LTM8027MPV#PBF

LTM8027 60V, 4A DC/DC µModule Regulator FEATURES DESCRIPTION Complete Switch Mode Power Supply n Wide Input Voltage Range: 4.5V to 60V (7.5V Minimum Voltage to Start) n Wide Output Voltage Range: 2.5V to 24V (See Table 2) n 4A Output Current n Programmable Soft-Start n 9µA Shutdown Supply Current n Selectable Switching Frequency Current Mode Control n Up to 95% Efficiency n SnPb (BGA) or RoHS Compliant (LGA and BGA) Finish n Surface Mount LGA (15mm × 15mm × 4.32mm) and (15mm × 15mm × 4.92mm) BGA Packages The LTM®8027 is a complete 4A, DC/DC step-down power supply. Included in the package are the switching controller, power switches, inductor and all support components. Operating over an input voltage range of 4.5V to 60V (7.5V minimum voltage to start), the LTM8027 supports output voltages up to 24V, and a switching frequency range of 100kHz to 500kHz, each set by a single resistor. Only the bulk input and output filter capacitors are needed to finish the design. n APPLICATIONS n n n n 12V and 42V Automotive and Heavy Equipment 48V Telecom Power Supplies Avionics and Industrial Control Systems Distributed Power Converters The low profile package (4.32mm) enables utilization of unused space on the bottom of PC boards for high density point of load regulation. A built-in soft-start timer is adjustable with a small capacitor. The LTM8027 is packaged in a compact (15mm × 15mm × 4.32mm) over-molded land grid array (LGA) and (15mm × 15mm × 4.92mm) BGA package suitable for automated assembly by standard surface mount equipment. The LTM8027 is available with SnPb (BGA) or RoHS compliant terminal finish. L, LT, LTC, LTM, Linear Technology, the Linear logo and µModule are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. TYPICAL APPLICATION 48W, 16VIN to 60VIN DC/DC µModule® Regulator 4.7µF ×2 VIN 1M 48.7k LTM8027 RUN BIAS1 SS BIAS2 SYNC AUX RT ADJ GND 100 VOUT 12V 4A VOUT 24VIN 90 80 22µF ×4 56.2k 3845 TA01a EFFICIENCY (%) VIN 16V TO 60V Efficiency vs Load 70 60 50 40 30 20 10 0 0 1 2 LOAD (A) 3 4 8027 TA01b 8027fd For more information www.linear.com/LTM8027 1 LTM8027 ABSOLUTE MAXIMUM RATINGS (Note 1) VIN Voltage.................................................................65V BIAS1.........................................................................15V BIAS2.........................................................................24V SYNC, ADJ, RT, RUN, SS Voltages...............................5V Current into RUN Pin (Note 2)...................................1mA VOUT, AUX..................................................................25V Current Out of AUX.............................................. 200mA Internal Operating Temperature (Note 3) E-, I-Grade.......................................... –40°C to 125°C MP-Grade........................................... –55°C to 125°C Peak Solder Reflow Body Temperature.................. 245°C Storage Temperature Range................... –55°C to 125°C PIN CONFIGURATION TOP VIEW 11 10 9 8 AUX 7 BIAS1 6 SS 5 RUN 4 BIAS2 3 ADJ 2 1 TOP VIEW 11 10 9 8 AUX 7 BIAS1 6 SS 5 RUN 4 BIAS2 3 ADJ 2 1 VOUT BANK 1 GND BANK 2 VIN BANK 3 VOUT BANK 1 GND BANK 2 VIN BANK 3 A B C D E F G H J K L SYNC RT LGA PACKAGE 113-LEAD (15mm × 15mm × 4.32mm) A B C D E F G H J K L SYNC RT BGA PACKAGE 113-LEAD (15mm × 15mm × 4.92mm) TJMAX = 125°C, θJA = 12.2°C/W, θJC(TOP) = 9.3°C/W, θJC(BOTTOM) = 3.6°C/W, θJBOARD = 7.54°C/W θ VALUES DETERMINED PER JESD 51-9 WEIGHT = 2.6 GRAMS TJMAX = 125°C, θJA = 12.2°C/W, θJC(TOP) = 9.3°C/W, θJC(BOTTOM) = 3.6°C/W, θJBOARD = 7.54°C/W θ VALUES DETERMINED PER JESD 51-9 WEIGHT = 2.6 GRAMS ORDER INFORMATION PART