LTC1871-7 High Input Voltage, Current Mode Boost, Flyback and SEPIC Controller Description Features n n n n n n n n n n n n n n Optimized for High Input Voltage Applications Wide Chip Supply Voltage Range: 6V to 36V Internal 7V Low Dropout Voltage Regulator Optimized for 6V-Rated MOSFETs Current Mode Control Provides Excellent Transient Response High Maximum Duty Cycle (92% Typ) ±2% RUN Pin Threshold with 100mV Hysteresis ±1% Internal Voltage Reference Micropower Shutdown: IQ = 10µA Programmable Operat
LTC1871-7 Absolute Maximum Ratings (Note 1) Pin Configuration VIN Voltage ................................................ – 0.3V to 36V INTVCC Voltage............................................ –0.3V to 9V INTVCC Output Current...........................................50mA GATE Voltage............................ –0.3V to VINTVCC + 0.3V ITH, FB Voltages........................................ –0.3V to 2.7V RUN Voltage................................................ –0.3V to 7V MODE/SYNC Voltage........
LTC1871-7 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 8V, VRUN = 1.5V, RFREQ = 80k, VMODE/SYNC = 0V, unless otherwise specified. SYMBOL PARAMETER VRUN+ Rising RUN Input Threshold Voltage VRUN – Falling RUN Input Threshold Voltage VRUN(HYST) RUN Pin Input Threshold Hysteresis IRUN RUN Input Current VFB Feedback Voltage CONDITIONS ● VITH = 0.2V (Note 5) VITH = 0.
LTC1871-7 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 8V, VRUN = 1.5V, RFREQ = 80k, VMODE/SYNC = 0V, unless otherwise specified.
LTC1871-7 Typical Performance Characteristics Shutdown Mode IQ vs VIN Shutdown Mode IQ vs Temperature 30 20 600 VIN = 8V Burst Mode IQ vs VIN 20 10 BURST MODE IQ (µA) SHUTDOWN MODE IQ (µA) SHUTDOWN MODE IQ (µA) 500 15 10 5 400 300 200 100 0 0 10 20 VIN (V) 30 0 –50 –25 40 0 18717 G04 18 500 TIME (ns) IQ (mA) 40 10 8 6 FALL TIME 10 2 0 25 50 75 100 125 150 TEMPERATURE (°C) 0 200 400 600 800 FREQUENCY (kHz) RUN Thresholds vs VIN 10 20 VIN (V) 30 40 18717 G10 2000 400
LTC1871-7 Typical Performance Characteristics Maximum Sense Threshold vs Temperature Frequency vs Temperature 325 SENSE Pin Current vs Temperature 35 160 GATE FREQUENCY (kHz) 315 310 305 300 295 290 285 155 SENSE PIN CURRENT (µA) MAX SENSE THRESHOLD (mV) 320 150 145 GATE HIGH VSENSE = 0V 30 280 275 –50 –25 0 140 –50 –25 25 50 75 100 125 150 TEMPERATURE (°C) 0 0 25 50 75 100 125 150 TEMPERATURE (°C) 18717 G14 18717 G13 INTVCC Load Regulation 18717 G15 INTVCC Dropout Voltage vs Curren
LTC1871-7 Pin Functions MODE/SYNC (Pin 5): This input controls the operating mode of the converter and allows for synchronizing the operating frequency to an external clock. If the MODE/ SYNC pin is connected to ground, Burst Mode operation is enabled. If the MODE/SYNC pin is connected to INTVCC, or if an external logic-level synchronization signal is applied to this input, Burst Mode operation is disabled and the IC operates in a continuous mode. GND (Pin 6): Ground Pin. GATE (Pin 7): Gate Driver Output.
LTC1871-7 operation Main Control Loop The LTC1871-7 is a constant frequency, current mode controller for DC/DC boost, SEPIC and flyback converter applications. With the LTC1871-7 the current control loop can be closed by sensing the voltage drop either across the power MOSFET switch or across a discrete sense resistor, as shown in Figure 2. L VIN D VOUT VIN + SENSE VSW COUT GATE GND GND 2a.
