LTC3867 Synchronous Step-Down DC/DC Controller with Differential Remote Sense and Nonlinear Control DESCRIPTION FEATURES n n n n n n n n n n n n n VIN Range: 4V to 38V VOUT Range: 0.6V to 14V Nonlinear Control Architecture Minimizes Output Transient Excursions (Optional) Programmable DCR Temperature Compensation ±0.75% 0.6V Voltage Reference Fixed Frequency Range of 200kHz to 1.
LTC3867 SW TG BST PGOOD MODE/PLLIN TOP VIEW FREQ 24 23 22 21 20 19 RUN 1 18 PGND DIFF+ 2 17 BG DIFF– 3 16 INTVCC 25 SGND DIFFOUT 4 15 VIN VFB 5 14 EXTVCC ITH 6 13 ITEMP ITSD 9 10 11 12 ILIM 8 SENSE– 7 SENSE+ VIN Voltage...................................................–0.3V to 40V BST Voltage..................................................–0.3V to 46V SW Voltage.....................................................–5V to 40V (BST-SW) Voltage......................................
LTC3867 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 15V, VRUN = 5V unless otherwise specified. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Main Control Loop/Whole System VIN Input Voltage Range VOUT Output Voltage Range VFB Regulated Feedback Voltage 4 38 0.6 ITH =1.2V, 0°C to 85°C ITH =1.2V, –40°C to 125°C (Note 3) l 595.
LTC3867 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 15V, VRUN = 5V unless otherwise specified.
LTC3867 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 15V, VRUN = 5V unless otherwise specified.
LTC3867 TYPICAL PERFORMANCE CHARACTERISTICS Load Step-Up (0A to 15A, 15A/µs) (Nonlinear Operation) VOUT 50mV/DIV Load Step-Up (0A to 15A, 15A/µs) (Normal Operation) VOUT 50mV/DIV 97.5mV SW 5V/DIV VOUT 500mV/DIV 137.5mV SW 5V/DIV IOUT 10A/DIV 3867 G01 5µs/DIV 5µs/DIV Efficiency and Output Current and Mode Burst Mode OPERATION DCM CCM 90 80 3.5 60 2.5 50 2.0 40 1.5 30 POWER LOSS (W) 70 1.0 20 0 0.01 VIN = 12V VOUT = 1.5V 0.
LTC3867 TYPICAL PERFORMANCE CHARACTERISTICS Current Sense Threshold vs ITH Voltage CURRENT SENSE THRESHOLD (mV) ILIM = 3.7V 45 VSENSE (mV) 80 ILIM = INTVCC 60 ILIM = FLOAT ILIM = 1.5V 30 ILIM = GND 15 0 –15 –30 0 0.25 0.5 0.75 1 1.25 1.5 1.75 ITH VOLTAGE (V) ILIM = INTVCC ILIM = FLOAT 50 ILIM = 1.5V 40 ILIM = GND 30 0 ILIM = GND 30 0.604 ON 1.20 0.602 1.15 OFF 1.10 100 400 300 500 200 FEEDBACK VOLTAGE (V) 0.596 1.00 –50 –25 600 50 25 75 0 TEMPERATURE (°C) 100 125 0.
LTC3867 TYPICAL PERFORMANCE CHARACTERISTICS TK/SS Pull-Up Current vs Temperature Shutdown Current vs Input Voltage 1.3 1.2 1.1 1.0 –50 –25 75 0 25 50 TEMPERATURE (°C) 100 125 50 60 40 50 SHUTDOWN CURRENT (µA) SHUTDOWN CURRENT (µA) TK/SS CURRENT (µA) 1.4 Shutdown Current vs Temperature 30 20 10 0 5 10 25 15 30 20 INPUT VOLTAGE (V) 3867 G13 35 40 3867 G19 40 30 20 10 –50 –25 50 0 75 25 TEMPERATURE (°C) 100 125 3867 G20 PIN FUNCTIONS RUN (Pin 1): Run Control Input.
LTC3867 PIN FUNCTIONS INTVCC (Pin 16): Internal 5.3V Regulator Output and Bottom MOSFET Driver Supply. The control circuits are powered from this voltage. Decouple this pin to PGND with a minimum of 4.7µF low ESR tantalum or ceramic capacitor. BG (Pin 17): Bottom Gate Driver Output. This pin drives the gate(s) of the bottom N-channel MOSFET(s) between PGND and INTVCC. PGND (Pin 18): Power Ground Pin.
