LT3695IMSETRPBF

LT3695 1A Fault Tolerant Micropower Step-Down Regulator FEATURES n DESCRIPTION The LT®3695 is an adjustable frequency (250kHz to 2.2MHz) monolithic buck switching regulator that accepts input voltages up to 36V and can safely sustain transient voltages up to 60V. The device includes a high efficiency switch, a boost diode, and the necessary oscillator, control and logic circuitry. Current mode topology is used for fast transient response and good loop stability. A SYNC pin allows the user to synchronize the part to an external clock, and to choose between low ripple Burst Mode operation and standard PWM operation. The LT3695 tolerates adjacent pin shorts or an open pin without raising the output voltage above its programmed value. Low ripple Burst Mode operation maintains high efficiency at low output currents while keeping output ripple below 15mV in a typical application. Shutdown reduces input supply current to less than 1μA while a resistor and capacitor on the RUN/SS pin provide a controlled output voltage ramp (soft-start). Protection circuitry senses the current in the power switch and external Schottky catch diode to protect the LT3695 against short-circuit conditions. Frequency foldback and thermal shutdown provide additional protection. The LT3695 is available in a thermally enhanced 16-Pin MSOP package. n n n n n n n n n Wide Input Range: Operation from 3.6V to 36V Overvoltage Lockout Protects Circuits Through 60V Transients FMEA Fault Tolerant: Output Stays at or Below Regulation Voltage During Adjacent Pin Short or When a Pin Is Left Floating 1A Output Current Low Ripple (< 15mVP-P) Burst Mode® Operation IQ = 75μA for 12VIN to 3.3VOUT with No Load Adjustable Switching Frequency: 250kHz to 2.2MHz Short-Circuit Protected Synchronizable Between 300kHz and 2.2MHz Output Voltage: 0.8V to 20V Power Good Flag Small Thermally Enhanced 16-Pin MSOP Package APPLICATIONS n n n n Automotive Battery Regulation Automotive Entertainment Systems Distributed Supply Regulation Industrial Supplies L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. TYPICAL APPLICATION Efficiency 5V Step-Down Converter VIN 6.9V TO 36V TRANSIENT TO 60V 2.2μF ON OFF VOUT 5V 0.9A, VIN > 6.9V 1A, VIN > 12V 0.22μF VC RT 16.2k 40.2k PG SYNC GND PGND f = 800kHz LT3695 DA FB 102k 3695 TA01a 100 VOUT = 5V 90 EFFICIENCY (%) VOUT = 3.3V VIN RUN/SS BD BOOST 10μH SW 80 70 536k 60 10μF 50 0 0.2 0.4 0.6 LOAD CURRENT (A) 0.8 1 3695 TA01b 470pF 100k VIN = 12V L = 10µH f = 800kHz 3695f 1 LT3695 ABSOLUTE MAXIMUM RATINGS (Notes 1, 2) PIN CONFIGURATION TOP VIEW PGND DA NC SW RUN/SS RT SYNC VIN 1 2 3 4 5 6 7 8 16 15 14 13 12 11 10 9 BOOST BD GND PG NC FB NC VC VIN, RUN/SS Voltage (Note 3) ...................................60V BOOST Pin Voltage ...................................................50V BOOST Pin Above SW Pin.........................................30V BD Voltage ................................................................30V RT , VC Voltage ............................................................5V RT Pin Current .........................................................1mA SYNC Voltage ............................................................20V FB Voltage ...................................................................5V PG Voltage ................................................................30V Operating Junction Temperature Range (Notes 4, 5) LT3695E ............................................. –40°C to 125°C LT3695I .............................................. –40°C to 125°C LT3695H ............................................ –40°C to 150°C Storage Temperature Range................... –65°C to 150°C Lead Temperature (Soldering, 10 sec) .................. 300°C 17 MSE PACKAGE 16-LEAD PLASTIC MSOP θJA = 40°C/W WITH EXPOSED PAD SOLDERED θJA = 110°C/W WITHOUT EXPOSED PAD SOLDERED ORDER INFORMATION LEAD FREE FINISH LT3695EMSE#PBF LT3695IMSE#PBF LT3695HMSE#PBF TAPE AND REEL LT3695EMSE#TRPBF LT3695IMSE#TRPBF LT3695HMSE#TRPBF PART MARKING* 3695 3695 3695 PACKAGE DESCRIPTION 16-Lead Plastic MSOP 16-Lead Plastic MSOP 16-Lead Plastic MSOP TEMPERATURE RANGE –40°C to 125°C –40°C to 125°C –40°C to 150°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/ 3695f 2 LT3695 ELECTRICAL CHARACTERISTICS PARAMETER Minimum Operating Voltage (Note 6) VIN Overvoltage Lockout Quiescent Current from VIN VRUN/SS = 0.2V VRUN/SS = 10V, VBD = 3.3V, Not Switching VRUN/SS = 10V, VBD = 0V, Not Switching VRUN/SS = 0.2V VRUN/SS = 10V, VBD = 3.3V, Not Switching VRUN/SS = 10V, VBD = 0V, Not Switching VBD = 3.3V VBD < 3V The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, VRUN/SS = 10V, unless otherwise noted. (Note 4) CONDITIONS l l l l MIN TYP 3.4 3.4 MAX 3.6 4.3 39.9 0.5 60 160 0.5 100 –5 3 808 815 –40 0.005 UNITS V V V μA μA μA μA μA μA V mV mV nA %/V μS V/V μA μA A/V 36 38 0.01 35 90 Quiescent Current from BD Pin l 35 0.01 55 0 2.8 800 800 –5 0.001 430 1300 50 50 1.25 Minimum BD Pin Voltage Feedback Voltage FB Pin Bias Current Reference Voltage Line Regulation Error Amp gm Error Amp Voltage Gain VC Source Current VC Sink Current VC Pin to Switch Current Gain VC Switching Threshold VC Clamp Voltage Switching Frequency RRT = 8.06k RRT = 29.4k RRT = 158k E- and I-Grades H-Grade SYNC = 0V SYNC = 3.3V or Clocked ISW = 1A 1.25 VSW = 0V, VIN = 36V l l l 792 785 FB Pin Voltage = 800mV 3.6V < VIN < 36V IVC = ±1.5μA l 0.4 1.98 0.9 225 0.6 2 2.2 1.0 250 130 130 0.8 2.42 1.1 275 210 250 2 1.66 1.95 1 V V MHz MHz kHz ns ns A A mV A μA Minimum Switch Off-Time Switch Current Limit (Note 7) Switch VCESAT DA Pin Current to Stop OSC Switch Leakage Current 1.45 1.18 1.7 1.4 350 1.6 0.01 3695f 3 LT3695 ELECTRICAL CHARACTERISTICS PARAMETER Boost Schottky Diode Voltage Drop Boost Schottky Diode Reverse Leakage Minimum Boost Voltage (Note 8) BOOST Pin Current RUN/SS Pin Current RUN/SS Input Voltage High RUN/SS Input Voltage Low PG Leakage Current PG Sink Current PG Threshold as % of VFB PG Threshold Hysteresis SYNC Threshold Voltage SYNC Input Frequency 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 the device reliability and lifetime. Note 2: Positive currents flow into pins, negative currents flow out of pins. Minimum and maximum values refer to absolute values. Note 3: Absolute maximum voltage at VIN and RUN/SS pins is 60V for nonrepetitive 1 second transients, and 36V for continuous operation. Note 4: The LT3695E 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 LT3695I is guaranteed over the full –40°C to 125°C operating junction temperature range. The LT3695H is guaranteed over the full –40°C to 150°C operating junction temperature range. VPG = 5V VPG = 0.4V Measured at FB Pin (Pin Voltage Rising) Measured at FB Pin 300 0.3 l The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, VRUN/SS = 10V, unless otherwise noted. (Note 4) CONDITIONS IBSD = 50mA VSW = 10V, VBD = 0V l MIN TYP 720 0.1 1.7 10.5 4.5 12 MAX 900 1 2.3 17.5 7.5 20 0.2 UNITS mV μA V mA μA μA V V μA μA % mV mV MHz ISW = 0.5A VRUN/SS = 2.5V VRUN/SS = 10V l 2.5 0.1 100 88 1000 90 12 550 800 2.2 92 1 Note 5: This IC includes overtemperature protection that is intended to protect the devices 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 6: This is the voltage necessary to keep the internal bias circuitry in regulation. Note 7: Current limit guaranteed by design and/or correlation to static test. Slope compensation reduces current limit at higher duty cycles. Note 8: This is the minimum voltage across the boost capacitor needed to guarantee full saturation of the switch. 