LT3971A/LT3971A-5 38V, 1.3A, 2MHz Step-Down Regulator with 2.2µA Quiescent Current DESCRIPTION FEATURES n n n n n n n n n n n n n n n Ultralow Quiescent Current: 2.8μA IQ Regulating 12VIN to 3.3VOUT Fixed Output Voltages: 5V 2.2μA IQ Regulating 12VIN to 5VOUT Low Ripple Burst Mode® Operation: Output Ripple < 15mVP-P Wide Input Voltage Range: 4.3V to 38V 1.3A Maximum Output Current Adjustable Switching Frequency: 200kHz to 2MHz Synchronizable Between 250kHz to 2MHz Fast Transient Response Accurate 1V Enable Pin Threshold Low Shutdown Current: IQ = 700nA Power Good Flag Soft-Start Capability Internal Compensation Output Voltage: 1.19V to 30V Small Thermally Enhanced 10-Lead MSOP Package The LT®3971A is an adjustable frequency monolithic buck switching regulator that accepts a wide input voltage range up to 38V. Low quiescent current design consumes only 2.8μA of supply current while regulating with no load. Low ripple Burst Mode operation maintains high efficiency at low output currents while keeping the output ripple below 15mV in a typical application. An internally compensated current mode topology is used for fast transient response and good loop stability. A high efficiency 0.33Ω switch is included on the die along with a boost Schottky diode and the necessary oscillator, control and logic circuitry. An accurate 1V threshold enable pin can be used to shut down the LT3971A, reducing the input supply current to 700nA. A capacitor on the SS pin provides a controlled inrush current (soft-start). A power good flag signals when VOUT reaches 91% of the programmed output voltage. The LT3971A is available in a small 10-lead MSOP package with an exposed pad for low thermal resistance. The LT3971A and LT3971A-5 have a more accurate reference voltage compared to the LT3971 and LT3971-5. The LT3971A-5 also has specified VOUT pin current and ABSMAX current. These characteristics are important for USB applications. APPLICATIONS n n n n USB VBUS Regulation Automotive Battery Regulation Power for Portable Products Industrial Supplies L, LT, LTC, LTM, Linear Technology, Burst Mode and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. TYPICAL APPLICATION No Load Supply Current 5.15V Step-Down Converter for USB Applications 3.0 OUTPUT IN REGULATION OFF ON EN VIN BOOST 0.47μF PG SS 4.7μF INPUT CURRENT (μA) VIN 6.3V TO 38V 10μH SW LT3971A-5 RT BD 20k, 1% 118k f = 400kHz SYNC GND VOUT 715k 1% 47μF VOUT 5.15V 1.3A 2.5 LT3971A-5 2.0 1.5 1.0 5 3971 TA01a 10 15 20 25 30 INPUT VOLTAGE (V) 35 3971A TA01b 3971af 1 LT3971A/LT3971A-5 ABSOLUTE MAXIMUM RATINGS (Note 1) VIN, EN Voltage .........................................................38V BOOST Pin Voltage ...................................................55V BOOST Pin Above SW Pin.........................................30V FB, RT, SYNC, SS Voltage ...........................................6V PG, BD Voltage .........................................................30V VOUT Pin Current ....................................................–2mA Operating Junction Temperature Range (Note 2) LT3971AI/LT3971AI-5 ........................ –40°C to 125°C Storage Temperature Range .............. –65°C to 150°C Lead Temperature (Soldering, 10 sec) (MSE Only) ....................................................... 300°C PIN CONFIGURATION LT3971A LT3971A-5 TOP VIEW TOP VIEW BD BOOST SW VIN EN 1 2 3 4 5 11 GND 10 9 8 7 6 SYNC PG RT SS FB BD BOOST SW VIN EN 1 2 3 4 5 11 GND 10 9 8 7 6 SYNC PG RT SS VOUT MSE PACKAGE 10-LEAD PLASTIC MSOP MSE PACKAGE 10-LEAD PLASTIC MSOP θJA = 45°C, θJC = 10°C/W EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB θJA = 45°C, θJC = 10°C/W EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT3971AIMSE#PBF LT3971AIMSE#TRPBF LTGFQ 10-Lead Plastic MSOP –40°C to 125°C LT3971AIMSE-5#PBF LT3971AIMSE-5#TRPBF LTGFR 10-Lead Plastic MSOP –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 The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VEN = 12V, VBD = 3.3V unless otherwise noted. (Note 2) PARAMETER Minimum Input Voltage Quiescent Current from VIN LT3971A FB Pin Current Internal Feedback Resistor Divider (LT3971A-5) CONDITIONS (Note 4) VEN Low VEN High, VSYNC Low VEN High, VSYNC Low VFB = 1.19V MIN l l l TYP 4 0.7 1.7 0.1 10 MAX 4.3 1.2 2.7 4.5 12 UNITS V μA μA μA nA MΩ 3971af 2 LT3971A/LT3971A-5 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VEN = 12V, VBD = 3.3V unless otherwise noted. (Note 2) VOUT Pin Current VOUT Pin Clamp Voltage Feedback Voltage VOUT = 5V VOUT = 5V IVOUT = –2mA l l LT3971A-5 Output Voltage FB Voltage Line Regulation Switching Frequency Minimum Switch On Time Minimum Switch Off Time Switch Current Limit Switch VCESAT Switch Leakage Current Boost Schottky Forward Voltage Boost Schottky Reverse Leakage Minimum Boost Voltage (Note 3) BOOST Pin Current EN Voltage Threshold EN Voltage Hysteresis EN Pin Current LT3971A PG Threshold Offset from VFB LT3971A PG Hysteresis LT3971A-5 PG Threshold Offset from VOUT LT3971A-5 PG Hysteresis PG Leakage PG Sink Current SYNC Threshold SYNC Pin Current SS Source Current l 4.3V < VIN < 38V (Note 4) RT = 11k RT = 35.7k RT = 255k –0.65 –0.90 9 1.176 1.173 4.94 4.93 1.8 0.8 160 2.0 ISW = 1A ISH = 100mA VREVERSE = 12V VIN = 5V ISW = 1A, VBOOST = 15V EN Rising l l 0.95 VFB Rising 60 VOUT Rising 5.5 VPG = 3V VPG = 0.4V VSS = 1V Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LT3971AI is guaranteed over the full –40°C to 125°C operating junction temperature range. High junction temperatures degrade operating lifetimes. Operating lifetime is derated at junction temperatures greater than 125°C. l 300 0.6 0.6 –0.5 –0.5 11 1.192 1.19 5.01 5 0.0002 2.2 1 200 80 110 2.5 330 0.02 770 0.02 1.4 20 1.01 30 0.2 100 20 9 1.3 0.02 570 0.8 0.1 1 –0.38 –0.32 13 1.204 1.207 5.06 5.07 0.01 2.6 1.2 240 150 3.2 1 1 1.8 28 1.07 20 140 12.5 1 1.0 1.6 μA μA V V V V V %/V MHz MHz kHz ns ns A mV μA mV μA V mA V mV nA mV mV % % μA μA V nA μA Note 3: This is the minimum voltage across the boost capacitor needed to guarantee full saturation of the switch. Note 4: Minimum input voltage depends on application circuit. 