Linear Technology Analog Devices LT8708 Скачать руководство пользователя страница 12

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LT8708

Rev 0

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TYPICAL PERFORMANCE CHARACTERISTICS

Discontinuous Mode (Page 59)

Continuous Mode (Page 59)

Continuous Mode (Page 59)

Load Step (Page 59)

Load Step (Page 59)

Continuous Mode (Page 59)

5μs/DIV

8708 G24

5A/DIV

SW1

20V/DIV

SW2

20V/DIV

BAT

= 38V

LOAD

= 47.4V

5μs/DIV

8708 G25

5A/DIV

SW1

20V/DIV

SW2

20V/DIV

BAT

= 48V

LOAD

= 47.4V

5μs/DIV

8708 G26

5A/DIV

SW1

20V/DIV

SW2

20V/DIV

BAT

= 52V

LOAD

= 47.4V

5μs/DIV

8708 G27

5A/DIV

SW1

20V/DIV

SW2

20V/DIV

BAT

= 38V

LOAD

= 47.4V

5ms/DIV

8708 G28

5A/DIV

LOAD

500mV/DIV

BAT

= 38V

LOAD

= 47.4V

LOAD STEP = 2A TO 4A

5ms/DIV

8708 G29

LOAD

500mV/DIV

5A/DIV

BAT

= 52V

LOAD

= 47.4V

LOAD STEP = 2A TO 4A

5ms/DIV

8708 G30

LOAD

500mV/DIV

5A/DIV

BAT

= 48V

LOAD

= 47.4V

LOAD STEP = 2A TO 4A

Содержание Analog Devices LT8708

Страница 1: ...rter Allfourcurrentlimits forwardinput reverseinput forward outputandreverseoutput canbesetindependentlyusing four resistors on the PCB The MODE pin can select between discontinuous conduc tion mode D...

Страница 2: ...nitoring ICP and ICN Pins 25 INTVCC EXTVCC GATEVCC LDO33 Power 25 CLKOUT and Temperature Sensing 25 Applications Information 26 Verify the Power Flow Conditions 26 Operating Frequency Selection 26 Int...

Страница 3: ...5 C to 150 C Note 1 15 16 17 18 TOP VIEW 41 GND UHG PACKAGE 40 LEAD 5mm 8mm PLASTIC QFN TJMAX 150 C JA 36 C W JC 3 8 C W EXPOSED PAD PIN 41 IS GND MUST BE SOLDERED TO PCB 19 20 21 40 39 38 37 36 35 34...

Страница 4: ...C Connected to INTVCC 170 mV INTVCC Regulator Dropout Voltage VINCHIP VINTVCC IINTVCC 20mA 220 mV LDO33 Pin Voltage 5mA from LDO33 Pin l 3 23 3 295 3 35 V LDO33 Pin Load Regulation ILDO33 0 1mA to 5mA...

Страница 5: ...2 V Regulation Voltage for FBIN Regulate VC to 1 2V l 1 184 1 205 1 226 V Line Regulation for FBOUT and FBIN Error Amp Reference Voltage VINCHIP 12V to 80V Not Switching 0 002 0 005 V FBOUT Pin Bias C...

Страница 6: ...or Amp EA1 gm FBIN 0V FBOUT 3 3V 190 mho IMON_INN Error Amp EA1 Voltage Gain FBIN 0V FBOUT 3 3V 130 V V CSPOUT Bias Current VCSPOUT 12V VCSPOUT 1 5V 0 01 0 01 A A CSNOUT Bias Current BOOST Capacitor C...

Страница 7: ...On Time for Synchronous Switch in Buck Operation tON M2 MIN Switch M2 CLOAD 3300pF 200 ns Minimum Off Time for Main Switch in Steady State Boost Operation Switch M3 CLOAD 3300pF 230 ns Minimum Off Ti...

Страница 8: ...and fall times are measured using 10 and 90 levels Delay times are measured using 50 levels Note 5 Do not apply a voltage or current source to these pins They must be connected to capacitive loads onl...

Страница 9: ...A 0 01 0 1 1 10 0 10 20 30 40 50 60 70 80 90 100 EFFICIENCY 8708 G03 HCM DCM CCM VIN 48V VOUT 47 4V VC 1 2V FBOUT FBIN IMON_INP IMON_INN IMON_ON IMON_OP TEMPERATURE C 45 20 5 30 55 80 105 130 155 1 1...

Страница 10: ...UCK REGION BOOST REGION TEMPERATURE C 45 20 5 30 55 80 105 130 155 120 100 80 60 40 20 0 CSP CSN mV 8708 G12 BUCK REGION BOOST REGION MINIMUM VC MAXIMUM VC TJ 25 C SS V 0 0 3 0 6 0 9 1 2 1 5 0 0 5 1 0...

