LT3690 36V, 4A, 1.5MHz Synchronous Step-Down Switching Regulator with 70µA Quiescent Current DESCRIPTION FEATURES Wide Input Range: – Operation from 3.9V to 36V – Overvoltage Lockout Protects Circuits Through 60V Transients n 4A Maximum Output Current n Integrated 30mΩ N-Channel Synchronous Switch ® n Low Ripple (<15mV P-P) Burst Mode Operation: IQ = 70µA at 12VIN to 3.3VOUT n Programmable Input Undervoltage Lockout n 0.8V Feedback Reference Voltage n Output Voltage: 0.8V to 20V n Programmable and Synchronizable Oscillator (170kHz to 1.5MHz) n Soft-Startup and Output Voltage Tracking n Short-Circuit Robust n Power Good Flag n Small Thermally Enhanced 4mm × 6mm QFN Package n APPLICATIONS Automotive Systems Industrial Supplies n Distributed Supply Regulation n n The LT®3690 is an adjustable frequency monolithic buck switching regulator that accepts input voltages up to 36V. A high efficiency 90mΩ switch is included on the device along with the boost diode and the necessary oscillator, control, and logic circuitry. The internal synchronous power switch of 30mΩ increases efficiency and eliminates the need for an external Schottky catch diode. Current mode topology is used for fast transient response and good loop stability. Shutdown reduces input supply current to less than 1µA. The low ripple Burst Mode maintains high efficiency at low output currents while keeping output ripple below 15mV in typical applications. The LT3690 features robust operation and is easily configurable. Using a resistor divider on the UVLO pin provides a programmable undervoltage lockout. A power good flag signals when VOUT reaches 90% of the programmed output voltage. Protection circuitry senses the current in the power switches to protect the LT3690 against short-circuit conditions. Frequency foldback and thermal shutdown provide additional protection. The LT3690 is available in a 4mm × 6mm QFN package with exposed pads for low thermal resistance. L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. TYPICAL APPLICATION 3.3V Step-Down Converter Efficiency and Power Loss VIN 4.5V TO 36V 100 VOUT = 5V 0.68µF VIN EN 90 BST UVLO SS 22k 680pF 0.47µF SW LT3690 3.3V 4A BIAS VC PG VCCINT FB SYNC RT GND ƒ = 600kHz 80 1.5 VOUT = 5V 70 60 100µF 102k 50 1.0 VOUT = 3.3V 316k 32.4k 2.0 VOUT = 3.3V 0.5 VIN = 12V L = 4.7µH ƒ = 600kHz 0 0.5 1 1.5 2 2.5 3 LOAD CURRENT (A) 3.5 POWER LOSS (W) 3.3µH EFFICIENCY (%) 10µF 2.5 4 0 3690 TA01b 3690 TA01a 3690f 1 LT3690 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) EN, UVLO, VIN Voltage (Note 2).................................60V BST Voltage ..............................................................55V BST Voltage Above SW Voltage ................................30V BIAS, PG Voltage ......................................................30V FB, RT, SS, SYNC, VC, VCCINT Voltage .........................6V SW 1 SW 2 SW 3 26 SW 25 SW 27 SW 24 SW SW 4 23 SW SYNC 5 22 BST GND 6 21 GND RT 7 VC 8 FB 9 18 PG GND 10 17 EN 20 VCCINT GND 16 19 BIAS VIN 15 VIN 14 VIN 13 SS 11 28 GND UVLO 12 Operating Junction Temperature Range (Notes 3 and 4) LT3690E ............................................ –40°C to 125°C LT3690I ............................................. –40°C to 125°C Storage Temperature Range .................. –65°C to 150°C TOP VIEW UFE PACKAGE 26-LEAD (4mm × 6mm) PLASTIC QFN θJA = 40°C/W, θJC = 2.7°C/W EXPOSED PAD (PIN 27) IS SW, MUST BE SOLDERED TO PCB EXPOSED PAD (PIN 28) IS GND, MUST BE SOLDERED TO PCB ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT3690EUFE#PBF LT3690EUFE#TRPBF 3690 26-Lead (4mm × 6mm) Plastic QFN –40°C to 125°C LT3690IUFE#PBF LT3690IUFE#TRPBF 3690 26-Lead (4mm × 6mm) Plastic QFN –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. 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/ ELECTRICAL CHARACTERISTICS l denotes the specifications which apply over the full operating The temperature range, otherwise specifications are at TA = 25°C, VIN = 12V unless otherwise noted (Notes 3, 7). PARAMETER CONDITIONS TYP MAX 3.0 3.9 38.2 40 V 0.1 1 µA 35 60 µA VBIAS = 0V, VFB = 0.85V Not Switching 110 150 µA VEN = 0.2V 0.1 1 µA 70 110 µA –3 –10 µA VIN Fixed Undervoltage Lockout VIN Overvoltage Lockout OVLO VIN Rising Quiescent Current from VIN VEN = 0.2V VBIAS = 3V, VFB = 0.85V Not Switching Quiescent Current from BIAS Pin MIN l VBIAS = 3V, VFB = 0.85V Not Switching VBIAS = 0V, VFB = 0.85V Not Switching l l l 36 UNITS V 3690f 2 LT3690 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C, VIN = 12V unless otherwise noted (Notes 3, 7). PARAMETER CONDITIONS Boost Schottky Diode Drop (VBIAS – VBST) IBIAS = 200mA BST Voltage (Note 5) (VBST – VSW) Minimum BOOST Voltage Above SW, ISW = 4A MIN l TYP MAX UNITS 820 950 mV 1.6 2.3 V BST Pin Current ISW = 4A 70 140 mA BST Pin Leakage VSW = 12V, VBIAS = 0V 0.1 6 µA HS Switch Drop (VIN – VSW) ISW = 4A 370 600 mV HS Switch Current Limit (Note 6) HS Switch Leakage Current 5.5 VSW = 0V HS Minimum Switch Off-Time 6.6 8 A 0.1 2 µA 210 ns LS Switch Off Voltage Drop ISW = 4A 700 850 mV LS Switch On-Resistance ISW = 4A, VCCINT = 5V 30 60 mΩ LS Switch On-Resistance ISW = 4A, VCCINT = 4V 30 90 mΩ 5 6.5 A 0.1 10 µA l LS Switch Current Threshold LS Switch Leakage Current 4 VEN = 0V, VSW = 12V, VBST =12V 95 µA VCCINT Pin Output Voltage VEN = 0V, VSW = 12V, VBST = 12V, TJ = 125°C IVCCINT = 0µA 4.3 4.9 5.3 V VCCINT Pin Output Voltage IVCCINT = –10mA 4.2 4.8 5.3 V EN Input Current VEN = 12V 8 15 µA VEN = 2.5V 2.5 6 µA EN Input Voltage, Enable 1.5 V EN Input Voltage, Disable 0.4 UVLO Threshold Voltage 1.1 V 1.33 V UVLO Pin Current VUVLO = 1.33V –2.0 –3.8 µA UVLO Pin Current VUVLO = 1.1V 0.1 1 µA UVLO Pin Current Hysteresis IUVH – IUVL 1.2 2 2.8 µA Pull-Up Current at SS Pin VSS = 0.8V –1.2 –2 –2.8 µA Tracking Offset (VSS – VFB) VSS = 0.4V –4 7 15 mV 0.4 V SYNC Input Voltage High 0.8 V SYNC Input Voltage Low SYNC Input Resistance to GND 150 SYNC Input Frequency 0.17 Feedback Reference Voltage l FB Pin Bias Current Flows Out of Pin VFB = 800mV FB Voltage Line Regulation 3.6V < VIN < 36V PG Threshold as Percentage of VFB VFB Rising 786 l 88 PG Hysteresis PG Sink Current VPG = 0.3V PG Leakage VPG = 5V l 100 300 800 600 kΩ 1.5 MHz 814 mV –8 –40 nA 0.001 0.01 %/V 90 92 % 12 mV 500 µA 0.1 1 µA Error Amplifier Transconductance 400 µA/V Error Amp Voltage Gain 60 dB 3690f 3 LT3690 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C, VIN = 12V unless otherwise noted (Notes 3, 7). PARAMETER CONDITIONS MIN TYP MAX UNITS VC Source Current –50 µA VC Sink Current 50 µA VC Pin to Switch Current Gain Transconductance 4.6 A/V VC Switching Threshold 0.7 V VC Clamp Voltage 2.0 V Programmable Switching Frequency RT = 10kΩ 1.32 1.5 1.68 MHz RT = 24.9kΩ 660 750 840 kHz RT = 180kΩ 122 138 154 kHz 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: Absolute Maximum Voltage at the EN, UVLO and VIN pins is 60V for non-repetitive 1 second transients, and 40V for continuous operation. Note 3: The LT3690E 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 LT3690I is guaranteed over the full –40°C to 125°C operating junction temperature range. Note 4: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed the maximum operating junction temperature when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability. Note 5: This is the minimum voltage across the boost capacitor needed to guarantee full saturation of the switch. Note 6: Current limit guaranteed by design and/or correlation to static test. Slope compensation reduces current limit at higher duty cycles. Current limit reduced when feedback voltage is below the reference voltage. Note 7: The voltages are referred to GND and currents are assumed positive, when the current flows into the pin. Negative magnitudes are shown as maximum. 