3695f 4 LT3695 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted. Efficiency (VOUT = 5V, SYNC = 0V) 90 80 70 EFFICIENCY (%) 60 50 VIN = 24V 40 30 20 10 0.1 1 100 LOAD CURRENT (mA) 10 1000 3695 G01 Efficiency (VOUT = 3.3V, SYNC = 0V) 90 L = 10μH 80 f = 800kHz VIN = 12V EFFICIENCY (%) VIN = 34V 100 Efficiency (VOUT = 3.3V, SYNC = 0V) VIN = 12V 90 VOUT = 3.3V L = 10μH 80 f = 800kHz 1 L = 10μH f = 800kHz VIN = 12V EFFICIENCY (%) VIN = 34V 70 60 50 POWER LOSS(W) 70 60 50 40 30 20 0.1 VIN = 24V 40 30 20 10 0.1 1 100 LOAD CURRENT (mA) 10 1000 3695 G02 0.01 10 0.1 1 100 LOAD CURRENT (mA) 10 0.001 1000 3695 G03 No-Load Supply Current 140 120 SUPPLY CURRENT (μA) 100 80 60 40 20 0 SUPPLY CURRENT (μA) VOUT = 3.3V No-Load Supply Current 1300 CATCH DIODE: DIODES, INC. B140 1200 V = 12V IN 1100 VOUT = 3.3V 1000 900 800 700 600 INCREASED SUPPLY CURRENT 500 DUE TO CATCH DIODE LEAKAGE 400 AT HIGH TEMPERATURE 300 200 100 0 25 50 75 100 125 150 –50 –25 0 TEMPERATURE (°C) 3695 G05 Maximum Load Current 1.75 TYPICAL 1.50 LOAD CURRENT (A) 1.25 1.00 0.75 0.50 SYNC = 0V SYNC = 3.3V 0.25 0 5 10 MINIMUM VOUT = 3.3V L = 10μH f = 800kHz 35 40 0 5 10 15 20 25 30 INPUT VOLTAGE (V) 35 40 15 20 25 30 INPUT VOLTAGE (V) 3695 G04 3695 G06 Maximum Load Current 1.50 TYPICAL LOAD CURRENT (A) 1.50 Maximum Load Current 1.75 TYPICAL 1.50 LOAD CURRENT (A) 1.25 1.00 0.75 Maximum Load Current TYPICAL 1.25 LOAD CURRENT (A) 1.25 1.00 1.00 0.75 MINIMUM 0.50 SYNC = 0V SYNC = 5V 0.25 5 10 15 20 25 30 INPUT VOLTAGE (V) VOUT = 5V L = 10μH f = 800kHz 35 40 0.75 MINIMUM 0.50 SYNC = 0V SYNC = 5V 0.25 8 10 14 16 INPUT VOLTAGE (V) 12 VOUT = 5V L = 4.7μH f = 2MHz 18 20 3695 G08 MINIMUM 0.50 0.25 SYNC = 0V SYNC = 3.3V 0 5 10 VOUT = 1.8V L = 10μH f = 500kHz 35 40 15 20 25 30 INPUT VOLTAGE (V) 3682 G07 3695 G09 3695f 5 LT3695 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted. Switch Current Limit 1.9 1.7 SWITCH CURRENT LIMIT (A) SWITCH CURRENT LIMIT (A) SYNC < 0.3V 1.5 1.3 1.1 0.9 0.7 0.5 SYNC > 0.8V OR CLOCKED 1.9 DC = 10% 1.7 DC = 90% VOLTAGE DROP (mV) 1.5 1.3 1.1 0.9 0.7 0.5 –50 –25 0 300 Switch Current Limit (SYNC Pin Grounded) 400 Switch Voltage Drop 200 100 0 20 60 DUTY CYCLE (%) 40 80 100 3695 G10 0 25 50 75 100 125 150 TEMPERATURE (°C) 3695 G11 0 0.25 0.50 0.75 1.00 SWITCH CURRENT (A) 1.25 3695 G28 BOOST Pin Current 35 30 BOOST PIN CURRENT (mA) FEEDBACK VOLTAGE (mV) 810 Feedback Voltage 1.20 1.15 800 FREQUENCY (MHz) 1.10 1.05 1.00 0.95 0.90 0.85 Switching Frequency RT = 29.4k 25 20 15 10 5 0 790 780 0 0.25 0.75 1.00 SWITCH CURRENT (A) 0.50 1.25 3695 G13 770 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3695 G14 0.80 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3695 G15 Frequency Foldback 1200 RRT = 29.4k MINIMUM SWITCH ON TIME (ns) 1000 FREQUENCY (kHz) 800 600 400 200 0 0 100 200 300 400 500 600 700 800 900 FB PIN VOLTAGE (mV) 3695 G16 Minimum Switch On Time 120 100 80 60 40 20 0 –50 –25 IOUT = 1A SWITCH CURRENT LIMIT (A) 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 25 50 75 100 125 150 TEMPERATURE (°C) 3695 G17 Soft-Start SYNC < 0.3V 0 0 0.5 1.0 1.5 2.0 2.5 3.0 RUN/SS PIN VOLTAGE (V) 3.5 3695 G20 3695f 6 LT3695 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted. RUN/SS Pin Current 12 10 RUN/SS PIN CURRENT (μA) 8 6 4 2 0 0 5 10 15 20 25 30 RUN/SS PIN VOLTAGE (V) 35 40 BOOST DIODE VF (V) 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 0 0.25 0.5 0.75 BOOST DIODE CURRENT (A) 1 3695 G21 Boost Diode Forward Voltage 60 50 40 VC PIN CURRENT (μA) 30 20 10 0 –10 –20 –30 –40 –50 Error Amplifier Output Current –60 –200 –100 0 100 FB PIN ERROR VOLTAGE (mV) 200 3659 G23 3695 G21 Minimum Input Voltage 5.0 6.5 VOUT = 3.3V L = 10μH 4.5 f = 800kHz INPUT VOLTAGE (V) Minimum Input Voltage VOUT = 5V L = 10μH f = 800kHz 6.0 INPUT VOLTAGE (V) 4.0 3.5 3.0 2.5 2.0 5.5 5.0 4.5 4 1 10 100 LOAD CURRENT (mA) 1000 3695 G26 1 10 100 LOAD CURRENT (mA) 1000 3695 G27 Maximum VIN for Full Frequency 40 35 30 VIN (V) 25 20 15 10 5 VOUT = 3.3V L = 10μH f = 800kHz SYNC = 3.3V 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 LOAD CURRENT(A) 1 TA = 25˚C TA = 85˚C VIN (V) 40 35 Maximum VIN for Full Frequency TA = 25˚C 30 25 20 15 10 5 VOUT = 5V L = 10μH f = 800kHz SYNC = 5V 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 LOAD CURRENT(A) 1 TA = 85˚C 3695 G29 3695 G30 3695f 7 LT3695 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted. Maximum VIN for Full Frequency 40 35 30 TA = 85˚C VIN (V) 25 20 15 10 5 VOUT = 5V L = 4.7μH f = 2MHz SYNC = 5V 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 LOAD CURRENT(A) 1 0.5 VC VOLTAGE (V) 1.5 TA = 25˚C 2.0 CURRENT LIMIT CLAMP VIN 20V/DIV VOUT 5V/DIV SWITCHING THRESHOLD 5ms/DIV VIN = 12V, FRONT PAGE APPLICATION ILOAD = 500mA 3695 G33 VC Voltages 2.5 VSW 10V/DIV Switching Waveforms, 60V Input Voltage Transient 1.0 0 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3695 G32 3695 G31 Switching Waveforms, Burst Mode Operation VSW 5V/DIV IL 0.2A/DIV VOUT 20mV/DIV 5μs/DIV VIN = 12V, FRONT PAGE APPLICATION ILOAD = 5mA 3695 G34 Switching Waveforms, Transition from Burst Mode Operation to Full Frequency VSW 5V/DIV IL 0.2A/DIV VOUT 20mV/DIV 1μs/DIV VIN = 12V, FRONT PAGE APPLICATION ILOAD = 55mA 3695 G35 Switching Waveforms, Full Frequency Continuous Operation VSW 5V/DIV IL 0.5A/DIV VOUT 20mV/DIV 1μs/DIV VIN = 12V, FRONT PAGE APPLICATION ILOAD = 500mA 3695 G36 3695f 8 LT3695 PIN FUNCTIONS PGND (Pin 1): This is the power ground used by the catch diode (D1 in the Block Diagram) when its anode is connected to the DA pin. DA (Pin 2): Connect the anode of the catch diode (D1) to this pin. Internal circuitry senses the current through the catch diode providing frequency foldback in extreme situations. NC (Pins 3, 10, 12): No Connects. These pins are not connected to internal circuitry and must be left floating to ensure fault tolerance. SW (Pin 4): The SW pin is the output of the internal power switch. Connect this pin to the inductor, catch diode and boost capacitor. RUN/SS (Pin 5): The RUN/SS pin is used to put the LT3695 in shutdown mode. Tie to ground to shut down the LT3695. Tie to 2.5V or more for normal operation. RUN/SS also provides a soft-start function; see the Applications Information section for more information. RT (Pin 6): Oscillator Resistor Input. Connect a resistor from this pin to ground to set the switching frequency. SYNC (Pin 7): This is the external clock synchronization input. Ground this pin with a 100k resistor for low ripple Burst Mode operation at low output loads. Tie to 0.8V or more for pulse-skipping mode operation. Tie to a clock source for synchronization. Clock edges should have rise and fall times faster than 1μs. Note that the maximum load current depends on which mode is chosen. See the Applications Information section for more information. VIN (Pin 8): The VIN pin supplies current to the internal regulator and to the internal power switch. This pin must be locally bypassed. VC (Pin 9): The VC pin is the output of the internal error amplifier. The voltage on this pin controls the peak switch current. Tie an RC network from this pin to ground to compensate the control loop. FB (Pin 11): The LT3695 regulates the FB pin to 0.8V. Connect the feedback resistor divider tap to this pin. PG (Pin 13): The PG pin is the open-collector output of an internal comparator. PG remains low until the FB pin is within 10% of the final regulation voltage. PG output is valid when VIN is above the minimum input voltage and RUN/SS is high. GND (Pin 14): The GND pin is the ground of all the internal circuitry. Tie directly to the local GND plane. BD (Pin 15): This pin connects to the anode of the boost Schottky diode and also supplies current to the LT3695’s internal regulator. BOOST (Pin 16): This pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar NPN power switch. Connect a capacitor (typically 0.22μF) between BOOST and SW. Exposed Pad (Pin 17): PGND. This is the power ground used by the catch diode (D1) when its anode is connected to the DA pin. The Exposed Pad may be soldered to the PCB in order to lower the thermal resistance. 