Страница 11: ...PIN A 8708 G19 RISING FALLING SHDN SWEN TEMPERATURE C 45 20 5 30 55 80 105 130 155 1 10 1 12 1 14 1 16 1 18 1 20 1 22 1 24 1 26 1 28 1 30 PIN THRESHOLD VOLTAGE V 8708 G20 TEMPERATURE C 45 20 5 30 55 8...

Страница 12: ...IV VBAT 38V VLOAD 47 4V 5 s DIV 8708 G25 IL 5A DIV SW1 20V DIV SW2 20V DIV VBAT 48V VLOAD 47 4V 5 s DIV 8708 G26 IL 5A DIV SW1 20V DIV SW2 20V DIV VBAT 52V VLOAD 47 4V 5 s DIV 8708 G27 IL 5A DIV SW1 2...

Страница 13: ...OUT voltage VC Pin 10 Error Amplifier Output Pin Tie external compensation network to this pin IMON_INP Pin 11 Positive VIN Current Monitor and Limit Pin The current out of this pin is 20 A plus a cur...

Страница 14: ...e VIN cur rent signals Connect this pin to VIN when not in use See ApplicationsInformationsectionforproperuseofthispin CSPIN Pin 33 The Input to the VIN Current Monitor Amplifier Connect this pin to V...

Страница 15: ...E CSN CSP SWEN VINCHIP CSNIN CSPIN IMON_INN MODE CLKOUT SYNC RT IMON_INP RVS DIR RVS VC EA5 EA6 EA4 EA3 1 209V IMON_INP EA1 EA2 1 21V IMON_INN 1 207V 1 205V 1 207V A6 A1 1 207V EA7 BOOST CAPACITOR CHA...

Страница 16: ...to the SW1 side Also refers to current that flows from VOUT and or into VIN Refer to the Block Diagram Figure 1 when reading the following sections about the operation of the LT8708 START UP Figure 2...

Страница 17: ...ry provides a gradual ramp of VC and the inductor current in the appropriate direction refer to the VC vs SS Voltage graph in the Typical Per formance Characteristics section This prevents abrupt surg...

Страница 18: ...GULATE EA1 IMON_INN Negative IIN EA2 IMON_ON Negative IOUT EA3 FBIN VIN Voltage EA4 FBOUT VOUT Voltage EA5 IMON_INP Positive IIN EA6 IMON_OP Positive IOUT TheVCvoltagetypicallyhasamin maxrangeofabout1...

Страница 19: ...and M4 turned on the controller first oper ates as if in the buck region When A5 trips switch M2 is turned off and M1 is turned on until the middle of the clock cycle Next switch M4 turns off and M3...

Страница 20: ...tch off time should be kept above 230ns typical see Electrical Characteristics to maintain steady stateoperationandavoiddutycyclejitter increased output ripple and reduction in maximum output current...

Страница 21: ...n be changed during operation as needed with the following restrictions 1 Before transitioning from MODE Burst Mode opera tion to MODE CCM the DIR pin must be driven to the Hi Forward state 2 Avoid co...

Страница 22: ...M light loadisdetectedwhenICN orIMON_INP isabove255mV typical As a result M4 or M1 is turned off to prevent average current flow opposite to the desired direction Heavy load is detected when ICN or IM...

Страница 23: ...lation VOUT is regulated subject to the priorities in Table 3 us ing a resistor divider between VOUT FBOUT and ground FBOUT connects to the EA4 amplifier to drive VC When FBOUT rises near or above the...

Страница 24: ...IN VINHIMON and ground is used to detect VIN overvoltage This function prevents reverse conduction from VOUT to VIN from forcing VIN higherthandesired WhenovervoltageisdetectedbyVIN HIMON RVSOFF is pu...

Страница 25: ...OWER PowerforthetopandbottomMOSFETdrivers theLDO33 pin and most internal circuitry is derived from the INTVCC pin INTVCC is regulated to 6 3V typical from either the VINCHIP or EXTVCC pin When the EXT...

Страница 26: ...o deactivate that pin function then the VIN VIN_FBIN row of Table 6 a is not applicable and no cells in that row should be circled Next for each cell identified in Table 6 a check that the operating c...

Страница 27: ...equency whether the internal clock is synchronized to the SYNC pin or is free runningbasedontheexternalRT resistor Therising edge of CLKOUT is approximately 180 out of phase from the internal clock s...

Страница 28: ...n operating in the boost region with forward conduction VIN to VOUT Skip this section and assume RSENSE MAX BOOST FWD when this operating condition does not apply to the application In the boost regio...

Страница 29: ...ondition does not apply to the application In the boost region the maximum reverse VIN current capability is the lowest when operating at the minimum duty cycle See Switch Control Boost Region VIN VOU...

