LT3470A Micropower Buck Regulator with Integrated Boost and Catch Diodes DESCRIPTION FEATURES n n n n n n n n n n n Low Quiescent Current: 35μA at 12VIN to 3.3VOUT Integrated Boost and Catch Diodes Input Range: 4V to 40V 3.3V at 250mA from 4V to 40V Input 5V at 250mA from 5.7V to 40V Input Low Output Ripple: <10mV < 1μA in Shutdown Mode Output Voltage: 1.25V to 16V Hysteretic Mode Control – Low Ripple Burst Mode® Operation at Light Loads – Continuous Operation at Higher Loads Solution Size as Small as 50mm2 Low Profile (0.75mm) 2mm × 3mm Thermally Enhanced 8-Lead DFN Package APPLICATIONS n n n n n Automotive Battery Regulation Power for Portable Products Distributed Supply Regulation Industrial Supplies Wall Transformer Regulation The LT®3470A is a micropower step-down DC/DC converter that integrates a 440mA power switch, catch diode and boost diode into low profile 2mm × 3mm DFN package. The LT3470A combines Burst Mode and continuous operation to allow the use of tiny inductor and capacitors while providing a low ripple output to loads of up to 250mA. With its wide input range of 4V to 40V, the LT3470A can regulate a wide variety of power sources, from 2-cell Li-Ion batteries to unregulated wall transformers and lead-acid batteries. Quiescent current in regulation is just 35μA in a typical application while a zero current shutdown mode disconnects the load from the input source, simplifying power management in battery-powered systems. Fast current limiting and hysteretic control protects the LT3470A and external components against shorted outputs, even at 40V input. The LT3470A has higher output current and improved start-up and dropout performance compared to the LT3470. L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. TYPICAL APPLICATION Efficiency and Power Loss vs Load Current 90 VIN 5.7V TO 40V 80 0.22μF BOOST OFF ON SHDN 70 33μH VOUT 5V 250mA SW BIAS 22pF 2.2μF 604k 1% FB GND 22μF 200k 1% 3470a TA01a 100 60 50 10 40 POWER LOSS (mW) LT3470A EFFICIENCY (%) VIN 1000 VIN = 12V 1 30 20 10 0.1 0.1 1 10 100 LOAD CURRENT (mA) 300 3470a TA01b 3470afb 1 LT3470A ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) VIN, SHDN Voltage ................................................... 40V BOOST Pin Voltage .................................................. 47V BOOST Pin Above SW Pin........................................ 25V FB Voltage .................................................................. 5V BIAS Voltage .............................................................15V SW Voltage ................................................................VIN Maximum Junction Temperature LT3470AE, LT3470AI ........................................ 125°C LT3470AH ......................................................... 150°C Operating Temperature Range (Note 2) LT3470AE ............................................– 40°C to 85°C LT3470AI ........................................... –40°C to 125°C LT3470AH .......................................... –40°C to 150°C Storage Temperature Range .................. –65°C to 150°C Lead Temperature (Soldering, 10 sec) .................. 300°C TOP VIEW FB 1 8 SHDN BIAS 2 7 NC BOOST 3 9 SW 4 6 VIN 5 GND DDB8 PACKAGE 8-LEAD (3mm × 2mm) PLASTIC DFN θJA = 80°C/W EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT3470AEDDB#PBF LT3470AEDDB#TRPBF LDPR 8-Lead (3mm × 2mm) Plastic DFN –40°C to 85°C LT3470AIDDB#PBF LT3470AIDDB#TRPBF LDPR 8-Lead (3mm × 2mm) Plastic DFN –40°C to 125°C LT3470AHDDB#PBF LT3470AHDDB#TRPBF LDPR 8-Lead (3mm × 2mm) Plastic DFN –40°C to 150°C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. 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/ 3470afb 2 LT3470A ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, VSHDN = 10V, VBOOST = 15V, VBIAS = 3V unless otherwise specified. PARAMETER CONDITIONS MIN TYP ● Minimum Input Voltage MAX UNITS 4 V Quiescent Current from VIN VSHDN = 0.2V VBIAS = 3V, Not Switching VBIAS = 0V, Not Switching ● 0.1 10 40 0.5 18 55 μA μA μA Quiescent Current from Bias VSHDN = 0.2V VBIAS = 3V, Not Switching VBIAS = 0V, Not Switching ● 0.1 30 0.1 0.5 60 1.5 μA μA μA FB Comparator Trip Voltage VFB Falling ● 1.250 1.265 V FB Pin Bias Current (Note 3) VFB = 1V 35 35 35 80 150 225 nA nA nA 0.0006 0.02 %/V H-Grade FB Voltage Line Regulation 1.228 ● ● 4V < VIN < 40V Minimum Switch Off-Time (Note 5) 500 ● Maximum Duty Cycle 90 Switch Leakage