LT3681 36V, 2A, 2.8MHz Step-Down Switching Regulator with Integrated Power Schottky Diode U FEATURES DESCRIPTIO ■ The LT®3681 is an adjustable frequency (300kHz to 2.8MHz) monolithic buck switching regulator that accepts input voltages up to 34V (36V maximum). A high efficiency 0.18Ω switch is included on the die along with a boost Schottky diode and the necessary oscillator, control, and logic circuitry. An undedicated power Schottky diode is integrated into the LT3681 to minimize the solution size. Current mode topology is used for fast transient response and good loop stability. Low ripple Burst Mode operation maintains high efficiency at low output currents while keeping output ripple below 15mV in a typical application. In addition, the LT3681 can further enhance low output current efficiency by drawing bias current from the output when VOUT is above 3V. Shutdown reduces input supply current to less than 1µA while a resistor and capacitor on the RUN/SS pin provide a controlled output voltage ramp (soft-start). A power good flag signals when VOUT reaches 90% of the programmed output voltage. The LT3681 is available in 14-Pin 4mm x 3mm DFN package with exposed pads for low thermal resistance. ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Wide Input Voltage Range: 3.6V to 34V Operating, 36V Maximum 2A Maximum Output Current Low Ripple Burst Mode® Operation 50µA IQ at 12VIN to 3.3VOUT Output Ripple < 15mVP-P Adjustable Switching Frequency: 300kHz to 2.8MHz Low Shutdown Current: IQ < 1µA Integrated Boost Diode Integrated Power Schottky Diode Power Good Flag Saturating Switch Design: 0.18Ω On-Resistance 1.265V Feedback Reference Voltage Output Voltage: 1.265V to 20V Soft-Start Capability Small 14-Pin Thermally Enhanced (4mm x 3mm) DFN Package U APPLICATIO S ■ ■ ■ ■ Automotive Battery Regulation Power for Portable Products Distributed Supply Regulation Industrial Supplies Wall Transformer Regulation , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. Burst Mode is a registered trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. U ■ TYPICAL APPLICATIO 5V Step-Down Converter BD VIN OFF ON Efficiency VOUT 5V 2A RUN/SS BOOST LT3681 L 6.8mH SW DC RT DA 330pF BIAS PG 60.4k 60 0.1000 50 40 0.0100 30 VIN = 12V VOUT = 3.3V L = 4.7mH F = 800 kHz 20 590k GND 1.0000 70 10 FB 22mF 200k 0 0.0001 POWER LOSS (W) VC 90 80 0.47mF 20k 4.7mF 10.0000 100 EFFICIENCY (%) VIN 6.3V TO 34V 0.0010 0.0001 0.001 0.01 0.1 ILOAD (A) 1 10 3681 TA01b L: NEC PLC-0745-6R8 3681 TA01 3681f 1 LT3681 U W W W ABSOLUTE AXI U RATI GS U W U PACKAGE/ORDER I FOR ATIO (Note 1) TOP VIEW VIN, RUN/SS Voltage .................................................36V BOOST Pin Voltage ...................................................56V BOOST Pin Above SW Pin.........................................30V FB, RT, VC Voltage .......................................................5V BIAS, PG, BD Voltage ................................................30V Maximum Junction Temperature .......................... 125°C DC above DA .............................................................40V Operating Temperature Range (Note 2) LT3681E............................................... –40°C to 85°C Storage Temperature Range................... –65°C to 150°C PG 1 BIAS 2 14 RUN/SS FB 3 13 VIN 12 SW GND 4 11 BOOST VC 5 RT 6 GND 7 15 10 BD 16 9 DC 8 DA DE14MA PACKAGE 14-LEAD (4mm ´ 3mm) PLASTIC DFN TJMAX = 125°C, θJA = 43°C/W EXPOSED PAD (PIN 15) IS GND, MUST BE SOLDERED TO PCB EXPOSED PAD PIN 16 IS DC ORDER PART NUMBER DE PART MARKING LT3681EDE 3681 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 = 10V, VRUNS/SS = 10V, VBOOST = 15V, VBIAS = 3.3V unless otherwise noted. (Note 2) PARAMETER CONDITIONS ● Minimum Input Voltage Quiescent Current from VIN MIN VRUN/SS = 0.2V VBIAS = 3V, Not Switching ● VBIAS = 0, Not Switching Quiescent Current from BIAS VRUN/SS = 0.2V VBIAS = 3V, Not Switching ● VBIAS = 0, Not Switching Minimum Bias Voltage Feedback Voltage ● FB Pin Bias Current (Note 3) VFB = 1.25V, VC = 0.4V FB Voltage Line Regulation 4V < VIN < 34V ● 1.25 1.24 TYP MAX UNITS 3 3.6 V 0.01 0.5 μA 22 60 μA 75 120 μA 0.01 0.5 μA 50 120 μA 0 5 μA 2.7 3 V 1.265 1.265 1.29 1.3 V V 30 100 nA 0.002 0.02 %/V μMho Error Amp GM 330 Error Amp Gain 800 VC Source Current 65 μA VC Sink Current 85 μA VC Pin to Switch Current Gain 3.5 A/V VC Clamp Voltage 2 V 3681f 2 LT3681 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, VRUNS/SS = 10V VBOOST = 15V, VBIAS = 3.3V unless otherwise noted. (Note 2) PARAMETER CONDITIONS Power Schottky Diode Forward Voltage IDA = 1A IDA = 2A Power Schottky Diode Leakage Current VDC-DA = 40V Switching Frequency RT = 8.66k RT = 29.4k RT = 187k MIN MAX 0.50 0.56 2.5 1.25 250 ● Minimum Switch Off-Time TYP 3.2 UNITS V V 100 μA 2.8 1.4 300 3.1 1.55 350 MHz MHz kHz 130 200 nS 3.8 4.4 A Switch Current Limit Duty Cycle = 5% Switch VCESAT ISW = 2A 360 Boost Schottky Reverse Leakage VSW = 10V, VBIAS = 0V 0.02 2 µA 1.5 2.1 V ● Minimum Boost Voltage (Note 4) mV BOOST Pin Current ISW = 1A 18 35 mA RUN/SS Pin Current VRUN/SS = 2.5V 5 10 μA RUN/SS Input Voltage High 2.5 V RUN/SS Input Voltage Low PG Threshold Offset from Feedback Voltage 0.2 VFB Rising 122 PG Leakage VPG = 5V 0.1 PG Sink Current VPG = 3V PG Hysteresis mV 5 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 LT3681E 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. ● 100 600 V mV 1 μA μA Note 3: Bias current flows into the FB pin. Note 4: This is the minimum voltage across the boost capacitor needed to guarantee full saturation of the switch. 