NUMBER PAD OR BALL FINISH http://www.linear.com/product/LTM8027#orderinfo PART MARKING* DEVICE FINISH CODE PACKAGE TYPE MSL RATING TEMPERATURE RANGE (See Note 3) LTM8027EV#PBF Au (RoHS) LTM8027V e4 LGA 3 –40°C to 125°C LTM8027IV#PBF Au (RoHS) LTM8027V e4 LGA 3 –40°C to 125°C LTM8027MPV#PBF Au (RoHS) LTM8027V e4 LGA 3 –55°C to 125°C LTM8027EY#PBF SAC305 (RoHS) LTM8027Y e1 BGA 3 –40°C to 125°C LTM8027IY#PBF SAC305 (RoHS) LTM8027Y e1 BGA 3 –40°C to 125°C LTM8027IY SnPb (63/37) LTM8027Y e0 BGA 3 –40°C to 125°C LTM8027MPY#PBF SAC305 (RoHS) LTM8027Y e1 BGA 3 –55°C to 125°C e0 BGA 3 –55°C to 125°C LTM8027MPY SnPb (63/37) LTM8027Y Consult Marketing for parts specified with wider operating temperature ranges. *Device temperature grade is indicated by a label on the shipping container. Pad or ball finish code is per IPC/JEDEC J-STD-609. • Recommended LGA and BGA PCB Assembly and Manufacturing Procedures: www.linear.com/umodule/pcbassembly • Pb-free and Non-Pb-free Part Markings: www.linear.com/leadfree • LGA and BGA Package and Tray Drawings: www.linear.com/packaging 2 8027fd For more information www.linear.com/LTM8027 LTM8027 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full internal operating temperature range, otherwise specifications are at TA = 25°C. VIN = 20V, BIAS1 = BIAS2 = 10V, RUN = 2V, unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP UNITS VIN Input DC Voltage (Note 5) VOUT Maximum Output DC Voltage 0A < IOUT ≤ 4A, VIN = 48V IOUT Output DC Current VIN ≤ 60V, VOUT = 12V, (Note 4) VIN(START) Minimum Start Voltage ∆VOUT/∆VIN Line Regulation VOUT = 12V, 15V< VIN < 60V, ILOAD = 4A 0.2 % ∆VOUT/∆ILOAD Load Regulation VOUT = 12V, VIN = 24V, 0A < ILOAD ≤ 4A 0.2 % VUVLO(RISING) Input Undervoltage Lockout Threshold (Rising) (Note 5) 4.6 V VUVLO(FALLING) Input Undervoltage Lockout Threshold (Falling) (Note 5) 3.7 V VADJ ADJ Voltage l 4.5 MAX 60 V 24 0 l 1.224 1.215 VBIAS = VAUX, VOUT = 12VDC, No Load VRUN = 0V V 4 A 7.5 V 1.238 1.245 V V IQ(VIN) Quiescent Current into IN VBIAS1 BIAS1 Undervoltage Lockout (Rising) BIAS1 Undervoltage Lockout (Falling) IBIAS1 Current into BIAS1 VBIAS2 Minimum BIAS2 Voltage IBIAS2 Current Into BIAS2 VBIAS1(MINOV) Minimum Voltage to Overdrive Internal Regulator (INTVCC) RFB Internal Feedback Resistor 499 kΩ VRUN(RISING) RUN Enable Voltage (Rising) 1.4 V VRUN(FALLING) RUN Enable Voltage (Falling) 1.2 V fSW Switching Frequency 100 500 kHz kHz RSYNC SYNC Input Resistance 40 kΩ VSYNC(TH) SYNC Voltage Threshold ISS Soft-Start Charging Current 2 µA No Load RUN = 0V RT = 187kΩ RT = 23.7kΩ fSYNC = 350kHz 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 RUN pin is internally clamped to 5V. Note 3: The LTM8027E is guaranteed to meet performance specifications from 0°C to 125°C internal operating temperature. Specifications over the full –40°C to 125°C internal operating temperature range are assured by design, characterization and correlation with statistical process controls. The LTM8027I is guaranteed to meet specifications over the full l 39 9 mA µA 6.5 6 V V 25 25 mA µA 3 V 1 µA 8.5 V 2.3 V –40°C to 125°C internal operating temperature range. The LTM8027MP is guaranteed to meet specifications over the full –55°C to 125°C internal operating range. Note that the maximum internal temperature is determined by specific operating conditions in conjunction with board layout, the rated package thermal resistance and other environmental factors. Note 4: The maximum continuous output current may be derated by the LTM8027 junction temperature. Note 5: VIN voltages below the start-up threshold (7.5V) are only supported when BIAS1 is externally driven above 