LTM8027MPV

LTM8027 60V, 4A DC/DC µModule Regulator FEATURES n n n n n n n n n n DESCRIPTION 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. 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 thermally enhanced, compact (15mm × 15mm) and low profile (4.32mm) over-molded land grid array (LGA) package suitable for automated assembly by standard surface mount equipment. The LTM8027 is Pb-free and RoHS compliant. L, LT, LTC, LTM, μModule, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Complete Switch Mode Power Supply Wide Input Voltage Range: 4.5V to 60V (7.5V Minimum Voltage to Start) Wide Output Voltage Range: 2.5V to 24V (See Table 2) 4A Output Current Programmable Soft-Start 10μA Shutdown Supply Current Selectable Switching Frequency Current Mode Control Up to 95% Efficiency Pb-Free (e4) RoHS Compliant Package with Gold Pad Finish Tiny, Low Profile (15mm × 15mm × 4.32mm) Surface Mount LGA Package APPLICATIONS n n n n 12V and 42V Automotive and Heavy Equipment 48V Telecom Power Supplies Avionics and Industrial Control Systems Distributed Power Converters TYPICAL APPLICATION 48W, 16VIN to 60VIN DC/DC μModule® Regulator VIN 16V TO 60V VIN 1M RUN SS SYNC RT 48.7k GND LTM8027 BIAS1 22μF 4 56.2k EFFICIENCY (%) BIAS2 AUX ADJ VOUT VOUT 12V 4A 100 90 80 70 60 50 40 30 3845 TA01a Efficiency vs Load 24VIN 4.7μF 2 20 10 0 0 1 2 LOAD (A) 3 4 8027 TA01b 8027f 1 LTM8027 ABSOLUTE MAXIMUM RATINGS (Note 1) PIN CONFIGURATION TOP VIEW 11 10 9 8 AUX 7 BIAS1 6 SS 5 RUN 4 BIAS2 3 ADJ 2 1 ABCDEFGHJKL SYNC RT LGA PACKAGE 113-LEAD (15mm 15mm 4.32mm) 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 VOUT BANK 1 VIN Voltage ................................................................65V BIAS1, BIAS2 ............................................................15V 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).................................................. –40°C to 125°C Maximum Soldering Temperature ......................... 245°C Storage Temperature Range .................. –55°C to 125°C GND BANK 2 VIN BANK 3 ORDER INFORMATION LEAD FREE FINISH LTM8027EV#PBF LTM8027IV#PBF LTM8027MPV#PBF TRAY LTM8027EV#PBF LTM8027IV#PBF LTM8027MPV#PBF PART MARKING LTM8027V LTM8027V LTM8027V PACKAGE DESCRIPTION INTERNAL TEMPERATURE RANGE 113-Lead (15mm × 15mm × 4.32mm) LGA –40°C to 125°C 113-Lead (15mm × 15mm × 4.32mm) LGA –40°C to 125°C 113-Lead (15mm × 15mm × 4.32mm) LGA –55°C to 125°C Consult LTC Marketing for parts specified with wider operating temperature ranges. Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ This product is only offered in trays. For more information go to: http://linear.com/packaging/ ELECTRICAL CHARACTERISTICS SYMBOL VIN VOUT IOUT VIN(START) ΔVOUT/ΔVIN ΔVOUT/ΔILOAD VUVLO(RISING) VUVLO(FALLING) PARAMETER Input DC Voltage Maximum Output DC Voltage Output DC Current Minimum Start Voltage Line Regulation Load Regulation Input Undervoltage Lockout Threshold (Rising) Input Undervoltage Lockout Threshold (Falling) The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 20V, BIAS1 = BIAS2 = 10V, RUN = 2V, unless otherwise noted. CONDITIONS (Note 5) 0 < IOUT ≤ 4A, VIN = 48V VIN ≤ 60V, VOUT = 12V, (Note 4) VOUT = 12V, 15V< VIN < 60V, ILOAD = 4A VOUT = 12V, VIN = 24V, 0A < ILOAD ≤ 4A (Note 5) (Note 5) 0 0.2 0.2 