LT3845

FEATURES ■ ■ ■ ■ ■ LT3845 High Voltage Synchronous Current Mode Step-Down Controller with Adjustable Operating Frequency DESCRIPTION The LT®3845 is a high voltage, synchronous, current mode controller used for medium to high power, high efficiency supplies. It offers a wide 4V to 60V input range (7.5V minimum start-up voltage). An onboard regulator simplifies the biasing requirements by providing IC power directly from VIN. Burst Mode® operation maintains high efficiency at light loads by reducing IC quiescent current to 120μA. Light load efficiency is also improved with the reverse inductor current inhibit function which supports discontinuous operation. Additional features include adjustable fixed operating frequency that can be synchronized to an external clock for noise sensitive applications, gate drivers capable of driving large N-channel MOSFETs, a precision undervoltage lockout, 10μA shutdown current, short-circuit protection and a programmable soft-start. The LT3845 is available in a 16-lead thermally enhanced TSSOP package. , LT LTC and LTM are registered trademarks of Linear Technology Corporation. , Burst Mode is a registered trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents, including 5481178, 6611131, 6304066, 6498466, 6580258. ■ ■ ■ ■ ■ ■ ■ ■ ■ High Voltage Operation: Up to 60V Synchronizable Up to 600kHz Adjustable Constant Frequency: 100kHz to 500kHz Output Voltages Up to 36V Adaptive Nonoverlap Circuitry Prevents Switch Shoot-Through Reverse Inductor Current Inhibit for Discontinuous Operation Improves Efficiency with Light Loads Programmable Soft-Start 120μA No Load Quiescent Current 10μA Shutdown Supply Current 1% Regulation Accuracy Standard Gate N-Channel Power MOSFETs Current Limit Unaffected by Duty Cycle Reverse Overcurrent Protection 16-Lead Thermally Enhanced TSSOP Package APPLICATIONS ■ ■ ■ ■ 12V and 42V Automotive and Heavy Equipment 48V Telecom Power Supplies Avionics and Industrial Control Systems Distributed Power Converters TYPICAL APPLICATION High Voltage Step-Down Regulator 48V to 12V at 75W VIN 20V TO 55V 100 2.2μF 100V 47μF 63V 82.5k 1500pF 0.1μF 1M VIN SHDN CSS LT3845 BOOST TG SW VCC BG PGND SENSE + Efficiency and Power Loss vs Load Current VIN = 48V 7 6 5 4 3 2 LOSS 70 1 0 1 LOAD CURENT (A) 10 3845 TA01b 95 Si7370DP 0.01Ω BAS521 Si7370DP 15μH B160 EFFICIENCY(%) VOUT 12V 75W 33μF ×3 90 85 80 75 POWER LOSS (W) BURST_EN 20k 16.2k 2200pF 143k 49.9k 100pF SYNC fSET SGND VFB VC SENSE– 1μF 1N4148 3845 TA01a 65 0.1 3845fc 1 LT3845 ABSOLUTE MAXIMUM RATINGS (Note 1) PIN CONFIGURATION TOP VIEW VIN SHDN CSS BURST_EN VFB VC SYNC fSET 1 2 3 4 5 6 7 8 17 16 BOOST 15 TG 14 SW 13 VCC 12 BG 11 PGND 10 SENSE+ 9 SENSE– Input Supply Voltage (VIN) ........................................65V Boosted Supply Voltage (BOOST) .............................80V Switch Voltage (SW) (Note 8) ....................... 65V to –2V Differential Boost Voltage (BOOST to SW).....................................................24V Bias Supply Voltage (VCC) .........................................24V SENSE+ and SENSE– Voltages ..................................40V Differential Sense Voltage (SENSE+ to SENSE–)................................... 1V to –1V BURST_EN Voltage ...................................................24V SYNC, VC, VFB, CSS, and SHDN Voltages ....................5V SHDN Pin Currents ..................................................1mA Operating Junction Temperature Range (Note 2) LT3845E (Note 3) ............................... –40°C to 125°C LT3845I .............................................. –40°C to 125°C Storage Temperature.............................. –65°C to 150°C Lead Temperature (Soldering, 10 sec) .................. 