Страница 30: ...then IL MIN BUCK can be calculated as follows IL MIN BUCK DC ABSMIN M2 BUCK 100 VOUT MIN BUCK L A where DC ABSMIN M2 BUCK is the minimum duty cycle per centage in the buck region as calculated previo...

Страница 31: ...VRSENSE MIN BUCK MAXDC is the minimum inductor current sense voltage at the maximum duty cycle This value is determined in a similar manner to VRSENSE MAX BOOST MAXDC discussedpreviouslyinthe RSENSE...

Страница 32: ...ercent age of the M3 switch see RSENSE Selection Max RSENSE in the Boost Region section is the switching frequency VRSENSE MAX BOOST MAXDC is the maximum current sense voltage in the boost region at m...

Страница 33: ...an otherwise occur when VIN VOUT is less than 0 5 or greater than 2 The slope compensation circuits will prevent these oscil lations provided that the inductance exceeds a minimum value see the earlie...

Страница 34: ...N MAX BUCK VOUT MIN BUCK VOUT MIN BUCK DC MAX M2 BUCK 100 2 L A where DC MAX M2 BUCK is the maximum duty cycle percent age of the M2 switch in the buck region see RSENSE Selection Max RSENSE in the Bu...

Страница 35: ...us the thermal resistance from the case to the ambient tem perature RTH CA Compare the calculated value of TJ to the manufacturer s data sheets to help choose MOSFETs that will not overheat Thepowerdi...

Страница 36: ...VIN and higher VOUT that cause the M1 switch to be on for the most amount of time Switch M2 In most cases of positive conduction the M2 switching power dissipation is quite small and I2R power losses...

Страница 37: ...A parallel combination of capacitors is typically used to achieve high capacitance and low ESR equivalent series resistance Dry tantalum special polymer aluminumelectrolyticandceramiccapacitorsare all...

Страница 38: ...eramic caps added in parallel the steady state VOUT ripple due to charging and discharging the ceramic COUT is given by the following equations V OUT BOOST CERAM IOUT ESRCERAM 1 exp VIN VOUT VOUT ESRC...

Страница 39: ...itor In the buck region when M4 is always on current is drawn as needed from the CSNIN and or BOOST1 pins to charge the CB2 capacitor Because of this function CSPIN and CSNIN should be connected acros...

Страница 40: ...switch to turn on if the inductor cur rent is negative In addition to the 24mV typical voltage hysteresis the VINHIMON pin will source 1 A typical current and the VOUTLOMON pin will sink 1 A typical c...

Страница 41: ...1 207 RIN3 VOVIN ROUT3 1 207 IFBDIV ROUT1 VOUT ROUT3 1 1 207 1 VUVOUT ROUT2 VOUT VUVOUT VUVOUT ROUT3 ROUT4 ROUT1 IHYSMON VUVOUT VUVOUT 1 207 VUVOUT IHYSMON VHYSMON IHYSMON VUVOUT 1 207 ROUT3 VUVOUT w...

Страница 42: ...circuits The remaining discussion refers to the IIN current monitor circuit of Figure 15 All discussion and equations are also applicable to the IOUT current monitor circuit substituting pin and devi...

Страница 43: ...capacitors CIMON_INP and CIMON_INN due to IIN ripple and discontinuities that can occur in various regions of operation A few nF of capacitance is usually sufficient APPLICATIONS INFORMATION CurrentL...

Страница 44: ...hen the average sensed current is low but contains high AC content ClippingmaydistorttheICNorIMON_INPvoltages that are used to select between heavy and light load HCM operation Once again the current...

Страница 45: ...ally derived from VINCHIP until VOUT EXTVCC rises above 6 4V after which the power is derived from VOUT EXTVCC This works well for example inacasewhereVOUT isregulatedto12Vandthemaximum VINCHIP voltag...

Страница 46: ...nd SWEN and in some cases FBIN pin The UVLO function sets the turn on off of the LT8708 at a desired minimum voltage For example a resistor divider can be connected between VIN SHDN and GND as shown i...

Страница 47: ...regulator s input voltage times the current represents lost power This loss can be reduced by supplying INTVCC current through the EXTVCC pin APPLICATIONS INFORMATION from a high efficiency source su...

Страница 48: ...le keeping the GND BG and SW traces short Minimize inductance from the sources of M2 and M3 to RSENSE by making the trace short and wide Keep the high dv dt nodes SW1 SW2 BOOST1 BOOST2 TG1 and TG2 awa...

Страница 49: ...voltage 2 ESR of the input or output capacitors 3 initial voltage of the capacitors and 4 cableimpedance Excessiveinrushcurrentcanlead to sparking that can compromise connector integrity and or voltag...

Страница 50: ...750 fOSC 1 k 43 750 150 1 290 7k We will choose 294k for RT resistor RSENSE Selection Start by calculating the maximum and minimum duty cycle in the boost region DC MAX M3 BOOST 1 VIN MIN BOOST VOUT M...