6.5V. 8027fd For more information www.linear.com/LTM8027 3 LTM8027 TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C unless otherwise noted) Efficiency vs Load, VOUT = 2.5V 100 Efficiency vs Load, VOUT = 3.3V 90 80 80 80 70 70 50 40 30 5VIN 12VIN 24VIN 36VIN 20 10 0 0 1 2 LOAD (A) 60 50 40 5VIN 12VIN 24VIN 36VIN 48VIN 60VIN 30 20 10 0 4 3 0 1 2 LOAD (A) 50 40 10 0 Efficiency vs Load, VOUT = 12V 100 90 80 80 70 70 70 12VIN 24VIN 36VIN 48VIN 60VIN 20 10 0 0 1 2 LOAD (A) 60 50 40 30 24VIN 36VIN 48VIN 60VIN 20 10 0 4 3 EFFICIENCY (%) 90 30 0 1 2 LOAD (A) 90 80 80 70 70 EFFICIENCY (%) 90 40 30 24VIN 36VIN 48VIN 60VIN 20 10 0 0 1 2 LOAD (A) 3 50 40 30 24VIN 36VIN 48VIN 60VIN 0 1 2 LOAD (A) 8027 G06 1.2 60 50 40 30 0 4 3 Input Current vs VIN Output Shorted 1.0 36VIN 48VIN 60VIN 10 8027 G07 Efficiency vs Load, VOUT = 15V 60 0 Efficiency vs Load, VOUT = 24V 20 4 4 3 10 INPUT CURRENT (A) 100 50 2 LOAD (A) 8027 G05 Efficiency vs Load, VOUT = 18V 60 1 20 4 3 8027 G04 100 0 8027 G03 80 40 12VIN 24VIN 36VIN 48VIN 60VIN 30 90 EFFICIENCY (%) EFFICIENCY (%) 100 50 EFFICIENCY (%) 70 60 8027 G02 Efficiency vs Load, VOUT = 8V 60 Efficiency vs Load, VOUT = 5V 20 4 3 8027 G01 100 EFFICIENCY (%) 90 60 4 100 90 EFFICIENCY (%) EFFICIENCY (%) 100 0 1 2 LOAD (A) 0.6 0.4 0.2 4 3 0.8 8027 G08 0 0 10 40 30 50 20 INPUT VOLTAGE (V) 60 70 8027 G09 8027fd For more information www.linear.com/LTM8027 LTM8027 TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C unless otherwise noted) 3000 3500 5VIN 12VIN 24VIN 36VIN 2000 1500 1000 2500 2000 0 3 4 1600 1400 1200 1000 0 2 LOAD (A) 200 0 1 2 1500 1000 500 500 2 3 0 4 0 1 2 3 8027 G13 1500 1000 500 1 2 LOAD (A) 0 1 3 4 8027 G16 2 LOAD (A) 3 8027 G15 15.50 36VIN 48VIN 60VIN 15.00 2500 2000 1500 1000 0 14.50 14.00 13.50 13.00 36VIN 24VIN 12VIN 12.50 0 1 4 Bias Current vs Load, VOUT = 2.5V 500 0 1000 0 4 BIAS CURRENT (mA) 2000 0 1500 Input Current vs Load, VOUT = 24V 3000 INPUT CURRENT (mA) INPUT CURRENT (mA) 2500 2000 8027 G14 3500 24VIN 36VIN 48VIN 60VIN 3000 24VIN 36VIN 48VIN 60VIN LOAD (A) Input Current vs Load, VOUT = 18V 4 3 500 LOAD (A) 3500 2 LOAD (A) Input Current vs Load, VOUT = 15V 2500 INPUT CURRENT (mA) INPUT CURRENT (mA) INPUT CURRENT (mA) 1000 1 1 8027 G12 3000 24VIN 36VIN 48VIN 60VIN 2000 1500 0 0 8027 G11 2500 2000 0 0 4 3 Input Current vs Load, VOUT = 12V 12VIN 24VIN 36VIN 48VIN 60VIN 2500 600 LOAD (A) Input Current vs Load, VOUT = 8V 3000 800 400 8027 G10 3500 12VIN 24VIN 36VIN 48VIN 60VIN 1800 1000 500 1 2000 1500 500 0 Input Current vs Load, VOUT = 5V 5VIN 12VIN 24VIN 36VIN 48VIN 60VIN 3000 INPUT CURRENT (mA) 2500 INPUT CURRENT (mA) Input Current vs Load, VOUT = 3.3V INPUT CURRENT (mA) Input Current vs Load, VOUT = 2.5V 2 LOAD (A) 3 4 12.00 0 1 2 3 4 LOAD (A) 8027 G17 8027 G18 8027fd For more information www.linear.com/LTM8027 5 LTM8027 TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C unless otherwise noted) Bias Current vs Load, VOUT = 3.3V Bias Current vs Load, VOUT = 5V 18.0 25.5 16.5 16.0 15.5 15.0 15.0 14.5 14.0 48VIN 36VIN 24VIN 12VIN 14.5 0 1 2 LOAD (A) 48VIN 36VIN 24VIN 12VIN 13.5 13.0 4 3 0 1 2 29.0 37 28.0 27.5 27.0 26.5 26.0 48VIN 36VIN 24VIN 25.5 34 33 32 41 40 39 38 37 48VIN 36VIN 24VIN 0 1 2 3 48VIN 36VIN 36 35 4 0 1 34 48VIN 36VIN 3 4 LOAD (A) 8027 G25 3 Minimum VIN vs Load, VOUT = 8V 6.0 10.0 5.9 9.8 5.8 9.6 5.7 9.4 5.6 9.2 5.5 5.4 9.0 8.8 5.3 8.6 5.2 8.4 5.1 8.2 5.0 0 1 2 LOAD (A) 4 8027 G24 VIN (V) VIN (V) 36 2 LOAD (A) 8027 G23 42 4 42 Minimum VIN vs Load, VOUT = 5V 38 3 8027 G21 LOAD (A) 40 2 43 35 30 4 3 44 BIAS CURRENT (mA) 1 44 31 46 6 0 Bias Current vs Load, VOUT = 18V 36 Bias Current vs Load, VOUT = 24V 2 22.0 4 