LT3431 High Voltage, 3A, 500kHz Step-Down Switching Regulator U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO The LT ®3431 is a 500kHz monolithic buck switching regulator that accepts input voltages up to 60V. A high efficiency 3A, 0.1Ω switch is included on the die along with all the necessary oscillator, control and logic circuitry. A current mode architecture provides fast transient response and good loop stability. Wide Input Range: 5.
LT3431 W W W AXI U U ABSOLUTE RATI GS U U W PACKAGE/ORDER I FOR ATIO (Note 1) Input Voltage (VIN) ................................................. 60V BOOST Pin Above SW ............................................ 35V BOOST Pin Voltage ................................................. 68V SYNC Voltage ........................................................... 7V SHDN Voltage ........................................................... 6V BIAS Pin Voltage ..........................................
LT3431 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TJ = 25°C. VIN = 15V, VC = 1.5V, SHDN = 1V, BOOST open circuit, SW open circuit, unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX Minimum Input Voltage (Note 3) Minimum Boost Voltage (Note 4) ISW ≤ 2.5A Boost Current (Note 5) BOOST = VIN + 5V, ISW = 0.75A BOOST = VIN + 5V, ISW = 2.
LT3431 U W TYPICAL PERFOR A CE CHARACTERISTICS FB Pin Voltage and Current Switch Peak Current Limit 6 SHDN Pin Bias Current 1.234 Tj = 25°C 250 2.0 4 GUARANTEED MINIMUM 3 1.5 1.224 VOLTAGE 1.219 1.0 CURRENT 1.214 CURRENT (µA) FEEDBACK VOLTAGE (V) TYPICAL CURRENT (µA) SWITCH PEAK CURRENT (A) 5 0 20 40 60 DUTY CYCLE (%) 3431 G01 0 125 AT 2.
LT3431 U W TYPICAL PERFOR A CE CHARACTERISTICS Minimum Input Voltage with 5V Output Switching Frequency 575 BOOST Pin Current 90 7.5 80 525 500 475 6.5 MINIMUM INPUT VOLTAGE TO START 6.0 MINIMUM INPUT VOLTAGE TO RUN 5.5 450 –25 0 25 50 75 100 5.0 125 60 50 40 30 20 0 0 JUNCTION TEMPERATURE (°C) 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 LOAD CURRENT (A) 3431 G10 2.1 600 400 SWITCH VOLTAGE (mV) 1.9 1.3 1.1 3 Switch Minimum ON Time vs Temperature 450 1.
LT3431 U U U PI FU CTIO S GND (Pins 1, 8, 9, 16): The GND pin connections act as the reference for the regulated output, so load regulation will suffer if the “ground” end of the load is not at the same voltage as the GND pins of the IC. This condition will occur when load current or other currents flow through metal paths between the GND pins and the load ground. Keep the paths between the GND pins and the load ground short and use a ground plane when possible.
LT3431 W BLOCK DIAGRA The LT3431 is a constant frequency, current mode buck converter. This means that there is an internal clock and two feedback loops that control the duty cycle of the power switch. In addition to the normal error amplifier, there is a current sense amplifier that monitors switch current on a cycle-by-cycle basis. A switch cycle starts with an oscillator pulse which sets the RS flip-flop to turn the switch on.
LT3431 U W U U APPLICATIO S I FOR ATIO FEEDBACK PIN FUNCTIONS The feedback (FB) pin on the LT3431 is used to set output voltage and provide several overload protection features. The first part of this section deals with selecting resistors to set output voltage and the second part talks about foldback frequency and current limiting created by the FB pin. Please read both parts before committing to a final design.
LT3431 U W U U APPLICATIO S I FOR ATIO LT3431 VSW TO FREQUENCY SHIFTING 1.4V – OUTPUT 5V Q1 ERROR AMPLIFIER + L1 R1 1.2V R4 2k R3 1k FB + C1 BUFFER Q2 R2 TO SYNC CIRCUIT VC GND 3431 F02 Figure 2. Frequency and Current Limit Foldback will increase and the protection accorded by frequency and current foldback will decrease. VOUT USING 47µF CERAMIC OUTPUT CAPACITOR CHOOSING THE INDUCTOR For most applications, the output inductor will fall into the range of 5µH to 33µH.
LT3431 U W U U APPLICATIO S I FOR ATIO importance, the subsequent suggestions in Peak Inductor and Fault Current and EMI will additionally help in the selection of the inductor value. Peak-to-peak output ripple voltage is the sum of a triwave (created by peak-to-peak ripple current (ILP-P) times ESR) and a square wave (created by parasitic inductance (ESL) and ripple current slew rate). Capacitive reactance is assumed to be small compared to ESR or ESL.
