LT3477 3A, DC/DC Converter with Dual Rail-to-Rail Current Sense U FEATURES DESCRIPTIO ■ Dual 100mV Rail-to-Rail Current Sense Amplifiers Wide Input Voltage Range: 2.5V to 25V 3A, 42V Internal Switch High Efficiency Power Conversion: Up to 93% Drives LEDs in Boost, Buck-Boost or Buck Mode Frequency Set by External Resistor: 200kHz to 3.5MHz Programmable Soft-Start Low VCESAT Switch: 0.3V at 2.5A Capable of Positive and Negative Output Voltages (Boost, Inverting, SEPIC, Flyback) Available in Thermally Enhanced 20-Lead (4mm × 4mm) QFN and 20-Lead TSSOP Packages The LT®3477 is a current mode, 3A DC/DC step-up converter with dual rail-to-rail current sense amplifiers and an internal 3A, 42V switch. It combines a traditional voltage feedback loop and two unique current feedback loops to operate as a constant-current, constant-voltage source. Both current sense voltages are set at 100mV and can be adjusted independently using the IADJ1 and IADJ2 pins. Efficiency of up to 91% can be achieved in typical applications. The LT3477 features a programmable soft-start function to limit inductor current during start-up. Both inputs of the error amplifier are available externally allowing positive and negative output voltages (Boost, Inverting, SEPIC, Flyback). The switching frequency is programmable from 200kHz to 3.5MHz through an external resistor. High Power LED Driver DSL Modems Distributed Power Input/Output Current Limited Boost, SEPIC, Inverting, Flyback Converters Constant-Voltage, Constant-Current Source Available in thermally enhanced 20-pin (4mm × 4mm) QFN and 20-pin TSSOP packages, the LT3477 provides a complete solution for both constant-voltage and constant-current applications. ■ ■ ■ ■ ■ ■ ■ ■ ■ U APPLICATIO S ■ ■ ■ ■ , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. U ■ TYPICAL APPLICATIO 330mA LED Driver with Open LED Protection Efficiency 10µH VIN 5V 90 3.3µF ISN1 VIN IADJ1 IADJ2 SHDN SHDN 200k SW 85 FBN 80 10k LT3477 EFFICIENCY (%) ISP1 3.3µF ISP2 0.3Ω VC ISN2 VREF 1k FBP 70 65 60 RT GND 75 55 SS 330mA 33nF 50 22k 4.7nF 0 0.1 0.2 0.3 0.4 IOUT (A) 3477 TA01b 3477 TA01a 3477fb 1 LT3477 W W U W ABSOLUTE MAXIMUM RATINGS (Note 1) SW Pin Voltage ....................................................... 42V VIN, SHDN Pin Voltage ............................................ 25V FBP, FBN Pin Voltage ................................................ 6V VREF Pin Voltage ....................................................... 6V RT, VC , SS Pin Voltage .............................................. 6V IADJ1, IADJ2 Pin Voltage ........................................... 25V ISP1, ISP2, ISN1, ISN2 Pin Voltage .............................. 42V Junction Temperature .......................................... 125°C Operating Temperature Range (Note 2) LT3477E ............................................. – 40°C to 85°C LT3477I ............................................ – 40°C to 125°C Storage Temperature Range ................. – 65°C to 125°C Lead Temperature (Soldering, 10 sec) TSSOP .............................................................. 300°C U W U PACKAGE/ORDER INFORMATION TOP VIEW 20 NC 20 19 18 17 16 RT 2 19 NC SHDN 3 18 NC SS 4 17 SW VC 5 FBN 6 FBP 7 14 ISN1 VREF 8 13 ISP1 SW 1 SW VIN NC ISN1 GND TOP VIEW NC 1 15 ISP1 NC 2 14 ISN2 13 ISP2 21 VIN 3 7 8 9 10 IADJ2 9 12 ISN2 IADJ1 10 11 ISP2 VREF 6 FBP 11 IADJ2 FBN SHDN 5 15 GND VC 12 IADJ1 16 SW SS RT 4 21 UF PACKAGE 20-LEAD (4mm × 4mm) PLASTIC QFN FE PACKAGE 20-LEAD PLASTIC TSSOP TJMAX = 125°C, θJA = 37°C/W EXPOSED PAD (PIN 21) IS PGND (MUST BE SOLDERED TO PCB) TJMAX = 150°C, θJA = 40°C/W EXPOSED PAD (PIN 21) IS PGND (MUST BE SOLDERED TO PCB) ORDER PART NUMBER UF PART MARKING ORDER PART NUMBER LT3477EUF LT3477IUF 3477 3477 LT3477EFE LT3477IFE SW PART MARKING 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 ● indicates specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 2.5V, VSHDN = 2.5V. PARAMETER CONDITIONS MIN ● Minimum Input Voltage Quiescent Current VSHDN = 0V VSHDN = 2.5V, VC = 0.3V (Not Switching) Reference Voltage E Grade I Grade Reference Voltage Line Regulation 2.5V < VIN < 25V, VC = 0.3V Maximum VREF Pin