3971af 3 LT3971A/LT3971A-5 TYPICAL PERFORMANCE CHARACTERISTICS Efficiency, VOUT = 5V Efficiency, VOUT = 3.3V 100 VIN = 12V VOUT = 5V 90 R1 = 1M R2 = 309k 80 VIN = 12V 70 VIN = 36V VIN = 24V 60 50 40 70 VIN = 36V 60 EFFICIENCY (%) 80 EFFICIENCY (%) EFFICIENCY (%) 100 90 80 20 Efficiency, VOUT = 5V 100 90 30 TA = 25°C, unless otherwise noted. VIN = 24V 50 0 0.2 0.4 0.6 0.8 LOAD CURRENT (A) 1 1.2 40 10 0 0.2 0.4 0.6 0.8 LOAD CURRENT (A) 3971A G01 1 0 0.01 1.2 0.1 1 10 100 LOAD CURRENT (mA) 3971A G02 Efficiency, VOUT = 3.3V 100 1000 3971A G03 No Load Supply Current 90 80 VIN = 36V VIN = 24V 50 20 30 20 60 30 40 VOUT = 5V R1 = 1M R2 = 309k VIN = 12V 70 No Load Supply Current 4.0 DIODES, INC. DFLS2100 VIN = 12V 3.5 VIN = 24V 50 VIN = 36V 40 30 INPUT CURRENT (μA) 60 INPUT CURRENT (μA) EFFICIENCY (%) 70 10 20 LT3971A VOUT = 3.3V 3.0 2.5 LT3971A-5 2.0 1.5 10 0 0.01 0.1 1 10 100 LOAD CURRENT (mA) 1 –55 1000 1.0 –25 5 35 65 95 TEMPERATURE (°C) 3971A G04 155 5 LT3971A-5 Output Voltage 5.04 2.5 1.180 1.175 –55 LOAD CURRENT (A) 1.200 OUTPUT VOLTAGE (V) 3.0 1.185 5.02 5.00 4.98 4.96 –25 5 35 65 95 TEMPERATURE (°C) 125 155 3971A G07 4.94 –55 35 Maximum Load Current 5.06 1.190 15 20 25 30 INPUT VOLTAGE (V) 3971A G06 1.205 1.195 10 3971A G05 LT3971A Feedback Voltage FEEDBACK VOLTAGE (V) 125 VOUT = 3.3V TYPICAL 2.0 MINIMUM 1.5 1.0 0.5 –25 5 35 65 95 TEMPERATURE (°C) 125 155 3971A G08 0 5 10 15 20 25 30 INPUT VOLTAGE (V) 35 40 3971A G09 3971af 4 LT3971A/LT3971A-5 TYPICAL PERFORMANCE CHARACTERISTICS 0.30 VOUT = 5V 950 0.20 1.5 MINIMUM 1.0 0.5 0.15 900 FREQUENCY (kHz) TYPICAL LOAD REGULATION (%) LOAD CURRENT (A) 1000 0.25 2.0 0.10 0.05 0 –0.05 –0.10 –0.15 –0.30 5 25 30 15 20 INPUT VOLTAGE (V) 10 35 40 850 800 750 700 –0.20 650 –0.25 0 Switching Frequency Load Regulation Maximum Load Current 2.5 TA = 25°C, unless otherwise noted. REFERENCED FROM VOUT AT 0.5A LOAD 0 200 400 600 800 1000 LOAD CURRENT (mA) 600 –55 1200 –25 5 35 65 95 TEMPERATURE (°C) 3971A G11 3971A G10 155 3971A G12 Switch VCESAT Switch Current Limit Switch Current Limit 3.0 125 2.5 600 2.0 1.5 1.0 0.5 500 2.3 2.2 400 VCESAT (mV) SWITCH CURRENT LIMIT (A) SWITCH CURRENT LIMIT (A) 2.4 2.5 2.1 2.0 1.9 300 200 1.8 1.7 100 1.6 0 0 20 40 60 DUTY CYCLE (%) 80 DUTY CYCLE = 30% 1.5 –55 –25 5 35 65 95 TEMPERATURE (°C) 100 125 3971A G13 Boost Pin Current 15 10 5 900 800 800 700 600 500 400 300 200 100 0 250 500 750 1000 1250 SWITCH CURRENT (mA) 1500 3971A G16 0 500 750 1000 1250 SWITCH CURRENT (mA) 1500 LT3971A-5 Frequency Foldback 900 SWITCHING FREQUENCY (kHz) SWITCHING FREQUENCY (kHz) BOOST PIN CURRENT (mA) 20 250 3971A G15 Frequency Foldback 25 0 3971A G14 30 0 0 155 700 600 500 400 300 200 100 0 0.2 0.6 0.4 0.8 FB PIN VOLTAGE (V) 1 1.2 3971A G17 0 0 20 60 40 80 VOUT (% OF REGULATION VOLTAGE) 100 3971A G18 3971af 5 LT3971A/LT3971A-5 TYPICAL PERFORMANCE CHARACTERISTICS Minimum Switch On-Time/ Switch Off-Time TA = 25°C, unless otherwise noted. Minimum Input Voltage Soft-Start 400 5.0 2.5 MIN TOFF 1A LOAD 250 200 MIN TOFF 0.5A LOAD 150 100 MIN TON 4.6 2.0 INPUT VOLTAGE (V) SWITCH CURRENT LIMIT (A) SWITCH ON/OFF TIME (ns) 300 1.5 1.0 4.2 TO START 4.0 3.8 TO RUN 3.6 3.2 –25 35 95 5 65 TEMPERATURE (°C) 125 0 155 3.0 0 0.25 0.5 0.75 1 1.25 1.5 1.75 SS PIN VOLTAGE (V) 3971A G19 VOUT = 5V THRESHOLD VOLTAGE (V) 6.0 TO START 5.8 5.6 5.4 1.6 1.04 1.4 1.03 1.02 RISING THRESHOLD 1.01 1.00 0.99 FALLING THRESHOLD 0.98 200 400 800 1000 600 LOAD CURRENT (mA) 1200 1.2 1.0 0.8 0.6 0.2 0.95 –55 –25 5 35 65 95 TEMPERATURE (°C) 3971A G22 125 155 0 0 250 500 750 1000 1250 BOOST DIODE CURRENT (mA) 3971A G23 1500 3971A G24 Transient Load Response, Load Current Stepped from 25mA (Burst Mode Operation) to 525mA Power Good Threshold 1200 0.4 0.96 0 600 400 800 1000 LOAD CURRENT (mA) Boost Diode Forward Voltage 1.05 0.97 TO RUN 200 3971A G21 EN Threshold 6.2 5.2 0 3971A G20 Minimum Input Voltage 6.4 2 BOOST DIODE VF (V) 0 –55 INPUT VOLTAGE (V) 4.4 3.4 0.5 50 5.0 VOUT = 3.3V 4.8 350 Transient Load Response, Load Current Stepped from 0.5A to 1A 95 THRESHOLD VOLTAGE (%) 94 93 VOUT 100mV/ DIV VOUT 100mV/DIV 92 91 90 89 IL 500mA/ DIV 88 87 IL 500mA/DIV 86 85 –55 –25 5 35 65 95 TEMPERATURE (°C) 125 155 3971A G25 10μs/DIV VIN = 12V, VOUT = 3.3V COUT = 47μF 3971A G26 10μs/DIV VIN = 12V, VOUT = 3.3V COUT = 47μF 3971A G27 3971af 6 LT3971A/LT3971A-5 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted. Switching Waveforms; Full Frequency Continuous Operation Switching Waveforms; Burst Mode Operation 3.3V Start-Up and Dropout 800kHz 3kΩ LOAD VSW 5V/DIV VSW 5V/DIV VIN 1V/DIV IL 500mA/DIV IL 500mA/DIV VOUT 20mV/DIV VOUT 20mV/DIV VOUT 3971A G28 5μs/DIV VIN = 12V, VOUT = 3.3V ILOAD = 10mA COUT = 22μF 3971A G29 1μs/DIV VIN = 12V, VOUT = 3.3V ILOAD = 1A COUT = 22μF 3.3V Start-Up and Dropout 5V Start-Up and Dropout 800kHz 6.7kΩ LOAD 800kHz 5kΩ LOAD 3971A G30 0.5s/DIV 5V Start-Up and Dropout 800kHz 10Ω LOAD VIN VIN VIN 1V/DIV 1V/DIV 1V/DIV VOUT VOUT VOUT 3971A G31 0.5s/DIV 3971A G32 0.5s/DIV 5 1.2 4 0.8 0.4 0 3971A G33 Minimum Input Voltage to Switch 1.6 INPUT VOLTAGE (V) FEEDBACK VOLTAGE (V) Feedback Regulation Voltage 0.5s/DIV 3 2 2 2.5 3 3.5 4 INPUT VOLTAGE (V) 4.5 5 3971A G34 1 –55 –25 5 35 65 95 TEMPERATURE (°C) 125 155 3971A G35 3971af 7 LT3971A/LT3971A-5 PIN FUNCTIONS BD (Pin 1): This pin connects to the anode of the boost diode. The BD pin is normally connected to the output. BOOST (Pin 2): This pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar NPN power switch. SW (Pin 3): The SW pin is the output of an internal power switch. Connect this pin to the inductor, catch diode, and boost capacitor. VIN (Pin 4): The VIN pin supplies current to the LT3971A’s internal circuitry and to the internal power switch. This pin must be locally bypassed. EN (Pin 5): The part is in shutdown when this pin is low and active when this pin is high. The hysteretic threshold voltage is 1.005V going