4.6 3.7 l MIN 4.5 TYP 24 MAX 60 4 7.5 UNITS V V A V % % V V 8027f 2 LTM8027 ELECTRICAL CHARACTERISTICS SYMBOL VADJ IQVIN VBIAS1 IBIAS1 VBIAS2 IBIAS2 VBIAS(MINOV) RFB VRUN(RISING) VRUN(FALLING) fSW RSYNC VSYNC(TH) ISS PARAMETER ADJ Voltage Quiescent Current into IN BIAS1 Undervoltage Lockout (Rising) BIAS1 Undervoltage Lockout (Falling) Current into BIAS1 Minimum BIAS2 Voltage Current Into BIAS2 Minimum Voltage to Overdrive INTVCC Regulator Internal Feedback Resistor RUN Enable Voltage (Rising) RUN Enable Voltage (Falling) Switching Frequency SYNC Input Resistance SYNC Voltage Threshold Soft-Start Charging Current fSYNC = 350kHz l The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 20V, BIAS1 = BIAS2 = 10V, RUN = 2V, unless otherwise noted. CONDITIONS l MIN 1.224 1.215 TYP MAX 1.238 1.245 UNITS V V mA μA V V mA μA V μA V kΩ V V kHz kHz kΩ V μA VBIAS = VAUX, VOUT = 12VDC, No Load VRUN = 0V 39 9 6.5 6 No Load RUN = 0V 25 25 6 1 8.5 499 1.4 1.2 RT = 187kΩ RT = 23.7kΩ 2.3 100 500 40 2 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 –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 the VCC is externally driven above 6.5V. 8027f 3 LTM8027 TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C unless otherwise noted) Efficiency vs Load, VOUT = 2.5V 100 90 80 EFFICIENCY (%) EFFICIENCY (%) 70 60 50 40 30 20 10 0 0 1 2 LOAD (A) 3 5VIN 12VIN 24VIN 36VIN 4 8027 G01 Efficiency vs Load, VOUT = 3.3V 100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0 1 2 LOAD (A) 3 5VIN 12VIN 24VIN 36VIN 48VIN 60VIN 4 8027 G02 Efficiency vs Load, VOUT = 5V 100 90 80 70 60 50 40 30 20 10 0 0 1 2 LOAD (A) 3 12VIN 24VIN 36VIN 48VIN 60VIN 4 8027 G03 Efficiency vs Load, VOUT = 8V 100 90 80 EFFICIENCY (%) EFFICIENCY (%) 70 60 50 40 30 20 10 0 0 1 2 LOAD (A) 3 12VIN 24VIN 36VIN 48VIN 60VIN 4 8027 G04 Efficiency vs Load, VOUT = 12V 100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0 1 2 LOAD (A) 3 24VIN 36VIN 48VIN 60VIN 4 8027 G05 Efficiency vs Load, VOUT = 15V 100 90 80 70 60 50 40 30 20 10 0 0 1 2 LOAD (A) 3 24VIN 36VIN 48VIN 60VIN 4 8027 G06 Efficiency vs Load, VOUT = 18V 100 90 80 EFFICIENCY (%) EFFICIENCY (%) 70 60 50 40 30 20 10 0 0 1 2 LOAD (A) 3 24VIN 36VIN 48VIN 60VIN 4 8027 G07 Efficiency vs Load, VOUT = 24V 100 90 80 70 60 50 40 30 20 10 0 0 1 2 LOAD (A) 3 36VIN 48VIN 60VIN 4 8027 G08 Input Current vs VIN Output Shorted 3000 2500 INPUT CURRENT (mA) 2000 1500 1000 500 0 0 10 20 30 VIN (V) 40 50 60 8027 G09 8027f 4 LTM8027 TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C unless otherwise noted) Input Current vs Load, VOUT = 2.5V 3000 2500 INPUT CURRENT (mA) 2000 1500 1000 500 0 0 1 2 LOAD (A) 3 4 8027 G43 Input Current vs Load, VOUT = 3.3V 3500 2000 5VIN 12VIN 24VIN 36VIN 48VIN 60VIN 1800 1600 INPUT CURRENT (mA) 1400 3000 INPUT CURRENT (mA) 2500 2000 1500 1000 500 0 0 1 2 LOAD (A) 8027 G10 Input Current vs Load, VOUT = 5V 12VIN 24VIN 36VIN 48VIN 60VIN 5VIN 12VIN 24VIN 36VIN 1200 1000 800 600 400 200 3 4 0 0 1 2 LOAD (A) 3 4 8027 G11 Input Current vs Load, VOUT = 8V 3500 3000 INPUT CURRENT (mA) 2500 2000 1500 1000 500 0 0 1 2 LOAD (A) 8027 G12 Input Current vs Load, VOUT = 12V 2500 24VIN 36VIN 48VIN 60VIN 3000 2500 INPUT CURRENT (mA) 2000 1500 1000 500 0 Input Current vs Load, VOUT = 15V 24VIN 36VIN 48VIN 60VIN INPUT