Current Out of Pin ● ● 1.216 1.210 TYP MAX UNITS 2.3 2.5 V 0.1 5.0 1.0 7.5 µA mA 1.235 1.235 1.250 1.260 V V 0.01 0.03 %/V 100 µA 3477fb 2 LT3477 ELECTRICAL CHARACTERISTICS The ● indicates specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 2.5V, VSHDN = 2.5V. PARAMETER CONDITIONS Soft-Start Pin Current SS = 0.5V, Out of Pin MIN TYP FBP Pin Bias Current FBN Pin Bias Current Feedback Amplifier Offset Voltage MAX FBP – FBN, VC = 1V –2 Feedback Amplifier Voltage Gain UNITS µA 9 25 100 25 100 nA 2 6 mV 500 nA V/V 500 µS Feedback Amplifier Sink Current VFBP = 1.25V, VFBN = 1.5V, VC = 1V 10 µA Feedback Amplifier Source Current VFBP = 1.25V, VFBN = 1V, VC = 0.5V 10 µA Current Sense Amplifier Sense Voltage Positive Rail, VCM = 25V, E Grade Positive Rail, VCM = 25V, I Grade Ground Switching Frequency RT = 17.2k RT = 107.4k RT = 2.44k Maximum Switch Duty Cycle RT = 17.2k Switch Current Limit (Note 3) 4 5 Switch VCESAT ISW = 1A (Note 3) 150 200 mV Switch Leakage Current SW = 40V 0.2 5 µA SHDN Pin Current VSHDN = 5V VSHDN = 0V 30 0.1 60 1 µA µA 1.5 2 V Voltage Feedback Amplifier Transconductance ● ● ● 97.5 97.5 88 100 100 100 102.5 103 112 mV mV mV 0.9 160 2.7 1 200 3.5 1.15 240 4.3 MHz kHz MHz 87 93 3 SHDN Pin Threshold 0.3 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 LT3477E is guaranteed to meet performance specifications from 0°C to 70°C. Specifications over the – 40°C to 85°C operating % A junction temperature range are assured by design, characterization and correlation with statistical process controls. The LT3477I is guaranteed over the full –40°C to 125°C operating junction temperature range. Note 3: Switch current limit and switch VCESAT for UF package guaranteed by design and/or correlation to static test. U W TYPICAL PERFOR A CE CHARACTERISTICS Switch Current Limit Switch VCE(SAT) VREF 5 0.50 1.27 0.45 CURRENT (A) 125°C 0.30 25°C 0.25 0.20 –50°C 0.15 1.25 3 VREF (V) 0.35 VCE(SAT) (V) 1.26 4 0.40 2 1 0.10 1.24 VIN = 25V 1.23 VIN = 2.5V 1.22 0.05 0 0 0.5 1.5 2 1 SWITCH CURRENT (A) 2.5 3 3477 G01 0 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 125 3477 G02 1.21 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3477 G03 3477fb 3 LT3477 U W TYPICAL PERFOR A CE CHARACTERISTICS SHDN Pin Turn-On Threshold SHDN Pin Current Quiescent Current 50 1.6 6 VC = 0.3V 1.4 1.2 40 QUIESCENT CURRENT (mA) SHDN PIN CURRENT (µA) SHDN THRESHOLD (V) –50°C 30 25°C 20 125°C 10 1.0 –50 0 50 25 75 0 TEMPERATURE (°C) –25 100 125 0 5 15 10 VSHDN (V) 20 4 3 2 –50 –25 25 Soft-Start Pin Current 25 50 75 100 125 150 TEMPERATURE (°C) 3477 G06 Oscillator Frequency Feedback Amplifier Offset Voltage 4 2.0 FREQUENCY (MHz) 15 3 RT = 10kΩ 1.6 1.2 OFFSET VOLTAGE (mV) 20 10 0 3477 G05 3477 G04 ISS (µA) 5 RT = 15kΩ RT = 20kΩ 0.8 5 2 VC = 1V 1 0 VC = 0.5V –1 –2 0.4 –3 0 –50 –25 0 0 –50 –25 25 50 75 100 125 150 TEMPERATURE (°C) 0 25 50 75 100 125 150 TEMPERATURE (°C) FBP Pin Bias Current 40 FBN PIN BIAS CURRENT (nA) FBP PIN BIAS CURRENT (nA) 25 50 75 100 125 150 TEMPERATURE (°C) 3477 G09 FBN Pin Bias Current 50 “+” INDICATES THE CURRENT FLOWS OUT OF PIN 30 20 10 0 –10 –50 –25 0 3477 G08 3477 G07 50 –4 –50 –25 “+” INDICATES THE CURRENT FLOWS OUT OF PIN 40 30 20 10 0 50 25 75 0 TEMPERATURE (°C) 100 125 3477 G10 –10 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 125 3477 G11 3477fb 4 LT3477 U W TYPICAL PERFOR A CE CHARACTERISTICS Current Sense Voltage vs IADJ Current Sense Voltage vs Temperature 120 103 101 VCM = 10V 100 99 VCM = 42V 98 80 60 40 20 97 96 –50 VCM = 10V 100 102 VOLTAGE SENSE (mV) CURRENT SENSE VOLTAGE (mV) 104 –25 0 50 75 25 TEMPERATURE (°C) 100 125 3477 G14 U U U PI FU CTIO S 0 0 100 200 300 400 500 600 700 800 IADJ VOLTAGE (mV) 2477 G13 (QFN/TSSOP) NC (Pins 1, 2, 20/Pins 18, 19, 20): No Connect Pin. Okay to connect to ground or VIN, or to float. VIN (Pin 3/Pin 1): Input Supply. Must be locally bypassed. Powers the internal control circuitry. RT (Pin 4/Pin 2): Timing Resistor Pin. Adjusts the switching frequency. Connect a 17.2k resistor between RT and GND for a 1MHz switching frequency. Do not leave this pin open. See Table 4 for additional RT values and switching frequencies. SHDN (Pin 