up and 0.975V going down. The EN threshold is only accurate when VIN is above 4.3V. If VIN is lower than 4.2V, ground EN to place the part in shutdown. Tie to VIN if shutdown feature is not used. FB (Pin 6, LT3971A Only): The LT3971A regulates the FB pin to 1.19V. Connect the feedback resistor divider tap to this pin. Also, connect a phase lead capacitor between FB and VOUT. Typically this capacitor is 10pF. SS (Pin 7): A capacitor is tied between SS and ground to slowly ramp up the peak current limit of the LT3971A on start-up. The soft-start capacitor is only actively discharged when EN is low. The SS pin is released when the EN pin goes high. Float this pin to disable soft-start. For applications with input voltages above 25V, add a 100k resistor in series with the soft-start capacitor. RT (Pin 8): A resistor is tied between RT and ground to set the switching frequency. PG (Pin 9): The PG pin is the open-drain output of an internal comparator. PGOOD remains low until the FB pin is within 9% of the final regulation voltage. PGOOD is valid when the LT3971A is enabled and VIN is above 4.3V. SYNC (Pin 10): This is the external clock synchronization input. Ground this pin for low ripple Burst Mode operation at low output loads. Tie to a clock source for synchronization, which will include pulse-skipping at low output loads. When in pulse-skipping mode, quiescent current increases to 1.5mA. GND (Exposed Pad Pin 11): Ground. The exposed pad must be soldered to PCB. VOUT (Pin 6, LT3971A-5 Only): The LT3971A-5 regulates the VOUT pin to 5V. This pin connects to the internal 10MΩ feedback divider that programs the fixed output voltage. 3971af 8 LT3971A/LT3971A-5 BLOCK DIAGRAM VIN C1 INTERNAL 1.19V REF 1V EN RT – + VIN + – Σ SHDN BD SWITCH LATCH SLOPE COMP BOOST R OSCILLATOR 200kHz TO 2MHz RT C3 Q S L1 VOUT SW Burst Mode DETECT SYNC PG ERROR AMP + – + – 1.09V D1 VC CLAMP VC 1μA SS C5 SHDN R2 GND R3 C4 R1 VOUT FB R2 C2 R1 LT3971A-5 ONLY 3971A BD LT3971A ONLY LT3971A-5: R1 = 7.62M, R2 = 2.38M C5 OPERATION The LT3971A is a constant frequency, current mode stepdown regulator. An oscillator, with frequency set by RT, sets 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 (see Block Diagram). An error amplifier measures the output voltage through an external resistor divider tied to the FB pin and servos the VC node. 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 node provides current limit. The VC node is also clamped by the voltage on the SS pin; soft-start is implemented by generating a voltage ramp at the SS pin using an external capacitor. If the EN pin is low, the LT3971A is shut down and draws 700nA from the input. When the EN pin exceeds 1V, the switching regulator will become active. The switch driver operates from either VIN 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. To further optimize efficiency, the LT3971A 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 1.7μA. In a typical application, 2.8μA will be consumed from the supply when regulating with no load. The oscillator reduces the LT3971A’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. The LT3971A contains a power good comparator which trips when the FB pin is at 91% of its regulated value. The PG output is an open-drain 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 LT3971A is enabled and VIN is above 4.3V. 3971af 9 LT3971A/LT3971A-5 APPLICATIONS INFORMATION Achieving Ultralow Quiescent Current To enhance efficiency at light loads, the LT3971A operates in low ripple Burst Mode, which keeps the output capacitor charged to the desired output voltage while minimizing the input quiescent current. In Burst Mode operation the LT3971A delivers single pulses of current to the output capacitor followed by sleep periods where the output power is supplied by the output capacitor. When in sleep mode the LT3971A consumes 1.7μA, but when it turns on all the circuitry to deliver a current pulse, the LT3971A consumes 1.5mA of input current in addition to the switch current. Therefore, the total quiescent current will be greater than 1.7μA when regulating. As the output load decreases, the frequency of single current pulses decreases (see Figure 1) and the percentage of time the LT3971A is in sleep mode increases, resulting in much higher light load efficiency. By maximizing the time between pulses, the converter quiescent current gets closer to the 1.7μA ideal. Therefore, to optimize the quiescent current performance at light loads, the current in the feedback resistor divider and the reverse current in the catch diode must be minimized, as these appear to the output as load currents. Use the largest possible feedback resistors and a low leakage Schottky catch diode in applications utilizing the ultralow quiescent current performance of the LT3971A. The feedback resistors should preferably be on the order of MΩ and the Schottky catch diode should have less than 1μA of typical reverse SWITCHING FREQUENCY (kHz) 1000 VIN = 12V VOUT = 3.3V 800 It is important to note that another way to decrease the pulse frequency is to increase the magnitude of each single current pulse. However, this increases the output voltage ripple because each cycle delivers more power to the output capacitor. The magnitude of the current pulses was selected to ensure less than 15mV of output ripple in a typical application. See Figure 2. VSW 5V/DIV IL 500mA/DIV VOUT 20mV/DIV 5μs/DIV 3971A F02 VIN = 12V VOUT = 3.3V ILOAD = 10mA Figure 2. Burst Mode Operation While in Burst Mode operation, the burst frequency and the charge delivered with each pulse will not change with output capacitance. Therefore, the output voltage ripple will be inversely proportional to the output capacitance. In a typical application with a 22μF output capacitor, the output ripple is about 10mV, and with a 47μF output capacitor the output ripple is about 5mV. The output voltage ripple can continue to be decreased by increasing the output capacitance. At higher output loads (above 92mA for the front page application) the LT3971A 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 will exhibit slight frequency jitter, but will not disturb the output voltage. 