LOAD (A) 8027 G22 1 48VIN 36VIN 24VIN 45 LOAD (A) 0 23.0 BIAS CURRENT (mA) BIAS CURRENT (mA) BIAS CURRENT (mA) 28.5 32 23.5 Bias Current vs Load, VOUT = 15V 38 2 24.0 8027 G20 29.5 1 24.5 LOAD (A) Bias Current vs Load, VOUT = 12V 0 25.0 22.5 3 8027 G19 25.0 BIAS CURRENT (mA) 15.5 17.0 BIAS CURRENT (mA) BIAS CURRENT (mA) 26.0 16.0 17.5 14.0 Bias Current vs Load, VOUT = 8V 4 3 8027 G26 8.0 0 1 2 LOAD (A) 3 4 8027 G27 8027fd For more information www.linear.com/LTM8027 LTM8027 TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C unless otherwise noted) Minimum VIN vs Load, VOUT = 15V 19.0 15.5 18.5 15.0 18.0 14.5 17.5 14.0 16.5 13.0 16.0 12.5 15.5 0 1 2 3 15.0 4 24 23 22 17.0 13.5 12.0 Minimum VIN vs Load, VOUT = 18V VIN (V) 16.0 VIN (V) VIN (V) Minimum VIN vs Load, VOUT = 12V 20 19 0 2 1 0 1 3 35 30 28 25 26 20 VIN (V) 30 Minimum VIN vs VOUT, IOUT = 4A Minimum VIN vs Load, VOUT = –3.3V 9 8 7 6 15 22 10 20 5 18 0 4 8027 G30 VIN (V) Minimum VIN vs Load, VOUT = 24V 24 2 LOAD (A) 8027 G29 8027 G28 VIN (V) 18 4 3 LOAD (A) LOAD (A) 32 21 5 4 3 2 0 1 2 3 4 1 0 5 10 LOAD (A) 15 20 1 3 30 10 25 8 20 Minimum VIN vs Load, VOUT = –8V Minimum VIN vs Load, VOUT = –12V 50 45 40 35 15 4 10 2 5 0 0 4 8027 G33 VIN (V) Minimum VIN vs Load, VOUT = –5V 6 2 LOAD (A) 8027 G32 VIN (V) VIN (V) 0 VOUT (V) 8027 G31 12 0 25 30 25 20 15 10 5 0 1 2 LOAD (A) 3 4 8027 G34 0 1 2 LOAD (A) 3 4 8027 G35 0 0 1 2 LOAD (A) 3 4 8027 G36 8027fd For more information www.linear.com/LTM8027 7 LTM8027 TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C unless otherwise noted) 42 45 36VIN 24VIN 12VIN 5VIN 32 27 22 17 12 50 60VIN 48VIN 36VIN 24VIN 12VIN 5VIN 40 TEMPERATURE RISE (°C) 37 TEMPERATURE RISE (°C) Temperature Rise vs Load, VOUT = 3.3V 35 30 25 20 15 10 25 20 15 10 5 0 0 2 3 4 0 1 2 4 3 80 50 40 30 20 10 0 90 60VIN 48VIN 36VIN 24VIN 16VIN 70 60 40 30 20 10 0 1 2 3 4 0 1 2 0 2 1 50 40 30 20 10 3 4 8027 G42 Temperature Rise vs Load, VOUT = 24V 60VIN 48VIN 36VIN 90 TEMPERATURE RISE (°C) TEMPERATURE RISE (°C) 20 LOAD (A) 100 60 0 30 8027 G41 60VIN 48VIN 36VIN 26VIN 80 40 LOAD (A) Temperature Rise vs Load, VOUT = 18V 70 50 0 4 3 8027 G40 90 70 60 10 LOAD (A) 100 60VIN 48VIN 36VIN 24VIN 20.5VIN 80 50 0 4 Temperature Rise vs Load, VOUT = 15V TEMPERATURE RISE (°C) 60 3 8027 G39 Temperature Rise vs Load, VOUT = 12V Temperature Rise vs Load, VOUT = 8V 60VIN 48VIN 36VIN 24VIN 12VIN 2 LOAD (A) 8027 G38 TEMPERATURE RISE (°C) 70 1 0 LOAD (A) 8027 G37 TEMPERATURE RISE (°C) 30 2 LOAD (A) 80 70 60 50 40 30 20 10 0 1 2 LOAD (A) 3 4 0 0 8027 G43 8 40 5 1 60VIN 48VIN 36VIN 24VIN 12VIN 35 7 0 Temperature Rise vs Load, VOUT = 5V 45 TEMPERATURE RISE (°C) Temperature Rise vs Load, VOUT = 2.5V 1 2 LOAD (A) 3 4 8027 G44 8027fd For more information www.linear.com/LTM8027 LTM8027 PIN FUNCTIONS PACKAGE ROW AND COLUMN LABELING MAY VARY AMONG µModule PRODUCTS. REVIEW EACH PACKAGE LAYOUT CAREFULLY. VIN (Bank 3): The VIN pins supply current to the LTM8027’s internal regulator and to the internal power switch. These pins must be locally bypassed with an external, low ESR capacitor (see Table 2). VOUT (Bank 1): Power Output Pins. Apply the output filter capacitor and the output load between these and the GND pins. AUX (Pin A7): Low Current Voltage Source for BIAS1 and BIAS2. In many designs, the BIAS pin is connected to VOUT by way of the AUX pin. The AUX pin is internally connected to VOUT and is placed near the BIAS pins to ease printed circuit board routing. Although this pin is internally connected to VOUT, do NOT connect this pin to the load. If this pin is not tied to BIAS1 and BIAS2, leave it floating. BIAS1 (Pin A6): The BIAS1 pin connects