5/Pin 3): Shutdown. Tie to 2V or greater to enable the device. Tie below 0.3V to turn off the device. SS (Pin 6/Pin 4): Soft-Start. Place a soft-start capacitor here. Leave floating if not in use. VC (Pin 7/Pin 5): Compensation Pin for Error Amplifier. Connect a series RC from this pin to GND. Typical values are 1kΩ and 4.7nF. FBN (Pin 8/Pin 6): The Inverting Input to the Error Amplifier. Connect resistive divider tap here for positive output voltage. FBP (Pin 9/Pin 7): The Noninverting Input to the Error Amplifier. Connect resistive divider tap here for negative output voltage. VREF (Pin 10/Pin 8): Bandgap Voltage Reference. Internally set to 1.235V. Connect this pin to FBP if generating a positive output or to an external resistor divider if generating a negative voltage. This pin can provide up to 100µA of current and can be locally bypassed with a 100pF capacitor. IADJ2 (Pin 11/Pin 9): Second Current Sense Adjustment. Setting IADJ2 to be less than 625mV leads to adjustment of the sensed voltage of the second current sense amplifier linearly. If IADJ2 is tied to higher than 650mV, the default current sense voltage is 100mV. If current sense amplifier 2 is not used, always tie IADJ2 to higher than 650mV. IADJ1 (Pin 12/Pin 10): First Current Sense Adjustment. Setting IADJ1 to be less than 625mV leads to adjustment of the sensed voltage of the first current sense amplifier linearly. If IADJ1 is tied to higher than 650mV, the default current sense voltage is 100mV. If current sense amplifier 1 is not used, always tie IADJ1 to higher than 650mV. 3477fb 5 LT3477 U U U PI FU CTIO S (QFN/TSSOP) ISP2 (Pin 13/ Pin 11): Second Current Sense (+) Pin. The noninverting input to the second current sense amplifier. Connect to ISN2 if not used. ISN2 (Pin 14/ Pin 12): Second Current Sense (–) Pin. The inverting input to the second current sense amplifier. Connect to ISP2 if not used. ISP1 (Pin 15/Pin 13): First Current Sense (+) Pin. The noninverting input to the first current sense amplifier. Connect to ISN1 if not used. GND (Pins 17/Pin 15): Ground. Tie directly to local ground plane. SW (Pins 18, 19/Pins 16, 17): Switch Pins. Collector of the internal NPN power switch. Connect the inductor and diode here and minimize the metal trace area connected to this pin to minimize electromagnetic interference. Exposed Pad (Pin 21/Pin 21): Power Ground. Must be soldered to PCB ground for electrical contact and rated thermal performance. ISN1 (Pin 16/Pin 14): First Current Sense (–) Pin. The inverting input to the first current sense amplifier. Connect to ISP1 if not used. W BLOCK DIAGRA SS ISP1 – VADJ – + + A1 VADJ – + + A2 IADJ1 ISP2 + IA2 ISN2 – IADJ2 FBP SW + IA1 ISN1 VC A3 – SLOPE + + A4 Q R VA + FBN Q1 S Σ – – VREF VREF 1.25V OSCILLATOR SHDN VIN RT 3477 F01 Figure 1. LT3477 Block Diagram 3477fb 6 LT3477 U OPERATIO The LT3477 uses a fixed frequency, current mode control scheme to provide excellent line and load regulation. Operation can be best understood by referring to the Block Diagram in Figure 1. The start of each oscillator cycle sets the SR latch and turns on power switch Q1. The signal at the noninverting input of the PWM comparator (A4 SLOPE) is proportional to the sum of the switch current and oscillator ramp. When SLOPE exceeds VC (the output of the feedback amplifier), the PWM comparator resets the latch and turns off the power switch. In this manner, the feedback amplifier and PWM comparators set the correct peak current level to keep the output in regulation. Amplifier A3 drives A4 inverting input. A3 has three inputs, one from the voltage feedback loop and the other two from the current feedback loop. Whichever feedback input is higher takes precedence, forcing the converter into either a constant-current or a constant-voltage mode. The LT3477 is designed to transition cleanly between the two modes of operation. Current sense amplifier IA1 senses the voltage between the ISP1 and ISN1 pins and provides a pre-gain to amplifier A1. When the voltage between ISP1 and ISN1 reaches 100mV, the output of IA1 provides VADJ to the inverting input of A1 and the converter is in constantcurrent mode. If