LT3431 U W U U APPLICATIO S I FOR ATIO there are no helpful guidelines to calculate when the magnetic field radiation will be a problem. Table 2 VENDOR/ PART NUMBER VALUE (µH) IDC (Amps) DCR (Ohms) HEIGHT (mm)MAX CDRH8D28-4R7 4.7 3.4 0.019 3 CDRH8D28-7R3 7.3 2.8 0.030 3 CDRH8D43-100 10 4 0.029 4.5 CDRH8D43-150 15 2.9 0.042 4.5 CEI122-100 10 3.4 0.029 3 CEI122(H)-150 15 3.6 0.071 3 CDRH104R-150 15 3.6 0.037 4 CDRH104R-220 22 2.9 0.054 4 CDRH124-330 33 2.
LT3431 U W U U APPLICATIO S I FOR ATIO To calculate actual peak switch current with a given set of conditions, use: ISW (PEAK) = IOUT + = IOUT + ILP-P 2 (VOUT + VF ) VIN − VOUT – VF ( 2(L)( f)(VIN ) ) Reduced Inductor Value and Discontinuous Mode If the smallest inductor value is of most importance to a converter design, in order to reduce inductor size/cost, discontinuous mode may yield the smallest inductor solution.
LT3431 U W U U APPLICATIO S I FOR ATIO time involved in both the current comparator and turnoff of the output switch. These result in a minimum on time tON(MIN). When combined with the large ratio of VIN to (VF + I • R), the diode forward voltage plus inductor I • R voltage drop, the potential exists for a loss of control.
LT3431 U W U U APPLICATIO S I FOR ATIO Many engineers have heard that solid tantalum capacitors are prone to failure if they undergo high surge currents. This is historically true, and type TPS capacitors are specially tested for surge capability, but surge ruggedness is not a critical issue with the output capacitor. Solid tantalum capacitors fail during very high turn-on surges, which do not occur at the output of regulators.
LT3431 U W U U APPLICATIO S I FOR ATIO If tantalum capacitors are used, values in the 22µF to 470µF range are generally needed to minimize ESR and meet ripple current and surge ratings. Care should be taken to ensure the ripple and surge ratings are not exceeded. The AVX TPS and Kemet T495 series are surge rated. AVX recommends derating capacitor operating voltage by 2:1 for high surge applications. CATCH DIODE Highest efficiency operation requires the use of a Schottky type diode.
LT3431 U W U U APPLICATIO S I FOR ATIO RFB L1 LT3431 2.38V IN INPUT OUTPUT VSW + STANDBY RHI – 5.5µA + SHDN C1 + TOTAL SHUTDOWN C2 RLO 0.4V – GND 3431 F04 Figure 4. Undervoltage Lockout Threshold voltage for lockout is about 2.38V. A 5.5µA bias current flows out of the pin at this threshold. The internally generated current is used to force a default high state on the shutdown pin if the pin is left open.
LT3431 U W U U APPLICATIO S I FOR ATIO nanosecond range. To prevent noise both radiated and conducted, the high speed switching current path, shown in Figure 5, must be kept as short as possible. This is implemented in the suggested layout of Figure 6. Shortening this path will also reduce the parasitic trace inductance of approximately 25nH/inch. At switch off, this parasitic inductance produces a flyback spike across the LT3431 switch.
LT3431 U W U U APPLICATIO S I FOR ATIO The VC and FB components should be kept as far away as possible from the switch and boost nodes. The LT3431 pinout has been designed to aid in this. The ground for these components should be separated from the switch current path. Failure to do so will result in poor stability or subharmonic like oscillation. Board layout also has a significant effect on thermal resistance. Pins 1, 8, 9 and 16, GND, are a continuous copper plate that runs under the LT3431 die.
LT3431 U W U U APPLICATIO S I FOR ATIO Switch loss: PSW = ( ) (VOUT ) + tEFF (1/2)(IOUT )(VIN)(f) RSW IOUT 2 VIN Boost current loss: 2 PBOOST = ( ) PDIODE = VIN Quiescent current loss: ( ) ( PDIODE = (VF )(VIN – VOUT )(ILOAD ) VIN VF = Forward voltage of diode (assume 0.52V at 2A) VOUT IOUT /36 PQ = VIN 0.0015 + VOUT 0.003 When estimating ambient, remember the nearby catch diode and inductor will also be dissipating power: ) RSW = Switch resistance (≈ 0.
