LT3684 36V, 2A, 2.8MHz Step-Down Switching Regulator FEATURES DESCRIPTION Wide Input Range: 3.6V to 34V Operating, 36V Maximum nn 2A Maximum Output Current nn Adjustable Switching Frequency: 300kHz to 2.8MHz nn Low Shutdown Current: I < 1µA Q nn Integrated Boost Diode nn Power Good Flag nn Saturating Switch Design: 0.18Ω On-Resistance nn 1.265V Feedback Reference Voltage nn Output Voltage: 1.265V to 20V nn Soft-Start Capability nn Small 10-Pin Thermally Enhanced MSOP and (3mm × 3mm) DFN Packages The LT®3684 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. Current mode topology is used for fast transient response and good loop stability. The LT3684’s high operating frequency allows the use of small, low cost inductors and ceramic capacitors resulting in low output ripple while keeping total solution size to a minimum. The low current shutdown mode 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 LT3684 is available in 10-Pin MSOP and 3mm × 3mm DFN packages with Exposed Pads for low thermal resistance. nn APPLICATIONS Automotive Battery Regulation Power for Portable Products nn Distributed Supply Regulation nn Industrial Supplies nn Wall Transformer Regulation nn L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. nn TYPICAL APPLICATION 3.3V Step-Down Converter Efficiency VIN 4.5V TO 34V 16.2k 4.7µF VIN BD RUN/SS BOOST 0.47µF VC RT 330pF LT3684 SW BIAS PG 60.4k 4.7µH GND 324k FB 200k 90 80 EFFICIENCY (%) OFF ON VOUT 3.3V 2A 70 60 50 22µF 40 3684 TA01a 30 VIN = 12V VOUT = 3.3V L = 4.7µH f = 800kHz 0 0.5 1 1.5 LOAD CURRENT (A) 2 3684 TA01b 3684fa For more information www.linear.com/LT3684 1 LT3684 ABSOLUTE MAXIMUM RATINGS (Note 1) 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 Operating Temperature Range (Note 2) LT3684E...............................................–40°C to 85°C LT3684I.............................................. –40°C to 125°C Storage Temperature Range................... –65°C to 150°C Lead Temperature (Soldering, 10 sec) (MSE Only)........................................................ 300°C PIN CONFIGURATION TOP VIEW BD 1 10 RT BOOST 2 9 VC SW 3 VIN 4 7 BIAS RUN/SS 5 6 PG 11 TOP VIEW BD BOOST SW VIN RUN/SS 8 FB 11 10 9 8 7 6 RT VC FB BIAS PG MSE PACKAGE 10-LEAD PLASTIC MSOP DD PACKAGE 10-LEAD (3mm × 3mm) PLASTIC DFN TJMAX = 125°C, θJA = 45°C/W, θJC = 10°C/W EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB ORDER INFORMATION 1 2 3 4 5 TJMAX = 125°C, θJA = 45°C/W, θJC = 10°C/W EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB http://www.linear.com/product/LT3684#orderinfo LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT3684EDD#PBF LT3684EDD#TRPBF LCVT 10-Lead (3mm × 3mm) Plastic DFN –40°C to 85°C LT3684IDD#PBF LT3684IDD#TRPBF LCVT 10-Lead (3mm × 3mm) Plastic DFN –40°C to 125°C LT3684EMSE#PBF LT3684EMSE#TRPBF LTCVS 10-Lead Plastic MSOP –40°C to 85°C LT3684IMSE#PBF LT3684IMSE#TRPBF LTCVS 10-Lead Plastic MSOP –40°C to 125°C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. 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/. Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix. 2 3684fa For more information www.linear.com/LT3684 LT3684 ELECTRICAL CHARACTERISTICS The l 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 MIN Minimum Input Voltage Quiescent Current from VIN l VRUN/SS = 0.2V VBIAS = 3V, Not Switching Quiescent Current from BIAS l MAX UNITS 3 3.6 V 0.01 0.5 µA 0.4 0.8 mA VBIAS = 0, Not Switching 1.2 2.0 mA VRUN/SS = 0.2V 0.01 0.5 µA 0.85 1.5 mA 0 0.1 mA VBIAS = 3V, Not Switching l VBIAS = 0, Not Switching Minimum Bias Voltage Feedback Voltage l FB Pin Bias Current (Note 3) FB Voltage Line Regulation TYP 1.25 1.24 l 4V < VIN < 34V 2.7 3 V 1.265 1.265 1.28 1.29 V V 30 100 nA 0.002 0.02 %/V Error Amp gm 330 Error Amp Gain 1000 µMho VC Source Current 75 VC Sink Current 100 µA VC Pin to Switch Current Gain 3.5 A/V VC Clamp Voltage Switching Frequency µA 2 RT = 8.66k RT = 29.4k RT = 187k 2.7 1.25 250 Minimum Switch Off-Time l 3.1 V 3.0 1.4 300 3.3 1.55 350 MHz MHz kHz 100 150 nS 3.6 4.0 A Switch Current Limit Duty Cycle = 5% Switch VCESAT ISW = 2A 360 Boost Schottky Reverse Leakage VSW = 10V, VBIAS = 0V 0.02 2 mV µA 1.6 2.1 V BOOST Pin Current ISW = 1A 18 30 mA RUN/SS Pin Current VRUN/SS = 2.5V 5 10 µA 0.2 V Minimum Boost Voltage (Note 4) l RUN/SS Input Voltage High 2.5 V RUN/SS Input Voltage Low PG Threshold Offset from Feedback Voltage VFB Rising PG Hysteresis PG Leakage VPG = 5V PG Sink Current VPG = 0.4V 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 LT3684E 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 100 mV 10 mV 0.1 l 100 300 1 µA µA with statistical process controls. The LT3684I specifications are guaranteed over the –40°C to 125°C temperature range. Note 3: Bias current measured in regulation. 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. 3684fa For more information www.linear.com/LT3684 3 LT3684 TYPICAL PERFORMANCE CHARACTERISTICS Efficiency (VOUT = 5.0V) VIN = 24V 75 70 65 55 L: NEC PLC-0745-4R7 f = 800kHz 0 50 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 LOAD CURRENT (A) 0 VOUT = 3.3V TA = 25°C L = 4.7µH f = 800kHz 5 10 25 20 15 INPUT VOLTAGE (V) 3.0 2.5 MINIMUM 2.0 1.0 30 Switch Current Limit VOUT = 5V TA = 25°C L = 4.7µH f = 800kHz 5 10 25 20 15 INPUT VOLTAGE (V) 1.5 1.0 100 125 3684 G07 20 60 40 DUTY CYCLE (%) 100 80 500 400 300 200 0 80 Boost Pin Current 100 0.5 0 90 BOOST PIN CURRENT (mA) VOLTAGE DROP (mV) SWITCH CURRENT LIMIT (A) 1.5 0 25 50 75 TEMPERATURE (°C) 2.0 3684 G06 600 DUTY CYCLE = 90 % –25 2.5 Switch Voltage Drop 2.0 0 –50 3.0 1.0 30 700 DUTY CYCLE = 10 % 3 3.5 3684 G05 4.5 2.5 1 2 2.5 1.5 SWITCHING FREQUENCY (MHz) Switch Current Limit TYPICAL 1.5 3.0 0.5 4.0 3684 G04 4.0 0 3684 G03 SWITCH CURRENT LIMIT (A) 2.0 1.5 4 50 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 LOAD CURRENT (A) 3.5 LOAD CURRENT (A) LOAD CURRENT (A) 3.5 MINIMUM VOUT = 3.3V L = 10µH LOAD = 1A 55 L: NEC PLC-0745-4R7 f = 800kHz 4.0 2.5 65 Maximum Load Current Maximum Load Current 3.0 70 3684 G02 4.0 TYPICAL VIN = 24V 75 60 3684 G01 1.0 80 VIN = 12V 60 60 VIN = 12V 85 EFFICIENCY (%) 70 50 90 VIN = 7V 80 VIN = 24V 80 Efficiency Efficiency (VOUT = 3.3V) 85 VIN = 12V 90 EFFICIENCY (%) 90 EFFICIENCY (%) 100 (TA = 25°C unless otherwise noted) 70 60 50 40 30 20 10 0 500 1000 1500 2000 2500 3000 3500 SWITCH CURRENT (mA) 3684 G08 0 0 500 1000 1500 2000 2500 3000 3500 SWITCH CURRENT (mA) 3684 G09 3684fa For more information www.linear.com/LT3684 LT3684 TYPICAL PERFORMANCE CHARACTERISTICS Switching Frequency 1.20 1.285 1.15 1.280 1.10 1.275 1.270 1.265 1.00 0.95 0.90 1.255 0.85 –25 0 25 50 75 TEMPERATURE (°C) 100 RT = 45.3k 1.05 1.260 1.250 –50 Frequency Foldback 1200 SWITCHING FREQUENCY (kHz) 1.290 FREQUENCY (MHz) FEEDBACK VOLTAGE (V) Feedback Voltage (TA = 25°C unless otherwise noted) 0.80 –50 125 –25 0 25 50 75 TEMPERATURE (°C) 100 3684 G10 600 400 200 0 125 3.5 80 60 40 100 2.5 2.0 1.5 1.0 0 125 10 8 6 4 2 0 0.5 1 2 2.5 1.5 RUN/SS PIN VOLTAGE (V) 3684 G13 3 0 3.5 Error Amp Output Current 1.4 80 1.2 60 VC PIN CURRENT (µA) 100 0.6 0.4 1.0 0.5 1.5 BOOST DIODE CURRENT (A) 2.0 4.0 40 20 0 –20 3684 G16 –80 1.065 35 Minimum Input Voltage 3.5 3.0 2.5 –60 0 20 30 15 25 10 RUN/SS PIN VOLTAGE (V) 4.5 –40 0.2 5 3684 G15 INPUT VOLTAGE (V) Boost Diode 0.8 0 3684 G14 1.6 1.0 400 600 800 1000 1200 1400 