3681f 3 LT3681 U W TYPICAL PERFOR A CE CHARACTERISTICS TA = 25°C unless otherwise noted. Efficiency (VOUT = 3.3V) Efficiency (VOUT = 5.0V) 90 VIN = 12V 80 EFFICIENCY (%) 50 40 40 20 20 0.001 0.01 0.1 LOAD CURRENT (A) 1 VIN = 24V 50 30 L: NEC PLC-0745-4R7 f: 800kHz VIN = 12V 60 30 0 0.0001 80 70 60 70 65 0 0.0001 50 0.001 0.01 0.1 LOAD CURRENT (A) 1 0 10 100 90 Maximum Load Current 90 30 20 3.5 LOAD CURRENT (A) SUPPLY CURRENT (µA) 40 INCREASED SUPPLY CURRENT DUE TO CATCH DIODE LEAKAGE AT HIGH TEMPERATURE 80 70 60 50 3.0 2.5 2.0 VOUT = 3.3V L = 4.7µH f = 800 kHz 1.5 10 FRONT PAGE APPLICATION 0 0 5 10 20 25 15 INPUT VOLTAGE (V) 30 40 –40 35 1.0 –20 60 0 20 40 TEMPERATURE (°C) 3.5 3.5 3.0 2.5 2.0 VOUT = 5.0V L = 4.7µH f = 800 kHz 10 20 25 15 INPUT VOLTAGE (V) 30 3681 G07 30 Switch Current Limit DUTY CYCLE = 10 % 4.0 3.0 2.5 2.0 3.0 2.5 DUTY CYCLE = 90 % 2.0 1.5 1.0 0.5 1.0 5 25 20 15 INPUT VOLTAGE (V) 4.5 1.5 1.0 10 3681 G06 SWITCH CURRENT LIMIT (A) 4.0 SWITCH CURRENT LIMIT(A) LOAD CURRENT (A) 5 Switch Current Limit Maximum Load Current 1.5 80 3681 G05 3681 G04 4.0 3 4.0 VIN = 12V VOUT = 3.3V 80 50 1 2 2.5 1.5 SWITCHING FREQUENCY (MHz) 3681 G03 No Load Supply Current vs Temperature 60 0.5 3681 G02 No Load Supply Current 70 VOUT = 3.3V L = 10µH LOAD = 1A 55 L: NEC PLC-0745-4R7 f: 800kHz 3681 G01 SUPPLY CURRENT (µA) VIN = 24V 75 60 10 10 VIN = 12V 85 VIN = 7V 80 VIN = 24V 70 10 90 EFFICIENCY (%) 90 EFFICIENCY (%) Efficiency vs Switching Frequency 100 100 0 20 60 40 DUTY CYCLE (%) 80 100 3681 G08 0 –50 –25 0 25 50 75 TEMPERATURE (°C) 100 125 3681 G09 3681f 4 LT3681 U W TYPICAL PERFOR A CE CHARACTERISTICS TA = 25°C unless otherwise noted. Switch Voltage Drop 400 300 200 1.290 80 1.285 FEEDBACK VOLTAGE (V) 500 90 70 BOOST PIN CURRENT (mA) VOLTAGE DROP (mV) 600 60 50 40 30 100 0 500 0 –25 0 25 50 75 TEMPERATURE (°C) Frequency Foldback RT = 45.3kΩ 100 125 4681 G12 Minimum Switch On-Time 140 1200 SWITCHING FREQUENCY (kHz) 1.05 1.00 0.95 0.90 0.85 100 MINIMUM SWITCH ON TIME (ns) RT = 45.3kΩ 1.10 FREQUENCY (MHz) 1.250 –50 500 1000 1500 2000 2500 3000 3500 SWITCH CURRENT (mA) 3681 G11 1.15 0 25 50 75 TEMPERATURE (°C) 1.265 1.255 1000 1500 2000 2500 3000 3500 SWITCH CURRENT (mA) –25 1.270 10 Switching Frequency 0.80 –50 1.275 1.260 3681 G10 1.20 1.280 20 0 0 1000 800 600 400 200 0 200 120 100 80 60 40 20 0 –50 0 125 400 600 800 1000 1200 1400 FB PIN VOLTAGE (mV) 4681 G13 Soft Start RUN/SS Pin Current 2.0 1.5 1.0 100 125 Boost Diode 1.6 1.4 10 BOOST DIODE Vf (V) RUN/SS PIN CURRENT (µA) 3.5 2.5 25 0 50 75 TEMPERATURE (˚C) 3681 G15 12 3.0 –25 3681 G14 4.0 SWITCH CURRENT LIMIT (A) Feedback Voltage Boost Pin Current 700 8 6 4 1.2 1.0 0.8 0.6 0.4 2 0.5 0.2 0 0 0 0.5 1 2 2.5 1.5 RUN/SS PIN VOLTAGE (V) 3 3.5 3681 G16 0 5 20 30 15 25 10 RUN/SS PIN VOLTAGE (V) 35 3681 G17 0 0 1.0 0.5 1.5 BOOST DIODE CURRENT (A) 2.0 3681 G18 3681f 5 LT3681 U W TYPICAL PERFOR A CE CHARACTERISTICS TA = 25°C unless otherwise noted. Error Amp Output Current Minimum Input Voltage Minimum Input Voltage 100 4.5 6.5 4.0 6.0 INPUT VOLTAGE (V) VC PIN CURRENT (µA) 60 40 20 0 –20 –40 3.5 3.0 2.5 –60 –80 1.065 1.165 1.365 1 .265 FB PIN VOLTAGE (V) INPUT VOLTAGE (V) 80 0.1 0.01 1 LOAD CURRENT (A) 1.200 2.00 1.180 THRESHOLD VOLTAGE (V) 2.50 1.00 SWITCHING THRESHOLD Switching Waveforms; Burst Mode VIN = 12V; FRONT PAGE APPLICATION ILOAD = 10mA IL 0.5A/DIV 1.160 VSW 5V/DIV 1.140 VOUT 10mV/DIV 1.120 0.50 10 3681 G20 Power Good Threshold CURRENT LIMIT CLAMP 0.1 0.01 1 LOAD CURRENT (A) 3681 G20 VC Voltages 1.50 VOUT = 5.0V L = 4.7m f = 800kHz 4.0 0.001 10 3681 G19 THRESHOLD VOLTAGE (V) 5.0 4.5 VOUT = 3.3V L = 4.7m f = 800kHz 2.0 0.001 1.465 5.5 PG RISING 0 –50 –25 0 50 25 75 TEMPERATURE (°C) 100 125 1.100 –50 –25 0 50 25 75 TEMPERATURE (°C) 100 125 2µs/DIV 3681 G24 3681 G23 3681 G22 Switching Waveforms; Transition from Burst Mode to Full Frequency Power Schottky Diode Forward Voltage vs Current Switching Waveforms; Full Frequency Continuous Operation 6000 IL 0.5A/DIV VRUN/SS 5V/DIV VRUN/SS 5V/DIV VOUT 10mV/DIV VOUT 10mV/DIV 3681 G25 4000 3000 2000 1000 VIN = 12V; FRONT PAGE APPLICATION ILOAD = 1A VIN = 12V; FRONT PAGE APPLICATION ILOAD = 140mA 1µs/DIV 5000 CURRENT (mA) IL 0.5A/DIV 1µs/DIV 0 3681 G26 0 0.1 0.2 0.3 0.4 0.5 FORWARD VOLTAGE (V) 0.6 0.7 3681 G29 3681f 6 LT3681 U U U PI FU CTIO S PG (Pin 1): The PG pin is the open collector output of an internal comparator. PG remains low until the FB pin is within 10% of the final regulation voltage. PG output is valid when VIN is above 3.5V and RUN/SS is high. BIAS (Pin 2): The BIAS pin supplies the current to the LT3681’s internal regulator. Tie this pin to the lowest available voltage source above 3V (typically VOUT). This architecture increases efficiency especially when the input voltage is much higher than the output. FB (Pin 3): The LT3681 regulates the FB pin to 1.265V. Connect the feedback resistor divider tap to this pin. VC (Pin 5): The VC pin is the output of the internal error amplifier. The voltage on this pin controls the peak switch current. Tie an RC network from this pin to ground to compensate the control loop. RT (Pin 6): Oscillator Resistor Input. Connecting a resistor to ground from this pin sets the switching frequency. DA (Pin 8): This is the anode of the integrated power Schottky diode. High frequency, large amplitude currents flow through this pin, so tie it to ground through a low impedance connection. DC (Pin 9, Exposed Pad 16): These pins connect to the cathode of the integrated power Schottky diode. High frequency, large amplitude currents flow through these pins, so tie them to SW through a low impedance connection. BD (Pin 10): This pin connects to the anode of the internal boost Schottky diode. BOOST (Pin 11): This pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar NPN power switch. Connect a capacitor between this pin and SW. SW (Pin 12): The SW pin is the output of the internal power switch. Connect this pin to the inductor, DC and boost capacitor. VIN (Pin 13): The VIN pin supplies current to the LT3681’s internal regulator and to the internal power switch. This pin must be locally bypassed. RUN/SS (Pin 14): The RUN/SS pin is used to put the LT3681 in shutdown mode. Tie to ground to shut down the LT3681. Tie to 2.3V or more for normal operation. If the shutdown feature is not used, tie this pin to the VIN pin. RUN/SS also provides a soft-start function; see the Applications Information section. GND (Pins 4, 7, Exposed Pad 15): All three of these terminals internally connect to the LT3681 control IC’s signal return, while exposed pad 15 performs the added function of providing a low thermal resistance heat flow path between the IC and the system heatsink. Tie all of these terminals to a copper pour on the top layer of the printed circuit board. Please refer to the Applications Information section for more details. 