LTC3867 BLOCK DIAGRAM EXTVCC ITEMP MODE/PLLIN ITSD 4.7V FREQ + – TEMPSNS 0.6V MODE/SYNC DETECT VIN F + 5.3V REG + – INTVCC F PLL-SYNC VIN CIN INTVCC BOOST BURSTEN S R Q + 3k ICOMP M1 SENSE+ SWITCH LOGIC AND ANTISHOOTTHROUGH IREV IFAST DB SENSE– L1 VOUT + BG RUN M2 OV IFAST CB SW ON – + – TG FCNT OSC COUT CVCC PGND ILIM PGOOD SLOPE COMPENSATION DIFFOUT + INTVCC 1 51k ITHB UVLO UV – 20k + SLEEP OV – + – – + + 0.5V SS RUN 1.25µA + + – 0.
LTC3867 OPERATION The VFB pin receives this feedback signal and compares it to the internal 0.6V reference. When the load current increases, it causes a slight decrease in the VFB pin voltage relative to the 0.6V reference, which in turn causes the ITH voltage to increase until the inductor’s average current equals the new load current.
LTC3867 OPERATION Light Load Current Operation (Burst Mode Operation, Pulse-Skipping or Continuous Conduction) The LTC3867 can be enabled to enter high efficiency Burst Mode operation, constant-frequency pulse-skipping mode or forced continuous conduction mode. To select forced continuous operation, tie the MODE pin to SGND. To select pulse-skipping mode of operation, tie the MODE/PLLIN pin to INTVCC. To select Burst Mode operation, float the MODE/PLLIN pin.
LTC3867 OPERATION load across the load capacitors directly greatly benefits regulation in high current, low voltage applications, where board interconnection losses can be a significant portion of the total error budget. Connect DIFF+ to the center tap of the feedback divider across the output load, and DIFF– to the load ground. See Figure 1 The LTC3867 differential amplifier has a typical output slew rate of 2V/µs.
LTC3867 APPLICATIONS INFORMATION The Typical Application on the first page of this data sheet is a basic LTC3867 application circuit. The LTC3867 can be configured to use either DCR (inductor resistance) sensing or low value resistor sensing. The choice between the two current sensing schemes is largely a design trade-off between cost, power consumption and accuracy.
LTC3867 APPLICATIONS INFORMATION VIN INTVCC VIN SENSE RESISTOR PLUS PARASITIC INDUCTANCE BOOST TG LTC3867 RS SW BG VOUT CF • 2 • RF ≤ ESL/RS POLE-ZERO CANCELLATION PGND RF SENSE+ ESL CF SENSE– SGND RF FILTER COMPONENTS PLACED NEAR SENSE PINS 3867 F03a (3a) Using a Resistor to Sense Current VIN INTVCC VIN BOOST OPTIONAL TEMP COMP NETWORK L SW VOUT BG PGND R1** SENSE+ RNTC DCR LTC3867 ITEMP RS INDUCTOR TG RP C1* R2 SENSE– SGND L R2 R = DCR *PLACE C1 NEAR SENSE+, R1||R2 × C1 = D
LTC3867 APPLICATIONS INFORMATION of the parasitic inductance was determined to be 0.5nH using the equation: ESL = VESL(STEP) tON • tOFF ∆IL tON + tOFF (1) If the RC time constant is chosen to be close to the parasitic inductance divided by the sense resistor (L/R), the resulting waveform looks resistive again, as shown in Figure 5. For applications using low maximum sense voltages, check the sense resistor manufacturer’s data sheet for information about parasitic inductance.
LTC3867 APPLICATIONS INFORMATION 20°C. Increase this value to account for the temperature coefficient of resistance, which is approximately 0.4%/°C. A conservative value for TL(MAX) is 100°C. To scale the maximum inductor DCR to the desired sense resistor value, use the divider ratio: RD = RSENSE(EQUIV) C1 is usually selected to be in the range of 0.047µF to 0.47µF. This forces R1|| R2 to around 2k, reducing error that might have been caused by the SENSE+ pin’s ±1µA current.
LTC3867 APPLICATIONS INFORMATION The NTC resistor has a negative temperature coefficient, meaning its value decreases as temperature rises. The VITEMP voltage, therefore, decreases as temperature increases and in turn, the VSENSEMAX(ADJ) will increase to compensate the DCR temperature coefficient. The NTC resistor, however, is nonlinear and the user can linearize its value by building a resistor network with regular resistors. Consult the NTC manufacturer’s data sheets for detailed information.