FB PIN VOLTAGE (mV) RUN/SS Pin Current 0.5 0 50 75 25 TEMPERATURE (°C) 200 12 3.0 20 –25 0 3684 G12 RUN/SS PIN CURRENT (µA) 120 0 –50 BOOST DIODE Vf (V) 800 Soft-Start 4.0 SWITCH CURRENT LIMIT (A) MINIMUM SWITCH ON-TIME (ns) Minimum Switch On-Time 0 1000 3684 G11 140 100 RT = 45.3k 1.165 1.365 1 .265 FB PIN VOLTAGE (V) 1.465 3684 G17 2.0 0.001 VOUT = 3.3V TA = 25°C L = 4.7µH f = 800kHz 0.1 0.01 1 LOAD CURRENT (A) 10 3684 G18 3684fa For more information www.linear.com/LT3684 5 LT3684 TYPICAL PERFORMANCE CHARACTERISTICS VC Voltages 2.50 6.0 2.00 5.5 5.0 VOUT = 5V TA = 25°C L = 4.7µH f = 800kHz 4.5 4.0 0.001 0.1 0.01 1 LOAD CURRENT (A) Power Good Threshold 1.200 CURRENT LIMIT CLAMP 1.50 1.00 SWITCHING THRESHOLD 0.50 0 –50 10 –25 25 75 0 50 TEMPERATURE (°C) 100 125 PG RISING 1.180 1.160 1.140 1.120 1.100 –50 –25 25 75 0 50 TEMPERATURE (°C) Switching Waveforms (Discontinuous Operation) 100 125 3684 G21 3684 G20 3684 G19 Switching Waveforms (Continuous Operation) IL 0.5A/DIV IL 0.5A/DIV VSW 5V/DIV VRUN/SS 5V/DIV VOUT 10mV/DIV VOUT 10mV/DIV VIN = 12V, FRONT PAGE APPLICATION ILOAD = 140mA 1µs/DIV 6 THRESHOLD VOLTAGE (V) 6.5 THRESHOLD VOLTAGE (V) INPUT VOLTAGE (V) Minimum Input Voltage (TA = 25°C unless otherwise noted) VIN = 12V, FRONT PAGE APPLICATION ILOAD = 1A 3684 G22 1µs/DIV 3684 G23 3684fa For more information www.linear.com/LT3684 LT3684 PIN FUNCTIONS BD (Pin 1): This pin connects to the anode of the boost Schottky diode. BOOST (Pin 2): This pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar NPN power switch. SW (Pin 3): The SW pin is the output of the internal power switch. Connect this pin to the inductor, catch diode and boost capacitor. VIN (Pin 4): The VIN pin supplies current to the LT3684’s internal regulator and to the internal power switch. This pin must be locally bypassed. RUN/SS (Pin 5): The RUN/SS pin is used to put the LT3684 in shutdown mode. Tie to ground to shut down the LT3684. 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. PG (Pin 6): 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 7): The BIAS pin supplies the current to the LT3684’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 8): The LT3684 regulates the FB pin to 1.265V. Connect the feedback resistor divider tap to this pin. VC (Pin 9): 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 10): Oscillator Resistor Input. Connecting a resistor to ground from this pin sets the switching frequency. Exposed Pad (Pin 11): Ground. The Exposed Pad must be soldered to PCB. 3684fa For more information www.linear.com/LT3684 7 LT3684 BLOCK DIAGRAM VIN 4 C1 7 5 10 VIN BIAS – + INTERNAL 1.265V REF RUN/SS ∑ RT SLOPE COMP OSCILLATOR 300kHz–2.8MHz RT SOFT-START 6 BD SWITCH LATCH BOOST 1 2 C3 R S Q SW L1 VOUT 3 VC CLAMP C2 D1 PG ERROR AMP + – + – 1.12V GND 11 VC 9 CC RC FB CF 8 R2 R1 3684 BD OPERATION The LT3684 is a constant frequency, current mode stepdown regulator. An oscillator, with frequency set by RT, enables 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. 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). 8 This improves efficiency. The RUN/SS pin is used to place the LT3684 in shutdown, disconnecting the output and reducing the input current to less than 1µA. The switch driver operates from either the input or from the BOOST pin. An external capacitor and diode are 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. The oscillator reduces the LT3684’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 LT3684 contains a power good comparator which trips when the FB pin is at 90% 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 LT3684 is enabled and VIN is above 3.6V. 3684fa For more