LTC3453 Synchronous Buck-Boost High Power White LED Driver U FEATURES DESCRIPTIO ■ The LTC®3453 is a synchronous buck-boost DC/DC converter optimized for driving up to 4 white LEDs at a combined current of up to 500mA from a single Li-Ion battery input. 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 using a proprietary architecture that determines which LED requires the largest forward voltage drop at its programmed current, and regulates the common output rail for lowest dropout. Efficiency of 90% can be achieved over the entire usable range of a Li-Ion battery (2.7V to 4.2V). ■ ■ ■ ■ ■ ■ ■ ■ ■ High Efficiency: 90% Typical Over Entire Li-Ion Battery Range Wide VIN Range: 2.7V to 5.5V Up to 500mA Continuous Output Current Internal Soft-Start Open/Shorted LED Protection LED Current Matching Typically <2% Constant Frequency 1MHz Operation Low Shutdown Current: 6µA Overtemperature Protection Small Thermally Enhanced 16-Lead (4mm x 4mm) QFN Package U APPLICATIO S ■ ■ ■ ■ LED current is programmable to one of four levels (including shutdown) with dual current setting resistors and dual enable pins. In shutdown, the supply current is only 6µA. Cell Phones Digital Cameras PDAs Portable Devices A high constant operating frequency of 1MHz allows the use of a small external inductor. The LTC3453 is offered in a low profile (0.75mm) thermally enhanced 16-lead (4mm x 4mm) QFN package. , LTC and LT are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. U TYPICAL APPLICATIO High Efficiency Torch/Flash LED Driver L1 4.7µH VIN 1-CELL Li-Ion 2.7V to 4.2V Torch Mode Efficiency vs VIN VIN 2.2µF PVIN SW1 SW2 VOUT 150mA/500mA 4.7µF LED2 VC 0.1µF EN1 (TORCH) EN2 (FLASH) LED3 1MHz BUCK-BOOST LED4 EN1 D1: LUMILEDS LXCL-PWF1 L1: VISHAY DALE IDCS-2512 EN2 ISET1 8.25k 1% ISET2 3.48k 1% LTC3453 GND GND PGND 3453 TA01a EN1 EN2 0 1 0 1 0 0 1 1 ILED 0 (SHUTDOWN) 150mA 350mA 500mA 180 90 160 EFFICIENCY 80 140 70 120 IIN 60 INPUT CURRENT (mA) D1 LED1 LED POWER EFFICIENCY PLED/PIN (%) 100 100 ILED = 150mA TA = 25°C 50 2.7 3.1 3.5 80 3.9 4.3 VIN (V) 4.7 5.1 5.5 3453 TA01b 3453fa 1 LTC3453 U W W W ABSOLUTE AXI U RATI GS U W U PACKAGE/ORDER I FOR ATIO (Note 1) VOUT SW2 PVIN SW1 TOP VIEW VIN, PVIN, SW1, SW2, VOUT Voltage ............ –0.3V to 6V LED1 to LED4 Voltage ...... –0.3V to (VOUT + 0.3V) or 6V VC, EN1, EN2, ISET1, ISET2 Voltage .......... –0.3V to (VIN + 0.3V) or 6V LED1 to LED4 Peak Current ................................ 250mA Storage Temperature Range .................. –65°C to 125°C Operating Temperature Range (Note 2) ... –40°C to 85°C Junction Temperature (Note 3) ............................. 125°C 16 15 14 13 VIN 1 12 VC EN1 2 11 EN2 17 ISET1 3 10 ISET2 6 7 8 GND LED3 GND 9 LED4 5 LED2 LED1 4 UF PACKAGE 16-LEAD (4mm × 4mm) PLASTIC QFN TJMAX = 125°C, θJA = 40°C/W, θJC = 2.6°C/W EXPOSED PAD (PIN 17) IS PGND, MUST BE SOLDERED TO PCB ORDER PART NUMBER LTC3453EUF UF PART MARKING 3453 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 the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C, VIN = VOUT = 3.6V unless otherwise noted. (Note 2) PARAMETER CONDITIONS ● Input Supply Voltage Input DC Supply Current Normal Operation Shutdown UVLO Undervoltage Lockout Threshold MIN MAX UNITS 5.5 V 0.6 6 3 1 18 5 mA µA µA 2 1.9 2.3 1.6 V V 0.65 1 0.63 2.7 2.7V ≤ VIN ≤ 5.5V, RISET1||RISET2 = 51.1k, ILEDx = 0 (Note 4) 2.7V ≤ VIN ≤ 5.5V; VEN1 = VEN2 = 0V VIN < UVLO