CURRENT (mA) 4 12VIN 24VIN 36VIN 48VIN 60VIN 2000 1500 1000 500 3 0 0 1 2 LOAD (A) 3 4 8027 G13 0 1 2 LOAD (A) 3 4 8027 G14 Input Current vs Load, VOUT = 18V 3500 3000 INPUT CURRENT (mA) 2500 2000 1500 1000 500 0 0 1 2 LOAD (A) 3 4 8027 G15 Input Current vs Load, VOUT = 24V 3500 3000 INPUT CURRENT (mA) 2500 2000 1500 1000 500 0 0 1 2 LOAD (A) 3 4 8027 G16 Bias Current vs Load, VOUT = 2.5V 15.50 15.00 BIAS CURRENT (mA) 14.50 14.00 13.50 13.00 12.50 12.00 36VIN 24VIN 12VIN 0 1 2 LOAD (A) 8027 G17 24VIN 36VIN 48VIN 60VIN 36VIN 48VIN 60VIN 3 4 8027f 5 LTM8027 TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C unless otherwise noted) Bias Current vs Load, VOUT = 3.3V 18.0 17.5 15.5 BIAS CURRENT (mA) BIAS CURRENT (mA) 15.0 14.5 14.0 13.5 13.0 48VIN 36VIN 24VIN 12VIN 0 1 2 LOAD (A) 8027 G18 8027 G19 Bias Current vs Load, VOUT = 5V 16.0 26.0 25.5 BIAS CURRENT (mA) 25.0 24.5 24.0 23.5 23.0 22.5 22.0 3 4 Bias Current vs Load, VOUT = 8V 17.0 16.5 16.0 15.5 15.0 14.5 14.0 0 1 2 LOAD (A) 3 48VIN 36VIN 24VIN 12VIN 4 48VIN 36VIN 24VIN 0 1 2 LOAD (A) 8027 G20 3 4 Bias Current vs Load, VOUT = 12V 29.5 29.0 28.5 BIAS CURRENT (mA) BIAS CURRENT (mA) 28.0 27.5 27.0 26.5 26.0 25.5 25.0 0 1 2 LOAD (A) 8027 G21 Bias Current vs Load, VOUT = 15V 38 37 BIAS CURRENT (mA) 48VIN 36VIN 24VIN 0 1 2 LOAD (A) 8027 G22 Bias Current vs Load, VOUT = 18V 45 44 43 36 35 34 33 32 42 41 40 39 38 37 36 48VIN 36VIN 0 1 2 LOAD (A) 3 4 8027 G23 48VIN 36VIN 24VIN 3 4 31 30 3 4 35 Bias Current vs Load, VOUT = 24V 46 44 BIAS CURRENT (mA) 42 VIN (V) 40 38 36 34 32 48VIN 36VIN 0 1 2 LOAD (A) 8027 G44 Minimum VIN vs Load, VOUT = 5V 6.0 5.9 5.8 5.7 VIN (V) 4 8027 G25 Minimum VIN vs Load, VOUT = 8V 10.0 9.8 9.6 9.4 9.2 9.0 8.8 8.6 8.4 8.2 5.6 5.5 5.4 5.3 5.2 5.1 5.0 3 4 0 1 2 LOAD (A) 8.0 0 1 3 2 LOAD (A) 3 4 8027 G26 8027f 6 LTM8027 TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C unless otherwise noted) Minimum VIN vs Load, VOUT = 12V 16.0 15.5 15.0 14.5 VIN (V) VIN (V) 14.0 13.5 13.0 12.5 12.0 0 1 2 LOAD (A) 8027 G27 Minimum VIN vs Load, VOUT = 15V 19.0 18.5 18.0 17.5 VIN (V) 17.0 16.5 16.0 15.5 3 4 15.0 0 1 2 LOAD (A) 8027 G28 Minimum VIN vs Load, VOUT = 18V 24 23 22 21 20 19 18 3 4 0 1 2 LOAD (A) 3 4 8027 G29 Minimum VIN vs Load, VOUT = 24V 32 30 28 VIN (V) VIN (V) 26 24 22 20 18 0 1 2 LOAD (A) 8027 G30 Minimum VIN vs VOUT, IOUT = 4A 35 30 25 VIN (V) 0 5 10 15 20 25 8027 G31 Minimum VIN vs Load, VOUT = –3.3V 9 8 7 6 5 4 3 2 20 15 10 5 0 1 0 0 1 2 LOAD (A) 8027 G45 3 4 3 4 VOUT (V) Minimum VIN vs Load, VOUT = –5V 12 10 8 VIN (V) 6 4 2 0 0 1 2 LOAD (A) 3 4 8027 G32 Minimum VIN vs Load, VOUT = –8V 30 25 20 VIN (V) VIN (V) 15 10 5 0 0 1 2 LOAD (A) 3 4 8027 G33 Minimum VIN vs Load, VOUT = –12V 50 45 40 35 30 25 20 15 10 5 0 0 1 2 LOAD (A) 3 4 8027 G34 8027f 7 LTM8027 TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C unless otherwise noted) Temperature Rise vs Load, VOUT = 2.5V 42 37 TEMPERATURE RISE (°C) 32 27 22 17 12 7 2 0 1 2 LOAD (A) 8027 G35 Temperature Rise vs Load, VOUT = 3.3V 45 50 60VIN 48VIN 36VIN 24VIN 12VIN 5VIN 45 TEMPERATURE RISE (°C) 40 35 30 25 20 15 10 5 0 1 2 LOAD (A) 8027 G36 Temperature Rise vs Load, VOUT = 5V 60VIN 48VIN 36VIN 24VIN 12VIN TEMPERATURE RISE (°C) 36VIN 24VIN 12VIN 5VIN 40 35 30 25 20 15 10 5 0 3 4 3 4 0 0 1 2 LOAD (A) 