3695f 9 LT3695 BLOCK DIAGRAM VIN C1 OVLO 8 VIN INTERNAL 0.8V REF SLOPE COMP R OUT S Q 6 RT RT OSCILLATOR 250kHz-2.2MHz OUTB 7 5 13 SYNC RUN/SS PG SYNC SOFT-START ERROR AMP + – GND 14 0.720V + – PGND 17 R1 1 FB 11 R2 10 + – Burst Mode DETECT VC CLAMP THERMAL SHUTDOWN BD BOOST 15 16 C3 L1 4 D1 2 C2 VOUT SW DA DISABLE + – VC 9 CC CF PGND RC 3695 BD 3695f LT3695 OPERATION The LT3695 is a constant frequency, current mode stepdown regulator. An oscillator, with frequency set by RT , enables an RS flip-flop, turning on the internal power switch. An amplifier and comparator monitor the current flowing between the VIN and SW pins, turning the switch off when this current reaches a level determined by the voltage at VC. An error amplifier measures the output voltage through an external resistor divider tied to the FB pin and servos the VC pin. If the error amplifier’s output increases, more current is delivered to the output; if it decreases, less current is delivered. An active clamp on the VC pin provides current limit. The VC pin is also clamped to the voltage on the RUN/SS pin; soft-start is implemented by generating a voltage ramp at the RUN/SS pin using an external resistor and capacitor. An internal regulator provides power to the control circuitry. The bias regulator normally draws power from the VIN pin, but if the BD pin is connected to an external voltage higher than 3V, bias power will be drawn from the external source. This improves efficiency. The RUN/SS pin is used to place the LT3695 in shutdown, disconnecting the output and reducing the input current to less than 1μA. The switch driver operates from either the input or from the BOOST pin. An external capacitor and the internal boost diode are used to generate a voltage at the BOOST pin that is higher than the input supply. This allows the driver to fully saturate the internal bipolar NPN power switch for efficient operation. To further optimize efficiency, the LT3695 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 75μA in a typical application. The oscillator reduces the LT3695’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. Internal circuitry monitors the current flowing through the catch diode via the DA pin and delays the generation of new switch pulses if this current is too high (above 1.6A nominal). This mechanism also protects the part during short-circuit and overload conditions by keeping the current through the inductor under control. The LT3695 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 LT3695 is enabled and VIN is above the minimum input voltage. The LT3695 has an overvoltage protection feature which disables switching action when VIN goes above 38V (typical) during transients. The LT3695 can then safely sustain transient input voltages up to 60V. 3695f 11 LT3695 APPLICATIONS INFORMATION FB Resistor Network The output voltage of the LT3695 is programmed with a resistor divider between the output and the FB pin. Choose the resistor values according to: ⎛V ⎞ R1= R2 ⎜ OUT − 1⎟ ⎝ 0.8 V ⎠ Reference designators refer to the Block Diagram of the LT3695. 1% resistors are recommended to maintain output voltage accuracy. Setting the Switching Frequency The LT3695 uses a constant frequency PWM architecture that can be programmed to switch from 250kHz to 2.2MHz by using a resistor tied from the RT pin to ground. A table showing the necessary RT value for a desired switching frequency is in Table 1. Table 1. Switching Frequency vs RT Value SWITCHING FREQUENCY (MHz) 0.25 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.2 1.4 1.6 1.8 2.0 2.2 RT VALUE (kΩ) 158 127 90.9 71.5 57.6 47.5 40.2 34 29.4 22.6 18.2 14.7 12.1 9.76 8.06 Operating Frequency Trade-Offs 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: fSW(MAX ) = VOUT + VD tON(MIN)( VIN − VSW + VD ) where VIN is the typical input voltage, VOUT is the output voltage, VD is the catch diode drop (~0.5V) and VSW is the internal switch drop (~0.5V at max load). This equation shows that lower switching frequency is necessary to safely 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 LT3695 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 (250ns for TJ > 125°C). 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 150ns over temperature. The minimum and maximum duty cycles that can be achieved taking minimum on and off times into account are: DCMIN = fSWtON(MIN) DCMAX = 1 – fSWtOFF(MIN) where fSW is the switching frequency, tON(MIN) is the minimum switch on time (150ns), and tOFF(MIN) is the minimum switch off time (210ns, 250ns for TJ > 125°C). These equations show that the duty cycle range increases when the switching frequency is decreased. 3695f 12 LT3695 APPLICATIONS INFORMATION A good choice of switching frequency should allow an adequate input voltage range (see Input Voltage Range section) and keep the inductor and capacitor values small. Input Voltage Range The minimum input voltage is determined by either the LT3695’s minimum operating voltage of ~3.6V (VBD > 3V) or by its maximum duty cycle (see equation in Operating Frequency Trade-Offs section). The minimum input voltage due to duty cycle is: VIN(MIN) = VOUT + VD −V +V 1− fSW tOFF(MIN) D SW 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. During these events, the inductor peak current might easily reach and even exceed the maximum current limit of the LT3695, especially in those cases where the switch already operates at minimum on-time. The circuitry monitoring the current through the catch diode via the DA pin prevents the switch from turning on again if the inductor valley current is above 1.6A nominal. In these cases, the inductor peak current is therefore the maximum current limit of the LT3695 plus the additional current overshoot during the turn off delay due to minimum on time: IL(PEAK ) = 2A + VIN(MAX ) − VOUT(OL ) L • tON(MIN) where VIN(MIN) is the minimum input voltage, and tOFF(MIN) is the minimum switch off time. Note that a 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 LT3695 applications depends on switching frequency, the Absolute Maximum Ratings of the VIN and BOOST pins and the operating mode. The LT3695 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 (overvoltage lockout, 38V typical), the LT3695 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: VIN(MAX ) = VOUT + VD −V +V fSW tON(MIN) D SW 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 VOUT(OL) 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 3.5A. Inductor current saturation and excessive junction temperature may further limit performance. Input voltage transients of up to VOVLO are acceptable regardless of the switching frequency. In this case, the LT3695 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. Input voltage transients above VOVLO and up to 60V can be tolerated. However, since the part will stop switching during these transients, the output will fall out of regulation and the output capacitor may eventually be completely discharged. This case must be treated then as a start-up condition as soon as VIN returns to values below VOVLO and the part starts switching again. where VIN(MAX) is the maximum operating input voltage, VOUT is the output voltage, VD is the catch