600 400 200 0 leakage at room temperature. These two considerations are reiterated in the FB Resistor Network and Catch Diode Selection sections. 0 20 40 60 80 LOAD CURRENT (mA) 100 120 3971A F01 Figure 1. Switching Frequency in Burst Mode Operation 3971af 10 LT3971A/LT3971A-5 APPLICATIONS INFORMATION To ensure proper Burst Mode operation, the SYNC pin must be grounded. When synchronized with an external clock, the LT3971A will pulse skip at light loads. The quiescent current will significantly increase to 1.5mA in light load situations when synchronized with an external clock. Holding the SYNC pin high yields no advantages in terms of output ripple or minimum load to full frequency, so is not recommended. FB Resistor Network The LT3971A output voltage is programmed with a resistor divider between the output and the FB pin. Choose the resistor values according to: ⎛ V ⎞ R1= R2 ⎜ OUT − 1⎟ ⎝ 1.19V ⎠ Reference designators refer to the Block Diagram. 1% resistors are recommended to maintain output voltage accuracy. The total resistance of the FB resistor divider should be selected to be as large as possible to enhance low current performance. The resistor divider generates a small load on the output, which should be minimized to optimize the low supply current at light loads. When using large FB resistors, a 10pF phase lead capacitor should be connected from VOUT to FB. VOUT Pin The LT3971A-5 contains an internal 10M feedback resistor divider as well as an internal phase lead capacitor to feedback the output voltage information to the internal circuitry. The output will be regulated to 5V when connected directly to the VOUT pin. An external resistor divider can be added to shift the output voltage higher than 5V. For example, a USB VBUS supply programmed to 5.15V allows some voltage drop through connectors and cables, while keeping the VBUS voltage within specification at the device. By using the LT3971A-5, two external 1% resistors program the output to 5.15V without degrading initial accuracy, which is determined primarily by the LT3971A-5 output voltage specification. The external resistor divider must be small compared to the internal 10M to maintain output voltage accuracy. The 0.5μA into the VOUT pin is process and temperature dependent, so will degrade the accuracy of the output voltage if it is not small compared to the total current in the external resistor divider. Choose the resistor values according to: R1 = VOUT – 5 5 + 0.0005 R2 R1 and R2 refer to the components in Figure 3 in kΩ. 1% resistors should be used to maintain output voltage accuracy. It should also be noted that the smaller the resistor values the more the input current will increase during no load conditions. OUTPUT LT3971A-5 R1 VOUT R2 3971A F03 Figure 3. Resistor Divider Used to Increase the Output Voltage Above 5V The VOUT pin has an internal 11V clamp. Output voltage transients above 11V can be tolerated as long as there is enough series resistance to limit the current into the 11V clamp to less than 2mA. 3971af 11 LT3971A/LT3971A-5 APPLICATIONS INFORMATION Setting the Switching Frequency The LT3971A uses a constant frequency PWM architecture that can be programmed to switch from 200kHz to 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. the typical minimum on and off curves to design for an application’s maximum temperature, while adding about 30% for part-to-part variation. The minimum and maximum duty cycles that can be achieved taking minimum on and off times into account are: DCMIN = fSW tON(MIN) DCMAX = 1− fSW tOFF(MIN) Table 1. Switching Frequency vs RT Value SWITCHING FREQUENCY (MHz) RT VALUE (kΩ) 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 255 118 71.5 49.9 35.7 28.0 22.1 17.4 14.0 11.0 where fSW is the switching frequency, the tON(MIN) is the minimum switch on-time, and the tOFF(MIN) is the minimum switch off-time. 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. Operating Frequency Tradeoffs Input Voltage Range Selection of the operating frequency is a tradeoff 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: The minimum input voltage is determined by either the LT3971A’s minimum operating voltage of 4.3V or by its maximum duty cycle (see equation in Operating Frequency Tradeoffs section). The minimum input voltage due to duty cycle is: 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 slower 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. The input voltage range depends on the switching frequency because the LT3971A switch has finite minimum on and off times. The minimum switch on and off times are strong functions of temperature. Use VIN(MIN) = VOUT + VD − VD + VSW 1− fSW tOFF(MIN) where VIN(MIN) is the minimum 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 tOFF(MIN) is the minimum switch off-time. 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 LT3971A applications depends on switching frequency, the Absolute Maximum Ratings of the VIN and BOOST pins, and the operating mode. For a given application where the switching frequency and the output voltage are already selected, the 3971af 12 LT3971A/LT3971A-5 APPLICATIONS INFORMATION maximum input voltage (VIN(OP-MAX)) that guarantees optimum output voltage ripple for that application can be found by applying the following equation: VIN(OP-MAX) = VOUT + VD –V +V fSW • tON(MIN) D SW where tON(MIN) is the minimum switch on-time. Note that a higher switching frequency will decrease the maximum operating input voltage. Conversely, a lower switching frequency will be necessary to achieve normal operation at higher input voltages. The circuit will tolerate inputs above the maximum operating input voltage and up to the Absolute Maximum Ratings of the VIN and BOOST pins, regardless of chosen switching frequency. However, during such transients where VIN is higher than VIN(OP-MAX), the LT3971A will enter pulse-skipping operation where some switching pulses are skipped to maintain output regulation. The output voltage ripple and inductor current ripple will be higher than in typical operation. Do not overload when VIN is greater than VIN(OP-MAX). the saturation current should be above 3.5A. 