300°C FE PACKAGE 16-LEAD PLASTIC TSSOP TJMAX = 125°C, θJA = 40°C/W, θJC = 10°C/W EXPOSED PAD (PIN 17) IS SGND, MUST BE SOLDERED TO PCB ORDER INFORMATION LEAD FREE FINISH LT3845EFE#PBF LT3845IFE#PBF TAPE AND REEL LT3845EFE#TRPBF LT3845IFE#TRPBF PART MARKING 3845FE 3845FE PACKAGE DESCRIPTION 16-Lead Plastic TSSOP 16-Lead Plastic TSSOP TEMPERATURE RANGE –40°C to 125°C –40°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/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ 3845fc 2 LT3845 ELECTRICAL CHARACTERISTICS PARAMETER VIN Operating Voltage Range (Note 4) VIN Minimum Start Voltage VIN UVLO Threshold (Falling) VIN UVLO Threshold Hysteresis VIN Supply Current VIN Burst Mode Current VIN Shutdown Current BOOST Operating Voltage Range BOOST Operating Voltage Range (Note 5) BOOST UVLO Threshold (Rising) BOOST UVLO Threshold Hysteresis BOOST Supply Current (Note 6) BOOST Burst Mode Current BOOST Shutdown Current VCC Operating Voltage Range (Note 5) VCC Output Voltage VCC UVLO Threshold (Rising) VCC UVLO Threshold Hysteresis VCC Supply Current (Note 6) VCC Burst Mode Current VCC Shutdown Current VCC Current Limit Error Amp Reference Voltage VFB Pin Input Current SHDN Enable Threshold (Rising) SHDN Threshold Hysteresis Sense Pins Common Mode Range Current Limit Sense Voltage Reverse Protect Sense Voltage Reverse Current Inhibit Offset Input Current (ISENSE+ + ISENSE–) VSENSE+ – VSENSE– VSENSE+ – VSENSE–, VBURST_EN = VCC VBURST_EN = 0V or VBURST_EN = VFB VSENSE(CM) = 0V VSENSE(CM) = 2V VSENSE(CM) > 4V ● The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 20V, VCC = BOOST = BURST_EN = 10V, SHDN = 2V, RSET = 49.9kΩ, SENSE – = SENSE+ = 10V, SGND = PGND = SW = SYNC = 0V, unless otherwise noted. CONDITIONS ● ● ● MIN 4 3.6 TYP MAX 60 7.5 4 UNITS V V V mV μA μA μA V V V mV mA μA μA 3.8 670 20 20 9 VCC > 9V VBURST_EN = 0V, VFB = 1.35V VSHDN = 0V VBOOST – VSW VBOOST – VSW VBOOST – VSW VBURST_EN = 0V VSHDN = 0V Over Full Line and Load Range ● ● ● 15 75 20 5 400 1.4 0.1 0.1 ● ● 8 6.25 500 3 100 20 –150 1.231 25 20 8.3 V V V mV mA μA μA mA V V nA V mV V mV mV mV μA μA μA ● 3.7 VBURST_EN = 0V VSHDN = 0V Measured at VFB Pin VFB = 1.231V ● ● –40 1.224 1.215 1.3 0 90 1.238 1.245 1.4 36 115 ● ● ● 1.35 120 100 –100 10 800 –20 –300 Operating Frequency Minimum Programmable Frequency Maximum Programmable Frequency External Sync Frequency Range SYNC Input Resistance SYNC Voltage Threshold Soft-Start Capacitor Control Current Error Amp Transconductance Error Amp DC Voltage Gain Error Amp Sink/Source Current ● ● ● ● ● 290 270 500 100 300 310 330 100 600 kHz kHz kHz kHz kHz kΩ V μA μS dB μA 40 1.4 2 270 340 62 ±30 410 2 3845fc 3 LT3845 ELECTRICAL CHARACTERISTICS PARAMETER TG, BG Drive On Voltage (Note 7) TG, BG Drive Off Voltage TG, BG Drive Rise/Fall Time Minimum TG Off Time Minimum TG On Time Gate Drive Nonoverlap Time TG Fall to BG Rise BG Fall to TG Rise The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 20V, VCC = BOOST = BURST_EN = 10V, SHDN = 2V, RSET = 49.9kΩ, SENSE – = SENSE+ = 10V, SGND = PGND = SW = SYNC = 0V, unless otherwise noted. CONDITIONS CLOAD = 3300pF CLOAD = 3300pF 10% to 90% or 90% to 10%, CLOAD = 3300pF ● ● MIN TYP 9.8 0.1 50 350 250 200 150 MAX UNITS V V ns 650 400 ns ns ns ns 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 LT3845 includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed 125°C when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability. Note 3: The LT3845E is guaranteed to meet performance specifications from 0°C to 125°C junction temperature. Specifications over the – 40°C to 125°C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LT3845I is guaranteed over the full –40°C to 125°C operating