3 4 8027 G37 Temperature Rise vs Load, VOUT = 8V 70 60 TEMPERATURE RISE (°C) 50 40 30 20 10 0 0 1 2 LOAD (A) 8027 G38 Temperature Rise vs Load, VOUT = 12V 80 70 TEMPERATURE RISE (°C) 60 50 40 30 20 10 90 60VIN 48VIN 36VIN 24VIN 16VIN 80 TEMPERATURE RISE (°C) 70 60 50 40 30 20 10 0 0 1 2 LOAD (A) 8027 G39 Temperature Rise vs Load, VOUT = 15V 60VIN 48VIN 36VIN 24VIN 20.5VIN 60VIN 48VIN 36VIN 24VIN 12VIN 3 4 0 3 4 0 1 2 LOAD (A) 3 4 8027 G40 Temperature Rise vs Load, VOUT = 18V 100 90 TEMPERATURE RISE (°C) 80 70 60 50 40 30 20 10 0 0 1 2 LOAD (A) 3 4 8027 G41 Temperature Rise vs Load, VOUT = 24V 100 90 TEMPERATURE RISE (°C) 80 70 60 50 40 30 20 10 0 0 1 2 LOAD (A) 3 4 8027 G42 60VIN 48VIN 36VIN 26VIN 60VIN 48VIN 36VIN 8027f 8 LTM8027 PIN FUNCTIONS VIN (Bank 3): The VIN pin supplies current to the LTM8027’s internal regulator and to the internal power switch. This pin 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 BIAS1and BIAS2. In many designs, the BIAS pins are simply connected to VOUT. 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. For proper operation, connect this pin to the same power source as BIAS2. BIAS2 (Pin A3): Internal Biasing Power. This pin must be connected to the same power source as BIAS1 for proper operation. Always connect this pin to a voltage source above 8.5V. 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 in to 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. If unused, the SYNC pin is connected 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. 8027f 9 LTM8027 BLOCK DIAGRAM VIN 6.8μH 499k 4.7pF 2.2μF VOUT RUN SS CURRENT MODE CONTROLLER INTERNAL CONNECTION TO VOUT AUX SYNC VIN INTERNAL LINEAR REGULATOR INTVCC BIAS1 BIAS2 GND RT 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. 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 BIAS1and BIAS2 pins are 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 10μA. 8027f 10 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. Ceramic capacitors are also piezoelectric. The LTM8027’s switching frequency depends on the load current, and at light loads it can excite a ceramic capacitor at audio frequencies, generating audible noise. 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). This pin is internally decoupled with a low ESR capacitor to GND. The VCC regulator generates an 8V output provided there is ample voltage on the VIN pin. The VCC 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 INTVCC above its UVLO threshold. The INTVCC regulator can only source current, so forcing the BIAS 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 deriving power for VCC from the converter output maximizes conversion efficiencies and is common practice. If VCC is maintained above 6.5V using an external source, the LTM8027 can continue to operate with VIN as low as 4V. 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 and BIAS2 pins, 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. 