3681f 7 LT3681 W BLOCK DIAGRA VIN 13 VIN C1 2 14 6 BIAS – + INTERNAL 1.265V REF RUN/SS 5 SLOPE COMP BD SWITCH LATCH BOOST 11 C3 R RT OSCILLATOR 300kHz–2.8MHz Q S SW RT DISABLE DC SOFT-START 1 10 BurstMode DETECT DC L1 VOUT 12 C2 9 16 PG ERROR AMP + – GND GND 4 7 + – 1.12V FB GND 15 VC CLAMP DA VC 8 5 CC RC CF 3 R2 R1 3681 BD OPERATION The LT3681 is a constant frequency, current mode stepdown regulator. An oscillator, with frequency set by RT, sets an RS flip-flop, turning on the internal power switch. An amplifier and comparator monitor the current flowing between the VIN and SW pins, turning the switch off when this current reaches a level determined by the voltage at VC. An error amplifier measures the output voltage through an external resistor divider tied to the FB pin and servos the VC pin. If the error amplifier’s output increases, more current is delivered to the output; if it decreases, less current is delivered. An active clamp on the VC pin provides current limit. The VC pin is also clamped to the voltage on the RUN/SS pin; soft-start is implemented by generating a voltage ramp at the RUN/SS pin using an external resistor and capacitor. The switch driver operates from either the input or from the BOOST pin. An external capacitor 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. An internal regulator provides power to the control circuitry. The bias regulator normally draws power from the VIN pin, but if the BIAS pin is connected to an external voltage higher than 3V bias power will be drawn from the external source (typically the regulated output voltage). This improves efficiency. The RUN/SS pin is used to place the LT3681 in shutdown, disconnecting the output and reducing the input current to less than 1µA. The LT3681 contains a power good comparator which trips when the FB pin is at 91% of its regulated value. The PG output is an open-collector transistor that is off when the output is in regulation, allowing an external resistor to pull the PG pin high. Power good is valid when the LT3681 is enabled and VIN is above 3.6V. To further optimize efficiency, the LT3681 automatically switches to Burst Mode operation in light load situations. Between bursts, all circuitry associated with controlling the output switch is shut down reducing the input supply current to 55µA in a typical application. The oscillator reduces the LT3681’s operating frequency when the voltage at the FB pin is low. This frequency foldback helps to control the output current during startup and overload. The LT3681 integrates a high quality, 36V, 2A power Schottky diode to reduce the overall solution size. 3681f 8 LT3681 APPLICATIONS INFORMATION FB Resistor Network The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the 1% resistors according to: ⎞ ⎛V R1= R2 ⎜ OUT – 1⎟ ⎝ 1.265 ⎠ Reference designators refer to the Block Diagram. Setting the Switching Frequency The LT3681 uses a constant frequency PWM architecture that can be programmed to switch from 300kHz to 2.8MHz by using a resistor tied from the RT pin to ground. A table showing the necessary RT value for a desired switching frequency is in Figure 1. SWITCHING FREQUENCY (MHz) RT VALUE (kΩ) 0.3 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 187 133 84.5 60.4 45.3 36.5 29.4 23.7 20.5 16.9 14.3 12.1 10.2 8.66 Figure 1. Switching Frequency vs. RT Value Operating Frequency Tradeoffs Selection of the operating frequency is a tradeoff between efficiency, component size, minimum dropout voltage, and maximum input voltage. The advantage of high frequency operation is that smaller inductor and capacitor values may be used. The disadvantages are lower efficiency, lower maximum input voltage, and higher dropout voltage. The highest acceptable switching frequency (fSW(MAX)) for a given application can be calculated as follows: fSW(MAX ) = VD + VOUT tON(MIN) ( VD + VIN – VSW ) where VIN is the typical input voltage, VOUT is the output voltage, VD is the power Schottky catch diode drop (~0.55V), VSW is the internal switch drop (~0.5V at max load). This equation shows that slower switching frequency is necessary to safely accommodate high VIN/VOUT ratio. Also, as shown in the next section, lower frequency allows a lower dropout voltage. The reason input voltage range depends on the switching frequency is because the LT3681 switch has finite minimum on and off times. The switch can turn on for a minimum of ~150ns and turn off for a minimum of ~150ns. This means that the minimum and maximum duty cycles are: DCMIN = fSW tON(MIN) DCMAX = 1– fSW tOFF(MIN) where fSW is the switching frequency, the tON(MIN) is the minimum switch on time (~150ns), and the tOFF(MIN) is the minimum switch off time (~150ns). These equations show that duty cycle range increases when switching frequency is decreased. A good choice of switching frequency should allow adequate input voltage range (see next section) and keep the inductor and capacitor values small. Input Voltage Range The maximum input voltage for LT3681 applications depends on switching frequency, the Absolute Maximum Ratings on VIN and BOOST pins, and on operating mode. If the output is in start-up or short-circuit operating modes, then VIN must be below 34V and below the result of the following equation: VIN(MAX ) = VOUT + VD –V +V fSW tON(MIN) D SW where VIN(MAX) is the maximum operating input voltage, VOUT is the output voltage, VD is the catch diode drop (~0.55V), VSW is the internal switch drop (~0.5V at max load), fSW is the switching frequency (set by RT), and tON(MIN) is the minimum switch on time (~150ns). Note that a higher switching frequency will depress the maximum operating input voltage. Conversely, a lower switching 3681f 9 LT3681 APPLICATIONS INFORMATION frequency will be necessary to achieve safe operation at high input voltages. If the output is in regulation and no short-circuit or start-up events are expected, then input voltage transients of up to 36V are acceptable regardless of the switching frequency. In this mode, the LT3681 may enter pulse skipping operation where some switching pulses are skipped to maintain output regulation. In this mode the output voltage ripple and inductor current ripple will be higher than in normal