LTC3454 1A Synchronous Buck-Boost High Current LED Driver FEATURES DESCRIPTION n The LTC®3454 is a synchronous buck-boost DC/DC converter optimized for driving a single high power LED at currents up to 1A from a single cell Li-Ion battery input. The regulator operates in either synchronous buck, synchronous boost, or buck-boost mode depending on input voltage and LED forward voltage. PLED/PIN efficiency greater than 90% can be achieved over the entire usable range of a Li-Ion battery (2.7V to 4.2V). n n n n n n n n n High Efficiency: >90% Typical in Torch Mode, >80% in Flash Mode Wide VIN Range: 2.7V to 5.5V Up to 1A Continuous Output Current 3.5% LED Current Programming Accuracy Internal Soft-Start Open/Shorted LED Protection Constant Frequency 1MHz Operation Zero Shutdown Current Overtemperature Protection Small Thermally Enhanced 10-Lead (3mm × 3mm) DFN Package LED current is programmable to one of four levels, including shutdown, with dual external resistors and dual enable inputs. In shutdown no supply current is drawn. A high constant operating frequency of 1MHz allows the use of small external components. The LTC3454 is offered in a low profile (0.75mm) thermally enhanced 10-lead (3mm × 3mm) DFN package. APPLICATIONS n n n n n Cell Phone Camera Flash Cell Phone Torch Lighting Digital Cameras PDAs Misc Li-Ion LED Drivers L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. TYPICAL APPLICATION High Efficiency Torch/Flash LED Driver VIN 2.7V TO 4.2V 1-CELL Li-Ion LED Power Efficiency vs VIN L1 5μH 10μF VIN SW1 SW2 10μF VOUT 100 ILED 95 VC D B C LED LED EN2 EN1 ILED 0 0 0 (SHUTDOWN) 0 1 150mA EN1 (TORCH) 1 0 850mA 1 1 1A ISET1 ISET2 0.1μF LTC3454 LED: LUMILEDS LXL-PWF1 L1: SUMIDA CDRH6D28-5RONC GND (EXPOSED PAD) RISET2 3.65k 1% 3454 TA01a 85 ILED = 1A 80 75 70 EN2 (FLASH) 1MHz BUCK-BOOST ILED = 150mA 90 EFFICIENCY (%) A RISET1 20.5k 1% 65 TA = 25°C EFFICIENCY = (VOUT – VLED)ILED/VINIIN 60 2.7 3.1 3.5 3.9 4.3 4.7 5.1 VIN (V) 5.5 3454 TA01b 3454fa 1 LTC3454 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) VIN, SW1, SW2, VOUT Voltage ...................... –0.3V to 6V VC, EN1, EN2, ISET1, ISET2 Voltage.............................–0.3V to (VIN + 0.3V) or 6V LED Peak Current ...................................................1.25A Storage Temperature Range...................–65°C to 125°C Operating Temperature Range (Note 2)....–40°C to 85°C Junction Temperature (Note 3) ............................. 125°C TOP VIEW 10 SW1 EN1 1 EN2 2 ISET1 3 ISET2 4 7 VOUT LED 5 6 SW2 9 VIN 11 8 VC DD PACKAGE 10-LEAD (3mm s 3mm) PLASTIC DFN TJMAX = 125°C, θJA = 40°C/W EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE LTC3454EDD#PBF LTC3454EDD#TRPBF LBQX 10-Lead (3mm × 3mm) Plastic DFN –40°C to 85°C Consult LTC Marketing for parts specified with wider operating temperature ranges. Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 3.6V, RISET = 20.5k unless otherwise noted. (Note 2) PARAMETER CONDITIONS MIN l Input Supply Voltage (VIN) Input DC Supply Current Normal Operation Shutdown UVLO (Typicals at VIN = 3.6V, RISET1 = RISET2 = 20.5k) 2.7V ≤ VIN ≤ 5.5V (Note 4) 2.7V ≤ VIN ≤ 5.5V, VEN1 = VEN2 = 0V VIN < UVLO Threshold, VEN1 = VEN2 = VIN l Undervoltage Lockout Threshold VIN Rising VIN Falling l TYP 2.7 1.75 VEN1, VEN2 DC Threshold for Normal Operation (VIH) l VEN1, VEN2 DC Threshold for Shutdown (VIL) l 0.2 VEN1, VEN2 Input Current l –1 MAX UNITS 5.5 V 825 0 5 1200 1 10 μA μA μA 2.05 1.90 2.3 V V 0.68 1.2 V 1 μA 0.66 V ISET1 and ISET2 Servo Voltage 3.08k ≤ RISET1||RISET2 ≤ 20.5k l 780 788 800 800 812 812 