Threshold VIN Rising VIN Falling ● 2.7V ≤ VIN ≤ 5.5V, VEN1,2 Rising ● EN1,2 DC Threshold for Shutdown (ILEDx = 0) 2.7V ≤ VIN ≤ 5.5V, VEN1,2 Falling ● 0.2 EN1,2 Input Current VEN1,2 = 3.6V ● –1 ISET1,2 Servo Voltage RISET1,2 = 4.12k, 0°C ≤ TA ≤ 85°C RISET1,2 = 4.12k, –40°C ≤ TA ≤ 85°C ● 788 780 ILED/(ISET1 + ISET2), ILEDx = 75mA, VLEDx = 300mV, 2.7V ≤ VIN ≤ 5.5V ● 365 357 EN1,2 DC Threshold for Normal Operation LED Output Current Ratio TYP V V 1 µA 800 800 812 812 mV mV 384 384 403 403 mA/mA mA/mA 2 6 LED Output Current Matching (MAX – MIN)/[(MAX + MIN)/2] • 100%, ILEDx = 75mA VLEDx = 300mV LED Pin Drain Voltage ILEDx = 75mA Regulated Maximum VOUT VLEDx = 0V PMOS Switch RON Switches A and D, @ 100mA 0.3 Ω NMOS Switch RON Switches B and C, @ 100mA 0.25 Ω Forward Current Limit Switch A Reverse Current Limit Switch D 130 ● 4.4 1125 4.5 1612 200 % mV 4.6 2100 V mA mA 3453fa 2 LTC3453 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C, VIN = VOUT = 3.6V unless otherwise noted. (Note 2) PARAMETER CONDITIONS PMOS Switch Leakage Switches A and D ● MIN 1 µA NMOS Switch Leakage Switches B and C ● 1 µA Oscillator Frequency TYP 0.9 Soft-Start Time 1 MAX UNITS 1.1 MHz 0.65 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 LTC3453E 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. ms 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. U W TYPICAL PERFOR A CE CHARACTERISTICS Input DC Supply Current in Shutdown vs Temperature 20 ISET1,2 Servo Voltage vs Temperature Undervoltage Lockout Threshold vs Temperature 2.2 FRONT PAGE APPLICATION 812 18 VIN = 4.2V 12 10 VIN = 3.6V 8 VIN = 2.7V 6 UVLO THRESHOLD (V) IIN (µA) 14 808 2.1 VIN = 5.5V VIN RISING VISET1,2 (mV) 16 2.0 1.9 VIN FALLING 804 800 796 1.8 4 VIN = 3.6V RISET1,2 = 8.25k 792 2 0 –55 –35 –15 1.7 –55 –35 –15 5 25 45 65 85 105 125 TEMPERATURE (°C) 3453 G01 1050 VIN = 3.6V 4.54 ALL LED PINS OPEN VOUT (V) 804 796 4.53 1030 4.52 1020 4.51 4.50 4.49 4.48 792 3.1 3.5 3.9 4.3 VIN (V) 4.7 5.1 5.5 3453 G05 1000 980 970 960 3453 G06 VIN = 4.2V VIN = 3.6V 990 4.46 5 25 45 65 85 105 125 TEMPERATURE (°C) VIN = 5.5V 1010 4.47 4.45 –55 –35 –15 VOUT = 3V 1040 FREQUENCY (kHz) 808 VISET1,2 (mV) Oscillator Frequency vs Temperature 4.55 TA = 25°C RISET1,2 = 8.25k 788 2.7 3453 G04 Regulated Maximum VOUT vs Temperature 800 5 25 45 65 85 105 125 TEMPERATURE (°C) 3453 G02 ISET1,2 Servo Voltage vs VIN 812 788 –55 –35 –15 5 25 45 65 85 105 125 TEMPERATURE (°C) 950 –55 –35 –15 VIN = 2.7V 5 25 45 65 85 105 125 TEMPERATURE (°C) 3453 G07 3453fa 3 LTC3453 U W TYPICAL PERFOR A CE CHARACTERISTICS Efficiency vs LED Current 100 Output Voltage Ripple Startup Transient FRONT PAGE APPLICATION PLED/PIN, VIN = 3.6V, TA = 25°C CH1, VOUT 2V/DIV EFFICIENCY (%) 90 20mV/DIV AC COUPLED 80 0V CH2, EN1 1V/DIV 70 0V 60 5µs/DIV 50 100 150 200 250 300 350 400 450 500 LED CURRENT (mA) FRONT PAGE APPLICATION VIN = 3.6V ILED = 150mA 3453 G08 1ms/DIV 3453 G09 FRONT PAGE APPLICATION VIN = 3.6V ILED = 150mA 3453 G07 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 16). EN1 (Pin 2): Enable Input Pin for ISET1 Current. ISET1 (Pin 3): White Led Current Programming Pin. A resistor to ground programs each current source output to ILED = 384(0.8V/RISET1). This amount of current adds to any amount set by EN2/ISET2 if also used. LED1 to LED4 (Pins 4, 6, 7, 9): Individual Low Dropout Current Source Outputs for White LED Current Biasing. Connect each