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| Final Electrical Specifications LTC1871-7 High Input Voltage, Current Mode Boost, Flyback and SEPIC Controller FEATURES s s s DESCRIPTIO July 2002 s s s s s s s s s s s 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 = 10A Programmable Operating Frequency (50kHz to 1MHz) with One External Resistor Synchronizable to an External Clock Up to 1.3 x fOSC User-Controlled Pulse Skip or Burst Mode(R) Operation Output Overvoltage Protection Can be Used in a No RSENSETM Mode for VDS < 36V Small 10-Lead MSOP Package The LTC(R)1871-7 is a current mode, boost, flyback and SEPIC controller optimized for driving 6V-rated MOSFETs in high voltage applications. The LTC1871-7 works equally well in low or high power applications and requires few components to provide a complete power supply solution. The switching frequency can be set with an external resistor over a 50kHz to 1MHz range, and can be synchronized to an external clock using the MODE/SYNC pin. Burst Mode operation at light loads, a low minimum operating supply voltage of 6V and a low shutdown quiescent current of 10A make the LTC1871-7 well suited for battery-operated systems. For applications requiring constant frequency operation, Burst Mode operation can be defeated using the MODE/SYNC pin. The LTC1871-7 is available in the 10-lead MSOP package. PARAMETER INTVCC INTVCC INTVCC UV + UV - LTC1871-7 7.0V 5.6V 4.6V LTC1871 5.2V 2.1V 1.9V APPLICATIO S s s s s Telecom Power Supplies 42V Automotive Systems 24V Industrial Controls IP Phone Power Supplies , LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode is a registered trademark of Linear Technology Corporation. No RSENSE is a trademark of Linear Technology Corporation. TYPICAL APPLICATIO VIN 36V TO 72V 604k 26.7k 2.2F 100V X7R D3 10BQ060 100k D1 9.1V Q1 FMMT625 10 3:1 T1 VP1-0076 47F 16V X5R D2 4148 RUN ITH 2.2nF 3.4k FB 12.4k 110k 120k FREQ MODE/SYNC SENSE VIN LTC1871-7 INTVCC GATE GND 4.7F X5R 0.1F X5R 0.12 M1 FDC2512 Figure 1. Small, Nonisolated 12V Flyback Telecom Housekeeping Supply 18717i 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. U VOUT 12V 0.4A 18717 F01 U U 1 LTC1871-7 ABSOLUTE (Note 1) AXI U RATI GS PACKAGE/ORDER I FOR ATIO ORDER PART NUMBER TOP VIEW RUN ITH FB FREQ MODE/ SYNC 1 2 3 4 5 10 9 8 7 6 SENSE VIN INTVCC GATE GND 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 ...................................- 0.3V to 9V FREQ Voltage ............................................- 0.3V to 1.5V SENSE Pin Voltage ................................... - 0.3V to 36V Operating Temperature Range (Note 2) .. - 40C to 85C Junction Temperature (Note 3) ............................ 125C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C LTC1871EMS-7 MS PACKAGE 10-LEAD PLASTIC MSOP MS PART MARKING LTG4 TJMAX = 125C, JA = 120C/ W Consult LTC Marketing for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 8V, VRUN = 1.5V, RFREQ = 80k, VMODE/SYNC = 0V, unless otherwise specified. SYMBOL VIN(MIN) IQ PARAMETER Minimum Input Voltage Input Voltage Supply Current Continuous Mode Burst Mode Operation, No Load Shutdown Mode Rising RUN Input Threshold Voltage Falling RUN Input Threshold Voltage q CONDITIONS MIN 6 TYP MAX UNITS V Main Control Loop (Note 4) VMODE/SYNC = 5V, VFB = 1.4V, VITH = 0.75V VMODE/SYNC = 0V, VITH = 0.2V (Note 5) VRUN = 0V 1.223 1.198 50 VITH = 0.2V (Note 5) q 600 280 12 1.348 1.248 100 5 1.218 1.212 1.230 18 0.002 q 1000 500 25 1.273 1.298 150 60 1.242 1.248 60 0.02 VRUN+ VRUN- VRUN(HYST) IRUN VFB IFB VFB VIN VFB VITH VFB(OV) gm VITH(BURST) ISENSE(ON) ISENSE(OFF) RUN Pin Input Threshold Hysteresis RUN Input Current Feedback Voltage FB Pin Input Current Line Regulation Load Regulation FB Pin, Overvoltage Lockout Error Amplifier