mV mV LED Output Current to Programming Current Ratio ILED/(IISET1 + IISET2), ILED = 500mA (Note 5) l 3725 3775 3850 3850 3975 3925 mA/mA mA/mA LED Pin Voltage ILED = 1A Regulated Maximum VOUT LED Pin Open, Programmed ILED = 1A PMOS Switch RON Switches A and D (VOUT = 3.6V) 170 mΩ NMOS Switch RON Switches B and C 130 mΩ Forward Current Limit Switch A Reverse Current Limit Switch D (VOUT = 3.6V) PMOS Switch Leakage Switches A, D –1 1 NMOS Switch Leakage Switches B, C –1 1 105 l 4.95 2.5 5.15 mV 5.35 V 3.4 A 275 mA μA μA 3454fa 2 LTC3454 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 3.6V, RISET = 20.5k unless otherwise noted. (Note 2) PARAMETER CONDITIONS Oscillator Frequency MIN TYP MAX UNITS 0.9 1.0 1.15 MHz Soft-Start Time 200 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 LTC3454 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. μs 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: This parameter is tested using a feedback loop which servos VC to 1.8V. TYPICAL PERFORMANCE CHARACTERISTICS Undervoltage Lockout Threshold vs Temperature 2.4 1200 2.3 1100 VIN RISING 2.0 1.9 1.8 VIN FALLING 1.7 1.6 Enable Thresholds vs VIN 1200 VIN = 3.6V 1000 900 800 700 VIH 600 VIL 500 400 1.5 300 1.4 –55 –35 –15 200 –55 –35 –15 5 25 45 65 85 105 125 TEMPERATURE (°C) 800 800 500 400 200 2.7 TA = 25°C RISET1 = RISET2 = 15k 808 808 804 804 800 3.9 4.3 VIN (V) 4.7 5.1 792 792 792 788 2.7 5.5 VIN = 3.6V TA = 25°C 800 796 3454 G04 3.5 ISET1,2 Servo Voltage vs RISET 812 796 5 25 45 65 85 105 125 TEMPERATURE (°C) 3.1 3454 G03 796 788 –55 –35 –15 VIL 600 5 25 45 65 85 105 125 TEMPERATURE (°C) VISET1,2 (mV) VIN = 3.6V RISET1,2 = 15k 804 VIH 700 ISET1,2 Servo Voltage vs VIN 812 VISET1,2 (mV) VISET1,2 (mV) 900 3454 G02 ISET1,2 Servo Voltage vs Temperature 808 1000 300 3454 G01 812 TA = 25°C 1100 ENABLE THRESHOLDS (mV) 2.1 ENABLE THRESHOLDS (mV) 2.2 UVLO THRESHOLD (V) Enable Thresholds vs Temperature 788 3.1 3.5 3.9 4.3 VIN (V) 4.7 5.1 5.5 3454 G05 3 7 11 19 15 RISET (kΩ) 23 27 31 3454 G06 3454fa 3 LTC3454 TYPICAL PERFORMANCE CHARACTERISTICS LED Current Programming Ratio vs VIN 4050 VIN = 3.6V 4000 4000 3950 3950 3900 3900 RATIO RATIO 4050 3850 3800 150 PROGRAMMED ILED = 500mA TA = 25°C PROGRAMMED ILED = 1A 3850 90 PROGRAMMED ILED = 500mA 60 3750 3650 –55 –35 –15 30 PROGRAMMED ILED = 1A PROGRAMMED ILED = 500mA PROGRAMMED ILED = 150mA 3700 PROGRAMMED ILED = 100mA 3700 5 25 45 65 85 105 125 TEMPERATURE (°C) 3650 2.7 3.1 3.5 3.9 4.3 VIN (V) 4.7 3454 G07 5.1 0 –55 –35 –15 5.5 3454 G09 Maximum Regulated VOUT vs Temperature 60 Maximum Regulated VOUT vs VIN PROGRAMMED ILED = 500mA 58 TA = 25°C 5.40 5.40 56 5.30 5.30 54 5.25 5.25 52 5.20 5.20 PROGRAMMED ILED = 1A 5.35 TA = 25°C 48 VOUT (V) VOUT (V) PROGRAMMED ILED = 1A 5.35 VIN = 3.6V 50 5.15 5.15 5.10 5.10 46 5.05 5.05 44 5.00 5.00 42 4.95 4.95 40 4.90 –55 –35 –15 4.90 2.7 2.7 3.1 3.5 3.9 4.3 VIN (V) 4.7 5.1 5.5 5 25 45 65 85 105 125 TEMPERATURE (°C) 3454 G10 VIN = 3.6V 5.35 TA = 25°C MEASURED AT 500mA RDS (mΩ) 210 VIN = 2.7V VIN = 3.6V 180 150 5.00 VIN = 5.5V 5.5 MEASURED AT 500mA 90 –55 –35 –15 140 VIN = 3.6V VIN = 5.5V 120 VIN = 4.2V 100 80 VIN = 4.2V 120 3454 G13 5.1 VIN = 2.7V RDS (mΩ) 5.25 4.90 100 200 300 400 500 600 700 800 900 1000 PROGRAMMED ILED (mA) 4.7 160 240 4.95 4.3 VIN (V) 180 5.30 5.05 3.9 NMOS RDS(ON) vs Temperature 200 270 5.10 3.5 3454 G12 PMOS RDS(ON) vs Temperature 300 5.40 5.15 3.1 3454 G11 Maximum Regulated VOUT vs Programmed LED Current 5.20 5 25 45 65 85 105 125 TEMPERATURE (°C) 3454 G08 VLED vs VIN VOUT (V) VIN = 3.6V 120 3800 3750 VLED (mV) VLED vs Temperature VLED (mV) LED Current Programming Ratio vs Temperature 60 5 25 45 65 85 105 125 TEMPERATURE (°C) 3454 G14 40 –55 –35 –15 5 25 45 65 85 105 125 TEMPERATURE (°C) 3454 G15 3454fa 4 LTC3454 TYPICAL PERFORMANCE CHARACTERISTICS Oscillator Frequency vs Temperature 1080 100 VOUT = 3V 95 FREQUENCY (kHz) 1060 VIN = 5.5V 1040 VIN = 4.2V 1020 1000 VIN = 2.7V 980 VIN = 3.6V 960 90 EFFICIENCY (%) 1100 LED Power Efficiency vs LED Current 85 80 75 70 940 920 900 –55 –35 –15 5 25 45 65 85 105 125 TEMPERATURE (°C) VIN = 3.6V TA = 25°C 65 EFFICIENCY = (VOUT – VLED)ILED/VINIIN FRONT PAGE APPLICATION 60 100 200 300 400 500 600 700 800 900 1000 ILED (mA) 3454 G17 3454 G16 Output Voltage Ripple Back Page Application Start-Up Transient Back Page Application CH1, VOUT 1V/DIV CH2, ILED 500mA FINAL VALUE 20mV/DIV 0V, 0A CH3, VEN1 1V/DIV 0V VIN = 3.6V ILED = 500mA 500ns/DIV 3454 G19 VIN = 3.6V ILED = 500mA 5ms/DIV 3454 G19 PIN FUNCTIONS EN1 (Pin 1): Enable Input Pin for ISET1 Current. EN2 (Pin 2): Enable Input Pin for ISET2 Current. ISET1 (Pin 3): LED Current Programming Pin. A resistor to ground programs the current through the LED to ILED = 3850(0.8V/RISET1). This amount of current adds to any amount set by EN2/ISET2 if used. ISET2 (Pin 4): LED Current Programming Pin. A resistor to ground programs the current through the LED to ILED = 3850(0.8V/RISET2). This amount of current adds to any amount set by EN1/ISET1 if used. LED (Pin 5): Low Dropout Output for LED Current Biasing. Connect the LED between VOUT and the LED pin. SW2 (Pin 6): Switching Node. External inductor connects between SW1 and SW2. Recommended value is 4.7μH/5μH. VOUT (Pin 7): Buck-Boost Output Rail. Bypass to GND with a ceramic capacitor. Recommended value is 10μF. VC (Pin 8): Compensation Point for the Internal Error Amplifier Output. Connect a ceramic capacitor from VC to GND. Recommended value is 0.1μF. VIN (Pin 9): Voltage Input Supply Pin (2.7V ≤ VIN ≤ 5.5V). Bypass to GND with a ceramic capacitor. Recommended value is 10μF. SW1 (Pin 10): Switching Node. External inductor connects between SW1 and SW2. Recommended value is 4.7μH/5μH. Exposed Pad (Pin 11): Ground Pin. Solder to PCB ground for electrical contact and optimal thermal performance. 3454fa 5 LTC3454 BLOCK DIAGRAM OPTIONAL 10 SW1 SWITCH A GATE DRIVERS AND ANTICROSSCONDUCTION SWITCH B UNDERVOLTAGE LOCKOUT UV 7 REVERSE CURRENT LIMIT OT BANDGAP REFERENCE – – 3.4A 275mA 1.23V LOGIC + – – AB PWM COMPARATOR CD PWM COMPARATOR UV 8 VOUT SWITCH D SWITCH C FORWARD CURRENT LIMIT + OVERTEMP PROTECTION 6 SW2 + 9 VIN + VIN 2.7V TO 5.5V OPTIONAL OT 1MHz OSCILLATOR VC VOUT 1.23V – SAFETY ERROR AMP AUTOZEROING ERROR AMP LED – 5 377k R + + 123k SOFTSTART CLAMP 1.23V 800mV + LED CURRENT SETTING AMP 1 CURRENT MIRROR IISET1 – 3 ∑ ISET1 I RISET1 800mV + 3850 I R LED CURRENT SETTING AMP 2 IISET2 – 4 RISET2 ISET2 EN1 1 EN2 2 SHUTDOWN 11 EXPOSED PAD (GND) 3454 BD 3454fa 6 LTC3454 OPERATION Buck-Boost DC/DC Converter The LTC3454 employs an LTC proprietary buck-boost DC/DC converter to generate the output voltage required to drive a high current LED. This architecture permits highefficiency, 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. Figure 1 shows a simplified diagram of how the four internal power switches are connected to the inductor, VIN, 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. Buck Mode (VIN > VOUT) 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 VIN VOUT 9 7 SW1 10 NMOS B 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 4-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 