white LED between VOUT and an individual LEDx pin. Unused LEDx outputs should be connected to VOUT. GND (Pins 5 and 8): Signal Ground Pin. Connect to PGND (Exposed Pad). ISET2 (Pin 10): White Led Current Programming Pin. A resistor to ground programs each current source output to ILED = 384(0.8V/RISET2). This amount of current adds to any amount set by EN1/ISET1 if also used. EN2 (Pin 11): Enable Input Pin for ISET2 Current. VC (Pin 12): Compensation Point for the Internal Error Amplifier Output. Recommended compensation capacitor to GND is 0.1µF ceramic or larger. VOUT (Pin 13): Buck-Boost Output Pin. Recommended bypass capacitor to GND is 4.7µF ceramic. SW2 (Pin 14): Switching Node Pin. Connected to internal power switches C and D. External inductor connects between SW1 and SW2. Recommended value is 4.7µH. SW1 (Pin 15): Switching Node Pin. Connected to internal power switches A and B. External inductor connects between SW1 and SW2. Recommended value is 4.7µH. PVIN (Pin 16): Power Voltage Input Supply Pin. Connect to VIN (Pin 1). Exposed Pad (Pin 17): Power Ground Pin. Connect to GND (Pin 8) and solder to PCB ground for optimum thermal performance. 3453fa 4 LTC3453 W BLOCK DIAGRA OPTIONAL OPTIONAL 15 16 VIN 2.7V TO 5.5V 1 PVIN SWITCH A SWITCH B UV VOUT SWITCH D GATE DRIVERS AND ANTICROSSCONDUCTION VIN UNDERVOLTAGE LOCKOUT 14 SW2 SW1 13 VOUT SWITCH C LED1 4 BANDGAP REFERENCE 1.23V 1612mA LED DETECT + – – OT REVERSE CURRENT LIMIT + OVERTEMP PROTECTION FORWARD CURRENT LIMIT 200mA LED2 6 LOGIC – – AB PWM COMPARATOR CD PWM COMPARATOR UV 12 LED DETECT + + LED3 7 OT LED DETECT 1MHz OSCILLATOR VC VBIAS – MAIN ERROR AMP SAFETY ERROR AMP VOUT – LED4 9 1.23V 327k VFB + LED CURRENT SETTING AMP 1 ILED 384 800mV + – 10 6 ISET1 RISET1 RISET2 4 ∑ + – 3 OR 123k SOFT START CLAMP 1.23V 800mV LED DETECT + 7 LED CURRENT SETTING AMP 2 ILED 384 9 ISET2 EN1 2 EN2 11 SHUTDOWN 5 GND 8 GND 17 EXPOSED PAD (PGND) 3453 BD 3453fa 5 LTC3453 U OPERATIO Buck-Boost DC-DC Converter Buck Mode (VIN > VOUT) The LTC3453 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, 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 16 13 SW1 SW2 15 14 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 3453 F01 Figure 1. Simplified Diagram of Internal Power Switches V1 (≈0.9V) 0% DUTY CYCLE 3453 F02 CONTROL VOLTAGE, VC Figure 2. Switch Control vs Control Voltage, VC 3453fa 6 LTC3453 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 – DC4SW) = 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. Soft-Start The LTC3453 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 0.65ms 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 If the current delivered from VIN through PMOS switch A exceeds 1612mA (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. 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 4 LED current sources to determine which LED to close the regulation loop on. Reverse Current Limit Safety Error Amp 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. 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. Forward Current Limit Undervoltage Lockout To prevent operation of the power switches at high RDS(ON), an undervoltage lockout is incorporated on the LTC3453. 