Transconductance Burst Mode Operation ITH Pin Voltage SENSE Pin Current (GATE High) SENSE Pin Current (GATE Low) VITH = 0.2V (Note 5) 6V VIN 30V VMODE/SYNC = 0V, VTH = 0.5V to 0.90V (Note 5) VFB(OV) - VFB(NOM) in Percent ITH Pin Load = 5A (Note 5) Falling ITH Voltage (Note 5) Duty Cycle < 20% VSENSE = 0V VSENSE = 30V (No RSENSE Mode) 120 -1 2.5 - 0.1 6 600 0.3 150 35 0.1 180 70 5 10 VSENSE(MAX) Maximum Current Sense Input Threshold 2 U A A A V V V mV nA V V nA %/V % % mho V mV A A 18717i W U U WW W LTC1871-7 The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 8V, VRUN = 1.5V, RFREQ = 80k, VMODE/SYNC = 0V, unless otherwise specified. SYMBOL Oscillator fOSC DMAX fSYNC/fOSC tSYNC(MIN) tSYNC(MAX) VIL(MODE) VIH(MODE) RMODE/SYNC VFREQ VINTVCC UVLO VINTVCC VIN1 VINTVCC VIN2 VLDO(LOAD) VDROPOUT GATE Driver tr tf GATE Driver Output Rise Time GATE Driver Output Fall Time CL = 3300pF (Note 7) CL = 3300pF (Note 7) 17 8 100 100 ns ns Oscillator Frequency Oscillator Frequency Range Maximum Duty Cycle Recommended Maximum Synchronized Frequency Ratio MODE/SYNC Minimum Input Pulse Width MODE/SYNC Maximum Input Pulse Width Low Level MODE/SYNC Input Voltage High Level MODE/SYNC Input Voltage MODE/SYNC Input Pull-Down Resistance Nominal FREQ Pin Voltage INTVCC Regulator Output Voltage INTVCC Undervoltage Lockout Thresholds VIN = 8V Rising INTVCC Falling INTVCC UVLO Hysteresis 8V VIN 15V 15V VIN 30V 0 IINTVCC 20mA, VIN = 8V VIN = 6V, INTVCC Load = 20mA -2 6.5 1.2 50 0.62 7 5.6 4.6 1.0 8 70 - 0.2 280 25 200 7.5 fOSC = 300kHz (Note 6) VSYNC = 0V to 5V VSYNC = 0V to 5V RFREQ = 80k 250 50 87 92 1.25 25 0.8/fOSC 0.3 300 350 1000 97 1.30 ns ns V V k V V V V V mV mV % mV kHz kHz % PARAMETER CONDITIONS MIN TYP MAX UNITS ELECTRICAL CHARACTERISTICS Low Dropout Regulator INTVCC Regulator Line Regulation INTVCC Regulator Line Regulation INTVCC Load Regulation INTVCC Regulator Dropout Voltage Note 1: Absolute Maximum Ratings are those values beyond which the life of the device may be impaired. Note 2: The LTC1871-7E is guaranteed to meet performance specifications from 0C to 70C. Specifications over the - 40C to 85C operating temperature range are assured by design, characterization and correlation with statistical process controls. Note 3: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formula: TJ = TA + (PD * 120C/W) Note 4: The dynamic input supply current is higher due to power MOSFET gate charging (QG * fOSC). See Applications Information. Note 5: The LTC1871-7 is tested in a feedback loop that servos VFB to the reference voltage with the ITH pin forced to a voltage between 0V and 1.4V (the no load to full load operating voltage range for the ITH pin is 0.3V to 1.23V). Note 6: In a synchronized application, the internal slope compensation gain is increased by 25%. Synchronizing to a significantly higher ratio will reduce the effective amount of slope compensation, which could result in subharmonic oscillation for duty cycles greater than 50%. Note 7: Rise and fall times are measured at 10% and 90% levels. 18717i 3 LTC1871-7 TYPICAL PERFOR A CE CHARACTERISTICS FB Voltage vs Temp 1.25 FB PIN CURRENT (nA) 0 5 10 15 20 VIN (V) 25 30 35 1.24 FB VOLTAGE (V) FB VOLTAGE (V) 1.23 1.22 1.21 -50 -25 0 25 50 75 100 125 150 TEMPERATURE (C) 1871 G01 Shutdown Mode IQ vs VIN 30 SHUTDOWN MODE IQ (A) SHUTDOWN MODE IQ (A) 20 Burst Mode IQ (A) 10 0 0 10 20 VIN (V) 30 Burst Mode IQ vs Temperature 500 18 16 400 14 12 Burst Mode IQ (A) TIME (ns) IQ (mA) 300 200 100 0 -50 -25 0 25 50 75 100 125 150 TEMPERATURE (C) 1871 G07 4 UW 1871 G04 FB Voltage Line Regulation 1.231 60 50 40 30 20 10 1.229 FB Pin Current vs Temperature 1.230 0 -50 -25 0 25 50 75 100 125 150 TEMPERATURE (C) 1871 G03 1871 G02 Shutdown Mode IQ vs Temperature 20 VIN = 8V 600 500 Burst