operation. The minimum input voltage is determined by either the LT3681’s minimum operating voltage of ~3.6V or by its maximum duty cycle (see equation in previous section). The minimum input voltage due to duty cycle is: VIN(MIN) = VOUT + VD –V +V 1– fSW tOFF(MIN) D SW where VIN(MIN) is the minimum input voltage, and tOFF(MIN) is the minimum switch off time (150ns). Note that higher switching frequency will increase the minimum input voltage. If a lower dropout voltage is desired, a lower switching frequency should be used. Inductor Selection For a given input and output voltage, the inductor value and switching frequency will determine the ripple current. The ripple current ΔIL increases with higher VIN or VOUT and decreases with higher inductance and faster switching frequency. A reasonable starting point for selecting the ripple current is: ΔIL = 0.4(IOUT(MAX)) where IOUT(MAX) is the maximum output load current. To guarantee sufficient output current, peak inductor current must be lower than the LT3681’s switch current limit (ILIM). The peak inductor current is: at least 3.5A at low duty cycles and decreases linearly to 2.5A at DC = 0.8. The maximum output current is a function of the inductor ripple current: IOUT(MAX) = ILIM – ΔIL/2 Be sure to pick an inductor ripple current that provides sufficient maximum output current (IOUT(MAX)). The largest inductor ripple current occurs at the highest VIN. To guarantee that the ripple current stays below the specified maximum, the inductor value should be chosen according to the following equation: ⎛V +V ⎞⎛ V +V ⎞ L = ⎜ OUT D ⎟ ⎜ 1– OUT D ⎟ VIN(MAX ) ⎟⎠ ⎝ f∆IL ⎠ ⎜⎝ where VD is the voltage drop of the integrated Schottky diode (~0.55V), VIN(MAX) is the maximum input voltage, VOUT is the output voltage, fSW is the switching frequency (set by RT), and L is in the inductor value. The inductor’s RMS current rating must be greater than the maximum load current and its saturation current should be about 30% higher. For robust operation in fault conditions (start-up or short circuit) and high input voltage (>30V), the saturation current should be above 3.5A. To keep the efficiency high, the series resistance (DCR) should be less than 0.1Ω, and the core material should be intended for high frequency applications. Table 1 lists several vendors and suitable types. Table 1. Inductor Vendors VENDOR URL PART SERIES TYPE Murata www.murata.com LQH55D Open TDK www.componenttdk.com SLF7045 SLF10145 Shielded Shielded Toko www.toko.com D75C Shielded D75F Open FDV0620 Shielded CDRH74 Shielded CDRH6D38 Shielded CR75 Open CDRH8D43 Shielded PLC-0745 Shielded Sumida www.sumida.com IL(PEAK) = IOUT(MAX) + ΔIL/2 where IL(PEAK) is the peak inductor current, IOUT(MAX) is the maximum output load current, and ΔIL is the inductor ripple current. The LT3681’s switch current limit (ILIM) is NEC www.nec.tokin.com 3681f 10 LT3681 APPLICATIONS INFORMATION Of course, such a simple design guide will not always result in the optimum inductor for your application. A larger value inductor provides a slightly higher maximum load current and will reduce the output voltage ripple. If your load is lower than 2A, then you can decrease the value of the inductor and operate with higher ripple current. This allows you to use a physically smaller inductor, or one with a lower DCR resulting in higher efficiency. There are several graphs in the Typical Performance Characteristics section of this data sheet that show the maximum load current as a function of input voltage and inductor value for several popular output voltages. Low inductance may result in discontinuous mode operation, which is okay but further reduces maximum load current. For details of maximum output current and discontinuous mode operation, see Linear Technology Application Note 44. Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5), there is a minimum inductance required to avoid subharmonic oscillations. See Application Note 19. Input Capacitor Bypass the input of the LT3681 circuit with a ceramic capacitor of X7R or X5R type. Y5V types have poor performance over temperature and applied voltage, and should not be used. A 4.7µF to 10µF ceramic capacitor is adequate to bypass the LT3681 and will easily handle the ripple current. Note that larger input capacitance is required when a lower switching frequency is used. If the input power source has high impedance, or there is significant inductance due to long wires or cables, additional bulk capacitance may be necessary. This can be provided with a low performance electrolytic 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 LT3681 and to force this very high frequency switching current into a tight local loop, minimizing EMI. A 4.7µF capacitor is capable of this task, but only if it is placed close to the LT3681 and the catch diode (see the PCB Layout section). A second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the LT3681. A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT3681 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3681’s voltage rating. This situation is easily avoided (see the Hot Plugging Safely section). For space sensitive applications, a 2.2µF ceramic capacitor can be used for local bypassing of the LT3681 input. However, the lower input capacitance will result in increased input current ripple and input voltage ripple, and may couple noise into other circuitry. Also, the increased voltage ripple will raise the minimum operating voltage of the LT3681 to ~3.7V. Output Capacitor and Output Ripple The output capacitor has two essential functions. Along with the inductor, it filters the square wave generated by the LT3681 to produce the DC output. In this role it determines the output ripple, and low impedance at the switching frequency is important. The second function is to store energy in order to satisfy transient loads and stabilize the LT3681’s control loop. Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance. A good starting value is: 100 COUT = VOUT fSW where fSW is in MHz, and COUT is the recommended output capacitance in µF. Use X5R or X7R types. This choice will provide low output ripple and good transient response. Transient performance can be improved with a higher value capacitor if the compensation network is also adjusted to maintain the loop bandwidth. A lower value of output capacitor can be used to save space and cost but transient performance will suffer. See the Frequency Compensation section to choose an appropriate compensation network. 3681f 11 LT3681 APPLICATIONS INFORMATION Table 2. Capacitor Vendors VENDOR PHONE URL PART SERIES Panasonic (714) 373-7366 www.panasonic.com Ceramic, Polymer, COMMENTS EEF Series Tantalum Kemet (864) 963-6300 www.kemet.com Ceramic, Tantalum Sanyo (408) 749-9714 www.sanyovideo.com T494, T495 Ceramic, Polymer, POSCAP Tantalum Murata (408) 436-1300 AVX www.murata.com Ceramic www.avxcorp.com Ceramic, Tantalum Taiyo Yuden (864) 963-6300 www.taiyo-yuden.com When choosing a capacitor, look carefully through the data sheet to find out what the actual capacitance is under operating conditions (applied voltage and temperature). A physically larger capacitor, or one with a higher voltage rating, may be required. High performance tantalum or electrolytic capacitors can be used for the output capacitor. Low ESR is important, so choose one that is intended for use in switching regulators. The ESR should be specified by the supplier, and should be 0.05Ω or less. Such a capacitor will be larger than a ceramic capacitor and will have a larger capacitance, because the capacitor must be large to achieve low ESR. Table 2 lists several capacitor vendors. TPS Series Ceramic frequencies, generating audible noise. Since the LT3681 operates at a lower current limit during Burst Mode operation, the noise is typically very quiet to a casual ear. If this is unacceptable, use a high performance tantalum or electrolytic capacitor at the output. A final precaution regarding ceramic capacitors concerns the maximum input voltage rating of the LT3681. A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT3681 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3681’s rating. This situation is easily avoided (see the Hot Plugging Safely section). Catch Diode The integral power Schottky catch diode conducts current only during switch off time. Average forward current in normal operation can be calculated from: ID(AVG) = IOUT (VIN – VOUT)/VIN where IOUT is the output load current. Ceramic Capacitors Ceramic capacitors are small, robust and have very low ESR. However, ceramic capacitors can cause problems when used with the LT3681 due to their piezoelectric nature. When in Burst Mode operation, the LT3681’s switching frequency depends on the load current, and at very light loads the LT3681 can excite the ceramic capacitor at audio Frequency Compensation The LT3681 uses current mode control to regulate the output. This simplifies loop compensation. In particular, the LT3681 does not require the ESR of the output capacitor for stability, so you are free to use ceramic capacitors to achieve low output ripple and small circuit size. Frequency compensation is provided by the components tied to the VC pin, as shown in Figure 2. Generally a capacitor (CC) and a resistor (RC) in series to ground are used. In addition, there may be a lower value capacitor in parallel. This capacitor (CF) is not part of the loop compensation but is used to filter noise at the switching frequency, and is required only if a phase-lead capacitor is used or if the output capacitor has high ESR. 3681f 12 LT3681 APPLICATIONS INFORMATION Loop compensation determines the stability and transient performance. Designing the compensation network is a bit complicated and the best values depend on the application and in particular the type of output capacitor. A practical approach is to start with one of the circuits in this data sheet that is similar to your application and tune the compensation network to optimize the performance. Stability should then be checked across all operating conditions, including load current, input voltage and temperature. The LT1375 data sheet contains a more thorough discussion of loop compensation and describes how to test the stability using a transient load. Figure 2 shows an equivalent circuit for the LT3681 control loop. The error amplifier is a transconductance amplifier with finite output impedance. The power section, consisting of the modulator, power switch and inductor, is modeled as a transconductance amplifier generating an output current proportional to the voltage at the VC pin. Note that the output capacitor integrates this current, and that the capacitor on the VC pin (CC) integrates the error amplifier output current, resulting in two poles in the loop. In most cases a zero is required and comes from either the output capacitor ESR or from a resistor RC in series with CC. This simple model works well as long as the value of the inductor is not too high and the loop crossover frequency is much lower than the switching frequency. A phase lead capacitor (CPL) across the feedback divider may improve the transient response. Figure 3 shows the transient response when the load current is stepped from 500mA to 1500mA and back to 500mA. VIN = 12V IL 1A/DIV VOUT 100mV/DIV 10ms/DIV 3681 F03 Figure 3. Transient Load Response of the LT3681 3.3V Application as the Load Current is Stepped from 500mA to 1500mA. Burst Mode Operation To enhance efficiency at light loads, the LT3681 automatically switches to Burst Mode operation which keeps the output capacitor charged to the proper voltage while minimizing the input quiescent current. During Burst Mode operation, the LT3681 delivers single cycle bursts of current to the output capacitor followed by sleep periods where the output power is delivered to the load by the output capacitor. In addition, VIN and BIAS quiescent currents are reduced to typically 20µA and 50µA respectively during the sleep time. As the load current decreases towards a no load condition, the percentage of time that the LT3681 operates in sleep mode increases and the average input current is greatly reduced resulting in higher efficiency. See Figure 4. LT3681 CURRENT MODE POWER STAGE gm = 3.5mho SW ERROR AMPLIFIER OUTPUT R1 IL 0.5A/DIV FB – gm = 330µmho ESR 1.265V C1 + + 3Meg C1 VC CF RC VIN = 12V; VOUT = 3.3V ILOAD = 10mA CPL