information www.linear.com/LT3684 LT3684 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 LT3684 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) 0.2 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 2.8 267 187 133 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 catch diode drop (~0.5V), 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 LT3684 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 LT3684 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 − VD + VSW 1− fSW tON(MIN) where VIN(MAX) is the maximum operating input voltage, VOUT is the output voltage, VD is the catch diode drop (~0.5V), 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 3684fa For more information www.linear.com/LT3684 9 LT3684 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 LT3684 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 LT3684’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 − VD + VSW 1− fSW tOFF(MIN) 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 ∆IL 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: ∆IL = 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 LT3684’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 – ∆IL/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 catch diode (~0.4V), 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 4A. 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 D62CB Shielded D63CB Shielded D75C Shielded D75F Open CR54 Open CDRH74 Shielded CDRH6D38 Shielded CR75 Open Sumida IL(PEAK) = IOUT(MAX) + ∆IL/2 www.sumida.com where IL(PEAK) is the peak inductor current, IOUT(MAX) is the maximum output load current, and ∆IL is the inductor ripple current. The LT3684’s switch current limit (ILIM) is 10 3684fa For more information www.linear.com/LT3684 LT3684 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 AN19. Input Capacitor Bypass the input of the LT3684 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 LT3684 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 LT3684 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 LT3684 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 LT3684. A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT3684 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3684’s voltage rating. This situation is easily avoided (see the Hot Plugging Safety section). For space sensitive applications, a 2.2µF ceramic capacitor can be used for local bypassing of the LT3684 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 LT3684 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 LT3684 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 LT3684’s control loop. Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance. A good starting value is: COUT = 100 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. 3684fa For more information www.linear.com/LT3684 11 LT3684 APPLICATIONS INFORMATION Table 2. Capacitor Vendors VENDOR PHONE URL PART SERIES Panasonic (714) 373-7366 www.panasonic.com Ceramic, COMMANDS Polymer, EEF Series Tantalum Kemet (864) 963-6300 www.kemet.com Sanyo (408) 749-9714 www.sanyovideo.com Ceramic, Tantalum 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 Table 3. Diode Vendors PART NUMBER VR (V) IAVE (A) VF AT 1A (mV) VF AT 2A (mV) On Semiconductor MBRM120E MBRM140 20 40 1 1 530 550 595 Diodes Inc. B120 B130 B220 B230 DFLS240L 20 30 20 30 40 1 1 2 2 2 500 500 International Rectifier 10BQ030 20BQ030 30 30 1 2 420 500 500 500 470 470 Catch Diode Frequency Compensation The catch diode conducts current only during switch off time. Average forward current in normal operation can be calculated from: The LT3684 uses current mode control to regulate the