Mode IQ vs VIN 15 400 300 200 100 10 5 40 0 -50 -25 0 25 50 75 100 125 150 TEMPERATURE (C) 1871 G05 0 0 10 20 VIN (V) 30 40 1871 G06 Dynamic IQ vs Frequency CL = 3300pF IQ(TOT) = 600A + Qg * f Gate Drive Rise and Fall Time vs CL 60 50 40 RISE TIME 30 20 FALL TIME 10 10 8 6 4 2 0 0 200 400 600 800 FREQUENCY (kHz) 1000 1200 0 0 2000 4000 6000 8000 CL (pF) 10000 12000 1871 G09 1871 G08 18717i LTC1871-7 TYPICAL PERFOR A CE CHARACTERISTICS RUN Thresholds vs VIN 1.5 1.40 RUN THRESHOLDS (V) 1.4 RUN THRESHOLDS (V) RT (k) 0 25 50 75 100 125 150 TEMPERATURE (C) 1871 G11 1.3 1.2 0 10 20 VIN (V) 30 Frequency vs Temperature 325 320 160 GATE FREQUENCY (kHz) 155 310 305 300 295 290 285 280 275 -50 -25 0 25 50 75 100 125 150 TEMPERATURE (C) 1871 G13 150 SENSE PIN CURRENT (A) 315 MAX SENSE THRESHOLD (mV) INTVCC Load Regulation 7.2 VIN = 8V DROPOUT VOLTAGE (mV) 7.0 INTVCC VOLTAGE (V) INTVCC VOLTAGE (V) 6.9 6.8 0 10 20 30 40 50 60 INTVCC LOAD (mA) 70 80 UW 1871 G10 1871 G16 RUN Thresholds vs Temperature 1000 RT vs Frequency 1.35 1.30 100 1.25 40 1.20 -50 -25 10 0 100 200 300 400 500 600 700 800 900 1000 FREQUENCY (kHz) 1871 G12 Maximum Sense Threshold vs Temperature 35 SENSE Pin Current vs Temperature GATE HIGH VSENSE = 0V 30 145 140 -50 -25 0 25 50 75 100 125 150 TEMPERATURE (C) 1871 G14 25 -50 -25 0 25 50 75 100 125 150 TEMPERATURE (C) 1871 G15 INTVCC Line Regulation 500 450 INTVCC Dropout Voltage vs Current, Temperature 150C 400 350 300 250 200 150 100 50 25C 125C 75C 7.1 7.0 0C -50C 6.9 0 5 10 15 20 25 VIN (V) 30 0 35 40 0 5 10 15 INTVCC LOAD (mA) 20 1871 G18 1871 G17 18717i 5 LTC1871-7 PI FU CTIO S RUN (Pin 1): The RUN pin provides the user with an accurate means for sensing the input voltage and programming the start-up threshold for the converter. The falling RUN pin threshold is nominally 1.248V and the comparator has 100mV of hysteresis for noise immunity. When the RUN pin is below this input threshold, the IC is shut down and the VIN supply current is kept to a low value (typ 10A). The Absolute Maximum Rating for the voltage on this pin is 7V. ITH (Pin 2): Error Amplifier Compensation Pin. The current comparator input threshold increases with this control voltage. Nominal voltage range for this pin is 0V to 1.40V. FB (Pin 3): Receives the feedback voltage from the external resistor divider across the output. Nominal voltage for this pin in regulaton is 1.230V. FREQ (Pin 4): A resistor from the FREQ pin to ground programs the operating frequency of the chip. The nominal voltage at the FREQ pin is 0.6V. 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. INTVCC (Pin 8): The Internal 7V Regulator Output. The gate driver and control circuits are powered from this voltage. Decouple this pin locally to the IC ground with a minimum of 4.7F low ESR tantalum or ceramic capacitor. This 7V regulator has an undervoltage lockout circuit with 5.6V and 4.6V rising and falling thresholds, respectively. VIN (Pin 9): Main Supply Pin. Must be closely decoupled to ground. SENSE (Pin 10): The Current Sense Input for the Control Loop. Connect this pin to a resistor in the source of the power MOSFET. Alternatively, the SENSE pin may be connected to the drain of the power MOSFET, in applications where the maximum VDS is less than 36V. Internal leading edge blanking is provided for both sensing methods. RUN SLOPE COMPENSATION FREQ 4 0.6V MODE/SYNC 5 85mV 1.230V IOSC PWM LATCH LOGIC OV 50k S Q R BURST COMPARATOR CURRENT COMPARATOR C1 GND V-TO-I OSC INTVCC GATE 7 BIAS AND START-UP CONTROL BLOCK DIAGRA 0.30V FB 3 EA gm 1.230V ITH 2 INTVCC 8 7V LDO UV 1.230V 6 + 5.6V UP 4.6V DOWN TO START-UP CONTROL + - + + - - - W U U U + C2 1 - 1.248V VIN 9 + SENSE + - 10 V-TO-I ILOOP SLOPE 1.230V GND BIAS VREF 6 1871 BD RLOOP VIN 18717i LTC1871-7 OPERATIO 