LTC3867 APPLICATIONS INFORMATION Generating the IMAX versus inductor temperature curve plot first using the above values as a starting point and then adjusting the RS and RP values as necessary is another approach. Figure 7 shows a typical curve of IMAX versus inductor temperature. 10000 THERMISTOR RESISTANCE RO = 100k TO = 25°C B = 4334 FOR 25°C/100°C RESISTANCE (kΩ) 1000 100 10 The same thermistor network can be used to correct for temperatures less than 25°C. But make sure VITEMP is greater than 0.
LTC3867 APPLICATIONS INFORMATION Pre-Biased Output Start-Up Thermal Protection There may be situations that require the power supply to start up with a pre-bias on the output capacitors. In this case, it is desirable to start up without discharging that output pre-bias. The LTC3867 can safely power up into a pre-biased output without discharging it. Excessive ambient temperatures, loads and inadequate airflow or heat sinking can subject the chip, inductor, FETs etc. to high temperatures.
LTC3867 APPLICATIONS INFORMATION Inductor Value Calculation Given the desired input and output voltages, the inductor value and operating frequency, fOSC, directly determine the inductor’s peak-to-peak ripple current: IRIPPLE = VOUT VIN – VOUT VIN fOSC •L Lower ripple current reduces core losses in the inductor, ESR losses in the output capacitors, and output voltage ripple. Thus, highest efficiency operation is obtained at low frequency with a small ripple current.
LTC3867 APPLICATIONS INFORMATION VIN MILLER EFFECT VGS a V b + QIN VGS – CMILLER = (QB – QA)/VDS +V DS – 3767 F09 Figure 9. Gate Charge Characteristic across the current source load. The upper sloping line is due to the drain-to-gate accumulation capacitance and the gate-to-source capacitance.
LTC3867 APPLICATIONS INFORMATION This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Note that capacitor manufacturers’ ripple current ratings are often based on only 2000 hours of life. This makes it advisable to further derate the capacitor, or to choose a capacitor rated at a higher temperature than required.
LTC3867 APPLICATIONS INFORMATION SP series of surface mount special polymer capacitors available in case heights ranging from 2mm to 4mm. Other capacitor types include Sanyo POSCAP, Sanyo OS-CON, Nichicon PL series and Sprague 595D series. Consult the manufacturers for other specific recommendations. Differential Amplifier The LTC3867 has true remote voltage sense capability.
LTC3867 APPLICATIONS INFORMATION the soft-start phase expires, the LTC3867 is forced into continuous mode of operation as soon as VFB is below the undervoltage threshold of 0.555V regardless of the setting on the MODE pin. However, the LTC3867 should always be set in forced continuous mode tracking down when there is no load. After TK/SS drops below 0.1V, the controller operates in discontinuous mode. The LTC3867 allows the user to program how its output ramps up and down by means of the TK/SS pin.
LTC3867 APPLICATIONS INFORMATION INTVCC (LDO) and EXTVCC The LTC3867 features a true PMOS LDO that supplies power to INTVCC from the VIN supply. INTVCC powers the gate drivers and much of the LTC3867’s internal circuitry. The LDO regulates the voltage at the INTVCC pin to 5.3V when VIN is greater than 5.8V. EXTVCC connects to INTVCC through a P-channel MOSFET and can supply the needed power when its voltage is higher than 4.7V.
LTC3867 APPLICATIONS INFORMATION VOUT LTC3867 VIN INTVCC RVIN 1Ω CINTVCC 4.7µF + 5V RB LTC3867 CFF VFB CIN RA 3867 F12 3867 F13 Figure 12. Setup for a 5V Input Figure 13. Setting Output Voltage Topside MOSFET Driver Supply (CB, DB) External bootstrap capacitor, CB, connected to the BOOST pin supplies the gate drive voltages for the topside MOSFET. Capacitor CB in the Functional Diagram is charged though external diode DB from INTVCC when the SW pin is low.
LTC3867 APPLICATIONS INFORMATION Phase-Locked Loop and Frequency Synchronization The output of the phase detector is a pair of complementary current sources that charge or discharge the internal filter network. There is a precision 20µA current flowing out of the FREQ pin. This allows the user to use a single resistor to SGND to set the switching frequency when no external clock is applied to the MODE/PLLIN pin.
LTC3867 APPLICATIONS INFORMATION If the duty cycle falls below what can be accommodated by the minimum on-time, the controller will begin to skip cycles. The output voltage will continue to be regulated, but the voltage ripple and current ripple will increase. The minimum on-time for the LTC3867 is approximately 65ns, with good PCB layout, minimum 30% inductor current ripple and at least 10mV ripple on the current sense signal.
