LT8705 80V VIN and VOUT Synchronous 4-Switch BuckBoost DC/DC Controller Features n n n n n n n n n n Description Single Inductor Allows VIN Above, Below, or Equal to Regulated VOUT VIN Range 2.8V (Need EXTVCC > 6.4V) to 80V VOUT Range: 1.
LT8705 Absolute Maximum Ratings (Note 1) VCSP-VCSN, VCSPIN-VCSNIN, VCSPOUT-VCSNOUT...................................... –0.3V to 0.3V SS, CLKOUT, CSP, CSN Voltage.................... –0.3V to 3V VC Voltage (Note 2).................................... –0.3V to 2.2V RT, LDO33, FBOUT Voltage........................... –0.3V to 5V IMON_IN, IMON_OUT Voltage...................... –0.3V to 5V SYNC Voltage............................................. –0.3V to 5.5V INTVCC, GATEVCC Voltage.........................
LT8705 Order Information LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT8705EUHF#PBF LT8705EUHF#TRPBF 8705 38-Lead (5mm × 7mm) Plastic QFN –40°C to 125°C LT8705IUHF#PBF LT8705IUHF#TRPBF 8705 38-Lead (5mm × 7mm) Plastic QFN –40°C to 125°C LT8705EFE#PBF LT8705EFE#TRPBF LT8705FE 38-Lead Plastic TSSOP –40°C to 125°C LT8705IFE#PBF LT8705IFE#TRPBF LT8705FE 38-Lead Plastic TSSOP –40°C to 125°C Consult LTC Marketing for parts specified with wider operat
LT8705 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, SHDN = 3V unless otherwise noted. (Note 3) PARAMETER CONDITIONS SHDN Pin Bias Current VSHDN = 3V VSHDN = 12V SWEN Rising Threshold Voltage (Note 5) MIN l TYP MAX 0 11 1 22 1.156 1.206 1.256 SWEN Threshold Voltage Hysteresis (Note 5) 22 MODE Pin Forced Continuous Mode Threshold l 0.4 MODE Pin Burst Mode Range l 1.
LT8705 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, SHDN = 3V unless otherwise noted. (Note 2) PARAMETER CONDITIONS CSPOUT, CSNOUT Bias Current BOOST Capacitor Charge Control Block Not Active ICSPOUT + ICSNOUT, VCSPOUT = VCSNOUT = 12V ICSPOUT + ICSNOUT, VCSPOUT = VCSNOUT = 1.
LT8705 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, SHDN = 3V unless otherwise noted. (Note 3) PARAMETER CONDITIONS MIN SYNC High Level for Synchronization l SYNC Low Level for Synchronization SYNC Clock Pulse Duty Cycle TYP 1.3 0.5 V 80 % 2.45 2.
LT8705 Typical Performance Characteristics Efficiency vs Output Current (Boost Region-Figure 14) Efficiency vs Output Current (Buck-Boost Region-Figure 14) 100 100 90 90 80 80 70 70 70 60 50 40 VIN = 36V VOUT = 48V BURST CCM DCM 20 10 10 60 50 40 20 10 0 10000 1000 100 LOAD CURRENT (mA) VIN = 48V VOUT = 48V BURST CCM DCM 30 10 8705 G01 0 1.20 1.19 Oscillator Frequency VC = 1.2V 300 1.21 1.20 1.19 1.
LT8705 Typical Performance Characteristics Minimum Inductor Current Sense Voltage in Forced Continuous Mode INTVCC Line Regulation (EXTVCC = 0V) 0 7.0 6.5 BUCK REGION –40 6.0 –60 –80 5.5 5.0 BOOST REGION –100 6.5 6.0 20 40 60 4.0 100 80 M2 OR M3 DUTY CYCLE (%) 0 4 6 8 10 12 14 VIN (V) 16 18 3.5 TJ = 25°C BOOST AND BUCK-BOOST REGIONS 1.6 GATEVCC CONNECTED TO INTVCC IIN (mA) 1.0 0.8 0.6 2.0 1.5 1.0 0.4 125°C 25°C –40°C 0.5 0.2 0 0.2 0.4 0.6 0.8 SS (V) 1.0 1.2 0 1.