to the internal power bus. Connect to a power source greater than 8.5V. If the output is greater than 8.5V, connect it to this pin. If the output voltage is less, connect this to a voltage source between 8.5V and 15V. BIAS2 (Pin A3): Internal Biasing Power. Connect to AUX (if 24V or less) or a voltage source above 3V. Do not leave BIAS2 floating. RUN (Pin A4): Tie the RUN pin to ground to shut down the LTM8027. Tie to 1.4V or more for normal operation. The RUN pin is internally clamped to 5V, so when it is pulled up, be sure to use a pull-up resistor that limits the current into the RUN pin to less than 1mA. If the shutdown feature is not used, tie this pin to the VIN pin through a pull-up resistor. GND (Bank 2): Tie these GND pins to a local ground plane below the LTM8027 and the circuit components. RT (Pin B1): The RT pin is used to program the switching frequency of the LTM8027 by connecting a resistor from this pin to ground. The Applications Information section of the data sheet includes a table to determine the resistance value based on the desired switching frequency. Minimize capacitance at this pin. SYNC (Pin C1): The SYNC pin provides an external clock input for synchronization of the internal oscillator. The RT resistor should be set such that the internal oscillator frequency is 10% to 25% below the external clock frequency. This external clock frequency must be between 100kHz and 500kHz. If unused, tie the SYNC pin to GND. For more information see Oscillator Sync in the Application Information section of this data sheet. ADJ (Pin A2): The LTM8027 regulates its ADJ pin to 1.23V. Connect the adjust resistor from this pin to ground. The value of RADJ is given by the equation: RADJ = 613.77/(VOUT – 1.23) where RADJ is in kΩ. SS (Pin A5): The soft-start pin is used to program the supply soft-start function. Use the following formula to calculate CSS for a given output voltage slew rate: CSS = 2µA(tSS/1.231V) The pin should be left unconnected when not using the soft-start function. 8027fd For more information www.linear.com/LTM8027 9 LTM8027 BLOCK DIAGRAM VIN VOUT 6.8µH 499k 4.7pF 2.2µF RUN SS INTERNAL CONNECTION TO VOUT CURRENT MODE CONTROLLER SYNC VIN GND INTERNAL LINEAR REGULATOR BIAS1 INTVCC RT AUX BIAS2 ADJ 8027 BD OPERATION The LTM8027 is a standalone nonisolated step-down switching DC/DC power supply with an input voltage range of 4.5V to 60V that can deliver up to 4A of output current. This module provides a precisely regulated output voltage up to 24V, programmable via one external resistor. Given that the LTM8027 is a step-down converter, make sure that the input voltage is high enough to support the desired output voltage and load current. A simplified block diagram is given above. The LTM8027 contains a current mode controller, power switching element, power inductor, power MOSFETs and a modest amount of input and output capacitance. 10 The LTM8027 is a fixed frequency PWM regulator. The switching frequency is set by simply connecting the appropriate resistor from the RT pin to GND. A linear regulator provides internal power (shown as INTVCC on the Block Diagram) to the control circuitry. The bias regulator normally draws power from the VIN pin, but if the BIAS1 pin is connected to an external voltage higher than 8.5V, bias power will be drawn from the external source (typically the regulated output voltage). This improves efficiency. The RUN pin is used to enable or place the LTM8027 in shutdown, disconnecting the output and reducing the input current to less than 9µA. 8027fd For more information www.linear.com/LTM8027 LTM8027 APPLICATIONS INFORMATION For most applications, the design process is straight forward, summarized as follows: 1. Look at Table 2 and find the row that has the desired input range and output voltage. 