junction temperature range. Note 4: VIN voltages below the start-up threshold (7.5V) are only supported when the VCC is externally driven above 6.5V. Note 5: Operating range is dictated by MOSFET absolute maximum VGS. Note 6: Supply current specification does not include switch drive currents. Actual supply currents will be higher. Note 7: DC measurement of gate drive output “ON” voltage is typically 8.6V. Internal dynamic bootstrap operation yields typical gate “ON” voltages of 9.8V during standard switching operation. Standard operation gate “ON” voltage is not tested but guaranteed by design. Note 8: The –2V absolute maximum on the SW pin is a transient condition. It is guaranteed by design and not subject to test. TYPICAL PERFORMANCE CHARACTERISTICS Shutdown Threshold (Rising) vs Temperature 1.38 SHUTDOWN THRESHOLD, FALLING (V) SHUTDOWN THRESHOLD, RISING (V) 1.37 1.36 1.35 1.34 1.33 1.32 –50 –25 1.26 1.25 1.24 1.23 1.22 1.21 1.20 –50 –25 VCC (V) Shutdown Threshold (Falling) vs Temperature 8.2 8.1 8.0 7.9 7.8 7.7 7.6 VCC vs Temperature ICC = 20mA 0 25 50 75 TEMPERATURE (°C) 100 125 0 25 50 75 TEMPERATURE (°C) 100 125 7.5 –50 –25 0 25 50 75 TEMPERATURE (°C) 100 125 3845 G01 3845 G02 3845 G03 3845fc 4 LT3845 TYPICAL PERFORMANCE CHARACTERISTICS VCC vs ICC(LOAD) 8.05 TA = 25°C 9 8 8.00 7 VCC (V) 7.95 VCC (V) 6 5 7.90 4 7.85 3 ICC CURRENT LIMIT (mA) VCC vs VIN ICC = 20mA TA = 25°C 225 200 175 150 125 100 75 ICC Current Limit vs Temperature 0 5 10 15 20 25 ICC(LOAD) (mA) 30 35 40 4 5 6 7 8 VIN (V) 9 10 11 12 50 –50 –25 0 25 50 75 TEMPERATURE (°C) 100 125 3845 G04 3845 G05 3845 G06 VCC UVLO Threshold (Rising) vs Temperature 6.5 VCC UVLO THRESHOLD, RISING (V) 25 ICC vs VCC (SHDN = 0V) ERROR AMP TRANSCONDUCTANCE (μS) TA = 25°C 350 345 340 335 330 325 Error Amp Transconductance vs Temperature 6.4 20 6.2 ICC (μA) 6.3 15 10 6.1 5 6.0 –50 –25 0 25 50 75 TEMPERATURE (°C) 100 125 0 0 2 4 6 8 10 12 14 16 18 20 VCC (V) 3845 G08 320 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 125 3845 G07 3845 G09 I(SENSE+ + SENSE–) vs VSENSE(CM) 800 600 I(SENSE+ + SENSE–) (μA) 400 200 0 –200 –400 TA = 25°C OPERATING FREQUENCY (kHz) 308 306 304 302 300 298 296 294 292 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 VSENSE(CM) (V) 3845 G10 Operating Frequency vs Temperature 1.234 1.233 ERROR AMP REFERENCE (V) 1.232 1.231 1.230 1.229 1.228 Error Amp Reference vs Temperature 290 –50 –25 0 25 50 75 TEMPERATURE (°C) 100 125 1.227 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 125 3845 G11 3845 G12 3845fc 5 LT3845 TYPICAL PERFORMANCE CHARACTERISTICS Maximum Current Sense Threshold vs Temperature 106 CURRENT SENSE THRESHOLD (mV) VIN UVLO THRESHOLD, RISING (V) 104 102 100 98 96 94 –50 –25 4.54 4.52 4.50 4.48 4.46 4.44 4.42 4.40 –50 –25 VIN UVLO Threshold (Rising) vs Temperature 3.86 VIN UVLO THRESHOLD, FALLING (V) 50 25 75 0 TEMPERATURE (°C) 100 125 VIN UVLO Threshold (Falling) vs Temperature 3.84 3.82 3.80 3.78 50 25 75 0 TEMPERATURE (°C) 100 125 3.76 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 125 3845 G13 3845 G14 3845 G15 TYPICAL APPLICATION VIN (Pin 1): The VIN pin is the main supply pin and should be decoupled to SGND with a low ESR capacitor (at least 0.1μF) located close to the pin. SHDN (Pin 2): The SHDN pin has a precision IC enable threshold of 1.35V (rising) with 120mV of hysteresis. It is used to implement an undervoltage lockout (UVLO) circuit. See Application Information section for implementing a UVLO function. When the SHDN pin is pulled below a transistor VBE (0.7V), a low current shutdown mode is entered, all internal circuitry is disabled and the VIN supply current is reduced to approximately 9μA. Typical pin input bias current is