LTC1871-1 Wide Input Range, No RSENSE™ Current Mode Boost, Flyback and SEPIC Controller DESCRIPTION FEATURES n n n n n n n n n n n n n n n High Efficiency (No Sense Resistor Required) Wide Input Voltage Range: 2.
LTC1871-1 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) VIN Voltage ............................................... – 0.3V to 36V INTVCC Voltage............................................ –0.3V to 7V INTVCC Output Current .......................................... 50mA GATE Voltage ............................ –0.3V to VINTVCC + 0.3V ITH, FB Voltages ....................................... –0.3V to 2.7V RUN, MODE/SYNC Voltages ....................... –0.3V to 7V FREQ Voltage ......................
LTC1871-1 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 5V, 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 = 0.4V (Note 5) VITH = 0.
LTC1871-1 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 5V, VRUN = 1.5V, RFREQ = 80k, VMODE/SYNC = 0V, unless otherwise specified. SYMBOL PARAMETER CONDITIONS MIN TYP MAX VIN = 7.5V 5.0 5.2 5.4 5.0 UNITS Low Dropout Regulator VINTVCC INTVCC Regulator Output Voltage 5.2 5.4 V ΔVINTVCC INTVCC Regulator Line Regulation 7.
LTC1871-1 TYPICAL PERFORMANCE CHARACTERISTICS Shutdown Mode IQ vs VIN Shutdown Mode IQ vs Temperature 20 30 Burst Mode IQ vs VIN 600 VIN = 5V 20 10 Burst Mode IQ (μA) SHUTDOWN MODE IQ (μA) SHUTDOWN MODE IQ (μA) 500 15 10 400 300 200 5 100 0 0 10 20 VIN (V) 30 0 –50 –25 40 0 18711 G04 10 0 20 VIN (V) 30 18711 G05 Burst Mode IQ vs Temperature 18 Gate Drive Rise and Fall Time vs CL 60 CL = 3300pF IQ(TOT) = 550μA + Qg • f 16 40 18711 G06 Dynamic IQ vs Frequency 500 50 400 14
LTC1871-1 TYPICAL PERFORMANCE CHARACTERISTICS SENSE Pin Current vs Temperature Maximum Sense Threshold vs Temperature Frequency vs Temperature 325 35 160 GATE HIGH VSENSE = 0V GATE FREQUENCY (kHz) 315 310 305 300 295 290 285 SENSE PIN CURRENT (μA) MAX SENSE THRESHOLD (mV) 320 155 150 145 30 280 275 –50 –25 0 140 –50 –25 25 50 75 100 125 150 TEMPERATURE (°C) 0 18711 G13 0 25 50 75 100 125 150 TEMPERATURE (°C) 18711 G14 INTVCC Load Regulation 18711 G15 INTVCC Dropout Voltage vs Curren
LTC1871-1 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. INTVCC (Pin 8): The Internal 5.20V Regulator Output.
LTC1871-1 OPERATION Main Control Loop The LTC1871-1 is a constant frequency, current mode controller for DC/DC boost, SEPIC and flyback converter applications. The LTC1871-1 is distinguished from conventional current mode controllers because the current control loop can be closed by sensing the voltage drop across the power MOSFET switch instead of across a discrete sense resistor, as shown in Figure 2.
LTC1871-1 OPERATION Programming the Operating Mode For applications where maximizing the efficiency at very light loads (e.g., <100μA) is a high priority, the current in the output divider could be decreased to a few microamps and Burst Mode operation should be applied (i.e., the MODE/SYNC pin should be connected to ground).
LTC1871-1 APPLICATIONS INFORMATION 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. 0.6V, and the current that flows into the FREQ pin is used to charge and discharge an internal oscillator capacitor. A graph for selecting the value of RT for a given operating frequency is shown in Figure 6.
LTC1871-1 APPLICATIONS INFORMATION INPUT SUPPLY 2.5V TO 30V VIN 1.230V – P-CH + CIN R2 R1 5.2V INTVCC + LOGIC DRIVER GATE CVCC 4.7μF M1 GND 18711 F07 GND PLACE AS CLOSE AS POSSIBLE TO DEVICE PINS Figure 7. Bypassing the LDO Regulator and Gate Driver Supply As a result, high input voltage applications in which a large power MOSFET is being driven at high frequencies can cause the LTC1871-1 to exceed its maximum junction temperature rating.
LTC1871-1 APPLICATIONS INFORMATION 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). Programming Turn-On and Turn-Off Thresholds with the RUN Pin The LTC1871-1 contains an independent, micropower voltage reference and comparator detection circuit that remains active even when the device is shut down, as shown in Figure 8.
LTC1871-1 APPLICATIONS INFORMATION Application Circuits A basic LTC1871-1 application circuit is shown in Figure 1. 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).
LTC1871-1 APPLICATIONS INFORMATION Remember that boost converters are not short-circuit protected. Under a shorted output condition, the inductor current is limited only by the input supply capability. For applications requiring a step-up converter that is shortcircuit protected, please refer to the applications section covering SEPIC converters.
