FOR ATIO Optional Schottky Diodes Schottky diodes across the synchronous switches B and D are not required, but provide a lower drop during the break-before-make time (typically 20ns) of the NMOS to PMOS transition, improving efficiency. Use a Schottky diode such as an MBRM120T3 or equivalent. Do not use ordinary rectifier diodes, since the slow recovery times will compromise efficiency. The unity-gain frequency of the error amplifier with the Type I compensation is given by: fUG = gm 2 • π • CVC where gm is the error amp transconductance (typically 1/5.2k) and CVC is the external capacitor to GND at the VC pin. For the white LED application, a 0.1µF or greater capacitor value is recommended. Closing the Feedback Loop The LTC3452 incorporates voltage mode PWM control. The control to output gain varies with operation region (Buck, Boost, Buck/Boost), but is usually no greater than 15. The output filter exhibits a double pole response given by: fFILTER _ POLE = 1 Hz 2 • π • L • COUT where COUT is the output filter capacitor. The output filter zero is given by: fFILTER _ ZERO = 1 2 • π • RESR • COUT Hz where RESR is the capacitor equivalent series resistance. A troublesome feature in Boost mode is the right-half plane zero (RHP), and is given by: 2 fRHPZ VIN = Hz 2 • π • IOUT • L • VOUT The loop gain is typically rolled off before the RHP zero frequency. A simple Type I compensation network can be incorporated to stabilize the loop but at a cost of reduced bandwidth and slower transient response. To ensure proper phase margin, the loop is required to be crossed over a decade before the LC double pole. Paralleling LED Outputs for Higher Current Two or more LED output pins can be connected together in parallel to achieve higher output current in fewer than 7 LEDs. For a very high power LED such as a LumiLED, all 7 outputs can be connected in parallel for maximum total output current, as shown in the back page application of this data sheet. Maximum LED Current As described in the Operation section, the maximum output LED currents are equal to: ⎛ 0.8 V ⎞ IMAXL = 256⎜ ⎟ ⎝ RISETL ⎠ and ⎛ 0.8 V ⎞ IMAXH = 768⎜ ⎟ ⎝ RISETH ⎠ Since the maximum LED current for the low power bank is 25mA, this sets a minimum limit on RISETL of: ⎛ 0.8 V ⎞ RMINL = 256 ⎜ = 8192Ω ⎝ 25mA ⎟⎠ Similarly, for the high power bank: ⎛ 0.8 V ⎞ = 4096Ω RMINH = 768 ⎜ ⎝ 150mA ⎟⎠ In addition, since the maximum continuous output current for the buck-boost is limited to 425mA, this may impose higher resistor value minimums if all outputs are used. 3452f 12 LTC3452 U W U U APPLICATIO S I FOR ATIO Although the LTC3452 can safely provide this current continuously, the external LED(s) may not be rated for this high a level of continuous current. Higher current levels in a single LED are generally reserved for pulsed applications, such as LED camera flash. This is accomplished by programming a high current with one or both of the RISET resistors and pulsing the appropriate enable pin or pins as shown in the back page application. VIN Varying LED Brightness Linearly Continuously variable LED brightness control can be achieved by interfacing directly to one or both of the ISET pins. Figure 3 shows four such methods employing a voltage DAC, a current DAC, a simple potentiometer or a PWM input applied to the ISETL pin for controlling the low power bank LED currents. These four techniques can be similarly applied to the ISETH pin for controlling the high power bank LED currents. VOUT ENL VIN VOUT ENL LEDL1 LTC3452 ISETL VOLTAGE DAC ISETL LEDL5 0.8V – VDAC ILED = 256 RSET RSET ≥ RMINL LEDL1 LTC3452 ILED = 256 • IDAC 0.8V RMINL IDAC ≤ CURRENT DAC VDAC LEDL5 (3a) VIN (3b) VOUT ENL VIN VOUT ENL LEDL1 LTC3452 ISETL ISETL LEDL5 0.8V ILED = 256 RMINL + RPOT RMINL LEDL1 LTC3452 RSET 100 RSET ≥ RMINL LEDL5 ILED = 256 VPWM RPOT = 256 1µF 0.8V – VPWM RSET 0.8V – (DC% • VDVCC) RSET DVCC fPWM ≥ 10kHz (3c) (3d) 3452 F03 Figure 3. Additional Brightness Control Methods: (3a) Using Voltage DAC, (3b) Using Current DAC, (3c) Using Potentiometer, (3d) Using PWM Input 3452f 13 LTC3452 U W U U APPLICATIO S I FOR ATIO Unused Outputs If fewer than 7 LED pins are to be used, unused LEDx pins should be connected to VOUT. The LTC3452 senses which current source outputs are not being used and shuts off the corresponding output currents to save power. A small trickle current (10µA: low power bank, 30µA: high power bank) is still applied to unused outputs to detect if a white LED is later