Страница 51: ...ical Performance Charac teristics section VRSENSE MAX BUCK MINDC 82mV VRSENSE MIN BUCK MAXDC 65mV Next estimate the inductor current ripples at maximum and minimum buck duty cycles IL MIN BUCK IOUT MA...

Страница 52: ...cified for operation with the availablegatevoltageamplitude Inthiscase theamplitude is 6 3V and MOSFETs with an RDS ON value specified at VGS 4 5V can be used SelectM1andM2 With25Vmaximuminputvoltage...

Страница 53: ...by choosing a slower switching frequency Since this calculation is approximate measure the actual rise and fall times on the PCB to obtain a better power estimate Select M3 and M4 With 12V output volt...

Страница 54: ...is calculated to be RIMON_ON 1 21 I OUT RVS LIMIT 1m A V RSENSE2 20 A 1 21 3 6A 1m A V 8m 20 A 24 9k VOUT Voltage VOUT voltage is 12V Select RFBOUT2 as 20k RFBOUT1 is RFBOUT1 VOUT 1 207V 1 RFBOUT2 Sel...

Страница 55: ...URTH 701014330 XOR DIODES INC 74AHC1G86SE 7 M1 M4 INFINEON BSC010N04LS POWER TRANFER DECISION LOGIC 100k 27 4k M6 M5 12 1k M7 XOR 68 1k 220pF 4 7 F 127k 100k 54 9k 680pF 10nF 10k 365k 1 F 4 7nF 23 7k...

Страница 56: ...ging Current Limit 15A IMON_INN VBAT1_DEAD 9V Falling or 9 4V Rising VBAT2 Charging Current Limit 15A IMON_OP VBAT2_DEAD 9 25V Falling or 9 4V Rising Frequency 120kHz VBAT1_UV to Stop Discharging 10 5...

Страница 57: ...CTIONAL CONDUCTION SECTION FOR MORE DETAILS D B1 D B2 CENTRAL SEMI CMMR1U 02 LTE L1 10 H SER2918H 103KL XOR DIODES INC 74AHC1G86SE 7 M1 M2 INFINEON BSC026N08NS5 POWER TRANFER DECISION LOGIC 100k 16 9k...

Страница 58: ...8708 TA04h VBAT1 Charge Voltage 48V FBIN in RHCM VBAT2_UV to Stop Discharging 10 5V VOUTLOMON Falling or 12 3V VOUTLOMON Rising VBAT2 Charge Voltage 14 5V FBOUT in FHCM VBAT1 Charging Current Limit 4A...

Страница 59: ...UT3 220 F 100V C IN2 C IN3h C OUT1 C OUT2 4 7 F 100V X7R M5 TOSHIBA T2N7002AK D IN APPROPRIATE 8A SCHOTTKY DIODE OR IDEAL DIODE SUCH AS LTC4357 D B1 D B2 CENTRAL SEMI CMMR1U 02 LTE V IN 52V D IN TO LO...

Страница 60: ...ell Into 45 9V to 47 4V Range VBAT_DEAD Rose Into 47 4V to 50 2V Range RHCM Hi Lo Rose Into 45 9V to 47 4V Range Power Flows from VBAT to VLOAD Backup Operation 45 9V VLOAD is powered from VIN VBAT Ch...

Страница 61: ...ND BG2 SW2 BOOST2 V IN 12V C IN1 C IN2 D IN LD033 C OUT1 1 2k 6 C OUT2 C SC 6 V OUT 15V 8708 TA06a 5 6nF 124k 1 F 220pF 15k 22nF 6 8nF 17 4k 22nF 26 7k 6 8nF 17 4k 17 4k D B1 D B2 TO BOOST1 TO BOOST2...

Страница 62: ...OUT Charging Current Limit 1A IMON_OP Frequency 350kHz VIN Current Limit 2A IMON_INP Table of Operation Modes and Power Flow Directions VBACKUP VOUT POWER FLOW CHIP OPERATES IN RVSOFF VIN_MIN NO POWER...

Страница 63: ...00 0 10 R 0 125 TYP UHG QFN 0116 REV 1 00 TYP 1 00 TYP 0 20 REF DETAIL A 0 40 0 05 0 25 0 05 0 50 BSC 0 00 0 05 0 75 0 05 NOTE 1 ALL DIMENSIONS ARE IN MILLIMETERS ANGLES IN DEGREES 2 COPLANARITY APPLI...

Страница 64: ...System 2 8V Need EXTVCC 6 4V VIN 80V 1 3V VOUT 80V 5mm 8mm QFN 40 LT8705A 80V VIN and VOUT Synchronous 4 Switch Buck Boost DC DC Controller 2 8V VIN 80V Input and Output Current Monitor 5mm 7mm QFN 3...

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