LTC1871-1 APPLICATIONS INFORMATION from a lithium-ion battery or a 3.3V logic supply), then sublogic-level threshold MOSFETs should be used. Pay close attention to the BVDSS specifications for the MOSFETs relative to the maximum actual switch voltage in the application. Many logic-level devices are limited to 30V or less, and the switch node can ring during the turn-off of the MOSFET due to layout parasitics.
LTC1871-1 APPLICATIONS INFORMATION for VSENSE(MAX) and the RDS(ON) of the MOSFET listed in the manufacturer’s data sheet. The power dissipated by the MOSFET in a boost converter is: 2 IO(MAX) PFET = • RDS(ON) • DMAX • T 1– DMAX IO(MAX) +k • VO1.85 • •C •f (1– DMAX ) RSS The first term in the equation above represents the losses in the device, and the second term, the switching losses. The constant, k = 1.
LTC1871-1 APPLICATIONS INFORMATION For many designs it is 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.
LTC1871-1 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 Diodes (805) 446-4800 diodes.com MOSFETs (408) 822-2126 fairchildsemi.
LTC1871-1 APPLICATIONS INFORMATION Burst Mode operations begins, since it is the peak current that is being clamped. The output voltage ripple can increase during Burst Mode operation if ΔIL is substantially less than IBURST. This can occur if the input voltage is very low or if a very large inductor is chosen. At high duty cycles, a skipped cycle causes the inductor current to quickly decay to zero. However, because ΔIL is small, it takes multiple cycles for the current to ramp back up to IBURST(PEAK).
LTC1871-1 APPLICATIONS INFORMATION dissipated in this resistor would be 514mW at maximum output current. Assuming an efficiency of 90%, this sense resistor power dissipation represents 1.3% of the overall input power. In other words, for this application, the use of VDS sensing would increase the efficiency by approximately 1.3%. For more details regarding the various terms in these equations, please refer to the section Boost Converter: Power MOSFET Selection. IOUT 2V/DIV VIN = 3.
LTC1871-1 APPLICATIONS INFORMATION The inductor ripple current is: IO(MAX) 7 IL = • = 0.4 • = 4.6A 1– DMAX 1– 0.39 And so the inductor value is: VIN(MIN) 3.3V L= • DMAX = • 0.39 = 0.93μH IL • f 4.6A • 300kHz The component chosen is a 1μH inductor made by Sumida (part number CEP125-H 1ROMH) which has a saturation current of greater than 20A. 5. With the input voltage to the IC bootstrapped to the output of the power supply (5V), a logic-level MOSFET can be used.
LTC1871-1 APPLICATIONS INFORMATION PC Board Layout Checklist the power MOSFET or the bottom terminal of the sense resistor, 4) the negative terminal of the input capacitor and 5) at least one via to the ground plane immediately adjacent to Pin 6. The ground trace on the top layer of the PC board should be as wide and short as possible to minimize series resistance and inductance. 1.
LTC1871-1 APPLICATIONS INFORMATION 2. Beware of ground loops in multiple layer PC boards. Try to maintain one central ground node on the board and use the input capacitor to avoid excess input ripple for high output current power supplies. If the ground plane is to be used for high DC currents, choose a path away from the small-signal components. 7. If a sense resistor is used in the source of the power MOSFET, minimize the capacitance between the SENSE pin trace and any high frequency switching nodes.
LTC1871-1 APPLICATIONS INFORMATION SEPIC Converter Applications The LTC1871-1 is also well suited to SEPIC (single-ended primary inductance converter) converter applications. The SEPIC converter shown in Figure 16 uses two inductors. The advantage of the SEPIC converter is the input voltage may be higher or lower than the output voltage, and the output is short-circuit protected. The first inductor, L1, together with the main switch, resembles a boost converter.
LTC1871-1 APPLICATIONS INFORMATION The constant ‘χ’ represents the fraction of 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. It is worth noting here that SEPIC converters that operate at high duty cycles (i.e., that develop a high output voltage from a low input voltage) can have very high input currents, relative to the output current.
LTC1871-1 APPLICATIONS INFORMATION devices are limited to 30V or less. Check the switching waveforms directly across the drain and source terminals of the power MOSFET to ensure the VDS remains below the maximum rating for the device. During the MOSFET’s on-time, the control circuit limits the maximum voltage drop across the power MOSFET to about 150mV (at low duty cycle). The peak inductor current is therefore limited to 150mV/RDS(ON).
LTC1871-1 APPLICATIONS INFORMATION and the diode junction temperature is: SEPIC Converter: Output Capacitor Selection For many designs it is 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.
LTC1871-1 APPLICATIONS INFORMATION choice is AVX TPS series of surface mount tantalum. Also, ceramic capacitors are now available with extremely low ESR, ESL and high ripple current ratings. SEPIC Converter: Input Capacitor Selection The input capacitor of a SEPIC converter is less critical than the output capacitor due to the fact that an inductor is in series with the input and the input current waveform is triangular in shape.