3690f 4 LT3690 TYPICAL PERFORMANCE CHARACTERISTICS Efficiency and Power Loss 100 3.0 90 2.5 90 2.5 80 2.0 80 2.0 70 1.5 70 1.5 60 VOUT = 3.3V 1.0 L = 3.3µH ƒ = 500kHz VIN = 12V 0.5 VIN = 24V VIN = 34V 0 3 3.5 4 60 50 40 0 0.5 1 1.5 2 2.5 LOAD CURRENT (A) VOUT = 5V L = 4.7µH 1.0 ƒ = 600kHz VIN = 12V 0.5 VIN = 24V VIN = 34V 0 3 3.5 4 EFFICIENCY (%) 3.0 50 40 0 0.5 1 1.5 2 2.5 LOAD CURRENT (A) 3690 G01 3690 G02 Efficiency and Power Loss 3.0 2.5 80 2.0 70 1.5 60 VOUT = 3.3V 1.0 L = 3.3µH ƒ = 600kHz VIN = 12V 0.5 VIN = 24V VIN = 34V 0 3 3.5 50 40 0 0.5 1 1.5 2 2.5 LOAD CURRENT (A) No Load Supply Current vs VIN VOUT = 3.3V 140 INPUT CURRENT (µA) 90 160 POWER LOSS (W) EFFICIENCY (%) 100 120 100 80 60 40 20 0 0 5 10 15 20 25 INPUT VOLTAGE (V) 7.0 Maximum Load Current vs VIN 100 50 7.0 VOUT = 5V L = 4.7µH ƒ = 600kHz 6.5 150 3690 G04 6.0 TYPICAL 5.5 5.0 MINIMUM 4.5 0 25 50 75 100 125 150 TEMPERATURE (°C) 3690 G05 3.5 VOUT = 3.3V L = 4.7µH ƒ = 600kHz 6.0 TYPICAL 5.5 5.0 MINIMUM 4.5 4.0 4.0 0 –50 –25 Maximum Load Current vs VIN 6.5 LOAD CURRENT (A) VIN = 12V VOUT = 3.3V LOAD CURRENT (A) SUPPLY CURRENT (µA) 200 35 30 3690 G03 No Load Supply Current vs Temperature POWER LOSS (W) 100 POWER LOSS (W) EFFICIENCY (%) Efficiency and Power Loss TA = 25°C, unless otherwise noted. 5 10 15 20 25 INPUT VOLTAGE (V) 30 35 3690 G06 3.5 5 10 15 20 25 INPUT VOLTAGE (V) 30 35 3690 G07 3690f 5 LT3690 TYPICAL PERFORMANCE CHARACTERISTICS Switch Current Limit vs Duty Cycle TA = 25°C, unless otherwise noted. Switch Current Limit vs Temperature 8 0.6 8.0 VFB > 0.75V 6 VFB = 0V 5 7.0 HS SWITCH VOLTAGE DROP (V) 7 HS SWITCH CURRENT LIMIT (A) HS SWITCH CURRENT LIMIT (A) 7.5 DUTY CYCLE = 10% 6.5 6.0 DUTY CYCLE = 90% 5.5 5.0 4.5 4.0 Switch Voltage Drop vs ISW 0.5 0.4 0.3 0.2 0.1 3.5 4 0 20 40 60 DUTY CYCLE (%) 80 3.0 –50 100 –25 0 25 50 75 TEMPERATURE (°C) 3690 G08 1.4 BST DIODE VOLTAGE DROP (V) BST PIN CURRENT (mA) 80 60 40 20 0 0 1 2 3 4 SWITCH CURRENT (A) 5 0.8 0.6 0.4 0.2 0 0.5 1 1.5 2 BST DIODE CURRENT (A) 4 3 2 0 2.5 40 1 10 100 1000 LOAD CURRENT (mA) 10000 3690 G14 0.4 0.6 0.8 1.0 1.2 VOLTAGE DROP (V) INPUT VOLTAGE (V) 30 4.0 3.5 2.5 10 100 1000 LOAD CURRENT (mA) 1.6 LIMITED BY TJ = 125°C 25 20 VOUT = 3.3V L = 3.3µH ƒ = 600kHz 15 10 1 1.4 Maximum VIN for Fixed Frequency VSYNC > 0.8V, TA = 25°C VSYNC > 0.8V, TA = 85°C VSYNC < 0.4V, TA = 25°C VSYNC < 0.4V, TA = 85°C 5 4.0 0.2 35 3.0 4.5 0 3690 G13 VOUT = 3.3V L = 4.7µH ƒ = 600kHz 4.5 INPUT VOLTAGE (V) INPUT VOLTAGE (V) 5 1 5.0 5.0 Catch Diode Voltage Drop (VGND – VSW) vs ISW 3690 G12 VOUT = 5V L = 4.7µH ƒ = 600kHz 6 5 6 Minimum Input Voltage vs Load Current 5.5 2 3 4 SWITCH CURRENT (A) 7 1.0 Minimum Input Voltage vs Load Current 6.0 8 1.2 0 6 Boost Diode Drop (VBIAS – VBST) vs IBST 3690 G11 6.5 1 3690 G09 BST Pin Current vs ISW 100 0 3690 G10 SWITCH CURRENT (A) 120 0 125 100 10000 3690 G15 0 0 1 2 3 SWITCH CURRENT (A) 4 3690 G16 3690f 6 LT3690 TYPICAL PERFORMANCE CHARACTERISTICS 2.0 EN THRESHOLD VOLTAGE (V) EN PIN CURRENT (µA) 6 4 2 0 5 10 15 20 25 EN PIN VOLTAGE (V) 2.5 2.5 8 0 UVLO Threshold Voltage vs Temperature EN Threshold Voltage 30 UVLO THRESHOLD VOLTAGE (V) 10 EN Pin Current vs VEN TA = 25°C, unless otherwise noted. 1.5 1.0 0.5 0 –50 35 –25 0 25 50 75 TEMPERATURE (°C) 100 1.5 1.0 0.5 0 –50 125 –25 0 25 50 75 TEMPERATURE (°C) UVLO Pin Current vs Temperature (VUVLO = 1.33V) 100 125 3690 G19 3690 G18 3690 G17 –1.4 2.0 Power Good Threshold vs Temperature Feedback Voltage vs Temperature 0.82 95 0.81 90 –2.0 –2.2 –2.4 –2.6 PG THRESHOLD (%) –1.8 FB PIN VOLTAGE (V) UVLO PIN CURRENT (µA) –1.6 0.80 0.79 85 80 –2.8 –25 0 25 50 75 TEMPERATURE (°C) 100 125 0.78 –50 –25 0 25 50 75 TEMPERATURE (°C) 100 3690 G20 60 2.5 VC Voltages vs Temperature 800 –20 125 Frequency Foldback vs VFB RT = 32.4k 600 1.5 1.0 SWITCHING THRESHOLD 500 400 300 200 0.5 –40 100 700 2.0 VC PIN VOLTAGE (V) VC PIN CURRENT (µA) 40 0 0 25 50 75 TEMPERATURE (°C) 3690 G22 CURRENT LIMIT CLAMP 20 –25 3690 G21 Error Amp Output Current vs VFB –60 0.6 75 –50 125 FREQUENCY (kHz) –3.0 –50 100 0.7 0.8 0.9 FB PIN VOLTAGE (V) 1.0 3690 G23 0 –50 –25 0 25 50 75 TEMPERATURE (°C) 100 125 3690 G24 0 0 0.1 0.2 0.3 0.4 0.5 0.6 FB PIN VOLTAGE (V) 0.7 0.8 3690 G25 3690f 7 LT3690 TYPICAL PERFORMANCE CHARACTERISTICS Minimum Switch On-Time vs Temperature Switching Frequency vs Temperature MINIMUM SWITCH ON-TIME (ns) RT = 32.4k FREQUENCY (kHz) 630 610 590 570 550 –50 –25 0 25 50 75 TEMPERATURE (°C) 100 –1.4 –1.6 200 160 120 60 40 0 –50 125 –1.8 –2.0 –2.2 –2.4 –2.6 –2.8 –25 0 25 50 75 TEMPERATURE (°C) 100 125 –3.0 –50 VCCINT vs VIN VVCCINT Current Limit –10 4 0 0 1 2 3 4 5 6 7 VIN PIN VOLTAGE (V) 8 9 10 –20 –30 –50 3690 G29 3 2 0 1 2 3 4 VCCINT PIN VOLTAGE (V) 5 0 –50 –25 IL 0.5A/DIV VOUT 10mV/DIV 3690 G32 25 50 75 100 125 150 TEMPERATURE (°C) 3690 G31 Switching Waveforms, Full Frequency Continuous Operation VSW 5V/DIV VSW 5V/DIV IL 0.5A/DIV IL 1A/DIV VOUT 10mV/DIV VOUT 10mV/DIV 1µs/DIV VIN = 12V, ILOAD = 200mA FRONT PAGE APPLICATION 0 3690 G30 Switching Waveforms, Transition from Burst Mode Operation to Full Frequency VSW 5V/DIV 125 1 –40 Switching Waveforms, Burst Mode Operation 5µs/DIV VIN = 12V, ILOAD = 20mA FRONT PAGE APPLICATION VCCINT PIN VOLTAGE (V) 4 VCCINT PIN CURRENT (mA) 5 1 100 VCCINT Pin Voltage 0 2 0 25 50 75 TEMPERATURE (°C) 3690 G28 5 3 –25 3690 G27 3690 G26 VCCINT PIN VOLTAGE (V) Soft-Start Pin Current 240 SOFT-START PIN CURRENT (µA) 650 TA = 25°C, unless otherwise noted. 