LTC3869/LTC3869-2 Dual, 2-Phase Synchronous Step-Down DC/DC Controllers Description Features n n n n n n n n n n n n n n n Dual, 180° Phased Controllers Reduce Required Input Capacitance and Power Supply Induced Noise Accurate Multiphase Current Matching RSENSE or DCR Current Sensing ±0.75% 0.6V Output Voltage Accuracy Phase-Lockable Fixed Frequency 250kHz to 780kHz High Efficiency: Up to 95% Dual N-channel MOSFET Synchronous Drive Wide VIN Range: 4V to 38V (40V Max) Operation Wide VOUT Range: 0.
LTC3869/LTC3869-2 Absolute Maximum Ratings (Note 1) Input Supply Voltage: VIN............................ 40V to –0.3V Top Side Driver Voltages: BOOST1, BOOST2....................................... 46V to –0.3V Switch Voltage: SW1, SW2............................ 40V to –5V INTVCC , RUN1, RUN2, PGOOD, EXTVCC, BOOST1-SW1, BOOST2-SW2........................ 6V to –0.3V SENSE1+, SENSE2+, SENSE1–, SENSE2– Voltages.......................................13V to –0.
LTC3869/LTC3869-2 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 = 15V, VRUN1,2 = 5V unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Main Control Loops VIN Input Voltage Range 4 38 0.6 V VOUT Output Voltage Range VFB1,2 Regulated Feedback Voltage (Notes 2, 4) ITH1,2 Voltage = 1.2V, 0°C to 85°C ITH1,2 Voltage = 1.
LTC3869/LTC3869-2 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 = 15V, VRUN1,2 = 5V unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Internal VCC Voltage 6V < VIN < 38V 4.8 5 5.2 V VLDO INT INTVCC Load Regulation ICC = 0mA to 20mA 0.5 2 % VEXTVCC EXTVCC Switchover Voltage EXTVCC Ramping Positive 4.5 4.
LTC3869/LTC3869-2 Typical Performance Characteristics 100 100 90 90 80 Burst Mode OPERATION 80 60 EFFICIENCY (%) 70 DCM VIN = 12V VOUT = 1.8V 50 40 CCM 30 20 90 5 1.8V EFFICIENCY Burst Mode OPERATION 60 DCM 50 40 VIN = 12V VOUT = 1.2V CCM 30 1.2V 85 4 1.8V POWER LOSS 80 3 1.2V 20 10 0 0.
LTC3869/LTC3869-2 Typical Performance Characteristics TA = 25°C, unless otherwise noted. Tracking Up and Down with External Ramp Coincident Tracking TK/SS1 TK/SS2 2V/DIV RUN 2V/DIV VOUT1 VOUT1 VOUT2 3869 G09 5ms/DIV VOUT1 = 1.8V, 1.5Ω LOAD VOUT2 = 1.2V, 1Ω LOAD Quiescent Current without EXTVCC vs Temperature Current Sense Threshold vs ITH Voltage 4.0 5.5 80 3.5 5.0 60 2.5 2.0 1.5 1.0 ILIM = INTVCC 4.5 VSENSE (mV) 3.0 4.0 3.5 0 –50 –25 0 50 25 75 TEMPERATURE (°C) 100 3.0 2.
LTC3869/LTC3869-2 Typical Performance Characteristics TA = 25°C, unless otherwise noted. Shutdown (RUN) Threshold vs Temperature TK/SS Pull-Up Current vs Temperature 1.6 1.24 RUN PIN THRESHOLD (V) TK/SS CURRENT (µA) 1.22 1.4 1.2 ON 1.20 1.18 1.16 1.14 1.12 OFF 1.10 1.0 –50 –25 0 50 25 75 TEMPERATURE (°C) 100 1.08 –50 125 –25 3869 G17 125 Oscillator Frequency vs Temperature 604 900 800 602 VFREQ = INTVCC 700 FREQUENCY (kHz) 600 598 596 594 600 500 VFREQ = 1.
LTC3869/LTC3869-2 Typical Performance Characteristics Shutdown Current vs Input Voltage Shutdown Current vs Temperature 40 30 20 10 45 3.8 40 3.6 35 3.4 SUPPLY CURRENT (mA) 50 SHUTDOWN CURRENT (µA) SHUTDOWN INPUT CURRENT (µA) 60 0 TA = 25°C, unless otherwise noted. 30 25 20 15 10 5 10 15 20 30 25 INPUT VOLTAGE (V) 35 40 3.0 2.8 2.6 2.4 2.0 –25 50 25 75 0 TEMPERATURE (°C) 100 125 3869 G24 3869 G23 Pin Functions 3.2 2.
LTC3869/LTC3869-2 Pin Functions (UFD/GN) FREQ (Pin 26/Pin 28): There is a precision 10µA current flowing out of this pin. Connect a resistor to ground set the controllers’ operating frequency. Alternatively, this pin can be driven with a DC voltage to vary the frequency of the internal oscillator. ILIM (Pin 11/NA): Current Comparator Sense Voltage Range Inputs. This pin is to be programmed to SGND, FLOAT or INTVCC to set the maximum current sense threshold to three different levels for each comparator.
LTC3869/LTC3869-2 Functional Block Diagram FREQ MODE/PLLIN EXTVCC VIN VIN 4.7V + – 10µA F 0.6V MODE/SYNC DETECT + 5V REG + – CIN INTVCC INTVCC F PLL-SYNC BOOST BURSTEN S OSC R 3k + ON – ICMP + – IREV CB TG FCNT Q M1 SW SWITCH LOGIC AND ANTISHOOT THROUGH L1 SENSE+ SENSE– + RUN COUT BG OV M2 CVCC SLOPE COMPENSATION ILIM VOUT DB PGND LTC3869UFD ONLY PGOOD INTVCC UVLO + 1 51k ITHB UV – 0.