output. This simplifies loop compensation. In particular, the LT3684 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 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. ID(AVG) = IOUT (VIN – VOUT)/VIN where IOUT is the output load current. The only reason to consider a diode with a larger current rating than necessary for nominal operation is for the worst-case condition of shorted output. The diode current will then increase to the typical peak switch current. Peak reverse voltage is equal to the regulator input voltage. Use a diode with a reverse voltage rating greater than the input voltage. Table 3 lists several Schottky diodes and their manufacturers. 12 3684fa For more information www.linear.com/LT3684 LT3684 APPLICATIONS INFORMATION VOUT = 12V, FRONT PAGE APPLICATION LT3684 CURRENT MODE POWER STAGE gm = 3.5mho SW ERROR AMPLIFIER OUTPUT R1 FB – gm = 330µmho VC CF RC + 3M IL 1A/DIV CPL ESR 1.265V C1 C1 + POLYMER OR TANTALUM GND VOUT 100mV/DIV CERAMIC 10µs/DIV R2 CC 3684 F02 Figure 2. Model for Loop Response 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 LT3684 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 3684 F03 Figure 3. Transient Load Response of the LT3684 Front Page Application as the Load Current Is Stepped from 500mA to 1500mA. VOUT = 3.3V 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. 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 4 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 3V and above, the standard circuit (Figure 4a) 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 4b). For lower output voltages the boost diode can be tied to the input (Figure 4c), or to another supply greater than 2.8V. The circuit in Figure 4a 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 LT3684 application is limited by the minimum input voltage (3.6V) and by the maximum duty cycle as outlined in a previous section. For 3684fa For more information www.linear.com/LT3684 13 LT3684 APPLICATIONS INFORMATION 6.0 VOUT BD BOOST VIN LT3684 GND 4.7µF C3 INPUT VOLTAGE (V) VIN TO START 5.5 SW 5.0 4.5 4.0 3.5 TO RUN 3.0 (4a) For VOUT > 2.8V 2.5 2.0 0.001 VOUT D2 BD BOOST VIN LT3684 GND 4.7µF 8.0 C3 (4b) For 2.5V < VOUT < 2.8V VOUT BD VIN LT3684 10 TO START 6.0 5.0 TO RUN 4.0 3.0 2.0 0.001 BOOST VIN 0.1 0.01 1 LOAD CURRENT (A) 7.0 SW INPUT VOLTAGE (V) VIN VOUT = 3.3V TA = 25°C L = 4.7µH f = 800kHz C3 VOUT = 5V TA = 25°C L = 4.7µH f = 800kHz 0.1 0.01 1 LOAD CURRENT (A) 10 3684 F05 4.7µF GND SW Figure 5. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit 3684 FO4 (4c) For VOUT < 2.5V Figure 4. Three Circuits For Generating The Boost Voltage proper start-up, the minimum input voltage is also limited by the boost circuit. If the input voltage is ramped slowly, or the LT3684 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. Figure 5 shows a plot of minimum load to start and to run as a function of input 14 voltage. In many cases the discharged output capacitor will present a load to the switcher and the minimum input to start will be the same as the minimum input to run. This occurs, for example, if RUN/SS is asserted after VIN is applied. The plots show the worst-case situation where VIN is ramping very slowly. 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 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 LT3684, requiring a higher input voltage to maintain regulation. 