Current 95 0.7 Switch VCESAT ISW = 100mA Switch VCESAT Without Boost VBOOST = VSW Switch Top Current Limit VFB = 0V ns % 1.5 150 320 μA mV 0.9 1.2 V 440 560 mA Switch Bottom Current Limit VFB = 0V 280 mA Catch Schottky Drop ISW = 100mA 600 mV Catch Schottky Reverse Leakage VSW = 10V 0.2 2 μA Boost Schottky Drop IBIAS = 50mA 690 775 mV Boost Schottky Reverse Leakage VSW = 10V, VBIAS = 0V 0.2 2 μA ● 1.7 2.2 BOOST Pin Current ISW = 100mA 2.3 5 Bias Pin Preload VBOOST = 10V 50 SHDN Pin Current VSHDN = 2.5V Minimum Boost Voltage (Note 4) SHDN Input Voltage High SHDN Input Voltage Low 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 LT3470AE is guaranteed to meet performance specifications from 0°C to 85°C. Specifications over the –40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls. The LT3470AI specifications are 1 V mA mA 5 2 μA V 0.2 V guaranteed over the –40°C to 125°C temperature range. LT3470AH specifications are guaranteed over the –40°C to 150°C temperature range. Note 3: Bias current flows out of the FB pin. Note 4: This is the minimum voltage across the boost capacitor needed to guarantee full saturation of the switch. Note 5: This parameter is assured by design and correlation with statistical process controls. 3470afb 3 LT3470A TYPICAL PERFORMANCE CHARACTERISTICS Efficiency, VOUT = 3.3V Efficiency, VOUT = 5V VFB vs Temperature 90 90 80 TA = 25°C unless otherwise noted. L = TOKO D52LC 47μH TA = 25°C VIN = 12V 1.260 L = TOKO D52LC 47μH TA = 25°C VIN = 7V VIN = 12V 80 60 50 70 VIN = 24V VIN = 36V VFB (V) VIN = 24V VIN = 36V EFFICIENCY (%) EFFICIENCY (%) 1.255 70 60 1.250 50 1.245 40 40 30 0.1 1 10 LOAD CURRENT (mA) 100 30 0.1 1 10 LOAD CURRENT (mA) 1.240 –50 –25 100 0 25 50 75 100 125 150 TEMPERATURE (°C) 3470a G02 3470a G01 3470a G03 VIN Quiescent Current vs Temperature Top and Bottom Switch Current Limits (VFB = 0V) vs Temperature 50 600 40 500 VIN CURRENT (μA) CURRENT LIMIT (mA) 550 450 400 350 300 BIAS < 3V 30 20 BIAS > 3V 10 250 200 –50 –25 0 0 –50 –25 25 50 75 100 125 150 TEMPERATURE (°C) 0 25 50 75 100 125 150 TEMPERATURE (°C) 3470a G04 3470a G05 SHDN Bias Current vs Temperature BIAS Quiescent Current (Bias > 3V) vs Temperature 30 9 VSHDN = 36V 8 25 SHDN CURRENT (μA) BIAS CURRENT (μA) 7 20 15 10 6 5 4 3 2 VSHDN = 2.5V 5 1 0 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3470a G06 0 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3470a G07 3470afb 4 LT3470A TYPICAL PERFORMANCE CHARACTERISTICS FB Bias Current (VFB = 0V) vs Temperature FB Bias Current (VFB = 1V) vs Temperature 60 120 50 100 FB CURRENT (μA) FB CURRENT (nA) TA = 25°C unless otherwise noted. 40 30 20 80 60 40 20 10 0 –50 –25 0 0 –50 –25 25 50 75 100 125 150 TEMPERATURE (°C) 0 25 50 75 100 125 150 TEMPERATURE (°C) 3470a G09 3470a G08 Boost Diode VF (IF = 50mA) vs Temperature Switch VCESAT (ISW = 100mA) vs Temperature 0.8 300 0.7 0.6 SCHOTTKY VF (V) SWITCH VCESAT (mV) 250 200 150 100 0.5 0.4 0.3 0.2 50 0.1 0 –50 –25 0 0 –50 –25 25 50 75 100 125 150 TEMPERATURE (°C) 0 25 50 75 100 125 150 TEMPERATURE (°C) 3470a G11 3470a G10 Catch Diode VF (IF = 100mA) vs Temperature Diode Leakage (VR = 36V) vs Temperature 60 0.7 55 SCHOTTKY DIODE LEAKAGE (mA) SCHOTTKY VF (V) 0.6 0.5 0.4 0.3 0.2 0.1 50 CATCH BOOST 45 40 35 30 25 20 15 10 5 0 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3470a G12 0 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3470a G13 3470afb 5 LT3470A TYPICAL PERFORMANCE CHARACTERISTICS BOOST Pin Current 700 14 600 12 BOOST PIN CURRENT (mA) SWITCH VCESAT (mV) Switch VCESAT TA = 25°C unless otherwise noted. 500 400 300 200 100 10 8 6 4 2 0 0 100 0 200 300 400 SWITCH CURRENT (mA) 500 0 100 200 300 400 SWITCH CURRENT (mA) 3470a G14 500 3470a G15 Catch Diode Forward Voltage Boost Diode Forward Voltage 1.0 900 800 700 SCHOTTKY VF (V) SCHOTTKY VF (V) 0.8 0.6 0.4 600 500 400 300 200 0.2 100 0 200 100 300 CATCH DIODE CURRENT (mA) 0 0 400 0 100 50 150 BOOST DIODE CURRENT (mA) 3470a G16 3470a G17 Minimum Input Voltage, VOUT = 3.3V 6.0 200 Minimum Input Voltage, VOUT = 5V 8 TA = 25°C TA = 25°C 7 INPUT VOLTAGE (V) INPUT VOLTAGE (V) 5.5 5.0 4.5 4.0 VIN TO RUN/START 6 VIN TO RUN/START 5 3.5 3.0 0 50 100 150 200 LOAD CURRENT (mA) 250 3470a G18 4 0 50 100 150 200 LOAD CURRENT (mA) 250 3470a G19 3470afb 6 LT3470A PIN FUNCTIONS SHDN (Pin 8): The SHDN pin is used to put the LT3470A in shutdown mode. Tie to ground to shut down