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 VIN SENSE VSW GATE GND GND D VOUT 2a. SENSE Pin Connection for Maximum Efficiency (VSW < 36V) L VIN VIN GATE VSW D VOUT GND 2b. SENSE Pin Connection for Precise Control of Peak Current or for VSW > 36V Figure 2. Using the SENSE Pin On the LTC1871-7 For circuit operation, please refer to the Block Diagram of the IC and Figure 1. In normal operation, the power MOSFET is turned on when the oscillator sets the PWM latch and is turned off when the current comparator C1 resets the latch. The divided-down output voltage is compared to an internal 1.230V reference by the error amplifier EA, which outputs an error signal at the ITH pin. The voltage on the ITH pin sets the current comparator C1 input threshold. When the load current increases, a fall in the FB voltage relative to the reference voltage causes the ITH pin to rise, which causes the current comparator C1 to trip at U a higher peak inductor current value. The average inductor current will therefore rise until it equals the load current, thereby maintaining output regulation. The nominal operating frequency of the LTC1871-7 is programmed using a resistor from the FREQ pin to ground and can be controlled over a 50kHz to 1000kHz range. In addition, the internal oscillator can be synchronized to an external clock applied to the MODE/SYNC pin and can be locked to a frequency between 100% and 130% of its nominal value. When the MODE/SYNC pin is left open, it is pulled low by an internal 50k resistor and Burst Mode operation is enabled. If this pin is taken above 2V or an external clock is applied, Burst Mode operation is disabled and the IC operates in continuous mode. With no load (or an extremely light load), the controller will skip pulses in order to maintain regulation and prevent excessive output ripple. The RUN pin controls whether the IC is enabled or is in a low current shutdown state. A micropower 1.248V reference and comparator C2 allow the user to program the supply voltage at which the IC turns on and off (comparator C2 has 100mV of hysteresis for noise immunity). With the RUN pin below 1.248V, the chip is off and the input supply current is typically only 10A. An overvoltage comparator OV senses when the FB pin exceeds the reference voltage by 6.5% and provides a reset pulse to the main RS latch. Because this RS latch is reset-dominant, the power MOSFET is actively held off for the duration of an output overvoltage condition. The LTC1871-7 can be used either by sensing the voltage drop across the power MOSFET or by connecting the SENSE pin to a conventional shunt resistor in the source of the power MOSFET, as shown in Figure 2. Sensing the voltage across the power MOSFET maximizes converter efficiency and minimizes the component count, but limits the output voltage to the maximum rating for this pin (36V). By connecting the SENSE pin to a resistor in the source of the power MOSFET, the user is able to program output voltages significantly greater than 36V. + COUT + SENSE GND COUT RS 1871 F02 18717i 7 LTC1871-7 OPERATIO Programming the Operating Mode For applications where maximizing the efficiency at very light loads (e.g., <100A) 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). 