diode drop (~0.5V), VSW is the internal switch drop (~0.5V at max load), fSW is the switching frequency (set by RT) and tON(MIN) is the minimum switch on time (~150ns). Note that a higher 3695f 13 LT3695 APPLICATIONS INFORMATION Inductor Selection and Maximum Output Current A good first choice for the inductor value is: L = ( VOUT + VD) • 1.8 fSW The current in the inductor is a triangle wave with an average value equal to the load current. The peak inductor and switch current is: ΔIL 2 where IL(PEAK) is the peak inductor current, IOUT(MAX) is the maximum output load current and ΔIL is the inductor ripple current. The LT3695 limits its switch current in order to protect itself and the system from overload faults. Therefore, the maximum output current that the LT3695 will deliver depends on the switch current limit, the inductor value and the input and output voltages. I SW(PEAK ) = IL(PEAK ) = IOUT(MAX ) + When the switch is off, the potential across the inductor is the output voltage plus the catch diode drop. This gives the peak-to-peak ripple current in the inductor: ΔIL = (1− DC) •( VOUT + VD) L • fSW where fSW is the switching frequency in MHz, VOUT is the output voltage, VD is the catch diode drop (~0.5V) and L is the inductor value in μH. The inductor’s RMS current rating must be greater than the maximum load current and its saturation current should be about 30% higher. To keep the efficiency high, the series resistance (DCR) should be less than 0.1Ω, and the core material should be intended for high frequency applications. Table 2 lists several vendors and suitable types. For robust operation in fault conditions (start-up or shortcircuit) and high input voltage (>30V), the saturation current should be chosen high enough to ensure that the inductor peak current does not exceed 3.5A. For example, an application running from an input voltage of 36V using a 10μH inductor with a saturation current of 2.5A will tolerate the mentioned fault conditions. 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 Linear Technology’s Application Note 44. Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5), a minimum inductance is required to avoid sub-harmonic oscillations: L MIN = ( VOUT + VD) • 1.2 fSW where fSW is the switching frequency of the LT3695, DC is the duty cycle and L is the value of the inductor. To maintain output regulation, the inductor peak current must be less than the LT3695’s switch current limit, ILIM. If the SYNC pin is grounded, ILIM is at least 1.45A at low duty cycles and decreases to 1.1A at DC = 90%. If the SYNC pin is tied to 0.8V or more or if it is tied to a clock source for synchronization, ILIM is at least 1.18A at low duty cycles and decreases to 0.85A at DC = 90%. The maximum output current is also a function of the chosen inductor value and can be approximated by the following expressions depending on the SYNC pin configuration: For the SYNC pin grounded: IOUT(MAX ) = ILIM − ΔIL ΔI = 1.45A •(1− 0.24 • DC) − L 2 2 For the SYNC pin tied to 0.8V or more, or tied to a clock source for synchronization: IOUT(MAX ) = ILIM − ΔIL ΔI = 1.18 A •(1− 0.29 • DC) − L 2 2 3695f 14 LT3695 APPLICATIONS INFORMATION Choosing an inductor value so that the ripple current is small will allow a maximum output current near the switch current limit. Table 2. Inductor Vendors VENDOR Murata TDk Toko URL www.murata.com www.componenttdk.com www.toko.com PART SERIES LQH55D SLF7045 SLF10145 D62CB D63CB D73C D75F MSS7341 MSS1038 CR54 CDRH74 CDRH6D38 CR75 TYPE Open Shielded Shielded Shielded Shielded Shielded Open Shielded Shielded Open Shielded Shielded Open switching current into a tight local loop, minimizing EMI. A 2.2μF capacitor is capable of this task, but only if it is placed close to the LT3695 (see the PCB Layout section for more information). A second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the LT3695. A ceramic input capacitor combined with trace or cable inductance forms a high-Q (underdamped) tank circuit. If the LT3695 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3695’s voltage rating. For details see Application Note 88. Output Capacitor and Output Ripple The output capacitor has two essential functions. Along with the inductor, it filters the square wave generated by the LT3695 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 LT3695’s control loop. Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance. A good starting value is: COUT = 50 f VOUT SW Coilcraft Sumida www.coilcraft.com www.sumida.com 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 LT3695 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 LT3695 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 2.2μF to 10μF ceramic capacitor is adequate to bypass the LT3695 and will easily handle the ripple current. Note that larger input capacitance is required when a lower switching frequency is used. If the input power source has high impedance, or there is significant inductance due to long wires or cables, additional bulk capacitance may be necessary. This can be provided with a 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 LT3695 and to force this very high frequency where fSW is in MHz, and COUT is the recommended output capacitance in μF Use X5R or X7R types. This . choice will provide low output ripple and good transient response. Transient performance can be improved with a higher value capacitor if the compensation network is also adjusted to maintain the loop bandwidth. A lower value of output capacitor can be used to save space and cost but transient performance will suffer. See the Frequency Compensation section to choose an appropriate compensation network. When choosing a capacitor, look carefully through the data sheet to find out what the actual capacitance is under operating conditions (applied voltage and temperature). A physically larger capacitor, or one with a higher voltage rating, may be required. High performance tantalum or electrolytic capacitors can be used for the output capacitor. 3695f 15 LT3695 APPLICATIONS INFORMATION Table 3. Capacitor Vendors VENDOR Panasonic Kemet Sanyo Murata AVX Taiyo Yuden (864) 963-6300 PHONE (714) 373-7366 (864) 963-6300 (408) 749-9714 (408) 436-1300 URL www.panasonic.com www.kemet.com www.sanyovideo.com www.murata.com www.avxcorp.com www.taiyo-yuden.com PART SERIES Ceramic, Polymer, Tantalum Ceramic, Tantalum Ceramic, Polymer, Tantalum Ceramic Ceramic, Tantalum Ceramic TPS Series COMMANDS EEF Series T494, T495 POSCAP Low ESR is important, so choose one that is intended for use in switching regulators. The ESR should be specified by the supplier, and should be 0.05Ω or less. Such a capacitor will be larger than a ceramic capacitor and will have a larger capacitance, because the capacitor must be large to achieve low ESR. Table 3 lists several capacitor vendors. Diode Selection The catch diode (D1 from Block Diagram) conducts current only during switch off time. Average forward current in normal operation can be calculated from: ID(AVG) = IOUT • (1 – DC) where DC is the duty cycle. The only reason to consider a diode with larger current rating than necessary for nominal operation is for the case of shorted or overloaded output conditions. For the worst case of shorted output the diode average current will then increase to a value that depends on the following internal parameters: switch current limit, catch diode (DA pin) current threshold and minimum on-time. The worst case (taking maximum values for the above mentioned parameters) is given by the following expression: 1V ID( AVG)MAX = 2A + • IN • 150ns 2L Peak reverse voltage is equal to the regulator input voltage if it is below the overvoltage protection threshold. This feature keeps the switch off for VIN > VOVLO (39.9V maximum). For inputs up to the maximum operating voltage of 36V, use a diode with a reverse voltage rating greater Table 4. Schottky Diodes PART NUMBER On-Semiconducor MBR0520L MBR0540 MBRM120E MBRM140 Diodes Inc. B0530W B0540W B120 B130 B140 B220 B230 B140HB DFLS240L DFLS140 B240 CMSH1-40M CMSH1-40ML CMSH2-40M CMSH2-40L CMSH2-40 30 40 20 30 40 20 30 40 40 40 40 40 40 40 40 40 0.5 0.5 1 1 1 2 2 1 2 1.1 2 1 1 2 2 2 500 400 550 400 500 510 500 530 500 620 500 500 500 500 500 20 40 20 40 0.5 0.5 1 1 620 530 550 595 VR (V) IAVE (A) VF at 1A (mV) VF at 2A (mV) Central Semiconductor than the input voltage. If transients at the input of up to 60V are expected, use a diode with a reverse voltage rating of 40V. Table 4 lists several Schottky diodes and their manufacturers. If operating at high ambient temperatures, consider using a Schottky with low reverse leakage. 