LT8705 Typical Performance Characteristics SRVO_xx Pin Activation Thresholds Internal VIN UVLO SHDN and MODE Pin Currents 3.0 16 MODE 14 125 VPIN-VREGULATION VPIN APPROACHING VREGULATION (mV) 18 2.5 SHDN 12 VIN UVLO (V) CURRENT INTO PIN (µA) TA = 25°C unless otherwise specified. 10 8 6 4 2 2.0 1.5 1.0 0.
LT8705 Typical Performance Characteristics Burst Mode Operation (Figure 14) TA = 25°C unless otherwise specified.
LT8705 Pin Functions (QFN/TSSOP) SHDN (Pin 1/Pin 4): Shutdown Pin. Tie high to enable device. Ground to shut down and reduce quiescent current to a minimum. Do not float this pin. RT (Pin 12/Pin 15): Timing Resistor Pin. Adjusts the switching frequency. Place a resistor from this pin to ground to set the free-running frequency. Do not float this pin. CSN (Pin 2/Pin 5): The (–) Input to the Inductor Current Sense and Reverse-Current Detect Amplifier.
LT8705 Pin Functions (QFN/TSSOP) CSPOUT (Pin 31/Pin 34): The (+) Input to the Output Current Monitor Amplifier. This pin and the CSNOUT pin measure the voltage across the sense resistor, RSENSE2, to provide the output current signals. Connect this pin to VOUT when not in use. See Applications Information section for proper use of this pin. CSNIN (Pin 32/Pin 36): The (–) Input to the Input Current Monitor Amplifier.
LT8705 Block Diagram VIN RSENSE1 RSENSE CSN SWEN CSP DB1 BOOST1 CSNIN – CSPIN + + + A7 M1 SW1 – A5 CB1 TG1 A8 M2 BUCK LOGIC – D1 (OPT) GATEVCC VIN BG1 IMON_IN GND MODE BOOST CAPACITOR CHARGE CONTROL BG2 CLKOUT SYNC OSC RT M3 SW2 BOOST LOGIC TG2 + CB2 BOOST2 A9 – DB2 2.5V OT OI_IN OI_OUT STARTUP AND FAULT LOGIC SS + CSPOUT – CSNOUT A6 FAULT_INT RSENSE2 VOUT IMON_OUT – UV_INTVCC M4 D2 (OPT) EA1 + UV_LDO33 RSHDN1 UV_VIN UV_GATEVCC 1.
LT8705 Operation Refer to the Block Diagram (Figure 1) when reading the following sections about the operation of the LT8705. Main Control Loop The LT8705 is a current mode controller that provides an output voltage above, equal to or below the input voltage. The LTC proprietary topology and control architecture employs a current-sensing resistor (RSENSE) in buck or boost modes. The inductor current is controlled by the voltage on the VC pin, which is the diode-AND of error amplifiers EA1-EA4.
LT8705 Operation SHDN < 1.184V OR VIN < 2.5V OR TJUNCTION > 165°C CHIP OFF • SWITCHER OFF • LDOs OFF TYPICAL VALUES TJUNCTION < 160°C AND SHDN > 1.234V AND VIN > 2.5V AND (SWEN* < 1.184V OR (INTVCC AND GATEVCC < 4.65V) OR LDO33 < 3.04V) TYPICAL VALUES SWITCHER OFF • SWITCHER DISABLED • INTVCC AND LDO33 OUTPUTS ENABLED SHDN > 1.234V AND VIN > 2.5V AND SWEN* > 1.206V AND (INTVCC AND GATEVCC > 4.81V) AND LDO33 > 3.
LT8705 Operation charging up to 2.5V and be held there in the case of a fault event that persists. After the fault condition had ended and SS is greater than 1.6V, SS will then slowly discharge to 50mV (post fault delay state). This timeout period relieves the part and other downstream power components from electrical and thermal stress for a minimum amount of time as set by the voltage ramp rate on the SS pin.
LT8705 Operation Power Switch Control: Buck-Boost (VIN ≅ VOUT) Power Switch Control: Boost Region (VIN << VOUT) When VIN is close to VOUT, the controller enters the buckboost region. Figure 6 shows typical waveforms in this region. Every cycle, if the controller starts with switches M2 and M4 turned on, the controller first operates as if in the buck region. When A8 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 turns on.
LT8705 Operation When VOUT is much higher than VIN the duty cycle of switch M3 will increase, causing the M3 switch off-time to decrease. The M3 switch off-time should be kept above 245ns (typical, see Electrical Characteristics) to maintain steady-state operation, avoid duty cycle jitter, increased output ripple and reduction in maximum output current.
