LTC3775 High Frequency Synchronous Step-Down Voltage Mode DC/DC Controller DESCRIPTION FEATURES n n n n n n n n n n n n n n Wide VIN Range: 4.5V to 38V Line Feedforward Compensation Low Minimum On-Time: tON(MIN) < 30ns Powerful Onboard MOSFET Drivers Leading Edge Modulation Voltage Mode Control ±0.75%, 0.6V Reference Voltage Accuracy Over Temperature VOUT Range: 0.6V to 0.
LTC3775 ABSOLUTE MAXIMUM RATINGS (Note 1) Supply Voltage VIN ......................................................... –0.3V to 40V BOOST ................................................... –0.3V to 46V BOOST-SW............................................... –0.3V to 6V SW ............................................................ –5V to 40V ILIMT .............................................................–0.3V to VIN SENSE.............................................................–5V to VIN INTVCC .
LTC3775 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 12V, VRUN = 5V, unless otherwise specified. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Input Supply VIN VIN Supply Voltage IVIN Input DC Supply Current l 4.5 VFB = 0.7V (Note 5) VRUN = 0V 38 3.
LTC3775 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 12V, VRUN = 5V, unless otherwise specified. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS fOSC Oscillator Frequency RSET = 39.
LTC3775 TYPICAL PERFORMANCE CHARACTERISTICS Efficiency vs Load Current Efficiency vs Input Voltage 100 90 70 60 EFFICIENCY (%) EFFICIENCY (%) 80 PULSE-SKIPPING MODE 50 40 CONTINUOUS MODE 30 1.206 20 1.202 80 1.200 70 1A LOAD 1.198 60 VOUT = 1.2V CONTINUOUS MODE SW FREQ = 500kHz FIRST PAGE CIRCUIT 50 40 0.1 1 10 LOAD CURRENT (A) 100 4 8 12 16 20 INPUT VOLTAGE (V) 3775 G01 1.196 1.194 24 28 Line Regulation FB VOLTAGE (mV) 602 1.200 1.198 1.
LTC3775 TYPICAL PERFORMANCE CHARACTERISTICS Pulse-Skipping Mode Waveform with 0.1A Load Switching Frequency vs Temperature Output Short-Circuit Waveform 550 IL 20A/DIV 0A LOAD IL 2A/DIV VSS 1V/DIV VSW 10V/DIV 3775 G10 20μs/DIV VIN = 12V VOUT = 1.2V CSS = 0.
LTC3775 TYPICAL PERFORMANCE CHARACTERISTICS Shutdown Current vs Temperature Quiescent Current vs INTVCC 20 14 12 10 8 6 4 –0.2 6 –0.3 5 $INTVCC (%) QUIESCENT CURRENT (mA) 16 4 3 50 25 0 75 TEMPERATURE (°C) 100 125 –1.0 0 3.6 4.0 4.4 5.2 4.8 INTVCC (V) 5.6 6.0 0 10 30 40 20 INTVCC LOAD CURRENT (mA) 50 3775 G20 3775 G19 INTVCC Dropout INTVCC Dropout vs Temperature 0 TA = 25°C LOAD CURRENT = 20mA –0.2 –0.2 –0.4 –0.4 –0.6 –0.8 –1.0 –0.6 –0.9 3775 G18 0 –0.5 –0.
LTC3775 PIN FUNCTIONS (QFN/MSOP) ILIMT (Pin 1/Pin 3): Topside Current Limit Set Point. This pin has an internal 100μA pull-down current, allowing the topside current limit threshold to be programmed by an external resistor connected to VIN. See Current Limit Applications. ILIMB (Pin 2/Pin 4): Bottom Side Current Limit Set Point. This pin has an internal 10μA pull-up current, allowing the bottom side current limit threshold to be programmed by an external resistor connected to SGND.
LTC3775 BLOCK DIAGRAM VIN VIN R5 RUN/SHDN 0.74V – + SHDN CHIP SHUTDOWN 1.22V INTVCC 3.6V – EN + REF – UVLO + CTLIM CBLIM FREQ MODE/SYNC 50k EXT SYNC MODE/SYNC DETECT FB R3 C3 OSC MODE SENSE 0.6V SS + + – ILIMB INTVCC + – + – PGND VILIMB 0.2 • VILIMB PGND INTVCC TG VIN EA LINE FEEDFORWARD FB 0.66V PWM – + RILIMB 10μA BOOST – + VIN RILIMT RSENSE + – MODE ISS COMP C2 100μA INTVCC CSS C1 IREV 1μA SS RA 0.