switched in and also to distinguish unused outputs from used outputs during start-up. LED Failure Modes If an individual LED fails as a short circuit, the current source biasing it is shut off to save power. This is the same operation as described previously (if the output were initially designated unused at power-up by connecting its LEDx pin to VOUT). Efficiency is not materially affected. If an individual LED fails as an open circuit, the control loop will initially attempt to regulate off of its current source feedback signal, since it will appear to be the one requiring the largest forward voltage drop to run at its programmed current. This will drive VOUT higher. As the open circuited LED will never accept its programmed current, VOUT must be voltage-limited by means of a secondary control loop. The LTC3452 limits VOUT to 4.5V in this failure mode. The other LEDs will still remain biased at the correct programmed current but the overall circuit efficiency will decrease. 3452f 14 LTC3452 U PACKAGE DESCRIPTIO UF Package 20-Lead Plastic QFN (4mm × 4mm) (Reference LTC DWG # 05-08-1710) 0.70 ±0.05 4.50 ± 0.05 3.10 ± 0.05 2.45 ± 0.05 (4 SIDES) PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS BOTTOM VIEW—EXPOSED PAD 4.00 ± 0.10 (4 SIDES) 0.75 ± 0.05 R = 0.115 TYP PIN 1 NOTCH R = 0.30 TYP 19 20 0.38 ± 0.10 PIN 1 TOP MARK (NOTE 6) 1 2 2.45 ± 0.10 (4-SIDES) (UF20) QFN 10-04 0.200 REF 0.00 – 0.05 0.25 ± 0.05 0.50 BSC NOTE: 1. DRAWING IS PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGD-1)—TO BE APPROVED 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 3452f Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 15 LTC3452 U TYPICAL APPLICATIO 4 × 20mA White LED Display + 2 × 150mA Camera Light Driver L 4.7µH VIN 3V TO 5.5V 2.2µF VIN PVIN SW1 SW2 150mA 150mA D1 D2 VOUT 4.7µF ENH ENH ISETH LEDH1 4.02k LEDH2 D3 LOW POWER LED BANK LEDL1, 20mA 1MHz BUCK/BOOST VC 0.1µF ENL D4 LEDL2, 20mA D5 LEDL3, 20mA ENL LEDL4, 20mA LTC3452 D6 LEDL5, UNUSED ISETL 10.2k GND GND PGND EXPOSED PAD D1, D2: AOT 2015 D3-D6: NICHIA NSCW100 L: COILCRAFT D03314-472 3452 TA02a RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1618 Constant Current, Constant Voltage 1.4MHz, High Efficiency Boost Regulator VIN: 1.6V to 18V, VOUT(MAX) = 34V, IQ = 1.8mA, ISD = <1µA, MS10 Package/EDD Package LT1930/LT1930A 1A (ISW), 1.2MHz/2.2MHz, High Efficiency Step-Up DC/DC Converter VIN: 2.6V to 16V, VOUT(MAX) = 34V, IQ = 4.2mA/5.5mA, ISD = <1µA, ThinSOT Package LT1932 Constant Current, 1.2MHz, High Efficiency White LED Boost Regulator VIN: 1V to 10V, VOUT(MAX) = 34V, IQ = 1.2mA, ISD = <1µA, ThinSOT Package LT1937 Constant Current, 1.2MHz, High Efficiency White LED Boost Regulator VIN: 2.5V to 10V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD = <1µA, ThinSOT Package/SC70 Package LTC3205 High Efficiency, Multi-Display LED Controller VIN: 2.8V to 4.5V, VOUT(MAX) = 6V, IQ = 50µA, ISD = <1µA, QFN-24 Package LTC3215 700mA Low Noise High Current LED Charge Pump VIN: 2.9V to 4.4V, VOUT(MAX) = 5.5V, IQ = 300µA, ISD = <2.5µA, DFN Package LTC3216 1A Low Noise High Current LED Charge Pump with Independent Flash/Torch Current VIN: 2.9V to 4.4V, VOUT(MAX) = 5.5V, IQ = 300µA, ISD = <2.5µA, DFN Package LTC3440/ LTC3441 600mA/1.2A IOUT, 2MHz/1MHz, Synchronous Buck-Boost DC/DC Converter VIN: 2.4V to 5.5V, VOUT(MAX) = 5.25V, IQ = 25µA/50µA, ISD = <1µA, MS-10 Package/DFN Package LTC3443 600mA/1.2A IOUT, 600kHz, Synchronous Buck-Boost DC/DC Converter VIN: 2.4V to 5.5V, VOUT(MAX) = 5.25V, IQ = 28µA, ISD = <1µA, DFN Package LTC3453 500mA Synchronous Buck-Boost High Power White LED Driver VIN: 2.7V to 5.5V, VOUT(MAX) = 4.5V, IQ = 600µA, ISD = 6µA, QFN-16 Package LTC3454 1A Synchronous Buck-Boost High Power White LED Driver VIN: 2.7V to 5.5V, VOUT(MAX) = 5.15V, IQ = 825µA, ISD = 0µA, DFN Package LT3465/LT3465A Constant Current, 1.2MHz/2.7MHz, High Efficiency White LED VIN: 2.7V to 16V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD = <1µA, Boost Regulator with Integrated Schottky Diode ThinSOT Package LT3466 Dual Constant Current, 2MHz, High Efficiency White LED Boost Regulator with Integrated Schottky Diode VIN: 2.7V to 24V, VOUT(MAX) = 40V, IQ = 5mA, ISD = <16µA, DFN Package LT3479 3A, Full Featured DC/DC Converter with Soft-Start and Inrush Current Protection VIN: 2.5V to 24V, VOUT(MAX) = 40V, IQ = 6.5mA, ISD = <1µA, DFN Package/TSOPP Package 3452f 16 Linear Technology Corporation LT 0406 • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2006