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 – DC4SW) = VOUT • [1 – (150ns • f)] 75% DMAX BOOST V4 (2.1V) A ON, B OFF BOOST REGION PWM CD SWITCHES DMIN BOOST PMOS D PMOS A 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: SW2 DMAX BUCK V3 (1.65V) FOUR SWITCH PWM BUCK/BOOST REGION V2 (1.55V) D ON, C OFF PWM AB SWITCHES BUCK REGION 6 NMOS C 3454 F01 Figure 1. Simplified Diagram of Internal Power Switches V1 (0.9V) 0% DUTY CYCLE 3454 F02 CONTROL VOLTAGE, VC Figure 2. Switch Control vs Control Voltage, VC 3454fa 7 LTC3454 APPLICATIONS INFORMATION 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 3.4A (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 275mA (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 LTC3454. When the input supply voltage drops below approximately 1.90V, the four power switches and all control circuitry are turned off except for the undervoltage block, which draws a few microamperes. Overtemperature Protection If the junction temperature of the LTC3454 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 LTC3454 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 200μs 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. Autozero Error Amp The error amplifier is an autozeroing transconductance amp with source and sink capability. The output of this 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 feedback signal to the error amp is developed across a resistor through which LED current flows. 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 the LED pin opencircuits, the output voltage will keep rising, and the 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 5.15V. LED Current Programming and Enable Circuit Two enable pins work in conjunction with dual external resistors to program LED current to one of three nonzero settings. The table below explains how the current can be set. EN1 EN2 ILOAD (A) GND GND 0 (SHUTDOWN) VIN GND 3850 • 0.8V/RISET1 GND VIN 3850 • 0.8V/RISET2 VIN VIN 3850 • (0.8V/RISET1 + 0.8V/RISET2) With either enable pin pulled high, the buck-boost will regulate the output voltage at the current programmed by RISET1 and/or RISET2. With both enable pins pulled to GND, the LTC3454 is in shutdown and draws zero current. The enable pins are high impedance inputs and should not be floated. 3454fa 8 LTC3454 APPLICATIONS INFORMATION COMPONENT SELECTION Inductor Selection The high frequency operation of the LTC3454 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 and in buck mode is: ( VIN(MAX) – VOUT ) • VOUT • 100% L> f • VIN(MAX ) • %Ripple • IOUT where f = operating frequency, Hz %Ripple = allowable inductor current ripple, % Input Capacitor 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 2 for a list of component suppliers. Table 2. 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 Output Capacitor Selection The bulk value of the capacitor is set to reduce the ripple due to charge into the capacitor each cycle. The steadystate ripple due to charge is given by: %Ripple _ Boost = 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 white LED application, a 4.7μH/5μH inductor value is recommended. See Table 1 for a list of component suppliers. Table 1. Inductor Vendor Information SUPPLIER WEB SITE Coilcraft www.coilcraft.com %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 operating frequency to unitygain bandwidth of the converter is the amount the output capacitance will have to increase from the above calculations in order to maintain desired transient response. The other component of ripple is