the current sense voltage exceeds 100mV, the output of IA1 will increase causing the output of A3 to decrease, thus reducing the amount of current delivered to the output. In this manner the current sense voltage is regulated to 100mV. The current sense level is also pin adjustable by IADJ1. Forcing IADJ1 to less than 625mV will overwrite VADJ voltage that’s set internally, thus providing current level control. The second current sense amplifier, IA2, works the same as the first current sense amplifier IA1. Both current sense amplifiers provide rail-to-rail current sense operation. Similarly, for positive output voltage operation where FBP is tied to VREF, if the FBN pin increases above VREF, the output of A3 will decrease to reduce the peak current level and regulate the output (constant-voltage mode). For negative output voltage operation where FBN is tied to GND, if the FBP pin decreases below GND level, the output of A3 will decrease to reduce the peak current level and regulate the output (constant-voltage mode). The LT3477 also features a soft-start function. During start-up, 9µA of current charges the external soft-start capacitor. The SS pin directly limits the rate of voltage rise on the VC pin, which in turn limits the peak switch current. The switch current is constantly monitored and not allowed to exceed the nominal value of 3A. If the switch current reaches 3A, the SR latch is reset regardless of the output of the PWM comparator. Current limit protects the power switch and external components. 3477fb 7 LT3477 U W U U APPLICATIO S I FOR ATIO Capacitor Selection Low ESR (equivalent series resistance) ceramic capacitors should be used at the output to minimize the output ripple voltage. Use only X5R or X7R dielectrics, as these materials retain their capacitance over wider voltage and temperature ranges better than other dielectrics. A 4.7µF to 10µF output capacitor is sufficient for most high output current designs. Converters with lower output currents may need only a 1µF or 2.2µF output capacitor. Table 1. Ceramic Capacitor Manufacturers MANUFACTURER PHONE WEB Taiyo Yuden (408) 573-4150 www.t-yuden.com AVX (803) 448-9411 www.avxcorp.com Murata (714) 852-2001 www.murata.com TDK (847) 803-6100 www.component.tdk.com Inductor Selection Several inductors that work well with the LT3477 are listed in Table 2. However, there are many other manufacturers and devices that can be used. Consult each manufacturer for more detailed information and their entire range of parts. Ferrite core inductors should be used to obtain the best efficiency. Choose an inductor that can handle the necessary peak current without saturating, and ensure that the inductor has a low DCR (copper-wire resistance) to minimize I2R power losses. A 4.7µH or 10µH inductor will suffice for most LT3477 applications. Inductor manufacturers specify the maximum current rating as the current where the inductance falls to some percentage of its nominal value—typically 65%. An inductor can pass a current larger than its rated value without damaging it. Aggressive designs where board space is precious will exceed the maximum current rating of the inductor to save board space. Consult each manufacturer to determine how the maximum inductor current is measured and how much more current the inductor can reliably conduct. Diode Selection Schottky diodes, with their low forward voltage drop and fast switching speed, are ideal for LT3477 applications. Table 3 lists several Schottky diodes that work well with the LT3477. The diode’s average current rating must exceed the average output current. The diode’s maximum reverse voltage must exceed the output voltage. The diode conducts current only when the power switch is turned off (typically less than 50% duty cycle), so a 3A diode is sufficient for most designs. The companies below also offer Schottky diodes with higher voltage and current ratings. Table 3. Suggested Diodes MANUFACTURER MAX MAX REVERSE PART NUMBER CURRENT (A) VOLTAGE (V) MANUFACTURER UPS340 UPS315 3 3 40 15 Microsemi www.microsemi.com B220 B230 B240 B320 B330 B340 SBM340 2 2 2 3 3 3 3 20 30 40 20 30 40 40 