3690 G33 1µs/DIV 3690 G34 VIN = 12V, ILOAD = 2A FRONT PAGE APPLICATION 3690f 8 LT3690 PIN FUNCTIONS SW (Pins 1-4, 23-26, Exposed Pad Pin 27): The SW pin is the emitter output of the internal highside NPN power switch (HS) and the drain output of the internal lowside power N-channel switch (LS). Connect this pin to the inductor and boost capacitor. This pin is driven up to the VIN voltage by the HS switch during the on-time of the PWM duty cycle. The inductor current drives the SW pin negative during the off-time. The on-resistance of the LS switch and the internal Schottky diode fixes the negative voltage. The exposed pad is connected internally with SW pins 1-4, 23-26 and should be soldered to a large copper area to reduce thermal resistance. SYNC (Pin 5): The SYNC pin is used to synchronize the internal oscillator to an external signal. It is directly logic compatible and can be driven with any signal between 20% and 80% duty cycle. The synchronizing range is from 170kHz to 1.5MHz. See the Synchronization section in the Applications Information section for details. When not used for synchronization, the SYNC pin can be tied to ground to select low ripple Burst Mode operation or tied to the output voltage to select standard PWM mode. RT (Pin 7): Oscillator Resistor Input. Connecting a resistor to ground (Pin 10) from this pin sets the switching frequency. VC (Pin 8): 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. FB (Pin 9): The LT3690 regulates the FB pin to 0.8V. Connect the feedback resistor divider tap to this pin. The adjacent ground pin (Pin 10) is recommended for the resistor divider. SS (Pin 11): The SS pin is used to provide a soft-start or tracking function. The internal 2µA pull-up current ISS in combination with an external capacitor tied to this pin creates a voltage ramp. The output voltage tracks to this voltage. For tracking, tie a resistor divider to this pin from the tracked output. In undervoltage, overvoltage and thermal shutdown, the SS pin pulls low if the output voltage is below the power good threshold to restart the output voltage with soft-start behavior. Leave this pin disconnected if unused. UVLO (Pin 12): Tie a resistor divider between VIN, UVLO, and GND to program an undervoltage lockout threshold. The UVLO pin has an accurate 1.25V threshold. Above the threshold, the part operates normally. Below the threshold, the part drops into a low quiescent current state. See the Undervoltage Lockout section in the Applications Information section for more details. VIN (Pins 13, 14, 15): The VIN pin supplies current to the LT3690’s internal regulator and to the internal power switch. This pin must be locally bypassed. EN (Pin 17): The EN input is used to put the LT3690 in shutdown mode. Pull to GND to shut down the LT3690. Tie to 1.5V or more for normal operation. PG (Pin 18): 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. The PG output is valid when VIN is above 3.9V and EN is high. BIAS (Pin 19): This pin connects to the anode of the internal boost Schottky diode. BIAS also supplies the current to the LT3690’s internal regulator. Tie this pin to the lowest available voltage source above 3V (typically VOUT). VCCINT (Pin 20): VCCINT is an output of the internally generated supply voltage for the synchronous power DMOS transistor driver. An external capacitor CVCC must be connected between this pin and ground (Pin 21) to buffer the internal supply voltage of the LS switch. BST (Pin 22): This pin is used to provide, with the external boost capacitor, a drive voltage higher than the input voltage VIN to the internal bipolar NPN power switch. GND (Exposed Pad Pin 28, Pin 6, Pin 10, Pin 16, Pin 21): Ground. The exposed pad is connected internally to GND Pins 6, 10, 16 and 21, and should be soldered to a large copper area to reduce thermal resistance. 3690f 9 LT3690 BLOCK DIAGRAM VIN 14 CIN 15 16 17 VIN CURRENT SENSE VIN VIN + – 13 GND 19 EN 2.5V REFERENCE 2µA 12 5 7 BST 0.8V 22 0.72V SLEEP VIN MONITOR OVLO TEMPERATURE MONITOR UVLO CBST TSD HS SWITCH UVLO SYNC OSCILLATOR 0.17MHz TO 1.5MHz SYNC – + SLOPE COMP R S FF Q SWITCH CONTROL + – CSS TSD OVLO UVLO R1 9 R2 10 POWER GOOD + – 0.72V VC CLAMP OVLO UVLO TSD L VOUT COUT VCCINT 20 LS SWITCH + – ERROR AMP + gm – SS PG 23, 24, 25, 26 ZERO 2µA 18 SW CVCC + – 0.8V 1, 2 3, 4 REG SOFT-START/TRACKING 2.5V SW VIN BURST MODE DETECT RT RT 11 BIAS 6 GND 21 OVERLOAD VC 8 RC CF CC FB GND 3690 BD 3690f 10 LT3690 OPERATION The LT3690 is a constant frequency, current mode stepdown regulator. An oscillator, with frequency set by RT, enables an RS flip-flop, turning on the internal high side (HS) power switch. An amplifier and comparator monitor the current flowing between the VIN and SW pins, turning the RS flip-flop and HS switch off when this current reaches a level determined by the voltage at VC. While the high side switch is off, the inductor current conducts through the catch diode and the turned on low side (LS) switch until either the next clock pulse of the oscillator starts the next cycle, or the inductor current becomes too low, as indicated by the zero crossing comparator. This prevents the inductor from running reverse current. 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 SS node acts as an auxiliary input to the error amplifier. The voltage at FB will servo to the SS voltage until SS goes above 0.8V. Soft-start is implemented by generating a voltage ramp at the SS pin using an external capacitor CSS which is charged by an internal constant current. Alternatively, connecting the SS pin to a resistive divider between the voltage to be tracked and ground provides a tracking function. An internal regulator provides power to the control circuitry. The bias regulator normally draws power from the VIN pin, but if the BIAS pin is connected to an external voltage higher than 3V, bias power will be drawn from the external source (typically the regulated output voltage). This improves efficiency. The EN pin is used to place the LT3690 in shutdown, disconnecting the output and reducing the input current to less than 1µA. A comparator monitors the voltage at the UVLO input. A external resistive divider connected to VIN programs the wake up threshold and hysteresis. If unused, connect the input to VIN or above 1.5V. The HS switch driver operates from either the input or from the BOOST pin. An external capacitor is used to generate a voltage at the BOOST pin that is higher than the input supply. This allows the driver to fully saturate the internal bipolar NPN power switch for efficient operation. The synchronously driven N-channel transistor (LS switch) in parallel with the catch diode reduces the overall solution size and improves efficiency. Internal overload comparator circuitry monitors the current through the LS switch and delays the generation of new switch pulses if this current is too high (above 5A nominal). This mechanism also protects the part during short-circuit and overload conditions by keeping the current through the inductor under control. A short-circuit protected regulator at VCCINT supplies the LS driver. The LS switch only operates at VCCINT voltages greater than 3.8V. To further optimize efficiency, the LT3690 automatically switches to Burst Mode operation in light load situations. Between bursts, all circuitry associated with controlling the output switch is shut down, reducing the input supply current to 70µA in a typical application. Pulling the SYNC pin above 0.8V prevents Burst Mode operation. The positive edge of an external clock signal at the SYNC pin synchronizes the internal oscillator and therefore switching. The oscillator reduces the LT3690’s operating frequency when the voltage at the FB pin is low. This frequency foldback helps to control the output current during start-up and overload conditions. The LT3690 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 LT3690 is enabled and VIN is above 3.9V. The LT3690 has an overvoltage protection feature which disables switching action when VIN goes above 38V (typical) during transients. When switching is disabled, the LT3690 can safely sustain transient input voltages up to 60V. 3690f 11 LT3690 APPLICATIONS INFORMATION FB Resistor Network Operating Frequency Trade-Offs The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the resistor values according to: Selection of the operating frequency is a trade-off 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: ⎛V ⎞ R1 = R2 ⎜ OUT − 1⎟ ⎝ 0.8V ⎠ Reference designators refer to the Block Diagram. 