LTC1871-7 Operation In applications where fixed frequency operation is more critical than low current efficiency, or where the lowest output ripple is desired, pulse-skip mode operation should be used and the MODE/SYNC pin should be connected to the INTVCC pin. This allows discontinuous conduction mode (DCM) operation down to near the limit defined by the chip’s minimum on-time (about 175ns). Below this output current level, the converter will begin to skip cycles in order to maintain output regulation.
LTC1871-7 operation external frequency (above 1.3fO) can result in inadequate slope compensation and possible subharmonic oscillation (or jitter). The external clock signal must exceed 2V for at least 25ns, and should have a maximum duty cycle of 80%, as shown in Figure 5. The MOSFET turn on will synchronize to the rising edge of the external clock signal. Programming the Operating Frequency The choice of operating frequency and inductor value is a tradeoff between efficiency and component size.
LTC1871-7 Operation INPUT SUPPLY 6V TO 30V VIN – 1.230V P-CH + CIN R2 R1 LOGIC 7V INTVCC DRIVER CVCC 4.7µF X5R GATE M1 GND 18717 F07 6V-RATED POWER MOSFET GND PLACE AS CLOSE AS POSSIBLE TO DEVICE PINS Figure 7. Bypassing the LDO Regulator and Gate Driver Supply can cause the LTC1871-7 to exceed its maximum junction temperature rating.
LTC1871-7 operation The resistors R1 and R2 are typically chosen so that the error caused by the current flowing into the FB pin during normal operation is less than 1% (this translates to a maximum value of R1 of about 250k). The turn-on and turn-off input voltage thresholds are programmed using a resistor divider according to the following formulas: R2 VIN(OFF) = 1.
LTC1871-7 applications information Application Circuits A basic LTC1871-7 application circuit is shown in Figure 9. External component selection is driven by the characteristics of the load and the input supply. The first topology to be analyzed will be the boost converter, followed by SEPIC (single-ended primary inductance converter).
LTC1871-7 applications information Boost Converter: Ripple Current ∆IL and the ‘χ’ Factor The constant ‘χ’ in the equation above represents the percentage peak-to-peak ripple current in the inductor, relative to its maximum value. For example, if 30% ripple current is chosen, then χ = 0.30, and the peak current is 15% greater than the average. For a current mode boost regulator operating in CCM, slope compensation must be added for duty cycles above 50% in order to avoid subharmonic oscillation.
LTC1871-7 applications information Sense Resistor Selection During the switch on-time, the control circuit limits the maximum voltage drop across the sense resistor to about 150mV (at low duty cycle). The peak inductor current is therefore limited to 150mV/RSENSE.
LTC1871-7 applications information The RTH(JA) to be used in this equation normally includes the RTH(JC) for the device plus the thermal resistance from the board to the ambient temperature in the enclosure. ρT NORMALIZED ON RESISTANCE 2.0 1.5 Remember to keep the diode lead lengths short and to observe proper switch-node layout (see Board Layout Checklist) to avoid excessive ringing and increased dissipation. 1.0 0.
LTC1871-7 applications information For some designs it may be possible to choose a single capacitor type that satisfies both the ESR and bulk C requirements for the design. In certain demanding applications, however, the ripple voltage can be improved significantly by connecting two or more types of capacitors in parallel. For example, using a low ESR ceramic capacitor can minimize the ESR step, while an electrolytic capacitor can be used to supply the required bulk C.
LTC1871-7 applications information Table 1. Recommended Component Manufacturers VENDOR AVX BH Electronics COMPONENTS TELEPHONE WEB ADDRESS Capacitors (207) 282-5111 avxcorp.com Inductors, Transformers (952) 894-9590 bhelectronics.com Coilcraft Inductors (847) 639-6400 coilcraft.com Coiltronics Inductors (407) 241-7876 coiltronics.com Diodes, Inc Fairchild General Semiconductor Diodes (805) 446-4800 diodes.com MOSFETs (408) 822-2126 fairchildsemi.