LT3431 U W U U APPLICATIO S I FOR ATIO Note: Some of the internal power dissipation in the IC, due to BOOST pin voltage, can be transferred outside of the IC to reduce junction temperature, by increasing the voltage drop in the path of the boost diode D2. (see Figure␣ 9). This reduction of junction temperature inside the IC will allow higher ambient temperature operation for a given set of conditions.
LT3431 U W U U APPLICATIO S I FOR ATIO FREQUENCY COMPENSATION Before starting on the theoretical analysis of frequency response, the following should be remembered—the worse the board layout, the more difficult the circuit will be to stabilize. This is true of almost all high frequency analog circuits, read the Layout Considerations section first.
LT3431 U W U U APPLICATIO S I FOR ATIO tantalum output capacitor with a ceramic output capacitor because of its very low ESR. The zero provided by the tantalum output capacitor must now be reinserted back into the loop. Alternatively there may be cases where, even with the tantalum output capacitor, an additional zero is required in the loop to increase phase margin for improved transient response.
LT3431 U W U U APPLICATIO S I FOR ATIO BUCK CONVERTER WITH ADJUSTABLE SOFT-START Dual Polarity Output Converter Large capacitive loads or high input voltages can cause high input currents at start-up. Figure 13 shows a circuit that limits the dv/dt of the output at start-up, controlling the capacitor charge rate. The buck converter is a typical configuration with the addition of R3, R4, CSS and Q1.
LT3431 U W U U APPLICATIO S I FOR ATIO VIN 9V TO 16V 36V TRANSIENT C3 2.2µF 50V CER D2 MMSD914T1 VIN L1 CDRH6D28-100 10µH C2 0.22µF BOOST SW SHDN VOUT1 LT3431EFE SYNC BIAS C4 10µF 6.3V 0805 X5R CER R2 15.4k FB GND VOUT1 5V AT 1.5A* R3 4.99k VC RC 1.5k CC 10nF C5 22µF 6.3V X5R CER D1 B140A CF 220pF C6 22µF 6.3V X5R CER L2 CDRH6D28-100 10µH † FOR LOAD CURRENT LESS THAN 25mA, VOUT2† –5V AT 0.9A* A PRELOAD OF 200Ω SHOULD BE USED TO IMPROVE LOAD REGULATION.
LT3431 U W U U APPLICATIO S I FOR ATIO POSITIVE-TO-NEGATIVE CONVERTER Inductor Value The circuit in Figure 15 is a positive-to-negative topology using a grounded inductor. It differs from the standard approach in the way the IC chip derives its feedback signal because the LT3431 accepts only positive feedback signals. The ground pin must be tied to the regulated negative output. A resistor divider to the FB pin then provides the proper feedback voltage for the chip.
LT3431 U W U U APPLICATIO S I FOR ATIO For a load current of 0.5A, this says that discontinuous mode can be used and the minimum inductor needed is found from: LMIN = 2(12)(0.5) (500 • 103 )(3)2 = 2.7µH In practice, the inductor should be increased by about 30% over the calculated minimum to handle losses and variations in value. This suggests a minimum inductor of 3.5µH for this application.
LT3431 U PACKAGE DESCRIPTIO FE Package 16-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663, Exposed Pad Variation BB) 4.90 – 5.10* (.193 – .201) 3.58 (.141) 3.58 (.141) 16 1514 13 12 1110 6.60 ±0.10 9 2.94 (.116) 4.50 ±0.10 SEE NOTE 4 2.94 6.40 (.116) BSC 0.45 ±0.05 1.05 ±0.10 0.65 BSC 1 2 3 4 5 6 7 8 RECOMMENDED SOLDER PAD LAYOUT 1.10 (.0433) MAX 4.30 – 4.50* (.169 – .177) 0° – 8° 0.09 – 0.20 (.0036 – .0079) 0.45 – 0.75 (.018 – .030) NOTE: 1.
LT3431 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1074/LT1074HV 4.4A (IOUT), 100kHz, High Efficiency Step-Down DC/DC Converter VIN = 7.3V to 45/64V, VOUT = 2.21V, IQ = 8.5mA, ISD = 10µA, DD-5/7, TO220-5/7 Packages LT1076/LT1076HV 1.6A (IOUT), 100kHz, High Efficiency Step-Down DC/DC Converter VIN = 7.3V to 45/64V, VOUT = 2.21V, IQ = 8.5mA, ISD = 10µA, DD-5/7, TO220-5/7 Packages LT1616 25V, 500mA (IOUT), 1.4MHz, High Efficiency Step-Down DC/DC Converter VIN = 3.6V to 25V, VOUT = 1.