the LT3470A. Apply 2V or more for normal operation. If the shutdown feature is not used, tie this pin to the VIN pin. NC (Pin 7): This pin can be left floating, connected to VIN, or tied to GND. VIN (Pin 6): The VIN pin supplies current to the LT3470A’s internal regulator and to the internal power switch. This pin must be locally bypassed. GND (Pin 5): Tie the GND pin to a local ground plane below the LT3470A and the circuit components. Return the feedback divider to this pin. BOOST (Pin 3): The BOOST pin is used to provide a drive voltage, which is higher than the input voltage, to the internal bipolar NPN power switch. BIAS (Pin 2): The BIAS pin connects to the internal boost Schottky diode and to the internal regulator. Tie to VOUT when VOUT > 2.5V or to VIN otherwise. When VBIAS > 3V the BIAS pin will supply current to the internal regulator. FB (Pin 1): The LT3470A regulates its feedback pin to 1.25V. Connect the feedback resistor divider tap to this pin. Set the output voltage according to VOUT = 1.25V (1 + R1/R2) or R1 = R2 (VOUT /1.25 – 1). Exposed Pad (Pin 9): Ground. Must be soldered to PCB. SW (Pin 4): The SW pin is the output of the internal power switch. Connect this pin to the inductor, catch diode and boost capacitor. BLOCK DIAGRAM VIN VIN BIAS C1 + – BOOST 500ns ONE SHOT R Qʹ S Q C3 SW L1 VOUT – C2 SHDN + ENABLE BURST MODE DETECT NC VREF 1.25V gm GND FB R2 R1 3470a BD 3470afb 7 LT3470A OPERATION The LT3470A uses a hysteretic control scheme in conjunction with Burst Mode operation to provide low output ripple and low quiescent current while using a tiny inductor and capacitors. Operation can best be understood by studying the Block Diagram. An error amplifier measures the output voltage through an external resistor divider tied to the FB pin. If the FB voltage is higher than VREF, the error amplifier will shut off all the high power circuitry, leaving the LT3470A in its micropower state. As the FB voltage falls, the error amplifier will enable the power section, causing the chip to begin switching, thus delivering charge to the output capacitor. If the load is light the part will alternate between micropower and switching states to keep the output in regulation (See Figure 1a). At higher loads the part will switch continuously while the error amp servos the top and bottom current limits to regulate the FB pin voltage to 1.25V (See Figure 1b). The switching action is controlled by an RS latch and two current comparators as follows: The switch turns on, and the current through it ramps up until the top current comparator trips and resets the latch causing the switch to turn off. While the switch is off, the inductor current ramps down through the catch diode. When both the bottom current comparator trips and the minimum off-time one-shot expires, the latch turns the switch back on thus completing a full cycle. The hysteretic action of this control scheme results in a switching frequency that depends on inductor value, input and output voltage. Since the switch only turns on when the catch diode current falls below threshold, the part will automatically switch slower to keep inductor current under control during start-up or short-circuit conditions. The switch driver operates from either the input or from the BOOST pin. An external capacitor and internal diode is used to generate a voltage at the BOOST pin that is higher than the input supply. This allows the driver to fully saturate the internal bipolar NPN power switch for efficient operation. If the SHDN pin is grounded, all internal circuits are turned off and VIN current reduces to the device leakage current, typically 100nA. 200mA LOAD NO LOAD VOUT 20mV/DIV VOUT 20mV/DIV IL 100mA/DIV IL 100mA/DIV 1μs/DIV 1ms/DIV 150mA LOAD 10mA LOAD VOUT 20mV/DIV VOUT 20mV/DIV IL 100mA/DIV IL 100mA/DIV 5μs/DIV (1a) Burst Mode Operation 3470a F01a 1μs/DIV 3470a F1b (1b) Continuous Operation Figure 1. Operating Waveforms of the LT3470A Converting 12V to 5V Using a 33μH Inductor and 10μF Output Capacitor 3470afb 8 LT3470A APPLICATIONS INFORMATION Input Voltage Range The minimum input voltage required to generate a particular output voltage in an LT3470A application is limited by either its 4V undervoltage lockout or by its maximum duty cycle. The duty cycle is the fraction of time that the internal switch is on and is determined by the input and output voltages: VOUT + VD DC = VIN – VSW + VD where VIN(MAX) is the maximum input voltage for the application, tON-TIME(MIN) is ~150ns and IMAX is the maximum allowable increase in switch current during a minimum switch on-time (150mA). While this equation provides a safe inductor value, the resulting application circuit may switch at too high a frequency to yield good efficiency. It is advised that switching frequency be below 1.2MHz during normal operation: (1– DC) ( VD + VOUT ) f= L • ΔIL where VD is the forward voltage drop of the catch diode (~0.6V) and VSW is the voltage drop of the internal switch at maximum load (~0.4V). Given DCMAX = 0.90, this leads to a minimum input voltage of: where f is the switching frequency, ΔIL is the ripple current in the inductor (~200mA), VD is the forward voltage drop of the catch diode, and VOUT is the desired output voltage. ⎛V +V ⎞ VIN(MIN) = ⎜ OUT D ⎟ + VSW – VD ⎝ DCMAX ⎠ This analysis assumes the part has started up such that the capacitor tied between the BOOST and SW pins is charged to more than 2V. For proper start-up, the minimum input voltage is limited by the boost circuit as detailed in the section BOOST Pin Considerations. The maximum input voltage is limited by the absolute maximum VIN rating of 40V, provided an inductor of sufficient value is used. Inductor Selection The switching action of the LT3470A during continuous operation produces a square wave at the SW pin that results in a triangle wave of current in the inductor. The hysteretic mode control regulates the top and bottom current limits (see Electrical Characteristics) such that the average inductor current equals the load current. For safe operation, it must be noted that the LT3470A cannot turn the switch on for less than ~150ns. If the inductor is small and the input voltage is high, the current through the switch may exceed safe operating limit before the LT3470A is able to turn off. To prevent this from happening, the following equation provides a minimum inductor value: LMIN = VIN(MAX) • tON-TIME(MIN) IMAX If the application circuit is intended to operate at high duty cycles (VIN close to VOUT), it is important to look at the calculated value of the switch off-time: 1– DC tOFF-TIME = f The calculated tOFF-TIME should be more than LT3470A’s minimum tOFF-TIME (See Electrical Characteristics), so the application circuit is capable of delivering full rated output current. If the full output current of 250mA is not required, the calculated tOFF-TIME can be made less than minimum tOFF-TIME possibly allowing the use of a smaller inductor. See Table 1 for an inductor value selection guide. Table 1. Recommended Inductors for Loads up to 250mA VOUT VIN Up to 16V VIN Up to 40V 2.5V 10μH 33μH 3.3V 10μH 33μH 5V 15μH 33μH 12V 33μH 47μH Choose an inductor that is intended for power applications. Table 2 lists several manufacturers and inductor series. For robust output short-circuit protection at high VIN (up to 40V) use at least a 33μH inductor with a minimum 450mA saturation current. If short-circuit performance is not required, inductors with ISAT of 300mA or more may be used. It is important to note that inductor saturation current is reduced at high temperatures—see inductor vendors for more information. 3470afb 9 LT3470A APPLICATIONS INFORMATION Table 2. Inductor Vendors VENDOR URL PART SERIES INDUCTANCE RANGE (μH) SIZE (mm) Coilcraft www.coilcraft.com DO1605 ME3220 DO3314 10 to 47 10 to 47 10 to 47 1.8 × 5.4 × 4.2 2.0 × 3.2 × 2.5 1.4 × 3.3 × 3.3 Sumida www.sumida.com CR32 CDRH3D16/HP CDRH3D28 CDRH2D18/HP 10 to 47 10 to 33 10 to 47 10 to 15 3.0 × 3.8 × 4.1 1.8 × 4.0 × 4.0 3.0 × 4.0 × 4.0 2.0 × 3.2 × 3.2 Toko www.tokoam.com DB320C D52LC 10 to 27 10 to 47 2.0 × 3.8 × 3.8 2.0 × 5.0 × 5.0 Würth Elektronik www.we-online.com WE-PD2 Typ S WE-TPC Typ S 10 to 47 10 to 22 3.2 × 4.0 × 4.5 1.6 × 3.8 × 3.8 Coiltronics www.cooperet.com SD10 10 to 47 1.0 × 5.0 × 5.0 Murata www.murata.com LQH43C LQH32C 10 to 47 10 to 15 2.6 × 3.2 × 4.5 1.6 × 2.5 × 3.2 Input Capacitor Step-down regulators draw current from the input supply in pulses with very fast rise and fall times. The input capacitor is required to reduce the resulting voltage ripple at the VIN pin of the LT3470A and to force this switching current into a tight local loop, minimizing EMI. The input capacitor must have low impedance at the switching frequency to do this effectively. A 1μF