3684fa For more information www.linear.com/LT3684 LT3684 APPLICATIONS INFORMATION Soft-Start The RUN/SS pin can be used to soft-start the LT3684, 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 6 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 VRUN/SS 2V/DIV RUN/SS 0.22µF GND VOUT 2V/DIV 2ms/DIV 3684 F06 Figure 6. To Soft-Start the LT3684, Add a Resistor and Capacitor to the RUN/SS Pin Shorted and Reversed Input Protection If the inductor is chosen so that it won’t saturate excessively, an LT3684 buck regulator will tolerate a shorted output. There is another situation to consider in systems where the output will be held high when the input to the LT3684 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 LT3684’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 LT3684’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 D4 MBRS140 VIN VIN BOOST LT3684 RUN/SS VOUT SW VC GND FB BACKUP 3684 F07 Figure 7. 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 LT3684 Runs Only When the Input Is Present LT3684 can pull large currents from the output through the SW pin and the VIN pin. Figure 7 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input. PCB Layout For proper operation and minimum EMI, care must be taken during printed circuit board layout. Figure 8 shows the recommended component placement with trace, ground plane and via locations. Note that large, switched currents flow in the LT3684’s VIN and SW pins, the catch diode (D1) and the input capacitor (C1). 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. The Exposed Pad on the bottom of the package must be soldered to ground 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 LT3684 to additional ground planes within the circuit board and on the bottom side. 3684fa For more information www.linear.com/LT3684 15 LT3684 APPLICATIONS INFORMATION L1 C2 VOUT CC RRT RC R2 R1 D1 C1 RPG GND 3684 F08 VIAS TO LOCAL GROUND PLANE VIAS TO RUN/SS VIAS TO VOUT VIAS TO PG VIAS TO VIN OUTLINE OF LOCAL GROUND PLANE Figure 8. A Good PCB Layout Ensures Proper, Low EMI Operation Hot Plugging Safely The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LT3684 circuits. However, these capacitors can cause problems if the LT3684 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 LT3684 can ring to twice the nominal input voltage, possibly exceeding the LT3684’s rating and damaging the part. If the input supply is poorly controlled or the user will be plugging the LT3684 into an energized supply, the input network should be designed to prevent this overshoot. Figure 9 shows the waveforms that result when an LT3684 circuit is connected to a 24V supply through six feet of 24-gauge twisted pair. The first plot 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 9b. A 0.7Ω resistor is added in series with the 16 input to eliminate the voltage overshoot (it also reduces the peak input 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. High Temperature Considerations The PCB must provide heat sinking to keep the LT3684 cool. The Exposed Pad on the bottom of the package must be soldered to a ground plane. This ground should be tied to large copper layers below with thermal vias; these layers will spread the heat dissipated by the LT3684. Place additional vias can 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 LT3684, it is possible to dissipate enough heat to raise the junction temperature beyond the absolute maximum of 125°C. When operating at high ambient temperatures, the 3684fa For more information www.linear.com/LT3684 LT3684 APPLICATIONS INFORMATION CLOSING SWITCH SIMULATES HOT PLUG IIN VIN + LOW IMPEDANCE ENERGIZED 24V SUPPLY DANGER RINGING VIN MAY EXCEED ABSOLUTE MAXIMUM RATING LT3684 4.7µF IIN 10A/DIV STRAY INDUCTANCE DUE TO 6 FEET (2 METERS) OF TWISTED PAIR 0.7Ω + VIN 20V/DIV 0.1µF 20µs/DIV (9a) LT3684 VIN 20V/DIV 4.7µF IIN 10A/DIV (9b) + 22µF 35V AI.EI. + LT3684 20µs/DIV VIN 20V/DIV 4.7µF