2. Apply the recommended CIN, COUT, RADJ and RT values. 3. Connect the BIAS pins as indicated. While these component and connection combinations have been tested for proper operation, it is incumbent upon the user to verify proper operation over the intended system’s line, load and environmental conditions. Capacitor Selection Considerations The CIN and COUT capacitor values in Table 2 are the minimum recommended values for the associated operating conditions. Applying capacitor values below those indicated in Table 2 is not recommended, and may result in undesirable operation. Using larger values is generally acceptable, and can yield improved dynamic response, if it is necessary. Again, it is incumbent upon the user to verify proper operation over the intended system’s line, load and environmental conditions. Ceramic capacitors are small, robust and have very low ESR. However, not all ceramic capacitors are suitable. X5R and X7R types are stable over temperature and applied voltage and give dependable service. Other types, including Y5V and Z5U have very large temperature and voltage coefficients of capacitance. In an application circuit they may have only a small fraction of their nominal capacitance resulting in much higher output voltage ripple than expected. A final precaution regarding ceramic capacitors concerns the maximum input voltage rating of the LTM8027. A ceramic input capacitor combined with trace or cable inductance forms a high Q (under damped) tank circuit. If the LTM8027 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the device’s rating. This situation is easily avoided; see the Hot-Plugging Safely section. Input Power Requirements The LTM8027 is biased using an internal linear regulator to generate operational voltages from the VIN pin. Virtually all of the circuitry in the LTM8027 is biased via this internal linear regulator output (INTVCC on the Block Diagram). This pin is internally decoupled with a low ESR capacitor to GND. The INTVCC regulator generates an 8V output provided there is ample voltage on the VIN pin. The INTVCC regulator has approximately 1V of dropout, and will follow the VIN pin with voltages below the dropout threshold. The LTM8027 has a typical start-up requirement of VIN > 7.5V. This assures that the onboard regulator has ample headroom to bring the internal regulator (INTVCC) above its UVLO threshold. The INTVCC regulator can only source current, so forcing the BIAS1 pin above 8.5V allows use of externally derived power for the IC. This effectively shuts down the internal linear regulator and reduces power dissipation within the LTM8027. Using the onboard regulator for start-up, then applying power to BIAS1 from the converter output or external supply maximizes conversion efficiencies and is a common practice. If BIAS1 is maintained above 6.5V using an external source, the LTM8027 can continue to operate with VIN as low as 4.5V. BIAS Power The internal circuitry of the LTM8027 is powered by the INTVCC bus, which is derived either from the afore mentioned internal linear regulator or the BIAS1 pin, if it is greater than 8.5V. Since the internal linear regulator is by nature dissipative, deriving INTVCC from an external source through the BIAS pins reduces the power lost within the LTM8027 and can increase overall system efficiency. 