to 2.2μF ceramic capacitor satisfies these requirements. If the input source impedance is high, a larger value capacitor may be required to keep input ripple low. In this case, an electrolytic of 10μF or more in parallel with a 1μF ceramic is a good combination. Be aware that the input capacitor is subject to large surge currents if the LT3470A circuit is connected to a low impedance supply, and that some electrolytic capacitors (in particular tantalum) must be specified for such use. Output Capacitor and Output Ripple The output capacitor filters the inductor’s ripple current and stores energy to satisfy the load current when the LT3470A is quiescent. In order to keep output voltage ripple low, the impedance of the capacitor must be low at the LT3470A’s switching frequency. The capacitor’s equivalent series resistance (ESR) determines this impedance. Choose one with low ESR intended for use in switching regulators. The contribution to ripple voltage due to the ESR is approximately ILIM • ESR. ESR should be less than ~150mΩ. The value of the output capacitor must be large enough to accept the energy stored in the inductor without a large change in output voltage. Setting this voltage step equal to 1% of the output voltage, the output capacitor must be: COUT ⎛I ⎞2 LIM > 50 • L • ⎜ ⎟ ⎝ VOUT ⎠ Where ILIM is the top current limit with VFB = 0V (see Electrical Characteristics). For example, an LT3470A producing 3.3V with L = 33μH requires 22μF. The calculated value can be relaxed if small circuit size is more important than low output ripple. Sanyo’s POSCAP series in B-case and provides very good performance in a small package for the LT3470A. Similar performance in traditional tantalum capacitors requires a larger package (C-case). With a high quality capacitor filtering the ripple current from the inductor, the output voltage ripple is determined by the delay in the LT3470A’s feedback comparator. This ripple can be reduced further by adding a small (typically 22pF) phase lead capacitor between the output and the feedback pin. 3470afb 10 LT3470A APPLICATIONS INFORMATION Ceramic Capacitors BOOST and BIAS Pin Considerations Ceramic capacitors are small, robust and have very low ESR. However, ceramic capacitors can cause problems when used with the LT3470A. Not all ceramic capacitors are suitable. X5R and X7R types are stable over temperature and applied voltage and give dependable service. Other types, including Y5V and Z5U have very large temperature and voltage coefficients of capacitance. In an application circuit they may have only a small fraction of their nominal capacitance resulting in much higher output voltage ripple than expected. Capacitor C3 and the internal boost Schottky diode (see Block Diagram) are used to generate a boost voltage that is higher than the input voltage. In most cases a 0.22μF capacitor will work well. Figure 2 shows two ways to arrange the boost circuit. The BOOST pin must be more than 2.5V above the SW pin for best efficiency. For outputs of 3.3V and above, the standard circuit (Figure 2a) is best. For outputs between 2.5V and 3V, use a 0.47μF. For lower output voltages the boost diode can be tied to the input Ceramic capacitors are piezoelectric. The LT3470A’s switching frequency depends on the load current, and at light loads the LT3470A can excite the ceramic capacitor at audio frequencies, generating audible noise. Since the LT3470A operates at a lower current limit during Burst Mode operation, the noise is typically very quiet to a casual ear. If this audible noise is unacceptable, use a high performance electrolytic capacitor at the output. The input capacitor can be a parallel combination of a 2.2μF ceramic capacitor and a low cost electrolytic capacitor. A final precaution regarding ceramic capacitors concerns the maximum input voltage rating of the LT3470A. A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT3470A circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3470A’s rating. This situation is easily avoided; see the Hot-Plugging Safely section. VIN VIN BOOST C3 0.22μF LT3470A VOUT SW BIAS GND VBOOST – VSW ≅ VOUT MAX VBOOST ≅ VIN + VOUT (2a) VIN VIN BOOST C3 0.22μF LT3470A BIAS SW VOUT GND 3470a F02 VBOOST – VSW ≅ VIN MAX VBOOST ≅ 2•VIN (2b) Figure 2. Two Circuits for Generating the Boost Voltage Table 2. Capacitor Vendors Vendor Phone URL Part Series Comments Panasonic (714) 373-7366 www.panasonic.com Ceramic, Polymer, Tantalum EEF Series Kemet (864) 963-6300 www.kemet.com Ceramic, Tantalum T494, T495 Sanyo (408) 749-9714 www.sanyovideo.com Ceramic, Polymer, Tantalum POSCAP Murata (404) 436-1300 www.murata.com Ceramic www.avxcorp.com Ceramic, Tantalum www.taiyo-yuden.com Ceramic AVX Taiyo Yuden (864) 963-6300 TPS Series 3470afb 11 LT3470A APPLICATIONS INFORMATION (Figure 2b). The circuit in Figure 2a is more efficient because the BOOST pin current and BIAS pin quiescent current comes from a lower voltage source. You must also be sure that the maximum voltage ratings of the BOOST and BIAS pins are not exceeded. The LT3470A monitors the boost capacitor for sufficient voltage such that the switch is allowed to fully saturate. When boost voltage falls below adequate levels (1.8V typical) the switch will operate with about 1V of drop, and an internal current source will begin to pull 50mA (typical) from the BIAS pin which is typically connected to the output. This current forces the LT3470A to switch more often and with more inductor current, which recharges Minimum Input Voltage, VOUT = 3.3V 6.0 TA = 25°C INPUT VOLTAGE (V) 5.5 5.0 4.5 4.0 VIN TO RUN/START 3.5 3.0 0 50 100 150 200 LOAD CURRENT (mA) 250 3470a F03a the boost capacitor. When the boost capacitor voltage is above 1.8V (typical) the current source turns off, and the part may enter BurstMode. This cycle will repeat anytime there is an undervoltage condition on the boost capacitor. See Figure 3 for minimum input voltage for outputs of 3.3V and 5V. Shorted Input Protection If the inductor is chosen so that it won’t saturate excessively at the top switch current limit maximum of 525mA, an LT3470A buck regulator will tolerate a shorted output even if VIN = 40V. There is another situation to consider in systems where the output will be held high when the input to the LT3470A is absent. This may occur in battery charging applications or in battery backup systems where a battery or some other supply is diode OR-ed with the LT3470A’s output. If the VIN pin is allowed to float and the SHDN pin is held high (either by a logic signal or because it is tied to VIN), then the LT3470A’s internal circuitry will pull its quiescent current through its SW pin. This is fine if your system can tolerate a few mA in this state. If you ground the SHDN pin, the SW pin current will drop to essentially zero. However, if the VIN pin is grounded while the output is held high, then parasitic diodes inside the LT3470A can pull large currents from the output through the SW pin and the VIN pin. Figure 4 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input. Minimum Input Voltage, VOUT = 5V 8 TA = 25°C VIN 7 INPUT VOLTAGE (V) D1 VIN 100k BOOST LT3470A SHDN VOUT SW BIAS 6 1M VIN TO RUN/START FB GND BACKUP 5 3470a F04 4 0 50 100 150 200 LOAD CURRENT (mA) 250 3470a F03b Figure 3. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit Figure 4. Diode D1 Prevents a Shorted Input from Discharging a Backup Battery Tied to the Output; It Also Protects the Circuit from a Reversed Input. The LT3470A Runs Only When the Input is Present Hot-Plugging Safely 3470afb 12 LT3470A APPLICATIONS INFORMATION PCB Layout For proper operation and minimum EMI, care must be taken during printed circuit board layout. Note that large, switched currents flow in the power switch, the internal catch diode and the input capacitor. The loop formed by these components should be as small as possible. Furthermore, the system ground should be tied to the regulator ground in only one place; this prevents the switched current from injecting noise into the system ground. These components, along with the inductor and output capacitor, should be placed on the same side of the circuit board, and their connections should be made on that layer. Place a local, unbroken ground plane below these components, and tie this ground plane to system ground at one location, ideally at the ground terminal of the output capacitor C2. Additionally, the SW and BOOST nodes should be kept as small as possible. Unshielded inductors can induce noise in the feedback path resulting in instability and increased output ripple. To avoid this problem, use vias to route the VOUT trace under the ground plane to the feedback divider (as shown in Figure 5). Finally, keep the FB node as small as possible so that the ground