LT8705 Operation When the input or output current causes the respective IMON_IN or IMON_OUT voltage to rise near or above 1.208V (typical), the VC pin voltage will be pulled down to maintain the desired maximum input and/or output current (see EA1 and EA2 on the Block Diagram). The input current limit function prevents overloading the DC input source, while the output current limit provides a building block for battery charger or LED driver applications.
LT8705 Applications Information The first page shows a typical LT8705 application circuit. After the switching frequency is selected, external component selection continues with the selection of RSENSE and the inductor value. Next, the power MOSFETs are selected. Finally, CIN and COUT are selected. The following examples and equations assume continuous conduction mode unless otherwise specified. The circuit can be configured for operation up to an input and/or output voltage of 80V.
LT8705 Applications Information The duty cycle of CLKOUT is proportional to the die temperature and can be used to monitor the die for thermal issues. See the Junction Temperature Measurement section for more information. Inductor Current Sensing and Slope Compensation The LT8705 operates using inductor current mode control.
LT8705 Applications Information Otherwise, if the inductor value is already known then ∆IL(MAX,BOOST) can be more accurately calculated as follows: ∆IL(MAX,BOOST) = DC(MAX,M3,BOOST) • VIN(MIN) 100% f •L A Before calculating the maximum RSENSE resistance, however, the inductor ripple current must be determined.
LT8705 Applications Information Final RSENSE Value: The final RSENSE value should be lower than both RSENSE(MAX,BOOST) and RSENSE(MAX,BUCK). A margin of 30% or more is recommended. Figure 8 shows approximately how the maximum output current and maximum inductor current would vary with VIN/VOUT while all other operating parameters remain constant (frequency = 350kHz, inductance = 10μH, RSENSE = 10mΩ).
LT8705 Applications Information In order to provide adequate load current at low VIN voltages in the boost region, L should be at least: L(MIN2,BOOST) L(MIN1,BOOST) ≅ DC(MAX,M3,BOOST) VIN(MIN) • 100% VRSENSE(MAX,BOOST,MAX) IOUT(MAX) • VOUT(MAX) 2• f • – RSENSE VIN(MIN) where: DC(MAX,M3,BOOST) is the maximum duty cycle percentage of the M3 switch (see RSENSE Selection and Maximum Current section).
LT8705 Applications Information The peak inductor current when operating in the buck region is: IL(MAX,BUCK) ≅IOUT(MAX) DC(MAX,M2,BUCK • V OUT(MIN) 100% A + 2 •L • f where DC(MAX,M2,BUCK) is the maximum duty cycle percentage of the M2 switch in the buck region given by: V DC(MAX,M2,BUCK ) ≅ 1– OUT(MIN) •100% V IN(MAX) Note that the inductor current can be higher during load transients and if the load current exceeds the expected maximum IOUT(MAX).
LT8705 Applications Information are greatest or in the boost region when VIN is smallest, VOUT is highest and M1 is always on. Switch M1 power consumption can be approximated as: PM1 =PI2R +PSWITCHING V 2 ≅ OUT •IOUT •RDS(ON) • ρτ VIN + ( VIN •IOUT • f • tRF1) W Switch M2: In most cases the switching power dissipation in the M2 switch is quite small and I2R power losses dominate. I2R power is greatest in the buck region where the switch operates as the synchronous rectifier.
LT8705 Applications Information tRF2 is the average of the SW2 pin rise and fall times and, similar to tRF1, is typically 20ns to 40ns. As with the M1 switch, the switching power (PSWITCHING) often dominates. Look for MOSFETs with lower CRSS or consider operating at a lower frequency to minimize power loss and increase efficiency. Switch M4: In most cases the switching power dissipation in the M4 switch is quite small and I2R power losses dominate.
LT8705 Applications Information The maximum input ripple due to the voltage drop across the ESR is approximately: switch must be as small as possible, mandating that these components be placed adjacently. VIN(MAX) •IOUT(MAX) For applications with high input or output voltages (typically >40V) avoid Schottky diodes with excessive reverseleakage currents particularly at high temperatures. Some ultralow VF diodes will trade off increased high temperature leakage current for reduced forward voltage.