8027fd For more information www.linear.com/LTM8027 11 LTM8027 APPLICATIONS INFORMATION PINTVCC = (VIN – 8) • IINTVCC or only 60mW. This has a small but probably acceptable effect on the operating temperature of the LTM8027. If the input rises to 60V, however, the power dissipation is a lot higher, over 780mW. This can cause unnecessarily high junction temperatures if the INTVCC regulator must dissipate this amount of power for very long. Connect BIAS2 to AUX (if 24V or less) or a voltage source above 3V. Soft-Start The soft-start function controls the slew rate of the power supply output voltage during start-up. A controlled output voltage ramp minimizes output voltage overshoot, reduces inrush current from the VIN supply, and facilitates supply sequencing. A capacitor connected from the SS pin to GND programs the slew rate. The capacitor is charged from an internal 2µA current source producing a ramped voltage that overrides the command reference to the controller, resulting in a smooth output voltage ramp. The soft-start circuit is disabled once the SS pin voltage has been charged to 200mV above the internal reference of 1.231V. During a VIN UVLO, RUN event, or undervoltage on internal bias, the SS pin voltage is discharged with a 50µA current. Therefore, the value of the SS capacitor determines how long one of these events must be in order to completely discharge the soft-start capacitor. In the case of an output overload or short circuit, the SS pin voltage is clamped to a diode drop above the ADJ pin. Once the short has been removed the VADJ pin voltage starts to recover. The softstart circuit takes control of the output voltage slew rate once the VADJ pin voltage has exceeded the slowly ramp- 12 ing SS pin voltage, reducing the output voltage overshoot during a short-circuit recovery. The desired soft-start time (tSS) is programmed via the CSS capacitor as follows: CSS = 2µA • tSS 1.231V The amount of time in which the power supply must be under a VIN, internal regulator (INTVCC) or VSHDN UVLO fault condition (tFAULT) before the SS pin voltage enters its active region is approximated by the following formula: CSS • 0.65V 50µA tFAULT = Operating Frequency Trade-offs The LTM8027 uses a constant frequency architecture that can be programmed over a 100kHz to 500kHz range with a single resistor from the RT pin to ground. The nominal voltage on the RT pin is 1V and the current that flows from this pin is used to charge an internal oscillator capacitor. The value of RT for a given operating frequency can be chosen from Figure 1 or Table 1. 600 500 FREQUENCY (kHz) For example, suppose the LTM8027 needs to provide 5V from an input voltage source that is nominally 12V. From Table 2, the recommended RT value is 75k, which corresponds to an operating frequency of 210kHz. From the graphs in the Typical Performance Characteristics, the typical internal regulator (INTVCC) current at 12VIN and 210kHz is 15mA. The power dissipated by the internal linear regulator at 12VIN is given by the equation: 400 300 200 100 0 0 50 100 RT (kΩ) 150 200 8027 F01 Figure 1. Timing Resistor (RT) Value 8027fd For more information www.linear.com/LTM8027 LTM8027 APPLICATIONS INFORMATION Table 1 lists typical resistor values for common operating frequencies. Table 1. RT Resistor Values vs Frequency RT (kΩ) fSW (kHz) 187 100 118 150 82.5 200 63.4k 250 48.7k 300 40.2k 350 31.6k 400 27.4k 450 23.7k 500 It is recommended that the user apply the RT value given in Table 2 for the input and output operating condition. System level or other considerations, however, may necessitate another operating frequency. While the LTM8027 is flexible enough to accommodate a wide range of operating frequencies, a haphazardly chosen one may result in undesirable