Diodes, Inc www.diodes.com Table 2. Suggested Inductors MANUFACTURER PART NUMBER IDC (A) INDUCTANCE (µH) MAX DCR (mΩ) L×W×H (mm) CDRH6D283R0 CDRH6D28100 CDRH4D284R7 3 1.7 1.32 3 10 4.7 24 65 72 6.7 × 6.7 × 3.0 6.7 × 6.7 × 3.0 5.0 × 5.0 × 3.0 Sumida www.sumida.com LM N 05D B4R7M LM N 05D B100K 2.2 1.6 4.7 10 49 10 5.9 × 6.1 × 2.8 5.9 × 6.1 × 2.8 Taiyo Yuden www.t-yuden.com LQH55DN4R7M01L LQH55DN100M01K 2.7 1.7 4.7 10 57 130 5.7 × 5.0 × 4.7 5.7 × 5.0 × 4.7 Murata www.murata.com FDV0630-4R7M 4.2 4.7 49 7.0 × 7.7 × 3.0 Toko www.toko.com MANUFACTURER 3477fb 8 LT3477 U W U U APPLICATIO S I FOR ATIO Setting Positive Output Voltages To set a positive output voltage, select the values of R1 and R2 (see Figure 2) according to the following equation: ⎛ R1⎞ VOUT = 1.235V⎜ 1 + ⎟ ⎝ R2 ⎠ FBP LT3477 For designs needing an adjustable current level, the IADJ1 and IADJ2 pins are provided for the first and the second current sense amplifiers, respectively. With the IADJ1 and IADJ2 pins tied higher than 650mV, the nominal current sense voltage is 100mV (appearing between the ISP1 and ISN2 or ISP2 and ISN2 pins). Applying a positive DC voltage less than 600mV to the IADJ1 and IADJ2 pins will decrease the current sense voltage according to the following formula: VOUT VREF R1 FBN ISENSE = 100mV VIADJ • R SENSE 618mV R2 3477 F02 Figure 2. Positive Output Voltage Feedback Connections Setting Negative Output Voltages To set a negative output voltage, select the values of R3 and R4 (see Figure 3) according to the following equation: ⎛ R3 ⎞ VOUT = –1.235V⎜ ⎟ ⎝ R4 ⎠ –VOUT R3 FBP R4 LT3477 VREF FBN 3477 F03 Figure 3. Negative Output Voltage Feedback Connections Selecting RSENSE/Current Sense Adjustment Using the following formula to choose the correct current sense resistor value (for constant current or fail-safe operation). RSENSE = 100mV ISENSE For example, if 309mV is applied to the IADJ1 pin and RSENSE is 0.5Ω, the current sense will be reduced from 200mA to 100mA. The adjustability allows the regulated current to be reduced without changing the current sense resistor (e.g., to adjust brightness in an LED driver or to reduce the charge current in a battery charger). Considerations When Sensing Input Current In addition to regulating the DC output current for currentsource applications, the constant-current loop of the LT3477 can also be used to provide an accurate input current limit. Boost converters cannot provide output short-circuit protection, but the surge turn-on current can be drastically reduced using the LT3477 current sense at the input. SEPICs, however, have an output that is DCisolated from the input, so an input current limit not only helps soft-start the output but also provides excellent short-circuit protection. When sensing input current, the sense resistor should be placed in front of the inductor (between the decoupling capacitor and the inductor). This will regulate the average inductor current and maintain a consistent inductor ripple current, which will, in turn, maintain a well regulated input current. Do not place the sense resistor between the input source and the input decoupling capacitor, as this may allow the inductor ripple current to vary widely (even though the average input current and the average inductor current will still be regulated). Since the inductor current 3477fb 9 LT3477 U W U U APPLICATIO S I FOR ATIO is a triangular waveform (not a DC waveform like the output current) some tweaking of the compensation values (RC and CC on the VC pin) may be required to ensure a clean inductor ripple current while the constant-current loop is in effect. For these applications, the constantcurrent loop response can usually be improved by reducing the RC value or by adding a capacitor (with a value of approximately CC/10) in parallel with the RC and CC compensation network. Frequency Compensation The LT3477 has an external compensation pin (VC), which allows the loop response to be optimized for each application. An external resistor and capacitor (or sometimes just a capacitor) are placed at the VC pin to provide a pole and a zero (or just a pole) to ensure proper loop compensation. Several