IIN 10A/DIV (9c) 20µs/DIV 3684 F09 Figure 9. A Well Chosen Input Network Prevents Input Voltage Overshoot and Ensures Reliable Operation when the LT3684 Is Connected to a Live Supply maximum load current should be derated as the ambient temperature approaches 125°C. Power dissipation within the LT3684 can be estimated by calculating the total power loss from an efficiency measurement and subtracting the catch diode loss and inductor loss. The die temperature is calculated by multiplying the LT3684 power dissipation by the thermal resistance from junction to ambient. Other Linear Technology Publications Application Notes 19, 35 and 44 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. 3684fa For more information www.linear.com/LT3684 17 LT3684 TYPICAL APPLICATIONS 5V Step-Down Converter VOUT 5V 2A VIN 6.3V TO 34V BD VIN RUN/SS ON OFF BOOST 0.47µF VC 4.7µF SW LT3684 D RT 20k BIAS PG 60.4k 330pF L 6.8µH 590k FB GND 22µF 200k f = 800kHz 3684 TA02 D: DIODES INC. DFLS240L L: TAIYO YUDEN NP06DZB6R8M 3.3V Step-Down Converter VOUT 3.3V 2A VIN 4.4V TO 34V BD VIN RUN/SS ON OFF BOOST 0.47µF VC 4.7µF LT3684 SW D RT 16.2k BIAS PG 60.4k 330pF f = 800kHz L 4.7µH GND 324k FB 22µF 200k 3684 TA03 D: DIODES INC. DFLS240L L: TAIYO YUDEN NP06DZB4R7M 18 3684fa For more information www.linear.com/LT3684 LT3684 TYPICAL APPLICATIONS 2.5V Step-Down Converter VOUT 2.5V 2A VIN 4V TO 34V BD VIN RUN/SS ON OFF D2 BOOST L 4.7µH 1µF VC 4.7µF SW LT3684 D1 RT 22.1k BIAS PG 84.5k 220pF 196k FB GND 47µF 200k f = 600kHz 3684 TA04 D1: DIODES INC. DFLS240L D2: MBR0540 L: TAIYO YUDEN NP06DZB4R7M 5V, 2MHz Step-Down Converter VIN 8.6V TO 22V TRANSIENT TO 36V BD VIN RUN/SS ON OFF VOUT 5V 2A BOOST 0.47µF VC 2.2µF LT3684 SW D RT 20k BIAS PG 16.9k 330pF f = 2MHz L 2.2µH GND 590k FB 10µF 200k 3684 TA05 D: DIODES INC. DFLS240L L: SUMIDA CDRH4D22/HP-2R2 3684fa For more information www.linear.com/LT3684 19 LT3684 TYPICAL APPLICATIONS 12V Step-Down Converter VOUT 12V 2A VIN 15V TO 34V BD VIN RUN/SS ON OFF BOOST L 10µH 0.47µF VC 10µF SW LT3684 D RT 30k BIAS PG 60.4k 330pF 845k FB GND 22µF 100k f = 800kHz 3684 TA06 D: DIODES INC. DFLS240L L: NEC/TOKIN PLC-0755-100 1.8V Step-Down Converter VOUT 1.8V 2A VIN 3.5V TO 27V BD VIN RUN/SS ON OFF BOOST L 3.3µH 0.47µF VC 4.7µF LT3684 SW D RT 15.4k BIAS PG 105k 330pF f = 500kHz GND 84.5k FB 47µF 200k 3684 TA07 D: DIODES INC. DFLS240L L: TAIYO YUDEN NP06DZB3R3M 20 3684fa For more information www.linear.com/LT3684 LT3684 PACKAGE DESCRIPTION Please refer to http://www.linear.com/product/LT3684#packaging for the most recent package drawings. DD Package DD Package 10-Lead PlasticPlastic DFN (3mm 3mm)× 3mm) 10-Lead DFN×(3mm (Reference LTC DWG # 05-08-1699 Rev C) Rev C) (Reference LTC DWG # 05-08-1699 0.70 ±0.05 3.55 ±0.05 1.65 ±0.05 2.15 ±0.05 (2 SIDES) PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC 2.38 ±0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 ±0.10 (4 SIDES) R = 0.125 TYP 6 0.40 ±0.10 10 1.65 ±0.10 (2 SIDES) PIN 1 NOTCH R = 0.20 OR 0.35 × 45° CHAMFER PIN 1 TOP MARK (SEE NOTE 6) 0.200 REF 5 0.75 ±0.05 0.00 – 0.05 1 (DD) DFN REV C 0310 0.25 ±0.05 0.50 BSC 2.38 ±0.10 (2 SIDES) BOTTOM VIEW—EXPOSED PAD NOTE: 1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2). CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 3684fa For more information www.linear.com/LT3684 21 LT3684 PACKAGE DESCRIPTION Please refer to http://www.linear.com/product/LT3684#packaging for the most recent package drawings. MSE Package 10-Lead Plastic MSOP MSE Package (Reference LTC DWG # 05-08-1664 Rev I) 10-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1664 Rev I) BOTTOM VIEW OF EXPOSED PAD OPTION 1.88 ±0.102 (.074 ±.004) 5.10 (.201) MIN 1 0.889 ±0.127 (.035 ±.005) 1.68 ±0.102 (.066 ±.004) 0.05 REF 10 0.305 ± 0.038 (.0120 ±.0015) TYP RECOMMENDED SOLDER PAD LAYOUT 3.00 ±0.102 (.118 ±.004) (NOTE 3) DETAIL “B” CORNER TAIL IS PART OF DETAIL “B” THE LEADFRAME FEATURE. FOR REFERENCE ONLY NO MEASUREMENT PURPOSE 10 9 8 7 6 DETAIL “A” 0° – 6° TYP 1 2 3 4 5 GAUGE PLANE 0.53 ±0.152 (.021 ±.006) DETAIL “A” 0.18 (.007) SEATING PLANE 0.86 (.034) REF 1.10 (.043) MAX 0.17 – 0.27 (.007 – .011) TYP 0.50 (.0197) BSC NOTE: 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX 6. EXPOSED PAD DIMENSION DOES INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD SHALL NOT EXCEED 0.254mm (.010") PER SIDE. 22 0.497 ±0.076 (.0196 ±.003) REF 3.00 ±0.102 (.118 ±.004) (NOTE 4) 4.90 ±0.152 (.193 ±.006) 0.254 (.010) 0.29 REF 1.68 (.066) 3.20 – 3.45 (.126 – .136) 0.50 (.0197) BSC 1.88 (.074) 0.1016 ±0.0508 (.004 ±.002) MSOP (MSE) 0213 REV I 3684fa For more information www.linear.com/LT3684 LT3684 REVISION HISTORY REV DATE DESCRIPTION A 05/16 Reformated the Package/Order Information section. PAGE NUMBER 2 Clarified VD for catch diode voltage drop. 10 Clarified saturation current from 3.5A to 4A in the Inductor Selection section. 11 Clarified from Figure 2 to Figure 4 in the Boost and BIAS Pin Considerations section. 14 Clarified from Figure 7 to Figure 6 in the Soft-Start section. Updated package drawings. 16 21, 22 3684fa 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. For more information www.linear.com/LT3684 23 LT3684 TYPICAL APPLICATION 1.265V Step-Down Converter VOUT 1.265V 2A VIN 3.6V TO 27V BD VIN RUN/SS ON OFF BOOST 0.47µF VC 4.7µF LT3684 SW D RT 13k BIAS PG 105k 330pF L 3.3µH GND FB 47µF f = 500kHz 3684 TA08 D: DIODES INC. DFLS240L L: TAIYO YUDEN NP06DZB3R3M RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1933 500mA (IOUT), 500kHz Step-Down Switching Regulator in SOT-23 VIN: 3.6V to 36V, VOUT(MIN) = 12V, IQ = 1.6mA, ISD < 1µA, ThinSOTTM Package LT3437 60V, 400mA (IOUT), MicroPower Step-Down DC/DC Converter with Burst Mode Operation VIN: 3.3V to 80V, VOUT(MIN) = 1.25V, IQ = 100µA, ISD < 1µA, DFN Package LT1936 36V, 1.4A (IOUT), 500kHz High Efficiency Step-Down DC/ DC Converter VIN: 3.6V to 36V, VOUT(MIN) = 1.2V, IQ = 1.9mA, ISD < 1µA, MS8E Package LT3493 36V, 1.2A (IOUT), 750kHz High Efficiency Step-Down DC/ DC Converter VIN: 3.6V to 40V, VOUT(MIN) = 0.8V, IQ = 1.9mA, ISD < 1µA, DFN Package LT1976/LT1977 60V, 1.2A (IOUT), 200kHz/500kHz, High Efficiency StepDown DC/DC Converter with Burst Mode Operation VIN: 3.3V to 60V, VOUT(MIN) = 1.20V, IQ = 100µA, ISD < 1µA, TSSOP16E Package LT1767 25V, 1.2A (IOUT), 1.1MHz, High Efficiency Step-Down DC/ DC Converter LT1940 Dual 25V, 1.4A (IOUT), 1.1MHz, High Efficiency Step-Down VIN: 3.6V to 25V, VOUT(MIN) = 1.20V, IQ = 3.8mA, ISD < 30µA, TSSOP16E DC/DC Converter Package LT1766 60V, 1.2A (IOUT), 200kHz, High Efficiency Step-Down DC/ DC Converter LT3434/LT3435 60V, 2.4A (IOUT), 200/500kHz, High Efficiency Step-Down DC/DC Converter with Burst Mode Operation LT3481 24 36V, 2A (IOUT), Micropower 2.8MHz, High Efficiency Step-Down DC/DC Converter VIN: 3V to 25V, VOUT(MIN) = 1.20V, IQ = 1mA, ISD < 6µA, MS8E Package VIN: 5.5V to 60V, VOUT(MIN) = 1.20V, IQ = 2.5mA, ISD < 25µA, TSSOP16E Package VIN: 3.3V to 60V, VOUT(MIN) = 1.20V, IQ = 100µA, ISD < 1µA, TSSOP16E Package VIN: 3.6V to 36V, VOUT(MIN) = 1.265V, IQ = 5µA, ISD < 1µA, 3mm × 3mm DFN and MS10E Packages 3684fa Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 For more information www.linear.com/LT3684 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LT3684 LT 0516 REV A • PRINTED IN USA LINEAR TECHNOLOGY CORPORATION 2007