LTC3867 APPLICATIONS INFORMATION 4. Transition losses apply only to the topside MOSFET(s), and become significant only when operating at high input voltages (typically 15V or greater). Transition losses can be estimated from: Transition Loss = (1.7) VIN2 • IO(MAX) • CRSS • f Other hidden losses such as copper trace and internal battery resistances can account for an additional 5% to 10% efficiency degradation in portable systems.
LTC3867 APPLICATIONS INFORMATION PC Board Layout Checklist 5. Keep the switching nodes, SW, BOOST and TG away from sensitive small-signal nodes (SENSE+, SENSE–, DIFF+, DIFF–, VFB). Ideally the SW, BOOST and TG printed circuit traces should be routed away and separated from the IC and especially the quiet side of the IC. Separate the high dv/dt traces from sensitive small-signal nodes with ground traces or ground planes.
LTC3867 APPLICATIONS INFORMATION 8. Are the signal and power grounds kept separate? The combined IC signal ground pin and the ground return of CINTVCC must return to the combined COUT (–) terminals. The VFB and ITH traces should be as short as possible. The path formed by the top N-channel MOSFET, Schottky diode and the CIN capacitor should have short leads and PC trace lengths.
LTC3867 TYPICAL APPLICATIONS 1.2V, 10A Output with RSENSE VIN 4.5V TO 14V INTVCC M1 100k VOSP RUN FREQ MODE/PLLIN PGOOD BOOST TG SW 49.9k 22pF RUN DIFF+ DIFF– 49.9k DIFFOUT LTC3867 VFB ITH PGND BG INTVCC VIN EXTVCC ITEMP IFAST TKSS SENSE+ SENSE– ILIM ITSD VOSN 100pF 0.1µF 37.4k VOSP 49.9k L1 0.4µH RSENSE 0.002Ω 10Ω 10Ω 10Ω M2 INTVCC 330µF ×2 VOUT 1.2V 10A 100µF 10Ω 4.7µF VOSN 330pF 0.
LTC3867 TYPICAL APPLICATIONS 1.5V, 15A Output with DCR Sense, Nonlinear Control and DCR Temperature Compensation VIN 4.5V TO 14V INTVCC M1 100k VOSP RUN 22pF RUN DIFF+ DIFF– 49.9k DIFFOUT LTC3867 VFB ITH PGND BG INTVCC VIN EXTVCC ITEMP IFAST TKSS SENSE+ SENSE– ILIM ITSD VOSN 49.9k 100pF 330pF 90.9k L1 0.4µH FREQ MODE/PLLIN PGOOD BOOST TG SW 75k 0.1µF 37.4k VOSP 0.1µF COUT1 330µF ×2 3.3k M2 4.7µF RS 10k VOUT 1.5V 15A COUT2 100µF I73 10Ω INTVCC NTC 100k VOSN RP 100k 0.
LTC3867 PACKAGE DESCRIPTION UF Package 24-Lead Plastic QFN (4mm × 4mm) (Reference LTC DWG # 05-08-1697) 0.70 ±0.05 4.50 ±0.05 2.45 ±0.05 3.10 ±0.05 (4 SIDES) PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 4.00 ±0.10 (4 SIDES) BOTTOM VIEW—EXPOSED PAD R = 0.115 TYP 0.75 ±0.05 PIN 1 NOTCH R = 0.20 TYP OR 0.35 × 45 CHAMFER 23 24 PIN 1 TOP MARK (NOTE 6) 0.40 ±0.10 1 2 2.45 ±0.10 (4-SIDES) (UF24) QFN 0105 0.200 REF 0.00 – 0.05 0.25 ±0.05 0.50 BSC NOTE: 1.
LTC3867 TYPICAL APPLICATION 12V, 4A Output with RSENSE Synchronized to 400kHz VIN 14V TO 24V INTVCC 100k PLLIN 400kHz M1 VOSP RUN FREQ MODE/PLLIN PGOOD BOOST TG SW 143k 0.1µF 37.4k VOSP 22pF RUN DIFF+ DIFF– DIFFOUT LTC3867 VFB ITH 7.5k 330pF IFAST TKSS SENSE+ SENSE– ILIM ITSD VOSN PGND BG INTVCC VIN EXTVCC ITEMP 43k L1 6.8µH RSENSE 0.011Ω 10Ω 10Ω VOUT 12V 4A 220µF 16V 10Ω M2 SDM10K45 10Ω INTVCC 4.7µF VOSN 470pF M1, M2: Si4816BDY 0.