operation under certain operating or fault conditions. A frequency that is too high can damage the LTM8027 if the output is overloaded or short-circuited. A frequency that is too low can result in a final design that has too much output ripple or too large of an output capacitor. The maximum frequency (fMAX) at which the LTM8027 should be allowed to switch and the minimum frequency set resistor value that should be used for a given set of input and output operating condition is given in Table 2 as RT(MIN). There are additional conditions that must be satisfied if the synchronization function is used. Please refer to the Synchronization section for details. Output Voltage Programming The LTM8027 regulates its ADJ pin to 1.23V. Connect the adjust resistor from this pin to ground. The value of RADJ is given by the equation RADJ = 613.77/(VOUT – 1.23), where RADJ is in kΩ. RUN Control The LTM8027 RUN pin uses a reference threshold of 1.4V. This precision threshold allows use of the RUN pin for both logic-level controlled applications and analog monitoring applications such as power supply sequencing. The LTM8027 operational status is primarily controlled by a UVLO circuit on internal power source. When the LTM8027 is enabled via the RUN pin, only the internal regulator (INTVCC) is enabled. Switching remains disabled until the UVLO threshold is achieved at the BIAS1 pin, when the remainder of the LTM8027 is enabled and switching commences. Because the LTM8027 high power converter is a power transfer device, a voltage that is lower than expected on the input supply could require currents that exceed the sourcing capabilities of that supply, causing the system to lock up in an undervoltage state. Input supply startup protection can be achieved by enabling the RUN pin using a resistive divider from the VIN supply to ground. Setting the divider output to 1.4V when that supply is at an adequate voltage prevents an LTM8027 converter from drawing large currents until the input supply is able to provide the required power. 200mV of input hysteresis on the RUN pin allows for about 15% of input supply droop before disabling the converter. Input UVLO and RUN The RUN pin has a precision voltage threshold with hysteresis which can be used as an undervoltage lockout threshold (UVLO) for the power supply. Undervoltage lockout keeps the LTM8027 in shutdown until the supply input voltage is above a certain voltage programmed by the user. The hysteresis voltage prevents noise from falsely tripping UVLO. Resistors are chosen by first selecting RB (refer to Figure 2). Then: ⎛ VIN(ON) ⎞ RA = RB • ⎜ – 1⎟ 1.4V ⎝ ⎠ where VIN(ON) is the input voltage at which the undervoltage lockout is disabled and the supply turns on. 8027fd For more information www.linear.com/LTM8027 13 LTM8027 APPLICATIONS INFORMATION VSUPPLY RA RUN PIN RB 8027 F02 Figure 2. Undervoltage Lockout Resistive Divider Example: Select RB = 49.9k, VIN(ON) = 14.5V (based upon a 15V minimum input voltage) ⎛ 14.5V ⎞ RA = 49.9k • ⎜ – 1⎟ = 464k ⎝ 1.4V ⎠ The VIN turn off voltage is 15% below turn on. In the example the VIN(OFF) would be 12.3V. The shutdown function can be disabled by connecting the RUN pin to the VIN pin through a large value pull-up resistor, (RPU). This pin contains a low impedance clamp at 6V, so the RUN pin will sink current from the RPU pull-up resistor: V – 6V IRUN = IN RPU Because this arrangement will clamp the RUN pin to 6V, it will violate the 5V absolute maximum voltage rating of the pin. This is permitted, however, as long as the absolute maximum input current rating of 1mA is not exceeded. Input RUN pin currents of