pin and ground traces will shield it from the SW and BOOST nodes. Figure 5 shows component placement with trace, ground plane and via locations. Include vias near the GND pin, or pad, of the LT3470A to help remove heat from the LT3470A to the ground plane. SHDN VIN GND VOUT 3470a F05 Figure 5. A Good PCB Layout Ensures Proper, Low EMI Operation 3470afb 13 LT3470A APPLICATIONS INFORMATION Hot-Plugging Safely The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LT3470A. However, these capacitors can cause problems if the LT3470A is plugged into a live supply (see Linear Technology Application Note 88 for a complete discussion). The low loss ceramic capacitor combined with stray inductance in series with the power source forms an under damped tank circuit, and the voltage at the VIN pin of the LT3470A can ring to twice the nominal input voltage, possibly exceeding the LT3470A’s rating and damaging the part. If the input supply is poorly controlled or the user will be plugging the LT3470A into an energized supply, the input network should be designed to prevent this overshoot. Figure 6 shows the waveforms that result when an LT3470A circuit is connected to a 24V supply through six feet of 24-gauge twisted pair. The first plot is the response with a 2.2μF ceramic capacitor at the input. The input voltage rings as high as 35V and the input current peaks at 20A. One method of damping the tank circuit is to add another capacitor with a series resistor to the circuit. In Figure 6b an aluminum electrolytic capacitor has been added. This capacitor’s high equivalent series resistance damps the circuit and eliminates the voltage overshoot. The extra capacitor improves low frequency ripple filtering and can slightly improve the efficiency of the circuit, though it is likely to be the largest component in the circuit. An alternative solution is shown in Figure 6c. A 1Ω resistor is added in series with the input to eliminate the voltage overshoot (it also reduces the peak input current). A 0.1μF capacitor improves high frequency filtering. This solution is smaller and less expensive than the electrolytic capacitor. For high input voltages its impact on efficiency is minor, reducing efficiency less than one half percent for a 5V output at full load operating from 24V. High Temperature Considerations The die junction temperature of the LT3470A must be lower than the maximum rating of 125°C (150°C for H-grade). This is generally not a concern unless the ambient temperature is above 85°C. For higher temperatures, care should be taken in the layout of the circuit to ensure good heat sinking of the LT3470A. The maximum load current should be derated as the ambient temperature approaches the maximum junction rating. The die temperature is calculated by multiplying the LT3470A power dissipation by the thermal resistance from junction to ambient. Power dissipation within the LT3470A can be estimated by calculating the total power loss from an efficiency measurement. Thermal resistance depends on the layout of the circuit board and choice of package. The DFN package with the exposed pad has a thermal resistance of approximately 80°C/W. Finally, be aware that at high ambient temperatures the internal Schottky diode will have significant leakage current (see Typical Performance Characteristics) increasing the quiescent current of the LT3470A converter. 3470afb 14 LT3470A APPLICATIONS INFORMATION CLOSING SWITCH SIMULATES HOT PLUG IIN VIN LT3470A + VIN 10V/DIV 2.2μF LOW IMPEDANCE ENERGIZED 24V SUPPLY IIN 10A/DIV STRAY INDUCTANCE DUE TO 6 FEET (2 METERS) OF TWISTED PAIR 10μs/DIV (6a) LT3470A 10μF 35V AI.EI. + VIN 10V/DIV 2.2μF IIN 10A/DIV 10μs/DIV (6b) 1Ω LT3470A 0.1μF VIN 10V/DIV 2.2μF IIN 10A/DIV 10μs/DIV 3470a F06 (6c) Figure 6. A Well Chosen Input Network Prevents Input Voltage Overshoot and Ensures Reliable Operation When the LT3470A is Connected to a Live Supply 3470afb 15 LT3470A APPLICATIONS INFORMATION 3.3V Step-Down Converter VIN 4V TO 40V VIN BOOST LT3470A OFF ON SHDN C3 0.22μF, 6.3V L1 33μH BIAS 22pF C1 1μF VOUT 3.3V 250mA SW R1 324k FB GND R2 200k C2 22μF 3470a TA02 C1: TDK C3216JB1H105M C2: CE JMK316 BJ226ML-T L1: TOKO A993AS-270M=P3 5V Step-Down Converter VIN 5.7V TO 40V VIN BOOST LT3470A OFF ON SHDN C3 0.22μF, 6.3V L1 33μH BIAS 22pF C1 1μF VOUT 5V 250mA SW R1 604k FB GND R2 200k C2 22μF 3470a TA03 C1: TDK C3216JB1H105M C2: CE