3695f 16 LT3695 APPLICATIONS INFORMATION Audible Noise Ceramic capacitors are small, robust and have very low ESR. However, ceramic capacitors can sometimes cause problems when used with the LT3695 due to their piezoelectric nature. When in Burst Mode operation, the LT3695’s switching frequency depends on the load current, and at very light loads the LT3695 can excite the ceramic capacitor at audio frequencies, generating audible noise. Since the LT3695 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 LT3695 uses current mode control to regulate the output. This simplifies loop compensation. In particular, the LT3695 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 used to filter noise at the switching frequency, and is required only if a phase-lead capacitor (CPL) is used or if the output capacitor has high ESR. LT3695 CURRENT MODE POWER STAGE gm = 1.25S SW R1 FB ESR 0.8V VOUT 100mV/DIV CPL Loop compensation determines the stability and transient performance. Optimizing the design of the compensation network depends on the application and 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 LT3695 control loop. The error amplifier is a transconductance amplifier with finite output impedance. The power section, consisting of the modulator, power switch and inductor, is modeled as a transconductance amplifier generating an output current proportional to the voltage at the VC pin. Note that the output capacitor integrates this current, and that the capacitor on the VC pin (CC) integrates the error amplifier output current, resulting in two poles in the loop. In most cases a zero is required and comes from either the output capacitor ESR or from a resistor RC in series with CC. This simple model works well as long as the value of the inductor is not too high and the loop crossover frequency is much lower than the switching frequency. A phase lead capacitor (CPL) across the feedback divider may improve the transient response. Figure 2 shows the transient response when the load current is stepped from 300mA to 650mA and back to 300mA. OUTPUT gm = 430µS 3M VC RC CC GND R2 POLYMER OR TANTALUM OR ELECTROLITIC CF Figure 1. Model for Loop Response – + + C1 C1 CERAMIC ILOAD 0.5A/DIV 20μs/DIV 3695 F02 3695 F01 Figure 2. Transient Load Response of the LT3695. A 3.3VOUT Typical Application with VIN = 12V as the Load Current is Stepped from 300mA to 650mA 3695f 17 LT3695 APPLICATIONS INFORMATION Low Ripple Burst Mode Operation The LT3695 is capable of operating in either low ripple Burst Mode operation or pulse-skipping mode which are selected using the SYNC pin. See the Synchronization section for more information. To enhance efficiency at light loads, the LT3695 can be operated in low ripple Burst Mode operation which keeps the output capacitor charged to the proper voltage while minimizing the input quiescent current. During Burst Mode operation, the LT3695 delivers single cycle bursts of current to the output capacitor followed by sleep periods where the output power is delivered to the load by the output capacitor. Because the LT3695 delivers power to the output with single, low current pulses, the output ripple is kept below 15mV for a typical application. In addition, VIN and BD quiescent currents are reduced to typically 35μA and 55μA respectively during the sleep time. As the load current decreases towards a no-load condition, the percentage of time that the LT3695 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 70mA for the front page application) the LT3695 will be running at the frequency programmed by the RT resistor, and will be operating in standard PWM mode. The transition between PWM and low ripple Burst Mode operation is seamless, and will not disturb the output voltage. If low quiescent current is not required, tie SYNC high to select pulse-skipping mode. The benefit of this mode is that the LT3695 will enter full frequency standard PWM operation at a lower output load current than when in Burst Mode operation. With the SYNC pin tied low, the front page application circuit will switch at full frequency VSW 5V/DIV IL 0.2A/DIV VOUT 20mV/DIV 5μs/DIV VIN = 12V, FRONT PAGE APPLICATION ILOAD = 5mA 3695 F03 at output loads higher than about 100mA. With the SYNC pin tied high, the front page application circuit will switch at full frequency at output loads higher than about 30mA. The maximum load current that the LT3695 can supply is reduced when SYNC is high. BOOST Pin Considerations Capacitor C3 and the internal boost Schottky diode (see the Block Diagram) are used to generate a boost voltage that is higher than the input voltage. In most cases a 0.22μF capacitor will work well. Figure 4 shows three ways to arrange the boost circuit for the LT3695. The BOOST pin must be more than 2.3V above the SW pin for best efficiency. For outputs of between 3V and 8V, the standard circuit (Figure 4a) is best. For outputs between 2.8V and 3V, use a 1μF boost capacitor. A 2.5V output presents a special case because it is marginally adequate to support the boosted drive stage while using the internal boost diode. For reliable BOOST pin operation with 2.5V outputs use a good external Schottky diode (such as the ON Semi MBR0540), and a 1μF boost capacitor (see Figure 4b). For lower output voltages the boost diode can be tied to the input (Figure 4c), or to another supply greater than 2.8V. Keep in mind that a minimum input voltage of 4.3V is required if the voltage at the BD pin is smaller than 3V. Tying BD to VIN reduces the maximum input voltage to 25V. The circuit in Figure 4a is more efficient because the BOOST pin current and BD pin quiescent current come from a lower voltage source. You must also be sure that the maximum voltage ratings of the BOOST and BD pins are not exceeded. As mentioned, a minimum of 2.5V across the BOOST capacitor is required for proper operation of the internal BOOST circuitry to provide the base current for the power NPN switch. For BD pin voltages higher than 3V, the excess voltage across the BOOST capacitor does not bring an increase in performance but dissipates additional power in the internal BOOST circuitry instead. The BOOST circuitry tolerates reasonable amounts of power, however excessive power dissipation on this circuitry may impair reliability. For reliable operation, use no more than 8V on the BD pin for the circuit in Figure 4a. For higher output voltages, make sure that there is no more than 8V at the BD pin either by connecting it to another available supply higher than 3V or 3695f Figure 3. Switching Waveforms, Burst Mode Operation 18 LT3695 APPLICATIONS INFORMATION VOUT BD BOOST C3 LT3695 SW D1 DA GND PGND 3695 F04a VIN VIN (4a) For VOUT > 2.8V, VIN(MIN) = 4.3V if VOUT < 3V VOUT BD BOOST C3 LT3695 SW D1 DA GND PGND 3695 F04b D2 VIN VIN running properly. This minimum load will depend on input and output voltages, and on the arrangement of the boost circuit. The minimum load generally goes to zero once the circuit has started. Figure 5 shows a plot of minimum load to start and to run as a function of input 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 BOOST pin. At light loads, the inductor current becomes discontinuous and the effective duty cycle can be very high. This reduces the minimum input voltage to approximately 300mV above VOUT . At higher load currents, the inductor current is continuous and the duty cycle is limited by the maximum duty cycle of the LT3695, requiring a higher input voltage to maintain regulation. 