other poles and zeroes are present in the closedloop transfer function of a switching regulator, so the VC pin pole and zero are positioned to provide the best loop response. A thorough analysis of the switching regulator control loop is not within the scope of this data sheet, and will not be presented here, but values of 1k and 4.7nF will be a good choice for many designs. For those wishing to optimize the compensation, use the 1k and 4.7nF as a starting point. Soft-Start For many applications, it is necessary to minimize the inrush current at start-up. The built-in soft-start circuit significantly reduces the start-up current spike and output voltage overshoot. A typical value for the soft-start capacitor is 10nF. Switching Frequency The switching frequency of the LT3477 is set by an external resistor attached to the RT pin. Do not leave this pin open. A resistor must always be connected for proper operation. See Table 4 and Figure 4 for resistor values and corresponding frequencies. Increasing switching frequency reduces output voltage ripple but also reduces efficiency. The user should set the frequency for the maximum tolerable output voltage ripple. Table 4. Switching Frequency SWITCHING FREQUENCY (MHz) RT (kΩ) 3.5 2.43 3 3.65 2.5 4.87 Board Layout 6.81 10.2 1 17.4 0.5 43.2 0.2 107 3.5 3.0 SWITCH FREQUENCY (MHz) As with all switching regulators, careful attention must be paid to the PCB board layout and component placement. To maximize efficiency, switch rise and fall times are made as short as possible. To prevent radiation and high frequency resonance problems, proper layout of the high frequency switching path is essential. Minimize the length and area of all traces connected to the SW pin and always use a ground plane under the switching regulator to minimize interplane coupling. The signal path including the switch, output diode D1 and output capacitor COUT, contains nanosecond rise and fall times and should be kept as short as possible. 2 1.5 2.5 2.0 1.5 1.0 0.5 0 0.1 10 RT (kΩ) 100 3477 F04 Figure 4. Switch Frequency 3477fb 10 LT3477 U W U U APPLICATIO S I FOR ATIO PWM Dimming For LED applications where a wide dimming range is required, two competing methods are available: analog dimming and PWM dimming. The easiest method is to simply vary the DC current through the LED—analog dimming—but changing LED current also changes its chromaticity, undesirable in many applications. The better method is PWM dimming, which switches the LED on and off, using the duty cycle to control the average current. PWM dimming offers several advantages over analog dimming and is the method preferred by LED manufacturers. By modulating the duty cycle of the PWM signal, the average LED current changes proportionally as illustrated in Figure 5. The chromaticity of the LEDs remains unchanged in this scheme since the LED current is either zero or at programmed current. Another advantage of PWM dimming over analog dimming is that a wider dimming range is possible. The LT3477 is a DC/DC converter that is ideally suited for LED applications. For the LT3477, analog dimming offers a dimming ratio of about 10:1; whereas, PWM dimming with the addition of a few external components results in a wider dimming range of 500:1. The technique requires a PWM logic signal applied to the gate of both NMOS (refer to Figure 7). When the PWM signal is taken high the part runs in normal operation and ILED = 100mV/RSENSE runs 100 PWM dimming can be used in Boost (shown in Figure 7), Buck mode (shown in Figure 8) and Buck-Boost mode (shown in Figure 9). For the typical boost topology, efficiency exceeds 80%. Buck mode can be used to increase the power handling capability for higher current LED applications. A Buck-Boost LED driver works best in applications where the input voltage fluctuates to higher or lower than the total LED voltage drop. In high temperature applications, the leakage of the Schottky diode D1 increases, which in turn, discharges the output capacitor during the PWM “off” time. This results in a smaller effective LED dimming ratio. Consequently, the dimming range decreases to about 200:1 at 85°C. 