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. The inductor value must be sufficient to supply the desired maximum output current (IOUT(MAX)), which is a function of the switch current limit (ILIM) and the ripple current. IOUT(MAX) =ILIM – ΔIL 2 The LT3971A limits its peak switch current in order to protect itself and the system from overload faults. The LT3971A’s switch current limit (ILIM) is typically 2.5A at low duty cycles and decreases linearly to 1.75A at DC = 0.8. Table 2. Inductor Vendors VENDOR URL PART SERIES TYPE Murata www.murata.com LQH55D Open TDK www.componenttdk.com SLF7045 SLF10145 Shielded Shielded Toko www.toko.com D62CB D63CB D73C D75F Shielded Shielded Shielded Open Coilcraft www.coilcraft.com MSS7341 MSS1038 Shielded Shielded Sumida www.sumida.com CR54 CDRH74 CDRH6D38 CR75 Open Shielded Shielded Open Inductor Selection and Maximum Output Current A good first choice for the inductor value is: L= VOUT + VD 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. For robust operation in fault conditions (start-up or short-circuit) and high input voltage (>30V), 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 3971af 13 LT3971A/LT3971A-5 APPLICATIONS INFORMATION Where fSW is the switching frequency of the LT3971A, DC is the duty cycle and L is the value of the inductor. Therefore, the maximum output current that the LT3971A will deliver depends on the switch current limit, the inductor value, and the input and output voltages. The inductor value may have to be increased if the inductor ripple current does not allow sufficient maximum output current (IOUT(MAX)) given the switching frequency, and maximum input voltage used in the desired application. 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, than 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 the 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 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. See Application Note 19. 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 the equations above to check that the LT3971A 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 ∆IL/2. Input Capacitor Bypass the input of the LT3971A 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 4.7μF to 10μF ceramic capacitor is adequate to bypass the LT3971A and will easily handle the ripple current. Note that larger input capacitance is required when a lower switching frequency is used (due to longer on-times). 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 low 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 LT3971A and to force this very high frequency switching current into a tight local loop, minimizing EMI. A 4.7μF capacitor is capable of this task, but only if it is placed close to the LT3971A (see the PCB Layout section). A second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the LT3971A. A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT3971A circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3971A’s voltage rating. This situation is easily avoided (see the Hot Plugging Safely section). Output Capacitor and Output Ripple The output capacitor has two essential functions. Along with the inductor, it filters the square wave generated by the LT3971A to produce the DC output. In this role it determines the output ripple, so low impedance (at the switching frequency) is important. The second function is to store energy in order to satisfy transient loads and stabilize the LT3971A’s control loop. Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance. A good starting value is: COUT = 100 VOUT fSW 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. Increasing the output capacitance will also decrease the output voltage ripple. A lower value of output capacitor can be used to save space and cost but transient performance will suffer. 3971af 14 LT3971A/LT3971A-5 APPLICATIONS INFORMATION 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. Table 3 lists several capacitor vendors. Table 4. Schottky Diodes. The Reverse Current Values Listed Are Estimates Based Off of Typical Curves for Reverse Current vs Reverse Voltage at 25°C. Table 3. Recommended Ceramic Capacitor Vendors On Semiconductor PART NUMBER VR (V) IAVE (A) MANUFACTURER WEBSITE MBR0520L 20 0.5 VF at 1A (mV) VF at 2A (mV) IR at VR = 20V 25°C (μA) 30 AVX www.avxcorp.com MBR0540 40 0.5 620 Murata www.murata.com MBRM120E 20 1 530 Taiyo Yuden www.t-yuden.com MBRM140 40 1 550 Vishay Siliconix www.vishay.com Diodes Inc. TDK www.tdk.com 30 0.5 B0540W 40 0.5 620 1 Catch Diode Selection B120 20 1 500 1.1 The catch diode (D1 from Block Diagram) conducts current only during switch off time. Average forward current in normal operation can be calculated from: B130 30 1 500 1.1 B140 40 1 500 1.1 B150 50 1 700 0.4 B220 20 2 500 20 B230 30 2 500 0.6 B140HB 40 1 V –V ID(AVG) =IOUT IN OUT VIN where IOUT is the output load current. The only reason to consider a diode with a larger current rating than necessary for nominal operation is for the worst-case condition of shorted output. The diode current will then increase to the typical peak switch current. Peak reverse voltage is equal to the regulator input voltage. Use a diode with a reverse voltage rating greater than the input voltage. B0530W 0.4 595 0.5 20 15 1 DFLS240L 40 2 DFLS140 40 1.1 510 500 DFLS160 60 1 500 DFLS2100 100 2 770 B240 40 2 4 1 2.5 860 0.01 500 0.45 Central Semiconductor CMSH1 - 40M 40 1 500 CMSH1 - 60M 60 1 700 CMSH1 - 40ML 40 1 400 CMSH2 - 40M 40 2 550 CMSH2 - 60M 60 2 700 CMSH2 - 40L 40 2 400 CMSH2 - 40 40 2 500 CMSH2 - 60M 60 2 700 3971af 15 LT3971A/LT3971A-5 APPLICATIONS INFORMATION