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 several microamperes. Overtemperature Protection If the junction temperature of the LTC3453 exceeds 130°C for any reason, all four switches are shut off immediately. The overtemperature protection circuit has a typical hysteresis of 11°C. LED Current Setting Amplifiers and Enable Circuit The LTC3453 includes two LED current setting amplifiers that work in conjunction with dual external current setting resistors and dual enable pins to program LED current to one of four levels (including shutdown). All four LED current source outputs are programmed to the same level. When both enable inputs are logic low, the LTC3453 is in shutdown, the buck-boost is disabled and all LED currents are zero. In shutdown, the input supply current is typically 6µA. If either enable input is logic high, the buck-boost will regulate the output voltage such that the LEDs are biased 3453fa 7 LTC3453 U OPERATIO at the current programmed by resistors RISET1 and/or RISET2. Individually enabled, each LED current setting amplifier programs the output LED current to ILED = 384 (0.8V/RISET1,2) If both enable inputs are logic high, the setting currents are summed internally and the output LED current will be given by ILED = 384 [0.8V/(RISET1 || RISET2) ] Thus three different (nonzero) current levels are programmable, optimal for low current LED torch and high current LED camera flash applications. LED Current Sources Each LED pin is driven by a current source specifically designed for low dropout. The LTC3453 employs a propri- etary architecture that determines which of the four 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 75mA of current. LED Detect Circuit If fewer than four LED outputs are required, unused ones 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 30µA current is employed to detect the presence of an LED at startup. U U W U APPLICATIO S I FOR ATIO Component Selection Inductor Selection The high frequency operation of the LTC3453 allows the use of small surface mount inductors. The inductor current ripple is typically set to 20% to 40% of the maximum inductor current. For a given ripple the inductance terms are given as follows: L> L> ( ) VIN(MIN)2 • VOUT – VIN(MIN) • 100% f • IOUT(MAX) • %Ripple • VOUT2 ( , ) VOUT • VIN(MAX) – VOUT • 100% f • IOUT(MAX) • %Ripple • VIN(MAX) where f = operating frequency, Hz %Ripple = allowable inductor current ripple, % 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 1 for a list of component suppliers. Table 1. 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 VIN(MIN) = minimum input voltage, V VIN(MAX) = maximum input voltage, V VOUT = output voltage, V IOUT(MAX) = maximum output load current 3453fa 8 LTC3453 U U W U APPLICATIO S I FOR ATIO Input Capacitor Selection Optional Schottky Diodes 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. 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. 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 Closing the Feedback Loop 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 2 COUT • VOUT • f % ( VIN(MAX) – VOUT ) • 100 % 8 • VIN(MAX ) • f 2 • L • COUT The LTC3453 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 COUT = output filter capacitor, F where RESR is the capacitor equivalent series resistance. 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. A troublesome feature in Boost mode is the right-half plane zero (RHP), and is given by: 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 2 for a list of component suppliers. 