1000 DIMMING RANGE: 1 LED CURRENT (mA) The LED current can be controlled by feeding a PWM signal with a broad range of frequencies. Dimming below 80Hz is possible, but not desirable, due to perceptible flashing of LEDs at lower PWM frequencies. The LED current can be controlled at higher frequencies, but the dimming range decreases with increasing PWM frequency, as seen in Figure 6. RT = 6.81k 10 1 0.1 through the LEDs. When the PWM input is taken low, the LEDs are disconnected and turn off. This unique external circuitry produces a fast rise time for the LED current, resulting in a wide dimming range of 500:1 at a PWM frequency of 100Hz. VIN = 5V BOOST 4 LEDs PWM FREQUENCY = 100Hz 0.01 0.1 1 10 PWM DUTY CYCLE (%) RT = 6.81k 100 10 1 100 0.1 1 10 100 PWM FREQUENCY (kHz) 3477 F05 Figure 5. LED Current vs PWM Duty Cycle Wide Dimming Range (500:1) 3477 F06 Figure 6. Dimming Range vs PWM Frequency 3477fb 11 LT3477 U W U U APPLICATIO S I FOR ATIO VIN 5V L1 2.0µH C1 3.3µF ISP1 OUT C2 10µF 1M SW ISN1 VIN IADJ1 IADJ2 SHDN D1 FBN 75k FBP LT3477 VREF ISP2 SS CSS 33nF RSENSE 0.33Ω ISN2 LED1 RT VC GND LED2 6.81k D2 PWM 5V 300mA LED3 NMOS1 0 LED4 RC 2.4k 100Hz 100k NMOS2 CC 10nF 3477 F07a C1: TAIYO YUDEN EMK316BJ335ML C2: TAIYO YUDEN UDK325BJ106MM L1: TOKO D53LC (PN# A915AY-2ROM) D1: ZETEX ZLLS1000 D2: DIODES INC 1N4148 NMOS1: ZETEX 2N7002 NMOS2: FAIRCHILD FDG327N LED1 TO LED4: LUMILEDS LXHL-BW02 Figure 7a. 5V to 4 White LEDs: Boost with PWM Dimming 350 85 EFFICIENCY EFFICIENCY (%) PWM 5V/DIV IL 1A/DIV 80 300 75 250 70 200 LED CURRENT 150 65 60 ILED 200mA/DIV 100 VIN = 5V BOOST 4 LEDs, 300mA PWM FREQUENCY = 100Hz 55 3477 F07b VIN = 5V 4 LEDs 300mA 10µs/DIV PWM FREQ = 100Hz BOOST Figure 7b. PWM Dimming Waveforms 50 0 20 40 60 80 50 0 100 PWM DUTY CYCLE (%) 3477 F07c Figure 7c. Efficiency and LED Current vs PWM Duty Cycle 3477fb 12 LT3477 U W U U APPLICATIO S I FOR ATIO PVIN 32V C1 2.2µF RSENSE 0.33Ω 300mA C1: NIPPON NTS40X5R1H225M C2: TAIYO YUDEN GMK316BJ105ML C3: TAIYO YUDEN LMK316BJ335KL L1: TOKO D53LC (PN# A915AY-100M) D1: ZETEX ZLLS400 D2: DIODES INC 1N4148 NMOS1, NM0S2: ZETEX 2N7002 PMOS: SILICONIX Si2303BDS LED1 TO LED6: LUMILEDS LXHL-BW02 LED1 • • • LED6 1k PMOS NMOS2 PWM C2 1µF L1 10µH ISP1 VIN 3.3V 1k ISN1 280k SW VIN IADJ1 IADJ2 C3 3.3µF SHDN D1 FBN 10k FBP LT3477 VREF ISP2 SS CSS 33nF ISN2 VC RT GND D2 6.81k 3477 F08a PWM 5V NMOS1 0 100Hz CC 0.1µF 100k Figure 8a. 32V to 6 White LEDs: Buck Mode with PWM Dimming PWM 5V/DIV IL 500mA/DIV ILED 500mA/DIV 2ms/DIV PVIN = 32V 6 LEDs 300mA 3477 F08b PWM FREQUENCY = 100Hz BUCK MODE Figure 8b. PWM Dimming Waveforms 3477fb 13 LT3477 U W U U APPLICATIO S I FOR ATIO 1k C1: TAIYO YUDEN LMK316BJ335ML C2: TAIYO YUDEN UDK325BJ106MM L1: TOKO D53LC (PN# A915AY-4R7M) D1: ZETEX ZLLS1000 D2: DIODES INC 1N4148 NMOS1, NMOS2: ZETEX 2N7002 PMOS: SILICONIX Si2303BDS LED1, LED2: LUMILEDS LXHL-BW02 NMOS2 PWM 1k LED2 PMOS VIN 10V C1 3.3µF ISP1 L1 4.7µH ISN1 RSENSE 0.33Ω D1 1M SW VIN IADJ1 IADJ2 300mA LED1 FBN 49.9k FBP SHDN VREF LT3477 ISP2 SS CSS 33nF ISN2 VC RT GND D2 6.81k C2 10µF 3477 F09a PWM 5V NMOS1 0 100Hz RC 1.5k CC 10nF 100k Figure 9a. 10V to 2 White LEDs: Buck-Boost Mode with PWM Dimming PWM 10V/DIV IL 1A/DIV ILED 500mA/DIV 3477 F09b VIN = 10V 2 LEDs 300mA 2ms/DIV PWM FREQUENCY = 100Hz BUCK-BOOST MODE Figure 9b. PWM Dimming Waveforms 3477fb 14 LT3477 U TYPICAL APPLICATIO S Efficiency 5.5V SEPIC Converter with Short-Circuit Protection C1 3.3µF ISP1 ISN1 SHDN FBN LT3477 SHDN VIN = 3V 85 5.5V 670mA R4 34.8k L2 4.7µH SW VIN IADJ1 IADJ2 R3 0.15Ω D1 80 EFFICIENCY (%) VIN 3V TO 16V 90 C2 10µF L1 4.7µH R1 0.04Ω ISP2 75 70 65 60 55 VC ISN2 VREF RC 1k 50 RT FBP GND 0 SS C4 33nF CC 4.7nF C3 10µF R2 18.2k 0.1 0.2 0.3 0.4 IOUT (A) 0.5 0.6 0.7 3477 TA02b R5 10k 3477 TA02a C1: TAIYO YUDEN LMK316BJ335ML C2: TAIYO YUDEN LMK325BJ106MN C3: TAIYO YUDEN LMK316BJ106ZL D1: DIODES INC. DFLS130L L1, L2: TOKO FDV0630-4R7M 800mA, 5V to 12V Boost Converter with Accurate Input Current Limit L1 4.7µH 12V 0.8A C1 2.2µF ISP1 ISN1 SHDN SHDN FBN R4 23.2k ISP2 85 80 C2 10µF LT3477 ISN2 VC 75 70 65 60 VREF RC 1k R3 200k SW VIN IADJ1 IADJ2 90 D1 EFFICIENCY (%) VIN 5V R1 0.033Ω Efficiency FBP RT