1% resistors are recommended to maintain output voltage accuracy. ƒSW(MAX) = VOUT + VLS tON(MIN) • ( VIN − VSW + VLS ) Setting the Switching Frequency The LT3690 uses a constant frequency PWM architecture that can be programmed to switch from 150kHz to 1.5MHz by using a resistor tied from the RT pin to ground. Table 1 shows the necessary RT value for a desired switching frequency. where VIN is the typical input voltage, VOUT is the output voltage, VLS is the LS switch drop (0.12V at maximum load) and VSW is the internal switch drop (0.37V at maximum load). This equation shows that slower switching frequency is necessary to accommodate high VIN /VOUT ratio. Also, as shown in the Input Voltage Range section, lower frequency allows a lower dropout voltage. Input voltage range depends on the switching frequency because the LT3690 switch has finite minimum on and off times. An internal timer forces the switch to be off for at least tOFF(MIN) per cycle; this timer has a maximum value of 210ns over temperature. On the other hand, delays associated with turning off the power switch dictate the minimum on-time tON(MIN) before the switch can be turned off; tON(MIN) has a maximum value of 210ns over temperature. The minimum and maximum duty cycles that can be achieved taking minimum on and off times into account are: Table 1. Switching Frequency vs RT Value SWITCHING FREQUENCY (MHz) RT VALUE (kΩ) 0.15 164 0.2 117 0.3 72.9 0.4 52.2 0.5 40.2 0.6 32.4 0.7 26.8 0.8 22.7 0.9 19.6 1.0 17.0 1.1 15.0 1.2 13.3 1.3 11.8 1.4 10.6 1.5 9.59 DCMIN = ƒSW • tON(MIN) DCMAX = 1 – ƒSW • tOFF(MIN) where ƒSW is the switching frequency, the tON(MIN) is the minimum switch on-time (210ns), and the tOFF(MIN) is the minimum switch off-time (210ns). 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 Input Voltage Range section) and keep the inductor and capacitor values small. 3690f 12 LT3690 APPLICATIONS INFORMATION Input Voltage Range The minimum input voltage is determined by either the LT3690’s minimum operating voltage of 3.9V (VBIAS > 3V) or by its maximum duty cycle (see equation in the Operating Frequency Trade-offs section). The minimum input voltage due to duty cycle limitation is: VIN(MIN) = VOUT + VLS − VLS + VSW 1− ƒSW • tOFF(MIN) where VIN(MIN) is the minimum input voltage, and tOFF(MIN) is the minimum switch off-time (210ns). 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. The maximum input voltage for LT3690 applications depends on switching frequency, the absolute maximum ratings of the VIN and BST pins, and the operating mode. The LT3690 can operate from continuous input voltages up to 36V. Input voltage transients of up to 60V are also safely withstood. However, note that while VIN > VOVLO (typically 38V), the LT3690 will stop switching, allowing the output to fall out of regulation. For a given application where the switching frequency and the output voltage are already fixed, the maximum input voltage that guarantees optimum output voltage ripple for that application can be found by applying the following expression: V +V VIN(MAX) = OUT LS − VLS + VSW ƒSW • tON(MIN) where VIN(MAX) is the maximum operating input voltage, VOUT is the output voltage, VLS is the LS switch drop (0.12V at maximum load), VSW is the internal switch drop (0.37V at maximum load), fSW is the switching frequency (set by RT), and tON(MIN) is the minimum switch on-time (210ns). Note that a higher switching frequency will reduce the maximum operating input voltage. Conversely, a lower switching frequency will be necessary to achieve optimum operation at high input voltages. Special attention must be paid when the output is in startup, short-circuit, or other overload conditions. In these cases, the LT3690 tries to bring the output in regulation by driving current into the output load. During these events, the inductor peak current might easily reach and even exceed the maximum current limit of the LT3690, especially in those cases where the switch already operates at minimum on-time. The circuitry monitoring the current through the LS switch prevents the HS switch from turning on again if the inductor valley current is above IPSDLIM (5A nominal). In these cases, the inductor peak current is therefore the maximum current limit of the LT3690 plus the additional current overshoot during the turn-off delay due to minimum on-time: IL(PEAK) = 8A + VIN(MAX) − VOUTOL L • tON(MIN) where IL(PEAK) is the peak inductor current, VIN(MAX) is the maximum expected input voltage, L is the inductor value, tON(MIN) is the minimum on-time and VOUTOL is the output voltage under the overload condition. The part is robust enough to survive prolonged operation under these conditions as long as the peak inductor current does not exceed 9A. Inductor current saturation and excessive junction temperature may further limit performance. If the output is in regulation and no short-circuit, startup, or overload events are expected, then input voltage transients of up to VOVLO are acceptable regardless of the switching frequency. In this case, the LT3690 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. 3690f 13 LT3690 APPLICATIONS INFORMATION Inductor Selection and Maximum Output Current A good first choice for the inductor value is: ( ) L = VOUT + VLS • 0.67MHz ƒSW where VLS is the voltage drop of the low side switch (0.12V), ƒSW is in MHz, and L is in μH. The inductor’s RMS current rating must be greater than the maximum load current and its saturation current should be at least 30% higher. For highest efficiency, the series resistance (DCR) should be less than 0.03Ω. Table 2 lists several vendors and types that are suitable. Table 2. Inductor Vendors VENDOR URL PART SERIES Murata www.murata.com LQH6P TDK www.tdk.com CLF10040T SLF10165T Toko www.toko.com DEM8045C FDVE1040 Coilcraft www.coilcraft.com MSS1048 Sumida www.sumida.com CDRH8D43 CDRH105R Vishay www.vishay.com IHLP-2525EZ The optimum inductor for a given application may differ from the one indicated by this simple design guide. A larger value inductor provides a higher maximum load current, and reduces the output voltage ripple. If your load is lower than the maximum load current, then you can relax 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. Be aware that if the inductance differs from the simple rule above, then the maximum load current will depend on input voltage. In addition, low inductance may result in discontinuous mode operation, which further reduces maximum load current. For details of maximum output current and discontinuous mode operation, see Application Note 44. Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5), a minimum inductance is required to avoid sub-harmonic oscillations: 0.42MHz LMIN = ( VOUT + VLS ) • ƒSW where VLS is the voltage drop of the low side switch (0.12V at maximum load), ƒSW is in MHz, and LMIN is in μH. The current in the inductor is a triangle wave with an average value equal to the load current. The peak switch current is equal to the output current plus half the peak-to-peak inductor ripple current. The LT3690 limits its switch current in order to protect itself and the system from overload faults. Therefore, the maximum output current that the LT3690 will deliver depends on the switch current limit, the inductor value, and the input and output voltages. When the switch is off, the potential across the inductor is the output voltage plus the low side switch drop. This gives the peak-to-peak ripple current in the inductor: ΔIL = (1− DC) ( VOUT + VLS ) (L • ƒSW ) where ƒSW is the switching frequency of the LT3690 and L is the value of the inductor. The peak inductor and switch current is: ΔIL ISW(PK) = IL(PK) = IOUT + 2 To maintain output regulation, this peak current must be less than the LT3690’s switch current limit ILIM. See the Typical Performance graphs for the change in current limit vs duty cycle. Choosing an inductor value so that the ripple current is small will allow a maximum output current near the switch current limit. 3690f 14 LT3690 APPLICATIONS INFORMATION One approach to choosing the inductor is to start with the simple rule given above, look at the available inductors, and choose one to meet cost or space goals. Then use these equations to check that the LT3690 will be able to deliver the required output current. Note again that these equations assume that the inductor current is continuous. Discontinuous operation occurs when IOUT is less than ΔIL/2. Input Capacitor Bypass the input of the LT3690 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 10µF ceramic capacitor is adequate to bypass the LT3690, and easily handles 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 lower 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 LT3690 and to force this very high frequency switching current into a tight local loop, minimizing EMI. A 10µF capacitor is capable of this task, but only if it is placed close to the LT3690 (see the PCB Layout section). A second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the LT3690. A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT3690 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3690’s maximum voltage rating. See Application Note 88 for more details. Output Capacitor and Output Ripple The output capacitor has two essential functions. Along with the inductor, it filters the square wave generated by the LT3690 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 LT3690’s control loop. Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance. A good starting value is: 150 COUT = VOUT • ƒSW where ƒSW is in MHz, and COUT is the recommended output capacitance in µF. Use X5R or X7R types, which will provide low output ripple and good transient response. Using a high value capacitor on the output can improve transient performance, but a phase lead capacitor across the feedback resistor R1 may be required to get the full benefit (see the Frequency Compensation section). High performance electrolytic capacitors can be used for the output capacitor. If using an electrolytic capacitor, choose one intended for use in switching regulators, and with a specified ESR of 0.03Ω 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 3 lists several capacitor vendors. Table 3. Capacitor Vendors VENDOR PART SERIES COMMENTS Panasonic Ceramic, Polymer, Tantalum EEF Series Kemet Ceramic, Tantalum T494, T495 Sanyo Ceramic, Polymer, Tantalum POSCAP Murata Ceramic AVX Ceramic, Tantalum Taiyo Yuden Ceramic TPS Series Ceramic Capacitors Ceramic capacitors are small, robust and have very low ESR. However, ceramic capacitors can sometimes cause problems when used with the LT3690 due to their piezoelectric nature. When in Burst Mode operation, the LT3690’s switching frequency depends on the load current, and at very light loads the LT3690 can excite the ceramic capacitor at audio frequencies, generating audible noise. 3690f 15 LT3690 APPLICATIONS INFORMATION Since the LT3690 operates at a lower current limit during Burst Mode operation, the noise is typically very quiet. If this is unacceptable, use a high performance tantalum or electrolytic capacitor at the output. Frequency Compensation The LT3690 uses current mode control to regulate the output. This simplifies loop compensation. In particular, the LT3690 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 1. Generally a capacitor (CC) and a resistor (RC) in series to ground are used. In addition, there may be a lower value capacitor in parallel. This capacitor (CF) is not part of the loop compensation but is used to filter noise at the switching frequency, and is required only if a phase-lead capacitor is used or if the output capacitor has high ESR. Loop compensation determines the stability and transient performance. The best values for the compensation network 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 1 shows an equivalent circuit for the LT3690 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 either from the output capacitor ESR or from a resistor RC in series with CC. This simple model works well as long as the value of the inductor is not too high and the loop crossover frequency is much lower than the switching frequency. A phase lead capacitor (CPL) across the feedback divider may improve the transient response. LT3690 CURRENT MODE POWER STAGE gm = 4.6S SW FB R1 CPL – gm = 400µS 0.8V + 3M VC CF GND RC CC VOUT 100mV/DIV OUTPUT ESR + C1 C1 20µs/DIV CERAMIC POLYMER OR TANTALUM R2 OR ELECTROLITIC IL 2A/DIV 3690 F02 VIN = 12V, ILOAD STEPPED BETWEEN 0.6A AND 3.5A FRONT PAGE APPLICATION Figure 2. Transient Load Response 3690 F01 Figure 1. Model for Loop Response 3690f 16 LT3690 APPLICATIONS INFORMATION Low-Ripple Burst Mode and Pulse-Skipping Mode The LT3690 is capable of operating in either low ripple Burst Mode operation or pulse-skipping mode, which is selected using the SYNC pin. See the Synchronization and Mode section for details. To enhance efficiency at light loads, the LT3690 can be operated in low ripple Burst Mode operation that keeps the output capacitor charged to the proper voltage while minimizing the input quiescent current. During Burst Mode operation, the LT3690 delivers single cycle bursts of current to the output capacitor followed by sleep periods where the output capacitor is delivers output power to the load. Because the LT3690 delivers power to the output with single, low current pulses, the output ripple stays below 15mV for a typical application. In addition, VIN and BIAS quiescent currents are reduced to 35µA and 70µA (typical), respectively, during the sleep time. As the load current decreases towards a no-load condition, the percentage of