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. Figures 3 and 4 show the light load switching waveforms for Burst Mode and pulse-skip mode operation for the converter in Figure 1. Burst Mode Operation Burst Mode operation is selected by leaving the MODE/ SYNC pin unconnected or by connecting it to ground. In normal operation, the range on the ITH pin corresponding to no load to full load is 0.30V to 1.2V. In Burst Mode operation, if the error amplifier EA drives the ITH voltage below 0.525V, the buffered ITH input to the current comparator C1 will be clamped at 0.525V (which corresponds to 25% of maximum load current). The inductor current peak is then held at approximately 30mV divided by the MODE/SYNC = 0V (Burst Mode OPERATION) VOUT 50mV/DIV IL 5A/DIV Figure 3. LTC1871-7 Burst Mode Operation (MODE/SYNC = 0V) at Low Output Current 8 U power MOSFET RDS(ON). If the ITH pin drops below 0.30V, the Burst Mode comparator B1 will turn off the power MOSFET and scale back the quiescent current of the IC to 250A (sleep mode). In this condition, the load current will be supplied by the output capacitor until the ITH voltage rises above the 50mV hysteresis of the burst comparator. At light loads, short bursts of switching (where the average inductor current is 20% of its maximum value) followed by long periods of sleep will be observed, thereby greatly improving converter efficiency. Oscilloscope waveforms illustrating Burst Mode operation are shown in Figure 3. Pulse-Skip Mode Operation With the MODE/SYNC pin tied to a DC voltage above 2V, Burst Mode operation is disabled. The internal, 0.525V buffered ITH burst clamp is removed, allowing the ITH pin to directly control the current comparator from no load to full load. With no load, the ITH pin is driven below 0.30V, the power MOSFET is turned off and sleep mode is invoked. Oscilloscope waveforms illustrating this mode of operation are shown in Figure 4. When an external clock signal drives the MODE/SYNC pin at a rate faster than the chip's internal oscillator, the oscillator will synchronize to it. In this synchronized mode, Burst Mode operation is disabled. The constant frequency associated with synchronized operation provides a more controlled noise spectrum from the converter, at the expense of overall system efficiency of light loads. MODE/SYNC = INTVCC (PULSE-SKIP MODE) VOUT 50mV/DIV IL 5A/DIV 10s/DIV 1871 F03 2s/DIV 1871 F04 Figure 4. LTC1871-7 Low Output Current Operation with Burst Mode Operation Disabled (MODE/SYNC = INTVCC) 18717i LTC1871-7 APPLICATIO S I FOR ATIO When the oscillator's internal logic circuitry detects a synchronizing signal on the MODE/SYNC pin, the internal oscillator ramp is terminated early and the slope compensation is increased by approximately 30%. As a result, in applications requiring synchronization, it is recommended that the nominal operating frequency of the IC be programmed to be about 75% of the external clock frequency. Attempting to synchronize to too high an 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. 2V TO 7V MODE/ SYNC tMIN = 25ns 0.8T T T = 1/fO RT (k) GATE D = 40% IL 1871 F05 Figure 5. MODE/SYNC Clock Input and Switching Waveforms for Synchronized Operation Programming the Operating Frequency The choice of operating frequency and inductor value is a tradeoff between efficiency and component size. Low frequency operation improves efficiency by reducing MOSFET and diode switching losses. However, lower frequency operation requires more inductance for a given amount of load current. The LTC1871-7 uses a constant frequency architecture that can be programmed over a 50kHz to 1000kHz range with a single external resistor from the FREQ pin to ground, as shown in Figure 1. The nominal voltage on the FREQ pin is 0.6V, and the current that flows into the FREQ pin is used to charge and discharge an internal oscillator U capacitor. A graph for selecting the value of RT for a given operating frequency is shown in Figure 6. 