LT8705 Applications Information to VIN. Tie both pins to VIN if they are not being used. Also, CSPOUT and CSNOUT should always be tied to a potential close to VOUT, or be tied directly to VOUT if not being used. Boost Diodes DB1 and DB2: Although Schottky diodes have the benefit of low forward voltage drops, they can exhibit high reverse current leakage and have the potential for thermal runaway under high voltage and temperature conditions. Silicon diodes are thus recommended for diodes DB1 and DB2.
LT8705 Applications Information Input/Output Current Monitoring and Limiting to the Boost Capacitor Charge Control block (also see Figure 1) and can draw current in certain conditions. In addition, all four of the current sense pins can draw bias current under normal operating conditions. As such, do not place resistors in series with any of the CSxIN or CSxOUT pins. The LT8705 has independent input and output current monitor circuits that can be used to monitor and/or limit the respective currents.
LT8705 Applications Information Also, because of their use with the Boost Capacitor Charge Control block, tie the CSPIN and CSNIN pins to VIN and tie the IMON_IN pin to ground when the input current sensing is not in use. Similarly, the CSPOUT and CSNOUT pins should be tied to VOUT and IMON_OUT should be grounded when not in use. The remaining discussion refers to the input current monitor circuit.
LT8705 Applications Information INTVCC Regulators and EXTVCC Connection The LT8705 features two PNP LDOs (low dropout regulators) that regulate the 6.35V (typical) INTVCC pin from either the VIN or EXTVCC supply pin. INTVCC powers the MOSFET gate drivers via the required GATEVCC connection and also powers the LDO33 pin regulator and much of the LT8705’s internal control circuitry. The INTVCC LDO selection is determined automatically by the EXTVCC pin voltage. When EXTVCC is lower than 6.
LT8705 Applications Information Loop Compensation Table 1: Voltage Lockout Conditions The loop stability is affected by a number of factors including the inductor value, output capacitance, load current, VIN, VOUT and the VC resistor and capacitors. The LT8705 uses internal transconductance error amplifiers driving VC to help compensate the control loop. For most applications a 3.3nF series capacitor at VC is a good value.
LT8705 Applications Information Similar calculations can be used to select a resistor divider connected to SWEN that would stop switching activity during an undervoltage condition. Make sure that the divider doesn’t cause SWEN to exceed 7V (absolute maximum rating) under maximum VIN conditions. Using the FBIN pin as an undervoltage lockout is discussed in the Input Voltage Regulation or Undervoltage Lockout section.
LT8705 Applications Information 3. INTVCC current. This is the sum of the MOSFET driver current, LDO33 pin current and control currents. The INTVCC regulator’s input voltage times the current represents lost power. This loss can be reduced by supplying INTVCC current through the EXTVCC pin from a high efficiency source, such as the output or alternate supply if available. Also, lower capacitance MOSFETs can reduce INTVCC current and power loss. 4. CIN and COUT loss.
LT8705 Applications Information • Except under the SW pin regions, flood all unused areas on all layers with copper. Flooding with copper will reduce the temperature rise of power components. Connect the copper areas to a DC net (e.g., quiet GND). • Partition the power ground from the signal ground. The small-signal component grounds should not return to the IC GND through the power ground path. • Place switch M2 and switch M3 as close to the controller as possible, keeping the GND, BG and SW traces short.
LT8705 Applications Information Now calculate the maximum RSENSE values in the boost and buck regions to be: RSENSE(MAX,BOOST) = 2 • VRSENSE(MAX,BOOST,MAX) • VIN(MIN) (2 •IOUT(MAX,BOOST) • VOUT(MIN) ) + ( ∆IL(MAX,BOOST) • VIN(MIN) ) = 2 •107mV • 8V (2 • 5A •12V ) + (3.75A • 8V ) RSENSE(MAX,BUCK) = Ω = 11.4mΩ 2 • 86mV (2 •IOUT(MAX,BUCK) ) – ∆IL(MIN,BUCK) Ω The inductance must be higher than all of the minimum values calculated above. We will choose a 10μH standard value inductor for improved margin.
LT8705 Applications Information The Fairchild FDMS7672 meets the specifications with a maximum RDS(ON) of ~6.9mΩ at VGS = 4.5V (~10mΩ at 125°C). Checking the power dissipation in the buck region with VIN maximum and VOUT minimum yields: PM1 =PI2R +PSWITCHING V 2 ≅ OUT •IOUT •RDS(ON) • ρτ + ( VIN •IOUT • f • tRF1) W VIN 12V 2 PM1 ≅ • 5A • 6.9mΩ •1.5 + ( 25V • 5A • 350k • 20ns) 25V = 0.06W +0.88W = 0.94W The maximum switching power of 0.