POLYMER OR TANTALUM GND CERAMIC VSW 5V/DIV VOUT 10mV/DIV R2 CC 5ms/DIV 3681 F04 3681 F02 Figure 2. Model for Loop Response Figure 4. Burst Mode Operation 3681f 13 LT3681 APPLICATIONS INFORMATION BOOST and BIAS Pin Considerations Capacitor C3 and the internal boost Schottky diode (see the 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 5 shows three ways to arrange the boost circuit. The BOOST pin must be more than 2.3V above the SW pin for best efficiency. For outputs of 2.8V and above, the standard circuit (Figure 5a) is best. For outputs between 2.8V and 3V, use a 1µF boost capacitor. A 2.5V output presents a special case because it is marginally adequate to support the boosted drive stage while using the internal boost diode. For reliable BOOST pin operation with 2.5V outputs use a good external Schottky diode (such as the ON Semi MBR0540), and a 1µF boost capacitor (see Figure 5b). For lower output voltages the boost diode can be tied to the input (Figure 5c), or to another supply greater than 2.8V. The circuit in Figure 5a 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 minimum operating voltage of an LT3681 application is limited by the minimum input voltage (3.6V) and by the maximum duty cycle as outlined in a previous section. For proper startup, the minimum input voltage is also limited by the boost circuit. If the input voltage is ramped slowly, or the LT3681 is turned on with its RUN/SS pin when the output is already in regulation, then the boost capacitor may not be fully charged. Because the boost capacitor is charged with the energy stored in the inductor, the circuit will rely on some minimum load current to get the boost circuit running properly. This minimum load will depend on input and output voltages, and on the arrangement of the boost circuit. The minimum load generally goes to zero once the circuit has started. If, however, the LT3681 is started by the RUN/SS pin and the output is discharged, the discharged output capacitance will often present enough of a load to allow the circuit to start. Figure 6 gives plots of the input voltage required for three different situations: the worst case situation where RUN/SS is tied to VIN and VIN is ramped up very slowly, the minimum input voltage at which the circuit will regulate when start-up is controlled by RUN/SS, and the minimum input voltage required to maintain output regulation. For lower start-up voltage, the boost diode can be tied to VIN; however, this restricts the input range to one-half of the absolute maximum rating of the BOOST pin. At light loads, the inductor current becomes discontinuous and the effective duty cycle at the BOOST pin (not the SW pin) can be very high. This reduces the minimum input voltage to approximately 300mV above VOUT. At higher load currents, the inductor current is continuous and the duty cycle is limited by the maximum duty cycle of the LT3681, requiring a higher input voltage to maintain regulation. VOUT BD BOOST VIN VIN 4.7mF LT3681 GND DC DA C3 SW COUT (5a) For VOUT > 2.8V VOUT D2 BD BOOST VIN VIN 4.7mF LT3681 GND DC DA C3 SW COUT (5b) For 2.5V < VOUT < 2.8V VOUT BD BOOST VIN 4.7mF VIN LT3681 GND DA DC C3 SW COUT (5c) For VOUT < 2.5V 3681 FO5 Figure 5. Three Circuits For Generating The Boost Voltage 3681f 14 LT3681 APPLICATIONS INFORMATION 6.0 INPUT VOLTAGE (V) 5.5 5.0 Synchronization VOUT = 3.3V TA = 25°C L = 4.7m f = 800 kHz The internal oscillator of the LT3681 can be synchronized to an external 275kHz to 475kHz clock by using a 5pF to 20pF capacitor to connect the clock signal to the RT pin. The resistor tying the RT pin to ground should be chosen such that the LT3681 oscillates 20% lower than the intended synchronization frequency (see Setting the Switching Frequency section). 4.5 4.0 3.5 3.0 2.5 2.0 0.001 TO START (RUN/SS = VIN) TO START (RUN/SS CONTROL) TO RUN 0.1 0.01 1 LOAD CURRENT (A) 10 The LT3681 should not be synchronized until its output is near regulation as indicated by the PG flag. This can be done with the system microcontroller/microprocessor or with a discrete circuit by using the PG output. If a sync signal is applied while the PG is low, the LT3681 may exhibit erratic operation. 8.0 7.5 7.0 INPUT VOLTAGE (V) 6.5 VOUT = 5.0V TA = 25°C L = 4.7m f = 800 kHz 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 0.001 TO START (RUN/SS = VIN) TO START (RUN/SS CONTROL) TO RUN 0.1 0.01 1 LOAD CURRENT (A) 10 3681 F06 Figure 6. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit Soft-Start The RUN/SS pin can be used to soft-start the LT3681, reducing the maximum input current during start-up. The RUN/SS pin is driven through an external RC filter to create a voltage ramp at this pin. Figure 7 shows the startup and shut-down waveforms with the soft-start circuit. By choosing a large RC time constant, the peak start-up current can be reduced to the current that is required to regulate the output, with no overshoot. Choose the value of the resistor so that it can supply 20µA when the RUN/SS pin reaches 2.3V. IL 1A/DIV RUN 15k RUN/SS 0.22µF VRUN/SS 2V/DIV GND VOUT 2V/DIV 2ms/DIV When applying a sync signal, positive clock transitions reset LT3681’s internal clock and negative transitions initiate a switch cycle. The amplitude of the sync signal must be at least 2V. The sync signal duty cycle can range from 5% up to a maximum value given by the following equation: ⎛ VOUT + VD ⎞ – f • 600ns DCSYNC(MAX ) = ⎜ 1 – VIN – VSW + VD ⎟⎠ SW ⎝ where VOUT is the programmed output voltage, VD is the diode forward drop, VIN is the typical input voltage, VSW is the switch drop, and fSW is the desired switching frequency. For example, a 24V input to 5V output at 300kHz can be synchronized to a square wave with a maximum duty cycle of 60%. For some applications, such as 12VIN to 5VOUT at 350kHz, the maximum allowable sync duty cycle will be less than 50%. If a low duty cycle clock cannot be obtained from the system, then a one-shot should be used between the sync signal and the LT3681. The value of the coupling capacitor which connects the clock signal to the RT pin should be chosen based on the clock signal amplitude. Good starting values for 3.3V and 5V clock signals are 10pF and 5pF, respectively. These values should be tested and adjusted for each individual application to assure reliable operation. 