due to 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 10μF capacitor value is recommended. See Table 2 for a list of component suppliers. Cooper/Coiltronics www.cooperet.com Optional Schottky Diodes Murata www.murata.com Sumida www.japanlink.com/sumida Toko www.toko.com Vishay-Dale www.vishay.com 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 3454fa 9 LTC3454 TYPICAL APPLICATIONS diode such as an MBRM120T3 or equivalent. Do not use ordinary rectifier diodes, since the slow recovery times will compromise efficiency. and slower transient response. To ensure proper phase margin, the loop is required to be crossed over a decade before the LC double pole. In applications in which VIN is greater than 4V and VOUT to GND short-circuit protection is needed, a Schottky diode such as MBRM120T3 or equivalent may be used from GND to SW1 and/or a 2Ω/1nF series snubber from SW1 to GND. The Schottky diode should be added as close to the pins as possible. Neither of these is required for shorted LED protection. The unity-gain frequency of the error amplifier with the Type I compensation is given by: gm fUG = 2 • π • CVC In applications in which VIN is greater than 4.5V, a Schottky diode such as MBRM120T3 or equivalent may be required from SW1 to VIN if the LTC3454 is enabled with an output voltage already present. The Schottky diode should be added as close to the pins as possible. Closing the Feedback Loop The LTC3454 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: 1 fFILTER _ POLE = 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 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. Maximum LED Current As described in the Operation section, the output LED current with both enable pins logic high is equal to ILED = 3850 [0.8V/(RISET1 || RISET2)] Since the maximum continuous output current is limited to 1A, this sets a minimum limit on the parallel combination of RISET1 and RISET2 equal to RMIN = (RISET1 || RISET2)|MIN = 3850(0.8V/1A) = 3080Ω Although the LTC3454 can safely provide this current continuously, the external LED may not be rated for this high a level of continuous current. Higher current levels are generally reserved for pulsed applications, such as LED camera flash. This is accomplished by programming a high current with one of the RISET resistors and pulsing the appropriate enable pin. Varying LED Brightness 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. It is not recommended to control brightness by PWMing the enable pins directly as this will toggle the LTC3454 in and out of shutdown and result in erratic operation. 3454fa 10 LTC3454 APPLICATIONS INFORMATION LED Failure Modes If the LED fails as an open circuit, the safety amplifier takes control of the regulation loop to prevent VOUT runaway. The VOUT threshold at which this occurs is about 5.15V. The safety amplifier has no effect on loop regulation at VOUT less than 5.15V. VIN If the LED fails as a short-circuit, the current limiting circuitry detects this condition and limits the peak input current to a safe level. VOUT VIN VOUT ENx ENx LTC3454 ISETx LTC3454 ISETx LED LED ILED = 3850 RSET ≥ RMIN VOLTAGE DAC 0.8V – VDAC RSET IDAC ≤ CURRENT DAC VDAC ILED = 3850 • IDAC 0.8V RMIN (3a) (3b) VIN VOUT VIN VOUT ENx ENx LTC3454 ISETx LTC3454 ISETx LED LED RMIN ILED = 3850 0.8V RMIN + RPOT RSET 100 ILED = 3850 RSET ≥ RMIN VPWM RPOT = 3850 0.8V – VPWM RSET 0.8V – (DC% • VDVCC) RSET DVCC fPWM ≥ 10kHz (3c) (3d) 3454 F03 Figure 3. Brightness Control Methods: (a) Using Voltage DAC, (b) Using Current DAC, (c) Using Potentiometer, (d) Using