GND 55 SS 50 CC 4.7nF C3 10nF R2 17.8k 0 0.1 0.2 0.3 0.4 0.5 IOUT (A) 0.6 0.7 0.8 3477 TA04a 3477 TA04b C1: TAIYO YUDEN LMK316BJ225MD C2: AVX 1206YD106MAT D1: DIODES INC. B320A L1: TOKO FDV0630-4R7M 3477fb 15 LT3477 U TYPICAL APPLICATIO S 87% Efficient, 4W LED Driver R4 0.05Ω VIN 5V L2 10µH Efficiency 90 D1 85 ISP1 ISN1 SW VIN IADJ1 IADJ2 SHDN C2 3.3µF R2 200k FBN R1 10k LT3477 SHDN ISP2 VC 330mA 80 EFFICIENCY (%) C1 3.3µF R6 0.3Ω RC 1k GND 55 SS 50 LED2 C3 33nF CC 4.7nF 65 LED1 RT FBP 70 60 ISN2 VREF 75 0.1 0.2 0.4 0.3 IOUT (A) LED3 R3 22k C1: TAIYO YUDEN LMK316BJ335ML C2: TAIYO YUDEN TMK325BJ335MN D1: DIODES INC. DFLS120L L1: TOKO A915AY-100M 0 3477 TA01b LED4 3477 TA03a 1A Buck Mode High Current LED Driver PVIN 32V C1 2.2µF R1 0.1Ω LED1 • • • 1A LED4 Efficiency LED STRING C2 1µF 100 90 ISP1 VIN 3.3V C3 3.3µF ISN1 R3 280k SW VIN IADJ1 IADJ2 FBN R4 10k EFFICIENCY (%) 80 L1 33µH D1 70 60 50 40 30 SHDN SHDN LT3477 ISP2 VC ISN2 VREF RC 1k FBP CC 4.7nF 20 RT GND 0 0.2 0.4 0.6 LED CURRENT (A) 0.8 1 3477 TA05b SS C4 33nF R2 22k 3477 TA05a C1: NIPPON UNITED CHEMICON NTS40X5R1H225M C2: TAIYO YUDEN GMK316BJ105ML C3: TAIYO YUDEN LMK316BJ475 L1: TOKO A814AY-330M D1: DIODES INC DFLS140 3477fb 16 LT3477 U TYPICAL APPLICATIO S Buck-Boost Mode LED Driver LED2 LED1 L1 4.7µH VIN 2.7V TO 10V C1 3.3µF ISP1 ISN1 SHDN SHDN R3 200k SW1 VIN IADJ1 IADJ2 LED BRIGHTNESS CONTROL 0mV TO 650mV R1 0.1Ω D1 FBN LT3477 ISP2 VC ISN2 VREF RT FBP GND SS C3 33nF CC 10nF R2 18k R4 10k C1: TAIYO YUDEN LMK316BJ335ML C2: MURATA GRM31CR71E475KA88L D1: DIODES, INC. B320A L1: TOKO FDV0630-4R7M C2 4.7µF 3477 TA06a Efficiency 90 85 EFFICIENCY (%) VIN = 8V 80 VIN (V) IOUT (A) 75 2.7 3.6 4.2 5 8 0.57 0.74 0.83 0.93 1.0 VIN = 4.2V 70 65 60 55 50 0 0.2 0.4 0.6 IOUT (A) 0.8 1.0 3477 TA06b 3477fb 17 LT3477 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 3477fb 18 LT3477 U PACKAGE DESCRIPTIO FE Package 20-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663) Exposed Pad Variation CB 6.40 – 6.60* (.252 – .260) 3.86 (.152) 3.86 (.152) 20 1918 17 16 15 14 13 12 11 6.60 ±0.10 2.74 (.108) 4.50 ±0.10 6.40 2.74 (.252) (.108) BSC SEE NOTE 4 0.45 ±0.05 1.05 ±0.10 0.65 BSC 1 2 3 4 5 6 7 8 9 10 RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.50* (.169 – .177) 0.09 – 0.20 (.0035 – .0079) 0.25 REF 0.50 – 0.75 (.020 – .030) NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS 2. DIMENSIONS ARE IN (INCHES) 3. DRAWING NOT TO SCALE 1.20 (.047) MAX 0° – 8° 0.65 (.0256) BSC 0.195 – 0.30 (.0077 – .0118) TYP 0.05 – 0.15 (.002 – .006) FE20 (CB) TSSOP 0204 4. RECOMMENDED MINIMUM PCB METAL SIZE FOR EXPOSED PAD ATTACHMENT *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.150mm (.006") PER SIDE 3477fb 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. 19 LT3477 U TYPICAL APPLICATIO Buck Mode High Current LED Driver PVIN 32V C1 2.2µF R1 0.1Ω Efficiency LED1 • • • 1A LED4 100 LED STRING C2 1µF 90 EFFICIENCY (%) 80 L1 33µH D1 ISP1 VIN 3.3V C3 3.3µF ISN1 R3 280k SW VIN IADJ1 IADJ2 70 60 50 40 FBN R4 10k 30 20 SHDN SHDN LT3477 ISP2 VC RC 1k FBP 0.2 0.4 0.6 LED CURRENT (A) 0.8 1 3477 TA05b ISN2 VREF 0 RT GND SS C4 33nF CC 4.7nF R2 22k 3477 TA07 C1: NIPPON UNITED CHEMICON NTS40X5R1H225M C2: TAIYO YUDEN GMK316BJ105ML C3: TAIYO YUDEN LMK316BJ475 L1: TOKO A814AY-330M D1: DIODES INC DFLS140 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1618 Constant Current, Constant Voltage 1.4MHz, High Efficiency Boost Regulator VIN: 1.6V to 18V, VOUT(MAX) = 5.5V, IQ = 2.5mA, ISD < 1µA, QFN16 Package LT3436 3A (ISW), 800kHz, 34V Step-Up DC/DC Converter VIN: 3V to 25V, VOUT(MAX) = 34V, IQ = 0.9mA, ISD < 6µA, TSSOP16E Package LTC 3453 Synchronous Buck-Boost High Power White LED Driver VIN: 2.7V to 5.5V, VOUT(MAX) = 5.5V, IQ = 2.5mA, ISD < 1µA, QFN16 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, 42V Full Featured Boost/Inverter Converter with Soft-Start VIN: 2.5V to 24V, VOUT(MAX) = 40V, IQ = 5mA, ISD < 1µA, DFN/TSSOP Packages LTC3490 Single Cell 350mA, 1.3MHz LED Driver VIN: 1V to 3.2V, VOUT(MAX) = 4.7V, ISD < 1µA, DFN/SO8 Packages 20 Linear Technology Corporation ® 3477fb LT 0207 REV B • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2005