3681 F07 Figure 7. To Soft-Start the LT3681, Add a Resistor and Capacitor to the RUN/SS Pin 3681f 15 LT3681 APPLICATIONS INFORMATION Caution should be used when synchronizing more than 50% above the initial switching frequency (as set by the RT resistor) because at higher clock frequencies the amplitude of the internal slope compensation used to prevent subharmonic switching is reduced. This type of subharmonic switching only occurs at input voltages less than twice output voltage. Higher inductor values will tend to reduce this problem. Reversed Input Protection In some systems, the output may be held high when the input to the LT3681 is absent. This may occur in battery charging applications or in battery backup systems where a battery or some other supply is diode ORed with the LT3681’s output. If the VIN pin is allowed to float and the RUN/SS pin is held high (either by a logic signal or because it is tied to VIN), then the LT3681’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 RUN/SS 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 LT3681 can pull large currents from the output through the SW pin and the VIN pin. Figure 8 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input. D4 MBRS140 VIN VIN BD BOOST LT3681 RUN/SS VC GND DA SW VOUT DC FB BACKUP 3681 F08 Figure 8. Diode D4 Prevents a Shorted Input from Discharging a Backup Battery Tied to the Output. It Also Protects the Circuit from a Reversed Input. The LT3681 Runs Only When the Input is Present 3681f 16 LT3681 APPLICATIONS INFORMATION PCB Layout For proper operation and minimum EMI, care must be taken during printed circuit board layout. Figure 9 shows the recommended component placement with trace, ground plane and via locations. Note that large, switched currents flow in the LT3681’s VIN and SW pins, the integrated Schottky diode the input capacitor (CIN) and the output capacitor (COUT). The loop formed by these components should be as small as possible. 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. The SW and BOOST nodes should be as small as possible. Finally, keep the FB and VC nodes small so that the ground traces will shield them from the SW and BOOST nodes. Each of the Exposed Pads on the bottom of the package must be soldered to copper pours so that the pad acts as a heat sink. To keep thermal resistance low, extend the ground plane as much as possible, and add thermal vias under and near the LT3681 to additional ground planes within the circuit board and on the bottom side. Keep in mind that the thermal design must keep the junctions of the IC and power diode below the specified absolute maximum temperature of 125°C. 1 14 2 13 3 12 4 11 5 10 6 9 7 8 VIN CIN COUT 3681 F11 VIAS TO GND VIAS TO VIN VIAS TO VOUT VIAS TO DC HEATSINK Figure 9. A Good PCB Layout Ensures Proper, Low EMI Operation 3681f 17 LT3681 APPLICATIONS INFORMATION The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LT3681 circuits. However, these capacitors can cause problems if the LT3681 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 LT3681 can ring to twice the nominal input voltage, possibly exceeding the LT3681’s rating and damaging the part. If the input supply is poorly controlled or the user will be plugging the LT3681 into an energized supply, the input network should be designed to prevent this overshoot. Figure 10 shows the waveforms that result when an LT3681 circuit is connected to a 24V supply through six feet of 24-gauge twisted pair. The first plot (10a) is the response with a 4.7µF ceramic capacitor at the input. The input voltage rings as high as 50V and the input current peaks at 26A. A good solution is shown in Figure 10b. A 0.7Ω resistor is added in series with the input to eliminate the voltage overshoot (it also reduces the peak inrush current). A 0.1µF capacitor improves high frequency filtering. For high input voltages its impact on efficiency is minor, reducing efficiency by 1.5 percent for a 5V output at full load operating from 24V. Another effective method of reducing the overshoot is to add a 22µF aluminum electrolytic capacitor, as shown in Figure 10c. High Temperature Considerations 125°C. When operating at high ambient temperatures, the maximum load current should be derated as the ambient temperature approaches 125°C. Power dissipation within the LT3681 can be estimated by calculating the total power loss from an efficiency measurement. The die temperature is calculated by multiplying the LT3681 power dissipation by the thermal resistance from junction to ambient. Also keep in mind that the leakage current of the integrated power Schottky diode, like all Schottky diodes, goes up with junction temperature. The curves in Figure 11 show how the leakage current in the power Schottky diode varies with temperature and reverse voltage. When the power switch is closed, the power Schottky diode is in parallel with the power converter’s output filter stage. As a result, an increase in a diode’s leakage current results in an effective increase in the load, and a corresponding increase in input power. 10000 LEAKAGE CURRENT (µA) Hot Plugging Safely VR = 10V VR = 25V VR = 40V 1000 100 10 1 –50 0 50 100 TEMPERATURE (°C) 150 3681 F12 The PCB must provide heat sinking to keep the LT3681 cool. The Exposed Pads on the bottom of the package must be soldered to copper pours, which in turn should be tied to large copper layers below with thermal vias; these layers will spread the heat dissipated by the LT3681. Place additional vias to reduce thermal resistance further. With these steps, the thermal resistance from die (or junction) to ambient can be reduced to θJA = 35°C/W or less. With 100 LFPM airflow, this resistance can fall by another 25%. Further increases in airflow will lead to lower thermal resistance. Because of the large output current capability of the LT3681, it is possible to dissipate enough power to raise the junction temperature beyond the absolute maximum of Figure 11. Like all Schottky Diodes, the LT3681 Integrated Power Diode Leakage Current Varies with Temperature and Applied Reverse Voltage VR. Other Linear Technology Publications Application Notes 19, 35, 44 and 76 contain more detailed descriptions and design information for buck regulators and other switching regulators. The LT1376 data sheet has a more extensive discussion of output ripple, loop compensation and stability testing. Design Note 100 shows how to generate a bipolar output supply using a buck regulator. 