An additional consideration is reverse leakage current. When the catch diode is reversed biased, any leakage current will appear as load current. When operating under light load conditions, the low supply current consumed by the LT3971A will be optimized by using a catch diode with minimum reverse leakage current. Low leakage Schottky diodes often have larger forward voltage drops at a given current, so a trade-off can exist between low load and high load efficiency. Often Schottky diodes with larger reverse bias ratings will have less leakage at a given output voltage than a diode with a smaller reverse bias rating. Therefore, superior leakage performance can be achieved at the expense of diode size. Table 4 lists several Schottky diodes and their manufacturers. For outputs of 3V and above, 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 (Figure 4b). For output voltages below 2.5V, the boost diode can be tied to the input (Figure 4c), or to another external supply greater than 2.8V. However, the circuit in Figure 4a is more efficient because the BOOST pin current comes from a lower voltage source. You must also be sure that the maximum voltage ratings of the BOOST and BD pins are not exceeded. Ceramic Capacitors Ceramic capacitors are small, robust and have very low ESR. However, ceramic capacitors can cause problems when used with the LT3971A due to their piezoelectric nature. When in Burst Mode operation, the LT3971A’s switching frequency depends on the load current, and at very light loads the LT3971A can excite the ceramic capacitor at audio frequencies, generating audible noise. Since the LT3971A operates at a lower current limit during Burst Mode operation, the noise is typically very quiet to a casual ear. If this is unacceptable, use a high performance tantalum or electrolytic capacitor at the output. A final precaution regarding ceramic capacitors concerns the maximum input voltage rating of the LT3971A. As previously mentioned, a ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT3971A circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3971A’s rating. This situation is easily avoided (see the Hot Plugging Safely section). BD VIN BOOST LT3971A 4.7μF GND C3 SW VOUT (4a) For VOUT > 2.8V D2 BD VIN VIN BOOST LT3971A 4.7μF GND C3 SW VOUT (4b) For 2.5V < VOUT < 2.8V BD VIN VIN BOOST LT3971A BOOST and BD 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 0.47μF capacitor will work well. Figure 4 shows three ways to arrange the boost circuit. The BOOST pin must be more than 2.3V above the SW pin for best efficiency. VIN 4.7μF GND C3 VOUT SW 3971A FO4 (4c) For VOUT < 2.5V; VIN(MAX) = 27V Figure 4. Three Circuits for Generating the Boost Voltage 3971af 16 LT3971A/LT3971A-5 APPLICATIONS INFORMATION The minimum operating voltage of an LT3971A application is limited by the minimum input voltage (4.3V) and by the maximum duty cycle as outlined in the Input Voltage Range section. For proper start-up, the minimum input voltage is also limited by the boost circuit. If the input voltage is ramped slowly, 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 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. 5.0 4.8 INPUT VOLTAGE (V) 4.6 4.4 TO START 4.2 4.0 TO RUN 3.8 3.6 3.4 VOUT = 3.3V TA = 25°C 3.2 L = 4.7μH f = 800kHz 3.0 10 100 LOAD CURRENT (mA) 1000 6.4 INPUT VOLTAGE (V) TO START 6.0 Enable Pin The LT3971A is in shutdown when the EN pin is low and active when the pin is high. The rising threshold of the EN comparator is 1.01V, with 30mV of hysteresis. The EN pin can be tied to VIN if the shutdown feature is not used. Adding a resistor divider from VIN to EN programs the LT3971A to regulate the output only when VIN is above a desired voltage (see Figure 6). Typically, this threshold, VIN(EN), 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. The VIN(EN) threshold prevents the regulator from operating at source voltages where the problems might occur. This threshold can be adjusted by setting the values R3 and R4 such that they satisfy the following equation: R3 +1 R4 where output regulation should not start until VIN is above VIN(EN). Due to the comparator’s hysteresis, switching will not stop until the input falls slightly below VIN(EN). 5.8 5.6 TO RUN 5.2 At light loads, the inductor current becomes discontinuous and this reduces the minimum input voltage to approximately 400mV above VOUT. At higher load currents, the inductor current is continuous and the duty cycle is limited by the maximum duty cycle of the LT3971A, requiring a higher input voltage to maintain regulation. VIN(EN) = 6.2 5.4 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. VOUT = 5V TA = 25°C L = 4.7μH f = 800kHz 5.0 R3 10 LT3971A VIN 100 LOAD CURRENT (mA) 1V 1000 EN 3971A F05 + – SHDN R4 Figure 5. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit 3971A F06 Figure 6. Programmed Enable Threshold 3971af 17 LT3971A/LT3971A-5 APPLICATIONS INFORMATION Be aware that when the input voltage is below 4.3V, the input current may rise to several hundred μA. And the part may be able to switch at cold or for VIN(EN) thresholds less than 7V. Figure 7 shows the magnitude of the increased input current in a typical application with different programmed VIN(EN). When operating in Burst Mode for light load currents, the current through the VIN(EN) resistor network can easily be greater than the supply current consumed by the LT3971A. Therefore, the VIN(EN) resistors should be large to minimize their effect on efficiency at low loads. 