6.0 5.5 INPUT VOLTAGE (V) TO START (WORST CASE) (4b) For 2.5V < VOUT < 2.8V, VIN(MIN) = 4.3V VIN VIN BD BOOST C3 LT3695 SW D1 DA VOUT 5.0 4.5 4.0 3.5 3.0 3695 F04c TO RUN VOUT = 3.3V TA = 25˚C L = 10μH f = 800kHz 1 10 100 LOAD CURRENT (mA) 1000 GND PGND 2.5 2.0 (4c) For VOUT < 2.5V, VIN(MAX) = 25V Figure 4. Three Circuits for Generating the Boost Voltage 8.0 7.5 7.0 INPUT VOLTAGE (V) 6.5 6.0 5.5 5.0 TO RUN 4.5 4.0 3.5 3.0 2.5 2.0 1 10 100 LOAD CURRENT(mA) TO START (WORST CASE) by using a Zener diode between VOUT and BD to maintain the BD pin voltage between 3V and 8V. The minimum operating voltage of the LT3695 application is limited by the minimum input voltage and by the maximum duty cycle as outlined previously. For proper start-up, the minimum input voltage is also limited by the boost circuit. If the input voltage is ramped slowly, or the LT3695 is turned on with its RUN/SS pin when the output is already in regulation, then the boost capacitor may not be fully charged. Because the boost capacitor is charged with the energy stored in the inductor, the circuit will rely on some minimum load current to get the boost circuit VOUT = 5V TA = 25˚C L = 10μH f = 800kHz 1000 3695 F05 Figure 5. The Minimum Input Voltage depends on Output Voltage, Load Current and Boost Circuit 3695f 19 LT3695 APPLICATIONS INFORMATION Soft-Start The RUN/SS pin can be used to soft-start the LT3695, reducing the maximum input current during start-up. The RUN/SS pin is driven through an external RC network to create a voltage ramp at this pin. Figure 6 shows the startup and shutdown waveforms with the soft-start circuit. By choosing a large RC time constant, the peak start-up current can be reduced to the current that is required to regulate the output, with no overshoot. Choose the value of the resistor so that it can supply 7.5μA when the RUN/SS pin reaches 2.5V. For fault tolerant applications, see the discussion of the RUN/SS resistor in the Fault Tolerance section. VRUN 5V/DIV VRUN/SS 5V/DIV VOUT 5V/DIV IL 1A/DIV 5ms/DIV 3695 F05 The LT3695 may be synchronized over a 300kHz to 2.2MHz range. The RT resistor should be chosen to set the LT3695 switching frequency 20% below the lowest synchronization input. For example, if the synchronization signal is 360kHz, the RT should be chosen for 300kHz. To assure reliable and safe operation the LT3695 will only synchronize when the output voltage is near regulation as indicated by the PG flag. 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 for more information. It is also important to note that slope compensation is set by the RT value; to avoid subharmonic oscillations, calculate the minimum inductor value using the frequency determined by RT . Shorted and Reversed Input Protection If the inductor is chosen so that it won’t saturate excessively, the LT3695 will tolerate a shorted output. When operating in short-circuit condition, the LT3695 will reduce its frequency until the valley current is at a typical value of 1.6A (see Figure 7). There is another situation to consider in systems where the output will be held high when the input to the LT3695 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 RUN 15k RUN/SS 0.22μF GND Figure 6. To Soft-Start the LT3695, Add a Resistor and Capacitor to the RUN/SS Pin Synchronization To select low ripple Burst Mode operation, tie the SYNC pin below 0.3V (this can be ground or a logic output). Synchronizing the oscillator of the LT3695 to an external frequency can be done by connecting a square wave (with 20% to 80% duty cycle) to the SYNC pin. The square wave amplitude should have valleys that are below 0.3V and peaks that are above 0.8V (up to 6V). The LT3695 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 maximum load current that the part can supply is reduced when a clock signal is applied to SYNC. VSW 20V/DIV 0V IL 500mA/DIV 0A 2μs/DIV 3695 F07 Figure 7. The LT3695 Reduces its Frequency to Protect Against Shorted Output with 36V Input 3695f 20 LT3695 APPLICATIONS INFORMATION LT3695’s output. If the VIN pin is allowed to float and the RUN/SS pin is held high (either by a logic signal or because it is tied to VIN), then the LT3695’s internal circuitry will pull its quiescent current through its SW pin. This is fine if your system can tolerate a few mA in this state. If you ground the RUN/SS pin, the SW pin current will drop to essentially zero. However, if the VIN pin is grounded while the output is held high, then parasitic diodes inside the LT3695 can pull large currents from the output through the SW pin and the VIN pin. Figure 8 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input. D4 MBRS140 VIN VIN GND VOUT C2 L C3 D1 R2 RT RC C1 CC GND R1 BD BOOST LT3695 RUN/SS VC GND PGND SW DA FB VOUT VIN 3695 F09 BACKUP Figure 9. A Good PCB Layout Ensures Proper, Low EMI Operation 3695 F09 Figure 8. 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 Regulator Runs Only when the Input is Present PCB Layout For proper operation and minimum EMI, care must be taken during printed circuit board layout. Figure 9 shows the recommended component placement with trace, ground plane and via locations. Note that large, switched currents flow in the LT3695’s VIN, SW and PGND pins, the catch diode 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 (COUT), should be placed on the same side of the circuit board, and their connections should be made on that layer. All connections to GND should be made at a common star ground point or directly to a local, unbroken ground plane below these components. The SW and BOOST nodes should be laid out carefully to avoid interference. Finally, keep the FB, RT and VC nodes small so that the ground traces will shield them from the SW and BOOST nodes. To keep thermal resistance low, extend the ground plane as much as possible and add thermal vias under and near the LT3695 to any additional ground planes within the circuit board and on the bottom side. Keep in mind that the thermal design must keep the junctions of the IC below the specified absolute maximum temperature. 