PWM Input PACKAGE DESCRIPTION DD Package 10-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1699) R = 0.115 TYP 6 0.38 p 0.10 10 0.675 p0.05 3.00 p0.10 (4 SIDES) 3.50 p0.05 1.65 p0.05 2.15 p0.05 (2 SIDES) PACKAGE OUTLINE 1.65 p 0.10 (2 SIDES) PIN 1 TOP MARK (SEE NOTE 6) (DD) DFN 1103 5 0.200 REF 0.25 p 0.05 0.50 BSC 2.38 p0.05 (2 SIDES) 1 0.25 p 0.05 0.50 BSC 0.75 p0.05 2.38 p0.10 (2 SIDES) 0.00 – 0.05 BOTTOM VIEW—EXPOSED PAD RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS NOTE: 1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2). CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT 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 3454fa 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. 11 LTC3454 TYPICAL APPLICATION 500mA LED Flashlight Driver L1 4.7μH 3-CELL ALKALINE 4.5V VIN 2.2μF LED Power Efficiency vs VIN SW2 SW1 VOUT 4.7μF 100 ILED = 500mA 95 EN1 SWD SWB LED 90 EFFICIENCY (%) SWA LED SWC EN2 VC 80 75 70 1MHz BUCK-BOOST ISET1 ISET2 0.1μF LTC3454 GND (EXPOSED PAD) LED: LUMILEDS, LXCL LW3C L1: TOKO A997AS-4R7M 85 RISET1 6.19k 1% 3453 TA02 65 ILED = 500mA TA = 25°C EFFICIENCY = (VOUT – VLED)ILED/VINIIN 60 2.7 3.1 3.5 3.9 4.3 VIN (V) 4.7 5.5 5.1 3454 TA02b RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1618 Constant Current, Constant Voltage 1.4MHz, High Efficiency Boost Regulator LT1930/LT1930A 1A (ISW), 1.2MHz/2.2MHz, High Efficiency Step-Up DC/DC Converter LT1932 Constant Current, 1.2MHz, High Efficiency White LED Boost Regulator LT1937 Constant Current, 1.2MHz, High Efficiency White LED Boost Regulator LTC3205 High Efficiency, Multi-Display LED Controller LTC3215 700mA Low Noise Charge Pump LED Driver LTC3216 LTC3490 1A Low Noise High Current Charge Pump LED Driver with Independent Flash/Torch Current 600mA/1.2A IOUT, 2MHz/1MHz, Synchronous Buck-Boost DC/DC Converter 600mA/1.2A IOUT, 600kHz, Synchronous Buck-Boost DC/DC Converter Single Cell 350mA LED Driver LTC3453 Synchronous Buck-Boost High Power White LED Driver LTC3440/ LTC3441 LTC3443 LT3465/LT3465A Constant Current, 1.2MHz/2.7MHz, High Efficiency White LED Boost Regulator with Integrated Schottky Diode LT3466 Dual Constant Current, 2MHz, High Efficiency White LED Boost Regulator with Integrated Schottky Diode LT3479 3A, Full Featured DC/DC Converter with Soft-Start and Inrush Current Protection VIN: 1.6V to 18V, VOUT(MAX) = 34V, IQ = 1.8mA, ISD < 1μA, MS10 Package/EDD Package VIN: 2.6V to 16V, VOUT(MAX) = 34V, IQ = 4.2mA/5.5mA, ISD < 1μA, ThinSOT Package VIN: 1V to 10V, VOUT(MAX) = 34V, IQ = 1.2mA, ISD < 1μA, ThinSOT Package VIN: 2.5V to 10V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD < 1μA, ThinSOT Package/SC70 Package VIN: 2.8V to 4.5V, VOUT(MAX) = 6V, IQ = 50μA, ISD < 1μA, QFN-24 Package VIN: 2.9V to 4.4V, VOUT(MAX) = 5.5V, IQ = 300μA, ISD < 2.5μA, DFN Package VIN: 2.9V to 4.4V, VOUT(MAX) = 5.5V, IQ = 300μA, ISD < 2.5μA, DFN Package VIN: 2.4V to 5.5V, VOUT(MAX) = 5.25V, IQ = 25μA/50μA, ISD <1 μA, MS-10 Package/DFN Package VIN: 2.4V to 5.5V, VOUT(MAX) = 5.25V, IQ = 28μA, ISD < 1μA, DFN Package VIN: 1V to 3.2V, VOUT(MAX) = 4V, IQ = 20μA, ISD = 20μA, DFN Package VIN: 2.7V to 5.5V, Up to 500mA Continuous Output Current, QFN-16 Package VIN: 2.7V to 16V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD < 1μA, ThinSOT Package VIN: 2.7V to 24V, VOUT(MAX) = 40V, IQ = 5mA, ISD < 16μA, DFN Package VIN: 2.5V to 24V, VOUT(MAX) = 40V, IQ = 6.5mA, ISD < 1μA, DFN Package/TSOPP Package 3454fa 12 Linear Technology Corporation LT 0309 REV A • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2005