time that the LT3690 operates in sleep mode increases and the average input current is greatly reduced, resulting in high efficiency even at very low loads (see Figure 3). At higher output loads (above about 385mA at VIN = 12V for the front page application) the LT3690 will run at the frequency programmed by the RT resistor, and operate in standard PWM mode. The transition between PWM and low ripple Burst Mode operation is seamless, and does not disturb the output voltage. If low quiescent current is not required, the LT3690 can operate in pulse-skipping mode. The benefit of this mode is that the LT3690 will enter full frequency standard PWM operation at a lower output load current than when in Burst Mode operation. The front page application circuit will switch at full frequency at output loads higher than about 64mA at VIN = 12V. Low Side Switch Considerations The operation of the internal low side switch is optimized for reliable, high efficiency operation. The low side switch is connected in parallel with a catch diode. When the top side switch turns off, the inductor current pulls the SW pin low, and forward biases the internal catch diode. In order to prevent shoot through currents, the internal low side switch only turns on after detecting the SW pin going low. Once the low side switch turns on, the voltage drop between SW and GND is very small, minimizing power loss and improving efficiency. At the end of the switching cycle, the low side switch turns off, and after a delay, the top side switch can turn on again. The switching sequence is shown in Figure 4. The overload comparator monitors the current flowing through the low side switch and helps protect the circuit. This comparator delays switching if the low side switch current goes higher than 5A (typical) during a fault condition such as a shorted output with high input voltage. VIN = 12V VOUT = 3.3V L = 3.3µH VSW 5V/DIV VSW 2V/DIV IL 0.5A/DIV VOUT 10mV/DIV 0V 5µs/DIV 3690 F03 200ns/DIV 3690 F04 VIN = 12V: ILOAD = 20mA FRONT PAGE APPLICATION Figure 3. Burst Mode Operation Figure 4. Switching Sequence of High Side, Catch Diode and Low Side Switch 3690f 17 LT3690 APPLICATIONS INFORMATION The switching will only resume once the low side switch current has fallen below the 5A limit. This way, the comparator regulates the valley current of the inductor to 5A during short-circuit. With properly chosen external components, this will ensure that the part will survive a short-circuit event. VCCINT Considerations The linear voltage regulator requires a capacitor of 0.47µF to deliver the peak current for the gate driver of the low side N-channel transistor. The output voltage is monitored by a comparator. To ensure proper operation, the low side driver only turns on if VCCINT is above 3.8V (typ). BST and BIAS Pin Considerations Capacitor CBST and the internal boost Schottky diode (see the Block Diagram) are used to generate boost voltages that are higher than the input voltage. In most cases a 0.68µF capacitor will work well. Figure 5 shows three ways to arrange the boost circuit. The BST pin must be more than 2.3V above the SW pin for best efficiency. For outputs of 3V and above, the standard circuit (Figure 5a) is best. For outputs between 2.8V and 3V, use a 1µF boost capacitor. A 2.5V output presents a special case because it is marginally adequate to support the boosted drive stage while using the internal boost diode. For reliable BST pin operation with 2.5V outputs, use a good external Schottky diode (such as the ON Semi MBR0540), and a 1µF boost capacitor (see Figure 5b). For lower output voltages, the boost diode can be tied to the input (Figure 5c), or to another supply greater than 2.8V. The circuit in Figure 5a is more efficient because the BST pin current and BIAS pin quiescent current comes from a lower voltage source. However, the full benefit of the BIAS pin is not realized unless it is at least 3V. Ensure that the maximum voltage ratings of the BST and BIAS pins are not exceeded. The minimum operating voltage of an LT3690 application is limited by the minimum input voltage (3.9V) and by the maximum duty cycle as outlined in the Input Voltage Range section. For proper start-up, the minimum input voltage VIN BIAS VIN LT3690 CBST VOUT BST SW GND 3690 F05a (5a) VOUT > 2.8 VIN VIN BIAS VIN LT3690 D2 BIAS VIN CBST VOUT BST LT3690 SW CBST VOUT BST SW GND GND 3690 F05b (5b) 2.5V < VOUT < 2.8V 3690 F05c (5c) VOUT < 2.5V, VIN(MAX) = 27V Figure 5. Three Circuits for Generating the Boost Voltage 3690f 18 LT3690 APPLICATIONS INFORMATION 6.0 TO START 5.0 4.5 4.0 VOUT = 5V L = 4.7µH ƒ = 600kHz TO START 7.0 INPUT VOLTAGE (V) INPUT VOLTAGE (V) 5.5 7.5 VOUT = 3.3V L = 4.7µH ƒ = 600kHz 6.5 6.0 5.5 TO RUN TO RUN 3.5 3.0 5.0 1 10 100 1000 LOAD CURRENT (mA) 10000 4.5 1 10 100 1000 LOAD CURRENT (mA) 3690 F06a 10000 3690 F06b Figure 6. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit is also limited by the boost circuit. If the input voltage is ramped slowly, or the LT3690 is turned on with its EN pin when the output is already in regulation, then the boost capacitor may not be fully charged. Because the boost capacitor charges with the energy stored in the inductor, the circuit relies on some minimum load current to get the boost circuit running properly. This minimum load depends on the 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 6 shows a plot of minimum load to start and to run as a function of input voltage. In many cases the discharged output capacitor will present a load to the switcher, which will allow it to start. 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 BST 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 LT3690, requiring a higher input voltage to maintain regulation. Soft-Start The SS (soft-start) pin provides a soft-start function. If a capacitor CSS is tied from the SS pin to ground, then the internal pull-up current will generate a voltage ramp on this pin. A good value for the soft-start capacitor is COUT /10000, where COUT is the value of the output capacitor. The soft-start function limits peak input current to the circuit during start-up. The output of the LT3690 regulates to the lowest voltage present at either the SS pin or an internal 0.8V reference. A capacitor from the SS pin to ground is charged by an internal 2μA current source resulting in a linear output ramp from 0V to the regulated output voltage. The ramp duration is given by: C • 0.8V t RAMP = SS 2µA At power-up, an internal open-collector output discharges the SS pin. The SS pin can be left floating if the soft-start feature is not used. The internal current sources will charge this pin to ~2V as shown in Figure 7. VEN 2V/DIV VSS 1V/DIV VOUT 2V/DIV IL 2A/DIV CSS = 0.22µF 50ms/DIV 3690 F07 Figure 7. Soft-Start Ramp 3690f 19 LT3690 APPLICATIONS INFORMATION LT3690 SS OUT1 5V VEN 2V/DIV 0.1µF VOUT1 2V/DIV LT3690 SS OUT2 3.3V 0.047µF VOUT2 2V/DIV 3690 F08a 5ms/DIV 3690 F08b 5ms/DIV 3690 F08d (8a) Independent Start-Up LT3690 SS OUT1 5V 0.22µF VOUT1 2V/DIV LT3690 SS OUT2 VEN 2V/DIV 3.3V VOUT2 2V/DIV 3690 F08c (8b) Ratiometric Start-Up Figure 8. Output Tracking and Sequencing 3690f 20 LT3690 APPLICATIONS INFORMATION LT3690 SS 5V OUT1 VEN 2V/DIV 0.1µF R1 28.7k VOUT1 2V/DIV LT3690 R2 10k SS OUT2 VOUT2 2V/DIV 3.3V 5ms/DIV 3690 F09b 5ms/DIV 3690 F09d 3690 F09a (9a) Coincident Start-Up SS LT3690 OUT1 5V VEN 2V/DIV PG1 0.1µF VOUT1 2V/DIV LT3690 SS OUT2 VOUT2 2V/DIV 3.3V 0.047µF 3690 F09b (9b) Output Sequencing Figure 9. Output Tracking and Sequencing 3690f 21 LT3690 APPLICATIONS INFORMATION Output Tracking and Sequencing Output tracking and sequencing between voltage regulators can be implemented using the LT3690’s SS and PG pins. Figures 8 and 9 show several configurations for output tracking and sequencing of the LT3690 and an additional regulator. Independent soft-start for each channel is shown in Figure 8a. The output ramp time for each output is set by the soft-start capacitor as described in the soft-start section. Ratiometric tracking is achieved in Figure 8b by connecting SS pins of two regulators together. In this configuration, the SS pin current is set by the sum of the SS pin currents of the two regulators, which must be taken into account when calculating the output rise time. By connecting a feedback network from OUT1 to the SS pin with the same ratio that set the OUT2 voltage, absolute tracking shown in Figure 9a is implemented. A small OUT2 voltage offset will be present due to the SS pin’s 2µA source current. This offset can be corrected by slightly reducing the value of R2. Figure 9b illustrates output sequencing. When VOUT1 is within 10% of its regulated voltage, PG releases the SS soft-start pin, allowing VOUT2 to soft-start. Synchronization To select low-ripple Burst Mode operation, tie the SYNC pin below 0.4V (this can be ground or a logic output). Synchronize the LT3690 oscillator to an external frequency by connecting a square wave (with positive and negative pulse width > 100ns) to the SYNC pin. The square wave amplitude should have valleys that are below 0.4V and peaks that are above 1V (up to 6V). and safe operation, the LT3690 will synchronize when the output voltage is above 90% of its regulated voltage. It is therefore necessary to choose a large enough inductor value to supply the required output current at the frequency set by the RT resistor (see the Inductor Selection section). It is also important to note that slope compensation is set by the RT value. When the synchronization frequency is much higher than the one set by RT, the slope compensation is significantly reduced, which may require a larger inductor value to prevent sub-harmonic oscillation. For duty cycles greater than 50% (VOUT /VIN > 0.5), a minimum inductance is required to avoid sub-harmonic oscillations: 0.42MHz LMIN = ( VOUT + VLS ) • ƒSW where VLS is the voltage drop of the low side switch (0.12V at maximum load), ƒSW is in MHz, and LMIN is in μH. For ƒSW in the above calculation, use the frequency programmed by RT, not the synchronization frequency. Undervoltage Lockout Figure 10 shows how to add undervoltage lockout (UVLO) to the LT3690. Typically, UVLO is used in situations where the input supply is current limited, or has a relatively high source resistance. A switching regulator draws constant power from the source, so source current increases as source voltage drops. This looks like a negative resistance load to the source and can cause the source to current limit or latch low under low source voltage conditions. VIN VIN 2µA The LT3690 will not enter Burst Mode operation at low output loads while synchronized to an external clock, but instead will skip pulses to maintain regulation. The LT3690 may be synchronized over a 170kHz to 1.5MHz range. The RT resistor should be chosen to set the LT3690 switching frequency 20% below the lowest synchronization input. For example, if the synchronization signal will be 350kHz and higher, choose RT for 280kHz. To assure reliable LT3690 R3 UVLO C4 R4 1.25V – + SLEEP 3690 F10 Figure 10. Undervoltage Lockout 3690f 22 LT3690 APPLICATIONS INFORMATION The UVLO circuitry prevents the regulator from operating at source voltages where the problems might occur. An internal comparator will force the part into shutdown below the fixed VIN UVLO threshold of 3.0V. This feature can be used to prevent excessive discharge of battery-operated systems. If an adjustable UVLO threshold is required, the UVLO pin can be used. The threshold voltage of the UVLO pin comparator is 1.25V. Current hysteresis is added above the UVLO threshold. This can be used to set voltage hysteresis of the UVLO using the following equations: V − VL R3 = H 2µA 1 VH R4 = R3 • 1.25V −1 Example: switching should not start until the input is above 4.4V, and is to stop if the input falls below 4V. R3 = 4.4V − 4.0V 2µA R4 = 200kΩ • = 200kΩ 1 4.4V 1.25V = 79.4kΩ −1 Keep the connection from the resistor to the UVLO pin short and minimize the interplane or surface capacitance to switching nodes. If high resistor values are used, the UVLO pin should be bypassed with a 1nF capacitor to prevent coupling problems from the switch node. Shorted and Reversed Input Protection If the inductor is chosen to prevent excessive saturation, the LT3690 will tolerate a shorted output. When operating in short-circuit condition, the LT3690 will reduce its frequency until the valley current is at a typical value of 5A (see Figure 11). There is another situation to consider in systems where the output is held high when the input to the LT3690 is absent. This may occur in battery charging applications or in battery backup systems where a battery or some other supply is diode ORed with the LT3690’s output. If the VIN pin is allowed to float and the EN pin is held high (either by a logic signal or because it is tied to VIN), then the LT3690’s internal circuitry will pull its quiescent current through its SW pin. This is acceptable if the system can tolerate a few mA in this state. If the EN pin is grounded, 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 LT3690 can pull large currents from the output through the SW pin and the VIN pin. Figure 12 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input. D4 MBRS540 VIN VIN VSW 10V/DIV BIAS UVLO 0V EN IL1 2A/DIV VOUT BST LT3690 GND SW FB BACKUP 3690 F12 2µs/DIV 3690 F11 Figure 11. The LT3690 Reduces its Frequency to Below 250kHz to Protect Against Shorted Output with 36V Input Figure 12. 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 LT3690 Runs Only When the Input Is Present 3690f 23 LT3690 APPLICATIONS INFORMATION PCB Layout High Temperature Considerations For proper operation and minimum EMI, care must be taken during printed circuit board layout. Figure 13 shows the recommended component placement with trace, ground plane and via locations. Note that large, switched currents flow in the LT3690’s VIN, SW and GND pins and the input capacitor (CIN). 