1000 100 10 0 100 200 300 400 500 600 700 800 900 1000 FREQUENCY (kHz) 1871 F06 W UU Figure 6. Timing Resistor (RT) Value INTVCC Regulator Bypassing and Operation An internal, P-channel low dropout voltage regulator produces the 7V supply which powers the gate driver and logic circuitry within the LTC1871-7, as shown in Figure 7. The INTVCC regulator can supply up to 50mA and must be bypassed to ground immediately adjacent to the IC pins with a minimum of 4.7F tantalum or ceramic capacitor. Good bypassing is necessary to supply the high transient currents required by the MOSFET gate driver. The LTC1871-7 contains an undervoltage lockout circuit which protects the external MOSFET from switching at low gate-to-source voltages. This undervoltage circuit senses the INTVCC voltage and has a 5.6V rising threshold and a 4.6V falling threshold. For input voltages that don't exceed 8V (the absolute maximum rating for INTVCC is 9V), the internal low dropout regulator in the LTC1871-7 is redundant and the INTVCC pin can be shorted directly to the VIN pin. With the INTVCC pin shorted to VIN, however, the divider that programs the regulated INTVCC voltage will draw 14A of current from the input supply, even in shutdown mode. For applications that require the lowest shutdown mode input supply current, do not connect the INTVCC pin to VIN. Regardless of whether the INTVCC pin is shorted to VIN or not, it is always necessary to have the driver circuitry bypassed with a 4.7F ceramic capacitor to ground immediately adjacent to the INTVCC and GND pins. 18717i 9 LTC1871-7 APPLICATIO S I FOR ATIO 1.230V R2 R1 LOGIC Figure 7. Bypassing the LDO Regulator and Gate Driver Supply In an actual application, most of the IC supply current is used to drive the gate capacitance of the power MOSFET. As a result, high input voltage applications in which a large power MOSFET is being driven at high frequencies can cause the LTC1871-7 to exceed its maximum junction temperature rating. The junction temperature can be estimated using the following equations: IQ(TOT) IQ + f * QG PIC = VIN * (IQ + f * QG) TJ = TA + PIC * RTH(JA) The total quiescent current IQ(TOT) consists of the static supply current (IQ) and the current required to charge and discharge the gate of the power MOSFET. The 10-pin MSOP package has a thermal resistance of RTH(JA) = 120C/W. As an example, consider a power supply with VIN =10V. The switching frequency is 200kHz, and the maximum ambient temperature is 70C. The power MOSFET chosen is the FDS3670(Fairchild), which has a maximum RDS(ON) of 35m (at room temperature) and a maximum total gate charge of 80nC (the temperature coefficient of the gate charge is low). IQ(TOT) = 600A + 80nC * 200kHz = 16.6mA PIC = 10V * 16.6mA = 166mW 10 U VIN INPUT SUPPLY 2.5V TO 30V P-CH CIN 7V INTVCC CVCC 4.7F X5R M1 DRIVER GATE 6V-RATED POWER MOSFET GND 1871 F07 W + - UU GND PLACE AS CLOSE AS POSSIBLE TO DEVICE PINS TJ = 70C + 120C/W * 166mW = 89.9C TJRISE = 19.9C This demonstrates how significant the gate charge current can be when compared to the static quiescent current in the IC. To prevent the maximum junction temperature from being exceeded, the input supply current must be checked when operating in a continuous mode at high VIN. A tradeoff between the operating frequency and the size of the power MOSFET may need to be made in order to maintain a reliable IC junction temperature. Prior to lowering the operating frequency, however, be sure to check with power MOSFET manufacturers for their latest-and-greatest low QG, low RDS(ON) devices. Power MOSFET manufacturing technologies are continually improving, with newer and better performance devices being introduced almost yearly. Output Voltage Programming The output voltage is set by a resistor divider according to the following formula: R2 VO = 1.230V * 1 + R1 18717i LTC1871-7 APPLICATIO S I FOR ATIO The external resistor divider is connected to the output as shown in Figure 1, allowing remote voltage sensing. 