LTC1871-1 APPLICATIONS INFORMATION And so the inductor value is: VIN(MIN) 5 L= • DMAX = • 0.714 = 4μH 2 • IL • f 2 • 1.5 • 300k 1 1 • IO(MAX) V +V 1+ 2 • T O D + 1 VIN(MIN) 1 1 0.12 = 12.7m • • = 1.5 1.2 • 1.5 12.5 5 + 1 The component chosen is a BH Electronics BH5101007, which has a saturation current of 8A. 5. With an minimum input voltage of 5V, only logic-level power MOSFETs should be considered. Because the maximum duty cycle is 71.
LTC1871-1 APPLICATIONS INFORMATION VIN = 4.5V VOUT = 12V VIN = 15V VOUT = 12V VOUT (AC) 200mV/DIV VOUT (AC) 200mV/DIV IOUT 0.5A/DIV IOUT 0.5A/DIV 50μs/DIV 50μs/DIV 18711 F19 Figure 19. LTC1871-1 SEPIC Converter Load Step Response 6. The diode for this design must handle a maximum DC output current of 2A and be rated for a minimum reverse voltage of VIN + VOUT, or 27V.
LTC1871-1 TYPICAL APPLICATIONS 2.5V to 3.3V Input, 5V/2A Output Boost Converter VIN 2.5V to 3.3V L1 1.8μH D1 1 2 RC 22k VIN ITH 10 CC1 R1 6.8nF 12.1k 1% R2 37.4k 1% 4 RT 80.6k 1% 5 FB + INTVCC FREQ MODE/SYNC GATE GND VOUT 5V 2A 9 LTC1871-1 3 8 7 6 M1 CVCC 4.7μF X5R + CIN 47μF 6.3V COUT1 150μF 6.3V ×2 COUT2 10μF 6.
LTC1871-1 TYPICAL APPLICATIONS 18V to 27V Input, 28V Output, 400W 2-Phase, Low Ripple, Synchronized RF Base Station Power Supply (Boost) VIN 18V to 27V R1 93.1k 1% L1 5.6μH 1 2 CC1 47pF SENSE RUN VIN ITH 10 RT1 150k 5% CFB1 47pF 5 FB INTVCC FREQ GATE MODE/SYNC GND 1 2 D1 VIN ITH 7 M1 6 CVCC1 4.7μF X5R CIN2 2.2μF 35V X5R CFB2 47pF R3 12.1k 1% R4 CC3 261k 6.8nF 1% 4 RT2 150k 5% 5 FB INTVCC FREQ GATE MODE/SYNC GND + COUT2 330μF 50V RS1 0.
LTC1871-1 TYPICAL APPLICATIONS 4.5V to 28V Input, 5V/2A Output SEPIC Converter with Undervoltage Lockout and Soft-Start R2 54.9k 1% R1 115k 1% C1 4.7nF • L1* 1 RC 12k SENSE RUN 2 VIN ITH 10 VIN 4.5V to 28V CDC 2.2μF 25V X5R ×3 D1 + LTC1871-1 3 CC1 8.2nF R3 154k 1% CC2 47pF R4 49.9k 1% FB 4 RT 162k 1% INTVCC FREQ 5 GATE MODE/SYNC GND VOUT 5V 2A (3A TO 4A PEAK) 9 8 7 6 CVCC 4.7μF 10V X5R CIN1 2.2μF 35V X5R M1 + CIN2 22μF 35V L2* • COUT1 330μF 6.3V COUT2 22μF 6.
LTC1871-1 TYPICAL APPLICATIONS 5V to 15V Input, – 5V/5A Output Positive-to-Negative Converter with Undervoltage Lockout and Level-Shifted Feedback • R1 154k 1% R2 68.1k C1 1% 1nF 2 SENSE RUN VIN ITH 4 5 CC2 330pF FB FREQ MODE/SYNC INTVCC GATE GND 9 COUT 100μF 6.3V X5R ×2 CDC 22μF 25V X7R M1 8 7 6 CIN 47μF 16V X5R CVCC 4.7μF 10V X5R RT 80.6k 1% R4 10k 1% 6 1 – R3 10k 1% L2* 10 LTC1871-1 3 RC 10k CC1 10nF • L1* 1 VIN 5V to 15V VOUT –5V 5A 4 D1 GND C2 10nF R5 40.
LTC1871-1 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” 0.86 (.
LTC1871-1 TYPICAL APPLICATION High Power SLIC Supply with Undervoltage Lockout (Also See the LTC3704 Data Sheet) GND • D2 10BQ060 4 R1 49.9k 1% VIN 7V TO 12V R2 150k 1% CR 1nF + • T1* 1, 2, 3 D4 10BQ060 C5 10μF 25V X5R • • FB INTVCC FREQ GATE MODE/SYNC GND IRL2910 + f = 200kHz C1 4.7μF X5R *COILTRONICS VP5-0155 (PRIMARY = 3 WINDINGS IN PARALLEL) C2 4.7μF 50V X5R 6 RF1 10k 1% RS 0.012Ω COUT 3.