LTC3775 APPLICATIONS INFORMATION Operation (Refer to Block Diagram) The LTC3775 is a constant frequency, voltage mode controller for DC/DC step-down converters. It is designed to be used in a synchronous switching architecture with two external N-channel MOSFETs. For circuit operation, please refer to the Block Diagram. The LTC3775 uses voltage mode control in which the duty cycle is controlled directly by the error amplifier output.
LTC3775 APPLICATIONS INFORMATION VCOMP VOUT – (1+ sR2C1) 1+ s(RA + R3)C3 = sRA (C1+ C2) (1+ s(C1|| C2)R2) ( 1+ sC3R3) The RC network across the error amplifier and the feedforward components R3 and C3 introduce two pole-zero pairs to obtain a phase boost at the system unity-gain frequency, fC. In theory, the zeros and poles are placed symmetrically around fC, and the spread between the zeros and the poles is adjusted to give the desired phase boost at fC.
LTC3775 APPLICATIONS INFORMATION Output Overvoltage Protection Soft-Start An overvoltage comparator, MAX, guards against transient overshoots (>10%) as well as other more serious conditions that may overvoltage the output. In such cases, the top MOSFET is turned off and the bottom MOSFET is turned on until the overvoltage condition is cleared. The LTC3775 includes a soft-start circuit that provides a smooth output voltage ramp during start-up.
LTC3775 APPLICATIONS INFORMATION To prevent discharging a pre-biased VOUT, the LTC3775 always starts switching in pulse-skipping mode up to SS = 0.54V, regardless of the mode selected by the MODE/SYNC pin. Thus if VOUT > 0V during power-up, VOUT will remain at the pre-biased voltage (if there is no load) until the SS voltage catches up with VOUT, after which VOUT will track the SS ramp. The LTC3775 reverts to the selected mode once SS > 0.54V.
LTC3775 APPLICATIONS INFORMATION the upper MOSFET ’s RDS(ON) is used to sense current, connect the SENSE pin to the source of QT (the SW node). Alternatively, for accurate current sensing, connect this pin to a sense resistor located at the drain of QT . The reference input of CTLIM is connected to the ILIMT pin. Connect an external resistor, RILIMT , from the ILIMT pin to VIN to set the the current limit threshold. The voltage at the SENSE pin drops as the inductor current increases.
LTC3775 APPLICATIONS INFORMATION Since the current limit comparator contains leading edge blanking, an external RC filter is not required for proper operation. However, an external filter can be designed by adding a capacitor across the SENSE and ILIMT pins (CF in Figure 7a). The filter component should be placed close to the SENSE and ILIMT pins.
LTC3775 APPLICATIONS INFORMATION PULSE-SKIPPING MODE FORCED CONTINUOUS 0A 0A 0A 0A 0A 0A DECREASING LOAD CURRENT 3775 F09 Figure 9. Comparison of Inductor Current Waveforms for Pulse-Skipping Mode and Forced Continuous Mode response at low load currents, constant frequency operation, and the ability to maintain regulation when sinking current. See Figure 8 for a comparison of the efficiency at light loads for each mode.
LTC3775 APPLICATIONS INFORMATION Care must be taken to ensure that the maximum junction temperature of the LTC3775 is never exceeded. The junction temperature can be estimated using the following equations: PDISS = VIN • IINTVCC TJ = TA + PDISS • RTH(JA) As an example of the required thermal analysis, consider a buck converter with a 24V input voltage and an output voltage of 3.3V at 15A. The switching frequency is 500kHz and the maximum ambient temperature is 70°C.
LTC3775 APPLICATIONS INFORMATION After the calculations have been completed, it is important to measure the gate drive waveforms and the gate driver supply voltage (INTVCC to PGND) over all operating conditions (low VIN, nominal VIN and high VIN, as well as from light load to full load) to ensure adequate power MOSFET enhancement.
LTC3775 APPLICATIONS INFORMATION High Duty Cycle Operation The maximum duty cycle is limited by the LTC3775 internal oscillator reset time, the propagation delay of the PWM comparator and the BOOST pin supply refresh rate. The minimum off-time is typically 300ns. The top MOSFET driver is biased from the floating bootstrap capacitor, CB, which normally recharges during each off cycle through an external diode when the top MOSFET turns off.