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). Programming Turn-On and Turn-Off Thresholds with the RUN Pin The LTC1871-7 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. This allows users to accurately program an input voltage at which the converter will turn on and off. The falling threshold voltage on the RUN pin is equal to the internal reference voltage of 1.248V. The comparator has 100mV of hysteresis to increase noise immunity. + R2 INPUT SUPPLY OPTIONAL FILTER CAPACITOR R1 1.248V POWER REFERENCE GND 1871 F8a - Figure 8a. Programming the Turn-On and Turn-Off Thresholds Using the RUN Pin RUN COMPARATOR RUN 6V EXTERNAL LOGIC CONTROL 1.248V + - 1871 F08b Figure 8b. On/Off Control Using External Logic U The turn-on and turn-off input voltage thresholds are programmed using a resistor divider according to the following formulas: R2 VIN(OFF) = 1.248V * 1 + R1 R2 VIN(ON) = 1.348V * 1 + R1 W UU The resistor R1 is typically chosen to be less than 1M. For applications where the RUN pin is only to be used as a logic input, the user should be aware of the 7V Absolute Maximum Rating for this pin! The RUN pin can be connected to the input voltage through an external 1M resistor, as shown in Figure 8c, for "always on" operation. VIN RUN COMPARATOR BIAS AND START-UP CONTROL RUN 6V + - + R2 1M INPUT SUPPLY VIN RUN COMPARATOR RUN 6V + - - GND 1.248V 1871 F08c Figure 8c. External Pull-Up Resistor On RUN Pin for "Always On" Operation 18717i 11 LTC1871-7 TYPICAL APPLICATIO S A Small, Nonisolated 12V Flyback Telecom Housekeeping Supply VIN 36V TO 72V R1 604k 1% C1 1nF OPTIONAL CIN 2.2F 100V X7R UV+ = 31.8V UV - = 29.5V RUN ITH LTC1871-7 RC 3.4k R4 110k 1% FB R3 12.4k 1% RT 120k FREQ MODE/SYNC f = 200kHz INTVCC GATE GND C2 4.7F X5R C3 0.1F X5R RS 0.12 M1 D2 R5 100k Q1 D1 9.1V R6 10 SENSE VIN T1 1, 2, 3 (SERIES) 4, 5, 6 (PARALLEL) COUT 47F X5R D3 VOUT 12V 0.4A R2 26.7k 1% CC2 47pF CC1 2.2nF T1: COILTRONICS VP1-0076 M1: FAIRCHILD FDC2512 (150V, 0.5) Q1: ZETEX FMMT625 (120V) PACKAGE DESCRIPTIO 5.23 (.206) MIN 0.889 0.127 (.035 .005) 3.2 - 3.45 (.126 - .136) 0.254 (.010) GAUGE PLANE 0.50 0.305 0.038 (.0197) (.0120 .0015) BSC TYP RECOMMENDED SOLDER PAD LAYOUT 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 RELATED PARTS PART NUMBER LT(R)1619 LTC1624 LTC1700 LTC1871 LTC1872 LT1930 LT1931 LTC3401/LTC3402 DESCRIPTION Current Mode PWM Controller Current Mode DC/DC Controller No RSENSE Synchronous Step-Up Controller Wide Input Range, No RSENSE Controller SOT-23 Boost Controller 1.2MHz, SOT-23 Boost Converter Inverting 1.2MHz, SOT-23 Converter 1A/2A 3MHz Synchronous Boost Converters COMMENTS 300kHz Fixed Frequency, Boost, SEPIC, Flyback Topology SO-8; 300kHz Operating Frequency; Buck, Boost, SEPIC Design; VIN Up to 36V Up to 95% Efficiency, Operation as Low as 0.9V Input Operation as Low as 2.5V Input, Boost Flyback,SEPIC Delivers Up to 5A, 550kHz Fixed Frequency, Current Mode Up to 34V Output, 2.6V VIN 16V, Miniature Design Positive-to-Negative DC/DC Conversion, Miniature Design Up to 97% Efficiency, Very Small Solution, 0.5V VIN 5V 18717i LT/TP 0702 1.5K * PRINTED IN USA 12 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 q FAX: (408) 434-0507 q U U D1: ON SEMICONDUCTOR MMBZ5239BLT1 (9.1V) D2: ON SEMICONDUCTOR MMSD4148T11 D3: INTERNATIONAL RECTIFIER 10BQ060 18717 F02 MS Package 10-Lead Plastic MSOP (Reference LTC DWG # 05-08-1661) 3.00 0.102 (.118 .004) (NOTE 3) 10 9 8 7 6 0.497 0.076 (.0196 .003) REF DETAIL "A" 0 - 6 TYP 4.88 0.10 (.192 .004) 3.00 0.102 (.118 .004) NOTE 4 12345 0.53 0.01 (.021 .006) DETAIL "A" 0.18 (.007) SEATING PLANE 0.17 - 0.27 (.007 - .011) 0.13 0.05 (.005 .002) MSOP (MS) 0402 1.10 (.043) MAX 0.86 (.034) REF 0.50 (.0197) TYP www.linear.com (c) LINEAR TECHNOLOGY CORPORATION 2002 |
Price & Availability of LTC1871-7
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