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 requires to be crossed over a decade before the LC double pole. 3453fa 9 LTC3453 U U W U APPLICATIO S I FOR ATIO The unity-gain frequency of the error amplifier with the Type I compensation is given by: fUG = gm 2 • π • CVC Since the maximum continuous output current is limited to 500mA, this sets a minimum limit on the parallel combination of RISET1 and RISET2 equal to RMIN = (RISET1 || RISET2)|MIN = 4(384[0.8V/500mA]) = 2458Ω 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. 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 4 LEDs. For a very high power LED such as a LumiLED, all four outputs can be connected in parallel for maximum total output current, as shown in the cover page application of this datasheet. Maximum LED Current As described in the Operation section, the output LED current with both enable pins logic high is equal to ILED = 384 [0.8V/(RISET1 || RISET2)] VIN Although the LTC3453 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 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 LTC3453 in and out of shutdown and result in erratic operation. VIN VOUT ENx VOUT ENx LED1 LTC3453 ISETx VOLTAGE DAC ISETx LED4 ILED = 384 RSET ≥ RMIN LED1 LTC3453 0.8V – VDAC RSET ILED = 384 • IDAC 0.8V RMIN IDAC ≤ CURRENT DAC VDAC LED4 (a) VIN (b) VIN VOUT ENx VOUT ENx LED1 LTC3453 ISETx ISETx LED4 RMIN LED1 LTC3453 ILED = 384 0.8V RMIN + RPOT RSET 100 RSET ≥ RMIN LED4 ILED = 384 VPWM RPOT = 384 1µF 0.8V – VPWM RSET 0.8V – (DC% • VDVCC) RSET DVCC fPWM ≥ 5kHz (c) (d) 3453 F03 Figure 3. Brightness Control Methods: (a) Using Voltage DAC, (b) Using Current DAC, (c) Using Potentiometer, (d) Using PWM Input 3453fa 10 LTC3453 U W U U APPLICATIO S I FOR ATIO Unused Outputs If fewer than 4 LED pins are to be used, unused LEDx pins should be connected to VOUT. The LTC3453 senses which current source outputs are not being used and shuts off the corresponding output currents to save power. A small trickle current (~30µA) 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 startup. 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 LTC3453 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. U PACKAGE DESCRIPTIO UF Package 16-Lead Plastic QFN (4mm × 4mm) (Reference LTC DWG # 05-08-1692) 0.72 ±0.05 4.35 ± 0.05 2.15 ± 0.05 2.90 ± 0.05 (4 SIDES) PACKAGE OUTLINE 0.30 ±0.05 0.65 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 15 PIN 1 NOTCH R = 0.20 TYP OR 0.35 × 45° CHAMFER 16 0.55 ± 0.20 PIN 1 TOP MARK (NOTE 6) 1 2.15 ± 0.10 (4-SIDES) 2 (UF16) QFN 1004 0.200 REF 0.00 – 0.05 0.30 ± 0.05 0.65 BSC NOTE: 1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGC) 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 3453fa 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 LTC3453 U TYPICAL APPLICATIO High Efficiency 4 White LED Driver 4.7µH VIN 1-CELL Li-Ion VIN 2.2µF SW1 PVIN SW2 VOUT 4.7µF 30mA 30mA 30mA 30mA D1 D2 D3 D4 LED1 LED2 1MHz BUCK-BOOST VC 0.1µF EN LED3 LED4 EN1 EN2 D1 TO D4: NICHIA NSCW100 L1: VISHAY DALE IDCS-2512 ISET1 10.2k ISET2 LTC3453 GND GND PGND 3453 TA02 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 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 LTC3454 1A Synchronous Buck-Boost High Power LED Driver VIN: 2.7V to 5.5V, 1MHz, ISD < 6µA, 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 3453fa 12 Linear Technology Corporation LT 0206 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