3681f 18 LT3681 APPLICATIONS INFORMATION CLOSING SWITCH SIMULATES HOT PLUG IIN VIN DANGER VIN 20V/DIV RINGING VIN MAY EXCEED ABSOLUTE MAXIMUM RATING LT3681 + LOW IMPEDANCE ENERGIZED 24V SUPPLY 4.7mF IIN 10A/DIV STRAY INDUCTANCE DUE TO 6 FEET (2 METERS) OF TWISTED PAIR 20ms/DIV (10a) 0.7W + 0.1mF LT3681 VIN 20V/DIV 4.7mF IIN 10A/DIV (10b) LT3681 + 22mF 35V AI.EI. + 20ms/DIV VIN 20V/DIV 4.7mF IIN 10A/DIV (10c) 20ms/DIV 3681 F10 Figure 10. A Well Chosen Input Network Prevents Input Voltage Overshoot and Ensures Reliable Operation when the LT3681 is Connected to a Live Supply 3681f 19 LT3681 TYPICAL APPLICATIONS 5V Step-Down Converter VOUT 5V 2A VIN 6.3V TO 34V VIN ON OFF BD RUN/SS BOOST 0.47mF VC 4.7mF L 6.8mH SW LT3681 DC RT DA 20k PG BIAS 590k 60.4k FB GND 330pF 22mF 200k f = 800kHz 3681 TA02 L: NEC PLC-0745-6R8 3.3V Step-Down Converter VOUT 3.3V 2A VIN 4.4V TO 34V BD VIN ON OFF RUN/SS BOOST 0.47mF VC 4.7mF LT3681 L 4.7mH SW DC RT DA 20k BIAS PG 324k 60.4k GND 330pF f = 800kHz FB 22mF 200k 3681 TA03 L: NEC PLC-0745-4R7 3681f 20 LT3681 TYPICAL APPLICATIONS 2.5V Step-Down Converter VOUT 2.5V 2A VIN 4V TO 34V VIN BD RUN/SS ON OFF D2 BOOST L 4.7mH 1mF VC 4.7mF SW LT3681 DC RT 22k DA BIAS PG 196k 84.5k FB GND 680pF 47mF 200k f = 600kHz 3681 TA04 D2: MBR0540 L: SUMIDA CDRH8D43-4R7 5V, 2MHz Step-Down Converter VIN 8.6V TO 22V TRANSIENT TO 36V VIN ON OFF VOUT 5V 2A BD RUN/SS BOOST 0.47mF VC 2.2mF LT3681 L 2.2mH SW DC RT 6.8k DA BIAS PG 590k 16.9k GND 470pF f = 2MHz FB 10mF 200k 3681 TA05 L: TOKO FDV0620-2R2 3681f 21 LT3681 TYPICAL APPLICATIONS 12V Step-Down Converter VIN 15V TO 34V BD VIN RUN/SS ON OFF VOUT 12V 2A BOOST 0.47mF VC 10mF L 10mH SW LT3681 DC RT DA 24k BIAS PG 845k 60.4k FB GND 470pF 22mF 100k f = 800kHz 3681 TA06 L: SUMIDA CDRH8D43-100 1.8V Step-Down Converter VOUT 1.8V 2A VIN 3.6V TO 27V BD VIN RUN/SS ON OFF BOOST 0.47µF VC 10µF LT3681 L 3.3µH SW DC RT DA 15k BIAS PG 84.5k 105k GND 330pF f = 500kHz FB 47µF 200k 3681 TA09 L: SUMIDA CDRH8D28-3R3 3681f 22 LT3681 PACKAGE DESCRIPTION DE14MA Package 14-Lead Plastic DFN, Multichip (4mm × 3mm) (Reference LTC DWG # 05-08-1731 Rev 0) 1.78 ±0.05 0.70 ±0.05 0.10 TYP 0.51 TYP 3.50 ±0.05 1.65 ± 0.05 2.10 ±0.05 1.07 ±0.05 1.65 ± 0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 3.00 REF RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 4.00 ±0.10 (2 SIDES) R = 0.05 TYP 3.00 ±0.10 (2 SIDES) R = 0.115 TYP 8 1.78 ±0.10 14 1.07 ±0.10 1.65 ± 0.10 0.10 TYP 0.51 TYP 1.65 ± 0.10 PIN 1 TOP MARK (SEE NOTE 6) 0.200 REF 0.75 ±0.05 0.40 ± 0.10 7 1 0.25 ± 0.05 0.50 BSC 3.00 REF 0.00 – 0.05 PIN 1 NOTCH R = 0.20 OR 0.25 × 45° CHAMFER (DE14MA) DFN 1106 REV Ø BOTTOM VIEW—EXPOSED PAD NOTE: 1. DRAWING PROPOSED IS NOT A JEDEC PACKAGE OUTLINE 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 3681f 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. 23 LT3681 U TYPICAL APPLICATIO 1.265V Step-Down Converter VOUT 1.265V 2A VIN 3.6V TO 27V BD VIN RUN/SS ON OFF BOOST 0.47mF VC 4.7mF L 3.3mH SW LT3681 DC RT DA 13k BIAS PG 105k GND FB 47mF 330pF f = 500kHz 3681 TA10 L: NEC PLC-0745-3R3 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1766 60V, 1.2A (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter VIN = 5.5V to 60V, VOUT = 1.20V, IQ = 2.5mA, ISD 25μA TSSOP16E Package LT1767 25V, 1.2A (IOUT), 1.2MHz, High Efficiency Step-Down DC/DC Converter VIN = 3.0V to 25V, VOUT = 1.20V, IQ = 1mA, ISD < 6μA MS8E Package LT1933 500mA (IOUT), 500kHz Step-Down Switching Regulator in SOT-23 VIN = 3.6V to 36V, VOUT = 1.2V, IQ = 1.6mA, ISD < 1μA ThinSOT Package LT1936 36V, 1.4A (IOUT), 500kHz High Efficiency Step-Down DC/DC Converter VIN = 3.6V to 36V, VOUT = 1.2V, IQ = 1.9mA, ISD < 1μA MS8E Package LT1940 Dual 25V, 1.4A (IOUT), 1.1MHz, High Efficiency Step-Down DC/DC Converter VIN = 3.6V to 25V, VOUT = 1.20V, IQ = 3.8mA, ISD < 30μA TSSOP16E Package LT1976/LT1977 60V, 1.2A (IOUT), 200kHz/500kHz, High Efficiency Step-Down DC/DC Converter with Burst Mode VIN = 3.3V to 60V, VOUT = 1.20V, IQ = 100μA, ISD < 1μA TSSOP16E Package LT3434/LT3435 60V, 2.4A (IOUT), 200/500kHz, High Efficiency Step-Down DC/DC Converter with Burst Mode VIN = 3.3V to 60V, VOUT = 1.20V, IQ = 100μA, ISD < 1μA TSSOP16E Package LT3437 60V, 400mA (IOUT), MicroPower Step-Down DC/DC Converter with Burst Mode VIN = 3.3V to 60V, VOUT = 1.25V, IQ = 100μA, ISD < 1μA (3mm × 3mm) DFN-10 TSSOP16E Package LT3480 36V with Transient Protection to 60V, 2A (IOUT), 2.4MHz, High Efficiency Step-Down DC/DC Converter with Burst Mode Operation VIN = 3.6V to 38V, VOUT = 0.78V, IQ = 70μA, ISD < 1μA (3mm × 3mm) DFN-10 MSOP10E Package LT3481 34V with Transient Protection to 36V, 2A (IOUT), 2.8MHz, High Efficiency Step-Down DC/DC Converter with Burst Mode Operation VIN = 3.6V to 34V, VOUT = 1.265V, IQ = 50μA, ISD < 1μA (3mm × 3mm) DFN-10 MSOP10E Package LT3493 36V, 1.4A (IOUT), 750kHz High Efficiency Step-Down DC/DC Converter VIN = 3.6V to 36V, VOUT = 0.8V, IQ = 1.9mA, ISD < 1μA (2mm × 3mm)DFN-6 Package LT3505 36V with Transient Protection to 40V, 1.4A (IOUT), 3MHz, High Efficiency Step-Down DC/DC Converter VIN = 3.6V to 34V, VOUT = 0.78V, IQ = 2mA, ISD < 2μA (3mm × 3mm) DFN-8 MSOP8E Package LT3508 36V with Transient Protection to 40V, Dual 1.4A (IOUT), 3MHz, High Efficiency Step-Down DC/DC Converter VIN = 3.7V to 37V, VOUT = 0.8V, IQ = 4.6mA, ISD < 1μA (4mm × 4mm) QFN-24 TSSOP16E Package LT3684 34V with Transient Protection to 36V, 2A (IOUT), 2.8MHz, High Efficiency Step-Down DC/DC Converter VIN = 3.6V to 34V, VOUT = 01.26V, IQ = 850mA, ISD < 1μA (3mm × 3mm) DFN-10 MSOP10E Package LT3685 36V with Transient Protection to 60V, 2A (IOUT), 2.4MHz, High Efficiency Step-Down DC/DC Converter VIN = 3.6V to 38V, VOUT = 0.78V, IQ = 70mA, ISD < 1μA (3mm × 3mm) DFN-10 MSOP10E Package 3681f 24 Linear Technology Corporation LT 0407 • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2006