12V VIN(EN) Input Current 500 The SS pin can be used to soft-start the LT3971A by throttling the maximum input current during start-up. An internal 1μA current source charges an external capacitor generating a voltage ramp on the SS pin. The SS pin clamps the internal VC node, which slowly ramps up the current limit. Maximum current limit is reached when the SS pin is about 1.5V or higher. By selecting a large enough capacitor, the output can reach regulation without overshoot. For applications with input voltages above 25V, a 100k resistor in series with the soft-start capacitor is recommended. Figure 8 shows start-up waveforms for a typical application with a 10nF capacitor on SS for a 3.3Ω load when the EN pin is pulsed high for 13ms. The external SS capacitor is only actively discharged when EN is low. With EN low, the external SS cap is discharged through approximately 150Ω. The EN pin needs to be low long enough for the external cap to completely discharge through the 150Ω pull-down prior to start-up. 400 INPUT CURRENT (μA) Soft-Start 300 200 100 0 0 1 2 3 4 5 6 7 8 9 10 11 12 INPUT VOLTAGE (V) VIN(EN) = 12V R3 = 11M R4 = 1M VSS 1V/DIV VOUT 2V/DIV 6V VIN(EN) Input Current IL 0.5A/DIV 500 2ms/DIV INPUT CURRENT (μA) 400 3971A F08 Figure 8. Soft-Start Waveforms for Front-Page Application with 10nF Capacitor on SS. EN is Pulsed High for About 13ms with a 3.3Ω Load Resistor 300 200 Synchronization 100 To select low ripple Burst Mode operation, tie the SYNC pin below 0.6V (this can be ground or a logic low output). 0 0 1 VIN(EN) = 6V R3 = 5M R4 = 1M 2 3 4 INPUT VOLTAGE (V) 5 6 3971A F07 Figure 7. Input Current vs Input Voltage for a Programmed VIN(EN) of 6V and 12V Synchronizing the LT3971A oscillator 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 0.6V and peaks above 1.0V (up to 6V). 3971af 18 LT3971A/LT3971A-5 APPLICATIONS INFORMATION The LT3971A will not enter Burst Mode operation at low output loads while synchronized to an external clock, but instead will pulse skip to maintain regulation. The LT3971A may be synchronized over a 250kHz to 2MHz range. The RT resistor should be chosen to set the LT3971A switching frequency 20% below the lowest synchronization input. For example, if the synchronization signal will be 250kHz and higher, the RT should be selected for 200kHz. To assure reliable and safe operation the LT3971A 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). The slope compensation is set by the RT value, while the minimum slope compensation required to avoid subharmonic oscillations is established by the inductor size, input voltage, and output voltage. Since the synchronization frequency will not change the slopes of the inductor current waveform, if the inductor is large enough to avoid subharmonic oscillations at the frequency set by RT, than the slope compensation will be sufficient for all synchronization frequencies. D4 MBRS140 VIN VIN BOOST EN SW VOUT LT3971A GND BD FB + BACKUP 3971A F09 Figure 9. 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 LT3971A 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 10 shows the recommended component placement with trace, ground plane and via locations. Note that large, switched currents flow in the LT3971A’s VIN and SW pins, the catch diode (D1), and the input capacitor (C1). The loop formed by Shorted and Reversed Input Protection If the inductor is chosen so that it won’t saturate excessively, a LT3971A buck regulator will tolerate a shorted output. There is another situation to consider in systems where the output will be held high when the input to the LT3971A 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 LT3971A’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 LT3971A’s internal circuitry will pull its quiescent current through its SW pin. This is fine if your system can tolerate a few μA in this state. If you ground the EN pin, the SW pin current will drop to essentially zero. However, if the VIN pin is grounded while the output is held high, regardless of EN, parasitic diodes inside the LT3971A can pull current from the output through the SW pin and the VIN pin. Figure 9 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input. L1 C2 VOUT RPG GND RT C3 C4 C5 D1 R2 R1 C1 GND 3971A F10 VIAS TO LOCAL GROUND PLANE VIAS TO VOUT VIAS TO SYNC VIAS TO RUN/SS VIAS TO PG VIAS TO VIN OUTLINE OF LOCAL GROUND PLANE Figure 10. A Good PCB Layout Ensures Proper, Low EMI Operation 3971af 19 LT3971A/LT3971A-5 APPLICATIONS INFORMATION 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 BOOST nodes should be as small as possible. Finally, keep the FB and RT nodes small so that the ground traces will shield them from the SW and BOOST nodes. The Exposed 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 LT3971A to additional ground planes within the circuit board and on the bottom side. Hot Plugging Safely The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LT3971A circuits. However, these capacitors can cause problems if the LT3971A is plugged into a live supply. The low loss ceramic capacitor, combined with stray inductance in series with the power source, forms an under damped tank circuit, and the voltage at the VIN pin of the LT3971A can ring to twice the nominal input voltage, possibly exceeding the LT3971A’s rating and damaging the part. If the input supply is poorly controlled or the user will be plugging the LT3971A into an energized supply, the input network should be designed to prevent this overshoot. See Linear Technology Application Note 88 for a complete discussion. High Temperature Considerations must be soldered to a ground plane. This ground should be tied to large copper layers below with thermal vias; these layers will spread heat dissipated by the LT3971A. Placing additional vias can reduce thermal resistance further. The maximum load current should be derated as the ambient temperature approaches the maximum junction rating. Power dissipation within the LT3971A can be estimated by calculating the total power loss from an efficiency measurement and subtracting the catch diode loss and inductor loss. The die temperature is calculated by multiplying the LT3971A power dissipation by the thermal resistance from junction to ambient. Also keep in mind that the leakage current of the power Schottky diode goes up exponentially with junction temperature. When the power switch is closed, the power Schottky diode is in parallel with the power converter’s output filter stage. As a result, an increase in a diode’s leakage current results in an effective increase in the load, and a corresponding increase in input power. Therefore, the catch Schottky diode must be selected with care to avoid excessive increase in light load supply current at high temperatures. 