JMK316 BJ226ML-T L1: TOKO A914BYW-330M=P3 2.5V Step-Down Converter VIN 4V TO 40V VIN BOOST LT3470A OFF ON SHDN C3 0.47μF, 6.3V L1 33μH BIAS 22pF C1 1μF VOUT 2.5V 250mA SW R1 200k FB GND R2 200k C2 22μF 3470a TA04 C1: TDK C3216JB1H105M C2: TDK C2012JB0J226M L1: SUMIDA CDRH3D28 3470afb 16 LT3470A TYPICAL APPLICATIONS 1.8V Step-Down Converter VIN 4V TO 23V VIN BOOST LT3470A OFF ON SHDN VOUT 1.8V 250mA SW BIAS C1 1μF C3 0.22μF, 25V L1 22μH 22pF R1 147k FB GND R2 332k C2 22μF 3470a TA05 C1: TDK C3216JB1H105M C2: TDK C2012JB0J226M L1: MURATA LQH32CN150K53 12V Step-Down Converter VIN 15V TO 34V VIN BOOST LT3470A OFF ON SHDN C3 0.22μF, 16V L1 33μH BIAS 22pF C1 1μF VOUT 12V 250mA SW R1 866k FB GND R2 100k C2 10μF 3470a TA06 C1: TDK C3216JB1H105M C2: TDK C3216JB1C106M L1: MURATA LQH32CN150K53 3470afb 17 LT3470A PACKAGE DESCRIPTION DDB Package 8-Lead Plastic DFN (3mm × 2mm) (Reference LTC DWG # 05-08-1702 Rev B) 0.61 ±0.05 (2 SIDES) 3.00 ±0.10 (2 SIDES) R = 0.115 TYP 5 R = 0.05 TYP 0.40 ± 0.10 8 0.70 ±0.05 2.55 ±0.05 1.15 ±0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 2.20 ±0.05 (2 SIDES) PIN 1 BAR TOP MARK (SEE NOTE 6) 0.200 REF RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 2.00 ±0.10 (2 SIDES) 0.56 ± 0.05 (2 SIDES) 0.75 ±0.05 0 – 0.05 4 0.25 ± 0.05 1 PIN 1 R = 0.20 OR 0.25 × 45° CHAMFER (DDB8) DFN 0905 REV B 0.50 BSC 2.15 ±0.05 (2 SIDES) BOTTOM VIEW—EXPOSED PAD NOTE: 1. DRAWING CONFORMS TO VERSION (WECD-1) IN JEDEC PACKAGE OUTLINE M0-229 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 3470afb 18 LT3470A REVISION HISTORY (Revision history begins at Rev B) REV DATE DESCRIPTION PAGE NUMBER B 01/11 Added H-grade part information. 2 to 5, 15 3470afb 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 LT3470A RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1616 25V, 500mA (IOUT), 1.4MHz, High Efficiency Step-Down DC/DC Converter VIN = 3.6V to 25V, VOUT = 1.25V, IQ = 1.9mA, ISD = <1μA, ThinSOT Package LT1676 60V, 440mA (IOUT), 100kHz, High Efficiency Step-Down DC/DC Converter VIN = 7.4V to 60V, VOUT = 1.24V, IQ = 3.2mA, ISD = 2.5μA, S8 Package LT1765 25V, 2.75A (IOUT), 1.25MHz, High Efficiency Step-Down DC/DC Converter VIN = 3V to 25V, VOUT = 1.2V, IQ = 1mA, ISD = 15μA, S8, TSSOP16E Packages LT1766 60V, 1.2A (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter VIN = 5.5V to 60V, VOUT = 1.2V, IQ = 2.5mA, ISD = 25μA, TSSOP16/E Package LT1767 25V, 1.2A (IOUT), 1.25MHz, High Efficiency Step-Down DC/DC Converter VIN = 3V to 25V; VOUT = 1.2V, IQ = 1mA, ISD = 6μA, MS8/E Packages LT1776 40V, 550mA (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter VIN = 7.4V to 40V; VOUT = 1.24V, IQ = 3.2mA, ISD = 30μA, N8, S8 Packages LTC®1877 600mA (IOUT), 550kHz, Synchronous Step-Down DC/DC Converter VIN = 2.7V to 10V; VOUT = 0.8V, IQ = 10μA, ISD = <1μA, MS8 Package LTC1879 1.2A (IOUT), 550kHz, Synchronous Step-Down DC/DC Converter VIN = 2.7V to 10V; VOUT = 0.8V, IQ = 15μA, ISD = <1μA, TSSOP16 Package LT1933 36V, 600mA, 500kHz, High Efficiency Step-Down DC/DC Converter VIN = 3.6V to 36V; VOUT = 1.25V, IQ = 2.5μA, ISD = <1μA, ThinSOT and 2mm × 3mm DFN-6 Package LT1934 34V, 250mA (IOUT), Micropower, Step-Down DC/DC Converter VIN = 3.2V to 34V; VOUT = 1.25V, IQ = 12μA, ISD = <1μA, ThinSOT and 2mm × 3mm DFN-6 Package LT1956 60V, 1.2A (IOUT), 500kHz, High Efficiency Step-Down DC/DC Converter VIN = 5.5V to 60V, VOUT = 1.2V, IQ = 2.5mA, ISD = 25μA, TSSOP16/E Package LTC3405/LTC3405A 300mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter VIN = 2.7V to 6V, VOUT = 0.8V, IQ = 20μA, ISD = <1μA, ThinSOT Package LTC3406/LTC3406B 600mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter VIN = 2.5V to 5.5V, VOUT = 0.6V, IQ = 20μA, ISD = <1μA, ThinSOT Package LTC3411 1.25A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter VIN = 2.5V to 5.5V, VOUT = 0.8V, IQ = 60μA, ISD = <1μA, MS Package LTC3412 2.5A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter VIN = 2.5V to 5.5V, VOUT = 0.8V, IQ = 60μA, ISD = <1μA, TSSOP16E Package LT3430 60V, 2.75A (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter VIN = 5.5V to 60V, VOUT = 1.2V, IQ = 2.5mA, ISD = 30μA, TSSOP16E Package LT3470 40V, 200mA, Micropower Step-Down DC/DC Converter VIN = 4V to 40V, VOUT = 1.25V, IQ = 35μA, ISD = <1μA, DFN-8, ThinSOT Packages 3470afb 20 Linear Technology Corporation LT 0111 REV B • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2008