LT8705 Applications Information VIN 36V TO 80V L1 22µH M1 ×2 + CIN2 4.7µF ×4 CIN1 220µF ×2 M3 ×2 M2 TO DIODE DB1 0.22µF 2Ω* TG1 BOOST1 SW1 BG1 CSP M4 1nF 10Ω 1nF 10Ω TO DIODE DB2 392k 2Ω** CSN GND BG2 SW2 BOOST2 TG2 CSNIN CSPOUT CSPIN CSNOUT EXTVCC SHDN FBOUT SWEN INTVCC LDO33 SRVO_IIN SS SRVO_IOUT IMON_IN 1µF 4.7µF VC CLKOUT IMON_OUT SYNC 1µF 220pF 3.
LT8705 Typical Applications Supercapacitor Backup Supply TO LOADS DIN VIN 12V VINP + 25mΩ L1 2.2µH M1 CIN1 ×2 CIN2 ×3 M2 TO DIODE DB2 3mΩ 2Ω TG1 BOOST1 SW1 BG1 CSP CSN GND 2Ω BG2 SW2 BOOST2 TG2 CSNIN CSPOUT CSPIN CSNOUT EXTVCC SHDN FBOUT SWEN INTVCC LDO33 SRVO_FBIN IMON_IN 20k 1k TO TO BOOST1 BOOST2 SRVO_IOUT 2N3904* 20k VC 124k CLKOUT IMON_OUT SYNC 24k 14.3k 4.7µF 1µF 15nF 220pF DB2 DB1 SRVO_IIN SS 71.5k 4.7µF 4Ω SRVO_FBOUT RT 1µF 10k 4.
LT8705 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. UHF Package 38-Lead Plastic QFN (5mm × 7mm) (Reference LTC DWG # 05-08-1701 Rev C) 0.70 ±0.05 5.50 ±0.05 5.15 ±0.05 4.10 ±0.05 3.00 REF 3.15 ±0.05 PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC 5.5 REF 6.10 ±0.05 7.50 ±0.05 RECOMMENDED SOLDER PAD LAYOUT APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 5.00 ±0.10 0.75 ±0.05 PIN 1 NOTCH R = 0.30 TYP OR 0.35 × 45° CHAMFER 3.
LT8705 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. FE Package Package Variation: FE38 (31) 38-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1665 Rev B) Exposed Pad Variation AB 4.75 REF 38 9.60 – 9.80* (.378 – .386) 4.75 REF (.187) 20 6.60 ±0.10 4.50 REF 2.74 REF SEE NOTE 4 6.40 2.74 REF (.252) (.108) BSC 0.315 ±0.05 1.05 ±0.10 0.50 BSC RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.50* (.169 – .177) 0.09 – 0.20 (.0035 – .
LT8705 Revision History REV DATE DESCRIPTION A 12/13 Changed 0.4V to 0.35V under Start-Up paragraph 14 Changed 9mΩ resistor value to 10mΩ 39 Added 1kΩ resistor in series with 15V Zener 40 B 02/14 PAGE NUMBER Added “Not Switching” to Line Regulation conditions, two places 4 Changed COUT2 and COUT3 44 8705fb Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use.
LT8705 Typical Application 12V Output Converter Accepts 4V to 80V Input (5.5V Minimum to Start) M1 ×2 VIN 4V TO 80V (INCREASED VOUT RIPPLE FOR VIN > 60V) + CIN2 ×6 CIN1 15µH M2 TO DIODE DB1 0.22µF 2Ω* TG1 BOOST1 SW1 BG1 CSP 1nF 10Ω 1nF 10Ω CSN 4mΩ + CSPOUT CSNOUT 4.7µF EXTVCC SHDN FBOUT SWEN INTVCC 11.3k GATEVCC LT8705 4.7µF SRVO_FBIN FBIN 4.7µF SRVO_FBOUT RT SRVO_IIN SS SRVO_IOUT 4Ω DB1 IMON_IN 1µF VC CLKOUT SYNC CIN1: 220µF, 100V CIN2: 4.