LTC3775 APPLICATIONS INFORMATION INTVCC when the switch node is low. When the top MOSFET turns on, the switch node rises to VIN and the BOOST pin rises to approximately VIN + INTVCC. The boost capacitor needs to store at least 100 times the gate charge required by the top MOSFET. In most applications a 0.1μF to 1μF X5R or X7R dielectric capacitor is adequate. The reverse breakdown of the Schottky diode, DB, must be greater than VIN(MAX).
LTC3775 APPLICATIONS INFORMATION CMILLER = Calculated Miller capacitance using the gate charge curve from the MOSFET data sheet fSW = Switching frequency Both MOSFETs have conduction losses (I2R) while the topside N-channel equation includes an additional term for transition losses, which peak at the highest input voltage.
LTC3775 APPLICATIONS INFORMATION ΔIL may be calculated using the equation: IL = VOUT VOUT 1– L • fSW VIN Since ΔIL increases with input voltage, the output ripple voltage is highest at maximum input voltage. Typically, once the ESR requirement is satisfied, the capacitance is adequate for filtering and has the necessary RMS current rating. Manufacturers such as Sanyo, Panasonic and Cornell Dublilier should be considered for high performance through-hole capacitors.
LTC3775 APPLICATIONS INFORMATION RILIMIT = T • RDS(ON)(QT)(MAX) • IO(MAX) + 0.5 • IL ILIMIT(MIN) RDS(ON)(QT)(MAX) is the maximum MOSFET on-resistance typically specified at 25°C. The ρT term is a normalization factor (unity at 25°C) accounting for the significant variation in on-resistance with temperature, typically about 0.5%/°C as shown in Figure 16. For a maximum junction temperature of 100°C, using a value ρT = 1.4 is reasonable.
LTC3775 APPLICATIONS INFORMATION MODE/SYNC Pin The MODE/SYNC pin is a dual function pin that can be used to program the operating mode or to synchronize the switching frequency to an external clock. Pulseskipping mode is enabled when the MODE/SYNC pin is above 1.2V. The mode is forced continuous when the pin is below 1.2V. If this pin is left floating, an internal 50k pull-down resistor defaults the selection to forced continuous mode.
LTC3775 APPLICATIONS INFORMATION ripple, and this could interfere with the operation of the LTC3775. A few inches of PC trace or wire (L ≅ 100nH) between CIN of the LTC3775 and the actual source VIN should be sufficient to prevent input noise interference problems. 8. The top current limit programming resistor, RILIMT , should be placed close to the LTC3775 and the other end of RILIMT should run parallel to the SENSE trace to the Kelvin sense connection underneath the sense resistor. 9.
LTC3775 APPLICATIONS INFORMATION Design Example As a design example, take a supply with the following specifications: VIN = 5V to 26V (12V nominal), VOUT = 1.2V ±5%, IOUT(MAX) = 15A, f = 500kHz. First, verify the minimum on-time which occurs at maximum VIN: tON(MIN) = 1.2V = 92.3ns (26V )(500kHz ) The minimum on-time is lower than the top current limit comparator blanking time of 100ns with sense resistor sensing. The controller will rely on the bottom MOSFET RDS(ON) sensing at high VIN.
LTC3775 APPLICATIONS INFORMATION The worst-case peak inductor current based on a sense resistor tolerance of ±1% is IL(SAT) 110μA • 732 = 27.1A 2.97m The input RMS current is highest at VIN(MIN) = 5V and IOUT(MAX) = 15A: IRMS ≈ 15A 1.2V ( 5V – 1.2V ) 5V = 6.4A CIN is chosen for an RMS current rating of >6.4A at 85°C. For the output capacitor, two low ESR OS-CON capacitors (470μF/5mΩ each) are used to minimize output voltage changes due to inductor current ripple and load steps.
LTC3775 TYPICAL APPLICATIONS 5V to 26V Input, 1.2V/15A Output at 500kHz CF 220pF DB RSENSE 0.003Ω RILIMT 732Ω VIN ILIMT TG + VIN 5V TO 26V CIN1 330μF 35V QT RILIMB 57.6k CVCC 4.7μF CSS 0.01μF SENSE INTVCC BOOST CB 0.1μF L1 0.36μH LTC3775 RSET 39.2k C2 330pF RA 10k ILIMB SS SW FREQ BG FB + QB COUT 470μF 2.5V s2 VOUT 1.2V 15A PGND MODE/SYNC COMP RUN/SHDN SGND RB 10k R2 C1 4.7k 3.