3695f 21 LT3695 APPLICATIONS INFORMATION High Temperature Considerations The PCB must provide heat sinking to keep the LT3695 cool. The Exposed Pad on the bottom of the package may be soldered to a copper area which should be tied to large copper layers below with thermal vias; these layers will spread the heat dissipated by the LT3695. Place additional vias to 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 LT3695, it is possible to dissipate enough heat to raise the junction temperature beyond the absolute maximum. When operating at high ambient temperatures, the maximum load current should be derated as the ambient temperature approaches these maximums. If the junction temperature reaches the thermal shutdown threshold, the part will stop switching to prevent internal damage due to overheating. Table 5: Effects of Pin Shorts PINS PGND-DA SW-RUN/SS RUN/SS-RT RT -SYNC SYNC-VIN PG-GND GND-BD EFFECT No effect if VIN < VIN(MAX). See Input Voltage Range section for description of VIN(MAX). The result of this short depends on the load resistance and on R3 (Figure 10). See the following discussion. No effect or VOUT will fall below regulation voltage if IR3 (Figure 10) < 1mA. No effect or VOUT will fall below regulation voltage if the current into the RT pin is less than 1mA. No effect if VIN does not exceed the absolute maximum voltage of SYNC (20V). No effect. VOUT may fall below regulation voltage, power dissipation of the power switch will be increased. Note that this short also grounds the voltage source supplying BD. Make sure it is safe to short the supply for BD to ground. For this reason BD should not be connected to VIN, but it is safe to connect it to VOUT . If diode D2 (see Figure 10) is used, no effect or VOUT may fall below regulation voltage. Otherwise the device may be damaged. Power dissipation within the LT3695 can be estimated by calculating the total power loss from an efficiency measurement. The die temperature rise is calculated by multiplying the power dissipation of the LT3695 by the thermal resistance from junction to ambient. Die temperature rise was measured on a 2-layer, 10cm × 10cm circuit board in still air at a load current of 1A (fSW = 800kHz). For a 12V input to 5V output the die temperature elevation above ambient was 22°C with the exposed pad soldered and 44°C without the exposed pad soldered. Fault Tolerance The LT3695 is designed to tolerate single fault conditions. Shorting two adjacent pins together or leaving one single pin floating does not raise VOUT or cause damage to the LT3695. However, the application circuit must meet the requirements discussed in this section in order to achieve this tolerance level. Tables 5 and 6 show the effects that result from shorting adjacent pins or from a floating pin, respectively. BD-BOOST 3695f 22 LT3695 APPLICATIONS INFORMATION Table 6: Effects of Floating Pins PIN PGND EFFECT No effect if the Exposed Pad is soldered. Otherwise: VOUT may fall below regulation voltage. Make sure that VIN < VIN(MAX) (see Input Voltage Range section for details) and provide a bypass resistor at the DA pin. See the following discussion. VOUT may fall below regulation voltage. Make sure that VIN < VIN(MAX) (see Input Voltage Range section for details) and provide a bypass resistor. See the following discussion. VOUT will fall below regulation voltage. VOUT will fall below regulation voltage. VOUT will fall below regulation voltage. VOUT may fall below regulation voltage. A floating SYNC pin configures the LT3695 for pulse-skipping mode. However, a floating SYNC pin is sensitive to noise which can degrade device performance. VOUT will fall below regulation voltage. VOUT may fall below regulation voltage. Disconnecting the VC pin alters the loop compensation and potentially degrades device performance. The output voltage ripple will increase if the part becomes unstable. VOUT will fall below regulation voltage. No effect. Output maintains regulation, but potential degradation of device performance. VOUT may fall below regulation voltage. If BD is not connected, the boost capacitor cannot be charged and thus the power switch cannot saturate properly, which increases its power dissipation. VOUT may fall below regulation voltage. If BOOST is not connected, the boost capacitor cannot be charged and thus the power switch cannot saturate properly, which increases its power dissipation. DA SW RUN/SS RT SYNC VIN VC FB PG GND BD BOOST For the best fault tolerance to inadvertent adjacent pin shorts, the RUN/SS pin must not be directly connected to either ground or VIN. If there was a short between RUN/SS and SW then connecting RUN/SS to VIN would tie SW to VIN and would thus raise VOUT . Likewise, grounding RUN/SS would tie SW to ground and would damage the power switch if this is done when the power switch is on. A short between RT and a RUN/SS pin that is connected to VIN would violate the absolute maximum ratings of the RT pin. Therefore, the current supplying the RUN/SS pin must be limited, for example, with resistor R3 in Figure 10. In case of a short between RUN/SS and SW this resistor charges C2 through the inductor L1 if the current it supplies from VIN R3 VIN BD RUN/SS BOOST C3 RSS 47Ω RT CSS 220nF RT LT3695 SW D1 DA FB R2 C2 3695 F10 D2 L1 VOUT R1 RLOAD Figure 10. The Dashed Lines Show where a Connection Would Occur if There Were an Inadvertent Short from RUN/SS to an Adjacent Pin or from BOOST to BD. In These Cases, R3 Protects Circuitry Tied to the RT or SW Pins, and D2 Shields BOOST from VOUT. If CSS is Used for Soft Start, RSS Isolates it from SW 3695f 23 LT3695 APPLICATIONS INFORMATION VIN is not completely drawn by RLOAD, R1 + R2, and the BD pin (if connected to VOUT). Since this causes VOUT to rise, the LT3695 stops switching. The resistive divider formed by R3, RLOAD, and R1 + R2 must be adjusted for VOUT not to exceed its nominal value at the required maximum input voltage VIN(MAX). R3 must supply sufficient current into RUN/SS at the required minimum input voltage VIN(MIN) for normal non-fault situations. Based on the maximum RUN/SS current of 7.5μA at VRUN/SS = 2.5V this gives R3 ≤ VIN(MIN) – 2.5V 7.5µA Table 7 shows example values for common applications. RSS must be included as the switch node would otherwise have to charge CSS if the SW pin and the RUN/SS pin are shorted, which may damage the power switch. If RUN/SS is controlled by an external circuitry, the current this circuitry can supply must be limited. This can be done as discussed above. In addition, it may be necessary to protect this external circuitry from the voltage at SW, for example by using a diode. Table 7. Example Values for R1, R2 and R3 for Common Combinations of VIN and VOUT . IR1+R2 is the Current Drawn by R1 + R2 in Normal Operation VIN(MAX) VIN(MIN) (V) (V) 16 36 16 36 16 36 16 36 16 36 27 36 3.8 3.8 4.5 4.5 5 5 7 7 10 10 14 14 VOUT (V) 1.8 1.8 2.5 2.5 3.3 3.3 5 5 8 8 12 12 R3 (kΩ) 169 169 261 261 365 365 274 590 200 475 301 442 R1 (kΩ) 11.5 4.75 93.1 16.9 432 43.2 536 221 562 280 511 511 R2 (kΩ) 9.09 3.74 43.2 7.87 137 13.7 102 42.2 61.9 30.9 36.5 36.5 IR1+R2 (μA) 87 212 18 101 6 58 8 19 13 26 22 22 The current through R3 is maximal at VIN(MAX) with RUN/SS shorted to SW: IR3 = VIN(MAX ) – VOUT R3 This current must be drawn by RLOAD, R1 + R2, and the BD pin, if connected to VOUT: IR3 ≤ VOUT +I RLOAD || (R1+ R2) BD Without load (RLOAD = ∞) and assuming the minimum current of 35μA into the BD pin, this leads to VOUT R1+ R2 ≤ VIN(MAX ) – VOUT R3 – 35µA as upper limit for the feedback resistors. For VOUT < 2.5V assume no current drawn by the BD pin, which gives R1+ R2 ≤ VOUT • R3 VIN(MAX ) – VOUT The BOOST pin must not be shorted to a low impedance node like VOUT that clamps its voltage. For best fault tolerance, supply current into the BD pin through the Schottky diode D2 as shown in Figure 10. Note that this diode must be able to handle the maximum output current in case there is a short between the BD pin and the GND pin. A short between RUN/SS and SW may also increase the output ripple. To suppress this, connect the soft-start 3695f 24 LT3695 APPLICATIONS INFORMATION network consisting of RSS and CSS to RUN/SS as shown . in Figure 10. CSS should not be smaller than 0.22μF The