8027f 11 LTM8027 APPLICATIONS INFORMATION 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 162k, which corresponds to an operating frequency of 210kHz. From the graphs in the Typical Performance Characteristics, the typical INTVCC current at 12VIN and 210kHz is 15mA. The power dissipated by the internal linear regulator at 12VIN is given by the equation: PINTVCC = (VIN – 8.5) • IINTVCC or only 54mW. This has a small but probably acceptable effect on the operating temperature of the LTM8027. FREQUENCY (kHz) 600 500 400 300 200 100 0 Operating Frequency Tradeoffs 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. If the input rises to 60V, however, the power dissipation is a lot higher, over 750mW. This can cause unnecessarily high junction temperatures if the INTVCC regulator must dissipate this amount of power for very long. 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, INTVCC undervoltage or RUN event, the SS pin voltage is discharged with a 50μA. 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 soft-start circuit takes control of the output voltage slew rate once the VADJ pin voltage has exceeded the slowly ramping SS pin voltage, reducing the output voltage overshoot during a short-circuit recovery. 0 50 100 RT (kΩ) 150 200 8027 F01 Figure 1. Timing Resistor (RT) Value Table 1 lists typical resistor values for common operating frequencies. Table 1. RT Resistor Values vs Frequency RT (kΩ) 187 118 82.5 63.4k 48.7k 40.2k 31.6k 27.4k 23.7k fSW (kHz) 100 150 200 250 300 350 400 450 500 8027f 12 LTM8027 APPLICATIONS INFORMATION 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 VCC regulator is enabled. Switching remains disabled until the UVLO threshold is achieved at the VCC 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. VSUPPLY RA RUN PIN RB 8027 F02 Figure 2. Undervoltage Lockout Resistive Divider 8027f 13 LTM8027 APPLICATIONS INFORMATION 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. This pin contains a low impedance clamp at 6V, so the RUN pin will sink current from the pull-up resistor (RPU): IRUN = VIN – 6V RPU Hot-Plugging Safely The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LTM8027. However, these capacitors can cause problems if the LTM8027 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 LTM8027 can ring to twice the nominal input voltage, possibly exceeding the LTM8027’s rating and damaging the part. If the input supply is poorly controlled or the user will be plugging the LTM8027 into an energized supply, the input network should be designed to prevent this overshoot by introducing a damping element into the path of current flow. This is often done by adding an inexpensive electrolytic bulk capacitor across the input terminals of the LTM8027. The criteria for selecting this capacitor is that the ESR is high enough to damp the ringing, and the capacitance value is several times larger than the LTM8027 ceramic input capacitor. The bulk capacitor does not need to be located physically close to the LTM8027; it should be located close to the application board’s input connector, instead. Synchronization The oscillator can be synchronized to an external clock. Choose the RT resistor such that the resultant frequency is at least 10% below the desired synchronization frequency. It is recommended that the SYNC pin be driven with a square wave that has amplitude greater than 2.3V, pulse width greater than 1μs and rise time less than 500ns. The rising edge of the sync wave form triggers the discharge of the internal oscillator capacitor. 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