LTC3775 TYPICAL APPLICATIONS 8V to 36V Input, 2.5V/10A Output at 500kHz CB 0.1μF R4 43.2k V BOOST IN RILIMB 133k R5 10k ILIMT DB ILIMB CVCC 4.7μF L1 1.2μH SW SS C2 330pF COUT 330μF 4V s3 + FREQ C3 1500pF VIN 8V TO 36V QT TG LTC3775 RSET 39.2k CIN1 330μF 35V SENSE INTVCC CSS 0.01μF + RILIMT 464Ω R SENSE 0.003Ω CF 220pF QB BG VOUT 2.5V 10A RUN/SHDN FB PGND MODE/SYNC COMP RA 10k R3 390Ω SGND RB 3.
LTC3775 TYPICAL APPLICATIONS 24V Input, 12V/5A Output at 500kHz RILIMT 1.24k R5 10k VIN ILIMT TG QT RILIMB 56.2k CVCC 4.7μF CSS 0.01μF C2 330pF C3 330pF ILIMB SENSE INTVCC BOOST CB 0.1μF L1 4.7μH LTC3775 RSET 39.2k R3 2.05k VIN 24V CIN1 330μF 35V + DB R4 69.8k SS SW FREQ BG + QB RUN/SHDN FB PGND COUT 68μF 16V s2 VOUT 12V 5A MODE/SYNC COMP RA 191k SGND RB 10k R2 C1 7.68k 3.
LTC3775 PACKAGE DESCRIPTION UD Package 16-Lead Plastic QFN (3mm × 3mm) (Reference LTC DWG # 05-08-1691) 0.70 p0.05 3.50 p 0.05 1.45 p 0.05 2.10 p 0.05 (4 SIDES) PACKAGE OUTLINE 0.25 p0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 p 0.10 (4 SIDES) BOTTOM VIEW—EXPOSED PAD PIN 1 NOTCH R = 0.20 TYP OR 0.25 s 45o CHAMFER R = 0.115 TYP 0.75 p 0.05 15 PIN 1 TOP MARK (NOTE 6) 16 0.40 p 0.10 1 1.45 p 0.10 (4-SIDES) 2 (UD16) QFN 0904 0.200 REF 0.00 – 0.05 NOTE: 1.
LTC3775 PACKAGE DESCRIPTION MSE Package 16-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1667 Rev A) BOTTOM VIEW OF EXPOSED PAD OPTION 2.845 p 0.102 (.112 p .004) 5.23 (.206) MIN 2.845 p 0.102 (.112 p .004) 0.889 p 0.127 (.035 p .005) 8 1 1.651 p 0.102 (.065 p .004) 1.651 p 0.102 3.20 – 3.45 (.065 p .004) (.126 – .136) 0.305 p 0.038 (.0120 p .0015) TYP 16 0.50 (.0197) BSC 4.039 p 0.102 (.159 p .004) (NOTE 3) RECOMMENDED SOLDER PAD LAYOUT 0.254 (.010) 0.35 REF 0.
LTC3775 REVISION HISTORY REV DATE DESCRIPTION PAGE NUMBER A 8/10 MSOP package added. Reflected throughout the data sheet. 1 to 34 3775fa 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.
LTC3775 TYPICAL APPLICATION Wide Input Range CPU Power Supply (Refer to Demo Board DC1290A-B) VIN = 5V TO 36V VOUT = 1.2V/10A FS = 350kHz VIN+ R15 0Ω CIN5 0.1μF 50V VOUT R8 0Ω RSNS1 0.004Ω R1 1.62k C4 4700pF R5 2k E1 VIN RSNS2 (OPT) CIN2 4.7μF 50V R6 10k 1% 2 3 R7 C2 0.022μF 1.82k R4 10k 1% 1 4 C3 330pF 16 15 MODE 6 5 C6 10nF R16 (OPT) INTVCC ILIMB LTC3775 14 BOOST 100μF 5V TO 36V 50V J2 GND 5 INFINEON BSC093NO4LSG R3 4.7 C1 0.1μF FB TG COMP SW RUN R9 56.