LTC1871-7 applications information During this inductor charging interval, the output capacitor must supply the load current and a significant droop in the output voltage can occur. Generally, it is a good idea to choose a value of inductor ∆IL between 25% and 40% of IIN(MAX). The alternative is to either increase the value of the output capacitor or disable Burst Mode operation using the MODE/SYNC pin.
LTC1871-7 applications information Checking Transient Response The regulator loop response can be verified by looking at the load transient response at minimum and maximum VIN. Switching regulators generally take several cycles to respond to an instantaneous step in resistive load current. When the load step occurs, VO immediately shifts by an amount equal to (∆ILOAD)(ESR), and then CO begins to charge or discharge (depending on the direction of the load step) as shown in Figure 14.
LTC1871-7 applications information The component chosen is a 6.8µH inductor made by Cooper (part number DR127-6R8) which has a saturation current of greater than 13.3A. 5. Because the duty cycle is 81%, the maximum SENSE pin threshold voltage is reduced from its low duty cycle typical value of 150mV to approximately 115mV. In addition, we need to apply a worst-case derating factor to this SENSE threshold to account for manufacturing tolerances within the IC.
LTC1871-7 applications information VOUT 1V/DIV 100 95 EFFICIENCY (%) IL 1A/DIV MOSFET DRAIN VOLTAGE 20V/DIV VIN = 8V VIN = 12V VIN = 28V 90 85 80 VIN = 28V IOUT = 0.5A VOUT = 42V D = 27% 1µs/DIV 18717 F16 Figure 16. Switching Waveforms for the Converter in Figure 9 at Maximum VIN (28V) PC Board Layout Checklist 1.
LTC1871-7 applications information 7. Minimize the capacitance between the SENSE pin trace and any high frequency switching nodes. The LTC1871‑7 contains an internal leading edge blanking time of approximately 180ns, which should be adequate for most applications. 8. For optimum load regulation and true remote sensing, the top of the output resistor divider should connect independently to the top of the output capacitor (Kelvin connection), staying away from any high dV/dt traces.
LTC1871-7 applications information 9. For applications with multiple switching power converters connected to the same input supply, make sure that the input filter capacitor for the LTC1871-7 is not shared with other converters. AC input current from another converter could cause substantial input voltage ripple, and this could interfere with the operation of the LTC1871-7.
LTC1871-7 applications information The maximum output voltage for a SEPIC converter is: D 1 VO(MAX) = ( VIN + VD ) MAX – VD 1– DMAX 1– DMAX The maximum duty cycle of the LTC1871-7 is typically 92%. SEPIC Converter: The Peak and Average Input Currents The control circuit in the LTC1871-7 is measuring the input current (using a sense resistor in the MOSFET source), so the output current needs to be reflected back to the input in order to dimension the power MOSFET properly.
LTC1871-7 applications information rating for the inductor should be checked at the minimum input voltage (which results in the highest inductor current) and maximum output current.
LTC1871-7 applications information From a known power dissipated in the power MOSFET, its junction temperature can be obtained using the following formula: TJ = TA + PFET • RTH(JA) The RTH(JA) to be used in this equation normally includes the RTH(JC) for the device plus the thermal resistance from the board to the ambient temperature in the enclosure. This value of TJ can then be used to check the original assumption for the junction temperature in the iterative calculation process.
LTC1871-7 applications information should be verified on a dedicated PC board (see Board Layout section for more information on component placement). Lab breadboards generally suffer from excessive series inductance (due to inter-component wiring), and these parasitics can make the switching waveforms look significantly worse than they would be on a properly designed PC board. The output capacitor in a SEPIC regulator experiences high RMS ripple currents, as shown in Figure 21.