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 BST nodes should be small as possible. If synchronizing the part externally using the SYNC pin, avoid routing this signal near sensitive nodes, especially VC and FB. Finally, keep the FB and VC nodes small so that the ground traces will shield them from the SW and BST nodes. The exposed GND 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 LT3690 to additional ground planes within the circuit board and on the bottom side. In addition, the exposed SW pad on the bottom of the package must be soldered to the PCB to act as a heat sink for the low side switch. Add thermal vias under the SW pad and to the bottom side. The PCB must provide heat sinking to keep the LT3690 cool. The GND exposed pad on the bottom of the package must be soldered to a ground plane and the SW exposed pad must be soldered to a SW plane. Tie the ground plane and SW plane to large copper layers below with thermal vias; these layers will spread the heat dissipated by the LT3690. Placing additional vias can reduce thermal resistance further. With these steps, the thermal resistance from die (or junction) to ambient can be reduced to θJA = 40°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 LT3690, 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 maximum load current should be derated as the ambient temperature approaches 125°C. Power dissipation within the LT3690 can be estimated by calculating the total power loss from an efficiency measurement. The die temperature is calculated by multiplying the LT3690 power dissipation by the thermal resistance from junctionto-ambient. Thermal resistance depends on the layout of the circuit board, but values from 20°C/W to 60°C/W are typical. Die temperature rise was measured on a 4-layer, 6cm • 6cm circuit board in still air at a load current of 4A (ƒSW = 600kHz). For a 12V input to 3.3V output the die temperature elevation above ambient was 43°C; for 24VIN to 3.3VOUT the rise was 52°C; for 12VIN to 5VOUT the rise was 55°C and for 24VIN to 5VOUT the rise was 62°C. CC CF R1 R2 L RC RT Other Linear Technology Publications CSS VOUT VIN CIN COUT CBST GND CVCC Application Notes 19, 35 and 44 contain 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 318 shows how to generate a bipolar output supply using a buck regulator. Figure 13. Top Layer PCB Layout and Component Placement in the LT3690 Demonstration Board 3690f 24 LT3690 TYPICAL APPLICATIONS 5V Step-Down Converter VIN 6.3V TO 36V VIN 10µF BIAS UVLO ON OFF PG EN LT3690 SS VC 1nF 0.47µF L 4.7µH SW 536k FB VCCINT 15k 0.68µF BST RT SYNC 47µF GND 680pF VOUT 5V 4A 32.4k 102k ƒ = 600kHz 3690 TA02 3.3V Step-Down Converter VIN 4.5V TO 36V VIN 10µF BIAS UVLO ON OFF PG EN LT3690 SS VC 1nF 0.47µF BST L 3.3µH SW 316k FB VCCINT 22k 0.68µF RT SYNC GND 680pF VOUT 3.3V 4A 100µF 32.4k 102k ƒ = 600kHz 3690 TA03 (FIXED FREQUENCY AT VIN < 26V) 2.5V Step-Down Converter VIN 3.9V TO 36V VIN 10µF BIAS UVLO ON OFF EN SS BST 1nF 160k FB RT SYNC GND 1nF L 3.3µH SW VCCINT 0.47µF 1µF LT3690 VC 15k MBR0540 PG VOUT 2.5V 4A 100µF 32.4k 75k ƒ = 600kHz 3690 TA04 (FIXED FREQUENCY AT VIN < 21V) 3690f 25 LT3690 TYPICAL APPLICATIONS 1.8V Step-Down Converter AUXILIARY SUPPLY 3.3V OR 5V 1µF VIN 3.9V TO 36V 100k VIN 10µF BIAS UVLO ON OFF PG EN LT3690 SS VC 1nF 0.47µF 0.68µF L 4.7µH VOUT 1.8V 4A BST SW 18.7k FB VCCINT 16k POWER GOOD RT SYNC 100µF GND 1nF 40.2k 15k ƒ = 500kHz 3690 TA05 (FIXED FREQUENCY AT VIN < 18.5V) 1.2V Step-Down Converter AUXILIARY SUPPLY 3.3V OR 5V 1µF VIN 3.9V TO 36V 100k 2× 10µF VIN BIAS UVLO ON OFF PG EN LT3690 SS VC 14k 0.47µF 0.68µF L 8.2µH BST VOUT 1.2V 4A SW 23.2k FB VCCINT 10nF POWER GOOD RT SYNC GND 2.2nF 100µF 140k + 46.4k 680µF LOW ESR ƒ = 170kHz 3690 TA06 5V Step-Down Converter with Undervoltage Lockout VIN 14V TO 36V 10µF 200k VIN BIAS UVLO 21k ON OFF PG EN SS LT3690 VC 1nF 15k 536k FB RT SYNC GND 1nF SLEEP: VIN < 12.3V WAKE UP: VIN > 13.4V L 4.7µH SW VCCINT 0.47µF 0.68µF BST VOUT 5V 4A 100µF 40.2k 102k ƒ = 500kHz 3690 TA07 3690f 26 LT3690 PACKAGE DESCRIPTION UFE Package UFEMA Package 26-Lead Plastic QFN (4mm × 6mm) 26-Lead PlasticLTC QFN (4mm × 6mm)Rev A) (Reference DWG # 05-08-1770 (Reference LTC DWG # 05-08-1770 Rev A) 0.70 ±0.05 2.64 ± 0.05 4.50 ± 0.05 2.64 ± 0.05 2.31 ± 0.05 3.10 ± 0.05 0.41 ± 0.05 2.18 ± 0.05 2.50 REF 2.36 ± 0.05 PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC 2.55 ± 0.05 3.25 ± 0.05 4.50 REF RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 4.00 ± 0.10 0.75 ± 0.05 PIN 1 TOP MARK (NOTE 6) PIN 1 NOTCH R = 0.30 OR 0.35 × 45° CHAMFER R = 0.10 TYP 26 25 1 2.64 ± 0.10 2.18 ± 0.10 0.41 ± 0.10 4.50 REF 6.00 ± 0.10 2 R = 0.125 TYP 0.50 BSC 2.36 ± 0.10 2.31 ± 0.10 2.64 ± 0.10 0.25 ± 0.05 0.40 ± 0.10 0.200 REF 0.00 – 0.05 2.50 REF (UFE26MA) QFN 0608 REV A BOTTOM VIEW—EXPOSED PAD NOTE: 1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.20mm 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 3690f 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. 27 LT3690 TYPICAL APPLICATION 3.3V Step-Down Converter VIN 4.5V TO 36V BIAS VIN 10µF UVLO ON OFF PG EN SS LT3690 VC 1nF 0.47µF 316k FB RT SYNC GND 680pF L 3.3µH SW VCCINT 22k 0.68µF BST VOUT 3.3V 4A 100µF 40.2k 102k ƒ = 500kHz 3690 TA08 (FIXED FREQUENCY AT VIN < 31V) RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT3680 36V, 3.5A, 2.4MHz High Efficiency MicroPower Step-Down DC/DC Converter VIN(MIN) = 3.6V, VIN(MAX) = 36V, VOUT(MIN) = 0.8V, IQ = 75µA, ISD <1µA, 3mm × 3mm DFN-10 and MSOP-10E Packages LT3972 Transients to 60V, 3.5A, 2.4MHz High Efficiency Step-Down DC/DC Converter VIN(MIN) = 3.6V, VIN(MAX) = 33V, VOUT(MIN) = 0.8V, IQ = 75µA, ISD <1µA, 3mm × 3mm DFN-10 and MSOP-10E Packages LT3971 38V, 1.2A (IOUT), 2MHz, High Efficiency Step-Down DC/DC Converter with Only 2.8µA of Quiescent Current VIN(MIN) = 4.3V, VIN(MAX) = 38V, VOUT(MIN) = 1.19V, IQ = 2.8µA, ISD <1µA, 3mm × 3mm DFN-10 and MSOP-10E Packages LT3991 55V, 1.2A (IOUT), 2MHz, High Efficiency Step-Down DC/DC Converter with Only 2.8µA of Quiescent Current VIN(MIN) = 4.3V, VIN(MAX) = 38V, VOUT(MIN) = 1.19V, IQ = 2.8µA, ISD <1µA, 3mm × 3mm DFN-10 and MSOP-10E Packages LT3480 36V with Transient Protection to 60V, 2A (IOUT), 2.4MHz, High Efficiency Step-Down DC/DC Converter with Burst Mode Operation VIN(MIN) = 3.6V, VIN(MAX) = 38V, VOUT(MIN) = 0.78V, IQ = 70µA, ISD <1µA, 3mm × 3mm DFN-10 and MSOP-10E Packages LT3685 36V with Transient Protection to 60V, 2A (IOUT), 2.4MHz, High Efficiency Step-Down DC/DC Converter VIN(MIN) = 3.6V, VIN(MAX) = 38V, VOUT(MIN) = 0.78V, IQ = 70µA, ISD <1µA, 3mm × 3mm DFN-10 and MSOP-10E Packages LT3500 36V, 40VMAX, 2A, 2.5MHz High Efficiency Step-Down DC/DC Converter and LDO Controller VIN(MIN) = 3.6V, VIN(MAX) = 36V, VOUT(MIN) = 0.8V, IQ = 2.5mA, ISD <10µA, 3mm × 3mm DFN-10 Package LT3507 36V 2.5MHz, Triple (2.4A + 1.5A + 1.5A (IOUT)) with LDO Controller High Efficiency Step-Down DC/DC Converter VIN(MIN) = 4.0V, VIN(MAX) = 36V, VOUT(MIN) = 0.8V, IQ = 7mA, ISD = 1µA, 5mm × 7mm QFN-38 Package LT3682 36V, 60VMAX, 1A, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter VIN(MIN) = 3.6V, VIN(MAX) = 36V, VOUT(MIN) = 0.8V, IQ = 75µA, ISD <1µA, 3mm × 3mm DFN-12 Package 3690f 28 Linear Technology Corporation LT 0211 • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2011