SYNC pin must not be directly connected to either ground or VIN. A short between RT and a SYNC pin that is connected to VIN could violate the absolute maximum ratings of the RT pin. A short between the SYNC pin and the VIN pin could damage an external driver circuit which may be connected to SYNC or would short VIN to ground if SYNC is grounded. The recommended connection for SYNC is shown in Figure 11. If SYNC is to be driven by an external circuitry, RS may be used to isolate this circuitry from VIN. CS must be used in this case to provide a low impedance path for the synchronization signal. If SYNC is pulled low, RS prevents VIN from being shorted to ground in case of an inadvertent short between SYNC and VIN. If SYNC is pulled high to VIN, then RS protects the RT pin during an inadvertent short between SYNC and RT. If the DA pin or the PGND pin are inadvertently left floating, the current path of the catch diode is interrupted unless a bypass resistor is connected from DA to ground. Use a 360mΩ (5% tolerance) resistor rated for a power dissipation of P = I2LOAD(MAX) • 0.36 • (1 – DCMIN) where ILOAD(MAX) is the maximum load current and DCMIN is the minimum duty cycle. For example, this would require a power rating of at least 219mW for an output current of 800mA and a minimum duty cycle of 5%. Make sure not to exceed VIN(MAX) (see Input Voltage Range section for details) during start-up or overload conditions. Other Linear Technology Publications Application Notes 19, 35 and 44 contain more detailed descriptions and design information for buck regulators and other switching regulators. The LT1376 data sheet has a more extensive discussion of output ripple, loop compensation and stability testing. Design Note 318 shows how to generate a bipolar output supply using a buck regulator. VIN RS 100k SYNC CS 100pF RT RT 3695 F11 VIN SYNC LT3695 Figure 11. The Dashed Lines Show Where a Connection Would Occur if There Were an Inadvertent Short from SYNC to an Adjacent Pin. In This Case, RS Protects Circuitry Connecting to SYNC 3695f 25 LT3695 TYPICAL APPLICATIONS Fully Tolerant 3.3V Step-Down Converter with Soft-Start VIN 5V TO 28.5V TRANSIENT TO 36V VOUT 3.3V 0.9A, VIN > 5V 1A, VIN > 6.5V D2 B140 0.22μF VC 2.2μF 47Ω 0.22μF 14k 40.2k RT PG SYNC GND PGND f = 800kHz LT3695 DA FB 0.36Ω 17.8k 10μF 3695 TA02 324k VIN RUN/SS BD BOOST SW L 10μH D1 B140 56.2k 470pF 100k 1.8V Step-Down Converter VIN 3.6V TO 25V 4.7μF ON OFF VIN RUN/SS VC RT 17.4k 71.5k PG SYNC GND PGND f = 500kHz LT3695 DA FB 102k 3695 TA03 BD BOOST SW L1 0.22μF 6.8μH D1 B140 127k VOUT 1.8V 1A 330pF 100k 22μF 5V, 2MHz Step-Down Converter VIN 10V TO 16.5V TRANSIENT TO 36V 2.2μF ON OFF VOUT 5V 0.9A VIN RUN/SS VC RT 13.3k 9.76k PG SYNC GND PGND f = 2MHz LT3695 DA FB 102k 3695 TA04 BD BOOST SW L 0.22μF 4.7μH D1 B140 536k 10μF 680pF 100k 3695f 26 LT3695 PACKAGE DESCRIPTION MSE Package 16-Lead Plastic MSOP Exposed Die Pad , (Reference LTC DWG # 05-08-1667 Rev A) BOTTOM VIEW OF EXPOSED PAD OPTION 2.845 (.112 0.102 .004) 0.889 (.035 0.127 .005) 2.845 (.112 1 0.102 .004) 8 0.35 REF 5.23 (.206) MIN 1.651 (.065 0.102 3.20 – 3.45 .004) (.126 – .136) 1.651 (.065 0.102 .004) 0.12 REF 0.305 0.038 (.0120 .0015) TYP 0.50 (.0197) BSC 16 4.039 0.102 (.159 .004) (NOTE 3) DETAIL “B” CORNER TAIL IS PART OF DETAIL “B” THE LEADFRAME FEATURE. FOR REFERENCE ONLY 9 NO MEASUREMENT PURPOSE RECOMMENDED SOLDER PAD LAYOUT 16151413121110 9 0.280 0.076 (.011 .003) REF 0.254 (.010) GAUGE PLANE DETAIL “A” 0 – 6 TYP 4.90 0.152 (.193 .006) 3.00 0.102 (.118 .004) (NOTE 4) 0.53 0.152 (.021 .006) DETAIL “A” 0.18 (.007) 12345678 1.10 (.043) MAX 0.86 (.034) REF SEATING PLANE NOTE: 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX 0.17 – 0.27 (.007 – .011) TYP 0.50 (.0197) BSC 0.1016 (.004 0.0508 .002) MSOP (MSE16) 0608 REV A 3695f 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 LT3695 TYPICAL APPLICATION 5V Step-Down Converter VIN 6.9V TO 36V TRANSIENT TO 60V 2.2μF ON OFF VOUT 5V 0.9A, VIN > 6.9V 1A, VIN > 12V 0.22μF VC RT 16.2k 40.2k PG SYNC GND PGND f = 800kHz LT3695 DA FB 102k 3695 TA05 VIN RUN/SS BD BOOST 10μH SW D1 B140 536k 470pF 100k 10μF RELATED PARTS PART NUMBER DESCRIPTION LT3970 LT3689 LT3685 LT3684 LT3682 LT3508 LT3507 LT3505 LT3500 LT3493 LT3481 LT3480 LT3437 40V, 350mA, 2MHz High Efficiency MicroPower Step-Down DC/DC Converter COMMENTS VIN: 4V to 40V Transient to 60V, VOUT(MAX) = 1.21V, IQ = 2μA, ISD < 1μA, 3mm × 2mm DFN-10, MSOP-10 Packages 36V, 60V Transient Protection, 800mA, 2.2MHz High Efficiency MicroPower VIN: 3.6V to 36V Transient to 60V, VOUT(MAX) = 0.8V, IQ = 75μA, ISD < 1μA, 3mm × 3mm QFN-16 Package Step-Down DC/DC Converter with POR Reset and Watchdog Timer 36V with Transient Protection to 60V, 2A (IOUT), 2.4MHz, High Efficiency Step-Down DC/DC Converter 34V with Transient Protection to 36V, 2A (IOUT), 2.8MHz, High Efficiency Step-Down DC/DC Converter 36V, 60VMAX, 1A, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter 36V with Transient Protection to 40V, Dual 1.4A (IOUT), 3MHz, High Efficiency Step-Down DC/DC Converter 36V 2.5MHz, Triple (2.4A + 1.5A + 1.5A (IOUT)) with LDO Controller High Efficiency Step-Down DC/DC Converter 36V with Transient Protection to 40V, 1.4A (IOUT), 3MHz, High Efficiency Step-Down DC/DC Converter VIN: 3.6V to 38V, VOUT(MAX) = 0.78V, IQ = 70μA, ISD < 1μA, 3mm × 3mm DFN-10, MSOP-10E Packages VIN: 3.6V to 34V, VOUT(MAX) = 1.26V, IQ = 850μA, ISD < 1μA, 3mm × 3mm DFN-10, MSOP-10E Packages VIN: 3.6V to 36V, VOUT(MAX) = 0.8V, IQ = 75μA, ISD < 1μA, 3mm × 3mm DFN-12 Package VIN: 3.7V to 36V, VOUT(MAX) = 0.8V, IQ = 4.6mA, ISD = 1μA, 4mm × 4mm QFN-24, TSSOP-16E Packages VIN: 4V to 36V, VOUT(MAX) = 0.8V, IQ = 7mA, ISD = 1μA, 5mm × 7mm QFN-38 Package VIN: 3.6V to 34V, VOUT(MAX) = 0.78V, IQ = 2mA, ISD = 2μA, 3mm × 3mm DFN-8, MSOP-8E Packages 36V, 40VMAX, 2A, 2.5MHz High Efficiency Step-Down DC/DC Converter and VIN: 3.6V to 36V, VOUT(MAX) = 0.8V, IQ = 2.5mA, ISD < 10μA, LDO Controller 3mm × 3mm DFN-10 Package 36V, 1.4A (IOUT), 750kHz High Efficiency Step-Down DC/DC Converter 34V with Transient Protection to 36V, 2A (IOUT), 2.8MHz, High Efficiency Step-Down DC/DC Converter with Burst Mode Operation 36V with Transient Protection to 60V, 2A (IOUT), 2.4MHz, High Efficiency Step-Down DC/DC Converter with Burst Mode Operation 60V, 400mA (IOUT), MicroPower Step-Down DC/DC Converter with Burst Mode Operation VIN: 3.6V to 36V, VOUT(MAX) = 0.8V, IQ = 1.9mA, ISD < 1μA, 2mm × 3mm DFN-6 Package VIN: 3.6V to 34V, VOUT(MAX) = 1.26V, IQ = 50μA, ISD < 1μA, 3mm × 3mm DFN-10, MSOP-10E Packages VIN: 3.6V to 38V, VOUT(MAX) = 0.78V, IQ = 70μA, ISD < 1μA, 3mm × 3mm DFN-10, MSOP-10E Packages VIN: 3.3V to 60V, VOUT(MAX) = 1.25V, IQ = 100μA, ISD < 1μA, 3mm × 3mm DFN-10, TSSOP-16E Package VIN: 3.3V to 60V, VOUT(MAX) = 1.2V, IQ = 100μA, ISD < 1μA, TSSOP-16E Package VIN: 3.3V to 60V, VOUT(MAX) = 1.2V, IQ = 100μA, ISD < 1μA, TSSOP-16E Package VIN: 3.6V to 36V, VOUT(MAX) = 1.2V, IQ = 1.9mA, ISD < 1μA, MS8E Package VIN: 5.5V to 60V, VOUT(MAX) = 1.2V, IQ = 2.5mA, ISD = 25μA, TSSOP-16/E Package 3695f LT 0709 • PRINTED IN USA LT3434/LT3435 60V, 2.4A (IOUT), 200kHz/500kHz, High Efficiency Step-Down DC/DC Converter with Burst Mode Operation LT1976/LT1977 60V, 1.2A (IOUT), 200kHz/500kHz, High Efficiency Step-Down DC/DC Converter with Burst Mode Operation LT1936 LT1766 36V, 1.4A (IOUT), 500kHz High Efficiency Step-Down DC/DC Converter 60V, 1.2A (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter 28 Linear Technology Corporation (408) 432-1900 ● FAX: (408) 434-0507 ● 1630 McCarthy Blvd., Milpitas, CA 95035-7417 www.linear.com © LINEAR TECHNOLOGY CORPORATION 2009