LTC1871-7 Typical Applications A 48V Input Flyback Converter Configurable to 3.3V or 5V Outputs VIN 36V TO 72V 0.1µF 100k 1 2 1nF 100k MMBTA42 R1 604k 26.7k 82.5k 4 5 12.4k 3 RUN VIN ITH GATE LTC1871-7 SENSE FREQ MODE/SYNC INTVCC VFB GND 10V CTX-002-15242 T1A • • 2.2µF 100V UPS840 100µF 6.3V ×3 T1B VOUT 3.3V 3A MAX 9 7 Q1 FDC2512 10 8 4.7µF R3 0.1Ω 6 ALL CAPACITORS ARE CERAMIC X5R TYPE R2* 21k *R2 = 38.3k FOR VOUT = 5V 18717 TA02a Output Efficiency at 3.
LTC1871-7 typical applications 1.2A Automotive LED Headlamp Boost Converter D3 IRF12CW10 L1 VIN + C5 47µF 20V ×2 GND R6 1M 1% 1 RUN INPUT R8 187k 1% 2 C8 100nF 3 4 5 R10 300k SENSE RUN VIN ITH LTC1871-7 FB INTVCC FREQ GATE MODE/SYNC GND R13 17.8k 0V TO 5V DIMMING INPUT 10 9 8 Q3 SILICONIX SUP75N08-9L 7 C9 4.7µF X5R 6 R12 4.02k C10 4.7µF R15 0.20Ω 0.5W TO LEDS C7 10µF 100V R7 4.7M R9 1k D4 USE 68V 33V OR 75V D5 SINGLE 33V ZENER R11 0.
LTC1871-7 typical applications Automotive SEPIC Converter T1 VP5-0155 R46 47k VBATT 8V TO 25V 4• Q6 FMMT451 CR4 BZX84C15V R37 75k 1% • 1 C50 C46 4µF 100pF X7R 12 5• 8 • 2 11 6• 7 • 3 C52 4.7µF X7R ×2 10 1 3 R45 33.2k C47 6800pF 4 R47 133k 1% 5 RUN VIN SENSE 10 ITH LTC1871-7 INTVCC FB FREQ MODE/SYNC GND 6 GATE CR22 1N4148 L7 150Ω 3A BEAD 1B (OPTIONAL HF FILTER) CR21 MBR10100 9 2 R43 13.3k 1% 9 Q9 Si4486EY SO-8 8 7 + C49 4.7µF R59 0.
LTC1871-7 Package Description MS Package 10-Lead Plastic MSOP (Reference LTC DWG # 05-08-1661 Rev E) 0.889 ± 0.127 (.035 ± .005) 5.23 (.206) MIN 3.20 – 3.45 (.126 – .136) 3.00 ± 0.102 (.118 ± .004) (NOTE 3) 0.50 0.305 ± 0.038 (.0197) (.0120 ± .0015) BSC TYP RECOMMENDED SOLDER PAD LAYOUT 0.254 (.010) 10 9 8 7 6 3.00 ± 0.102 (.118 ± .004) (NOTE 4) 4.90 ± 0.152 (.193 ± .006) DETAIL “A” 0.497 ± 0.076 (.0196 ± .003) REF 0° – 6° TYP GAUGE PLANE 1 2 3 4 5 0.53 ± 0.152 (.021 ± .006) DETAIL “A” 0.
LTC1871-7 Revision History (Revision history begins at Rev D) REV DATE DESCRIPTION D 11/11 Corrected part numbers from LT to LTC in the Order Information section. PAGE NUMBER 2 18717fd 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.
LTC1871-7 Typical Application A Small, Nonisolated 12V Flyback Telecom Housekeeping Supply D3 VIN 36V TO 72V R5 100k D1 9.1V UV+ = 31.8V – = 29.5V UV CC2 47pF T1 1, 2, 3 (SERIES) Q1 4, 5, 6 (PARALLEL) • C1 1nF OPTIONAL CIN 2.2µF 100V X7R • R2 26.7k 1% R1 604k 1% R6 10Ω SENSE RUN COUT 47µF X5R VOUT 12V 0.4A D2 VIN ITH LTC1871-7 FB RC 3.4k CC1 2.2nF R4 110k 1% R3 12.4k 1% INTVCC FREQ GATE MODE/SYNC GND RT 120k f = 200kHz T1: COILTRONICS VP1-0076 M1: FAIRCHILD FDC2512 (150V, 0.