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. For higher ambient temperatures, care should be taken in the layout of the PCB to ensure good heat sinking of the LT3971A. The Exposed Pad on the bottom of the package 3971af 20 LT3971A/LT3971A-5 TYPICAL APPLICATIONS 5V Step-Down Converter VIN 7V TO 38V VIN EN OFF ON BOOST 0.47μF PG 4.7μF 4.7μH SW SS LT3971A RT BD 10pF VOUT 5V 1.3A 1M 49.9k SYNC FB GND 22μF 309k f = 800kHz 3971A TA02 3.3V Step Down Converter No Load Supply Current VIN 4.5V TO 38V 4.0 VIN 3.5 BOOST 0.47μF PG SS 4.7μF 4.7μH VOUT 3.3V 1.3A SW LT3971A RT BD 10pF 1.78M 49.9k SYNC GND INPUT CURRENT (μA) EN OFF ON 3.0 2.5 2.0 1.5 FB 22μF 1M f = 800kHz 1.0 10 0 3971A TA11 20 30 INPUT VOLTAGE (V) 40 3971A TA11b 5V Step-Down Converter 2.5V Step-Down Converter VIN 7V TO 38V VIN 4.3V TO 38V VIN EN OFF ON VIN BOOST 0.47μF PG 4.7μF BOOST 1μF PG 4.7μH SW SS EN OFF ON 4.7μF LT3971A-5 SS RT 4.7μH SW LT3971A RT BD BD 49.9k SYNC f = 800kHz GND VOUT 22μF 3971A TA03 VOUT 5V 1.3A 10pF 1M 118k SYNC f = 400kHz GND FB 909k 47μF VOUT 2.5V 1.3A 3971A TA04 3971af 21 LT3971A/LT3971A-5 TYPICAL APPLICATIONS 1.8V Step-Down Converter 12V Step-Down Converter VIN 15V TO 38V VIN 4.3V TO 27V VIN VIN BD EN OFF ON 0.47μF PG BOOST 0.47μF PG 4.7μH SW SS EN OFF ON BOOST 10μF LT3971A SW SS 4.7μF 10μH LT3971A RT RT 10pF BD 118k SYNC GND 100μF 1M f = 400kHz 1M VOUT 1.8V 1.3A 511k FB 10pF 49.9k GND SYNC FB 110k f = 800kHz 10μF VOUT 12V 1.3A 3971A TA06 3971A TA05 3.3V Step-Down Converter with Undervoltage Lockout, Soft-Start, and Power Good VIN 6V TO 38V 5M VIN BOOST EN 0.47μF 4.7μH SW 4.7μF SS 100k 150k LT3971A RT PG BD 1M PGOOD 10pF 1nF 49.9k 1M SYNC VOUT 3.3V 1.3A FB GND 562k f = 800kHz 22μF 3971A TA07 5V, 2MHz Step-Down Converter with Soft-Start VIN 10V TO 25V VIN EN OFF ON BOOST 0.47μF PG SS 2.2μH SW LT3971A 2.2μF RT BD 1nF 10pF 11k 1M SYNC f = 2MHz GND FB 309k 22μF VOUT 5V 1.3A 3971A TA08 3971af 22 LT3971A/LT3971A-5 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. MSE Package 10-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1664 Rev H) BOTTOM VIEW OF EXPOSED PAD OPTION 1.88 t 0.102 (.074 t .004) 5.23 (.206) MIN 1 0.889 t 0.127 (.035 t .005) 0.05 REF 10 DETAIL “B” CORNER TAIL IS PART OF DETAIL “B” THE LEADFRAME FEATURE. FOR REFERENCE ONLY NO MEASUREMENT PURPOSE 3.00 t 0.102 (.118 t .004) (NOTE 3) 10 9 8 7 6 DETAIL “A” 0s – 6s TYP 1 2 3 4 5 GAUGE PLANE 0.53 t 0.152 (.021 t .006) DETAIL “A” 0.18 (.007) 0.497 t 0.076 (.0196 t .003) REF 3.00 t 0.102 (.118 t .004) (NOTE 4) 4.90 t 0.152 (.193 t .006) 0.254 (.010) 0.29 REF 1.68 (.066) 1.68 t 0.102 3.20 – 3.45 (.066 t .004) (.126 – .136) 0.50 0.305 t 0.038 (.0197) (.0120 t .0015) BSC TYP RECOMMENDED SOLDER PAD LAYOUT 1.88 (.074) SEATING PLANE 0.86 (.034) REF 1.10 (.043) MAX 0.17 – 0.27 (.007 – .011) TYP 0.50 (.0197) BSC 0.1016 t 0.0508 (.004 t .002) MSOP (MSE) 0911 REV H 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 6. EXPOSED PAD DIMENSION DOES INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD SHALL NOT EXCEED 0.254mm (.010") PER SIDE. 3971af 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. 23 LT3971A/LT3971A-5 TYPICAL APPLICATION 4V Step-Down Converter with a High Impedance Input Source + 11M 24V – + VIN EN CBULK 100μF 1M BOOST 0.47μF PG * AVERAGE OUTPUT POWER CANNOT EXCEED THAT WHICH CAN BE PROVIDED BY HIGH IMPEDANCE SOURCE. NAMELY, V2 POUT(MAX)tη 4R 4.7μH SW SS LT3971A 4.7μF RT BD 1nF 10pF 49.9k 1M SYNC GND FB f = 800kHz 412k VOUT 4V 1.3A* 100μF WHERE V IS VOLTAGE OF SOURCE, R IS INTERNAL SOURCE IMPEDANCE, AND η IS LT3971 EFFICIENCY. MAXIMUM OUTPUT CURRENT OF 1.2A CAN BE SUPPLIED FOR A SHORT TIME BASED ON THE ENERGY WHICH CAN BE SOURCED BY THE BULK INPUT CAPACITANCE. 3971A TA09a Sourcing a Maximum Load Pulse VOUT 200mV/DIV Start-Up from High Impedance Input Source VIN 1V/DIV VIN 5V/DIV VOUT 2V/DIV IL 1A/DIV IL 500mA/DIV 3971A TA09b 500μs/DIV 2ms/DIV 3971A TA09c RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT3970 40V, 350mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter with IQ = 2.5μA VIN: 4.2V to 40V, VOUT(MIN) = 1.21V, IQ = 2.5μA, ISD <1μA, 3mm × 2mm DFN-10 and MSOP-10 Packages LT3971 38V, 1.2A, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter with IQ = 2.8μA VIN: 4.3V to 38V, VOUT(MIN) = 1.2V, IQ = 2.8μA, ISD < 1μA, 3mm × 3mm DFN-10, MSOPE-10 LT3990 62V, 350mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter with IQ = 2.5μA VIN: 4.2V to 62V, VOUT(MIN) = 1.21V, IQ = 2.5μA, ISD <1μA, 3mm × 2mm DFN-10 and MSOP-10 Packages LT3991 55V, 1.2A, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter with IQ = 2.8μA VIN: 4.3V to 38V, VOUT(MIN) = 1.2V, IQ = 2.8μA, ISD <1μA, 3mm × 3mm DFN-10 and MSOP-10E Packages LT3682 36V, 60VMAX, 1A, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter VIN: 3.6V to 36V, VOUT(MIN) = 0.8V, IQ = 75μA, ISD <1μA, 3mm × 3mm DFN-12 Package LT3689 36V, 60V with Transient Protection 800mA, 2.2MHz, High Efficiency Micropower Step-Down DC/DC Converter with POR Reset Watchdog Timer VIN: 3.6V to 36V, Transient to 60V, VOUT(MIN) = 0.8V, IQ = 75μA, ISD <1μA, 3mm × 3mm QFN-16 LT3480 36V with Transient Protection to 60V, 2A (IOUT), 2.4MHz, High Efficiency VIN: 3.6V to 36V, Transient to 60V, VOUT(MIN) = 0.78V, IQ = 70μA, Step-Down DC/DC Converter with Burst Mode Operation ISD <1μA, 3mm × 3mm DFN-10 and MSOP-10E Packages LT3980 58V with Transient Protection to 80V, 2A (IOUT), 2.4MHz High Efficiency VIN: 3.6V to 58V, Transient to 80V, VOUT(MIN) = 0.78V, IQ = 85μA, Step-Down DC/DC Converter with Burst Mode Operation ISD <1μA, MSOP-16E 3mm × 4mm DFN-16 Package and MSOP-16E Packages 3971af 24 Linear Technology Corporation LT 0212 • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2012