LTC3869/LTC3869-2 Operation Main Control Loop The LTC3869 is a constant-frequency, current mode stepdown controller with two channels operating 180 degrees out-of-phase. During normal operation, each top MOSFET is turned on when the clock for that channel sets the RS latch, and turned off when the main current comparator, ICMP, resets the RS latch. The peak inductor current at which ICMP resets the RS latch is controlled by the voltage on the ITH pin, which is the output of each error amplifier EA.
LTC3869/LTC3869-2 Operation Light Load Current Operation (Burst Mode Operation, Pulse-Skipping, or Continuous Conduction) The LTC3869 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/PLLIN pin to a DC voltage below 0.6V (e.g., 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.
LTC3869/LTC3869-2 Operation The PLL loop filter network is integrated inside the LTC3869. The phase-locked loop is capable of locking any frequency within the range of 250kHz to 780kHz. The frequency setting resistor should always be present to set the controller’s initial switching frequency before locking to the external clock. Power Good (PGOOD Pin) When VFB pin voltage is not within ±10% of the 0.6V reference voltage, the PGOOD pin is pulled low.
LTC3869/LTC3869-2 Applications Information The Typical Application on the first page is a basic LTC3869 application circuit. LTC3869 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. DCR sensing is becoming popular because it saves expensive current sensing resistors and is more power efficient, especially in high current applications.
LTC3869/LTC3869-2 Applications Information For previous generation current mode controllers, the maximum sense voltage was high enough (e.g., 75mV for the LTC1628 / LTC3728 family) that the voltage drop across the parasitic inductance of the sense resistor represented a relatively small error. For today’s highest current density solutions, however, the value of the sense resistor can be less than 1mΩ and the peak sense voltage can be as low as 20mV.
LTC3869/LTC3869-2 Applications Information The filter components need to be placed close to the IC. The positive and negative sense traces need to be routed as a differential pair and Kelvin connected to the sense resistor. VESL(STEP) VSENSE 20mV/DIV 500ns/DIV 3869 F03 Figure 3. Voltage Waveform Measured Directly Across the Sense Resistor VSENSE 20mV/DIV 500ns/DIV 3869 F04 Figure 4. Voltage Waveform Measured After the Sense Resistor Filter.
LTC3869/LTC3869-2 Applications Information To scale the maximum inductor DCR to the desired sense resistor value, use the divider ratio: RD = RSENSE(EQUIV) DCR(MAX) at TL(MAX) C1 is usually selected to be in the range of 0.047µF to 0.47µF. This forces R1|| R2 to around 2kΩ, reducing error that might have been caused by the SENSE pins’ ±1µA current. TL(MAX) is the maximum inductor temperature.
LTC3869/LTC3869-2 Applications Information duty cycles up to the maximum of 95%, use the following equation to find the minimum inductance. L MIN > fSW VOUT • 1.4 • ILOAD(MAX) where LMIN is in units of µH fSW is in units of MHz Inductor Core Selection Once the inductance value is determined, the type of inductor must be selected. Core loss is independent of core size for a fixed inductor value, but it is very dependent on inductance selected. As inductance increases, core losses go down.
LTC3869/LTC3869-2 Applications Information voltage when the top switch duty factor is low or during a short-circuit when the synchronous switch is on close to 100% of the period. The term (1 + d) is generally given for a MOSFET in the form of a normalized RDS(ON) vs Temperature curve, but d = 0.005/°C can be used as an approximation for low voltage MOSFETs. The optional Schottky diodes conduct during the dead time between the conduction of the two power MOSFETs.
LTC3869/LTC3869-2 Applications Information Output Voltage Tracking The LTC3869 allows the user to program how its output ramps up and down by means of the TK/SS pins. Through these pins, the output can be set up to either coincidentally or ratiometrically track another supply’s output, as shown in Figure 5. In the following discussions, VOUT1 refers to the LTC3869’s output 1 as a master channel and VOUT2 refers to the LTC3869’s output 2 as a slave channel.
LTC3869/LTC3869-2 Applications Information INTVCC Regulators and EXTVCC The LTC3869 features a true PMOS LDO that supplies power to INTVCC from the VIN supply. INTVCC powers the gate drivers and much of the LTC3869’s internal circuitry. The linear regulator regulates the voltage at the INTVCC pin to 5V when VIN is greater than 5.5V. EXTVCC connects to INTVCC through a P-channel MOSFET and can supply the needed power when its voltage is higher than 4.7V.
LTC3869/LTC3869-2 Applications Information For applications where the main input power is below 5V, tie the VIN and INTVCC pins together and tie the combined pins to the 5V input with a 1Ω or 2.2Ω resistor as shown in Figure 7 to minimize the voltage drop caused by the gate charge current. This will override the INTVCC linear regulator and will prevent INTVCC from dropping too low due to the dropout voltage.
LTC3869/LTC3869-2 Applications Information This makes it advisable to further derate the capacitor, or to choose a capacitor rated at a higher temperature than required. Several capacitors may be paralleled to meet size or height requirements in the design. Due to the high operating frequency of the LTC3869, ceramic capacitors can also be used for CIN. Always consult the manufacturer if there is any question.
LTC3869/LTC3869-2 Applications Information Phase-Locked Loop and Frequency Synchronization 900 800 The LTC3869 has a phase-locked loop (PLL) comprised of an internal voltage-controlled oscillator (VCO) and a phase detector. This allows the turn-on of the top MOSFET of controller 1 to be locked to the rising edge of an external clock signal applied to the MODE/PLLIN pin. The turn-on of controller 2’s top MOSFET is thus 180 degrees outof-phase with the external clock.
LTC3869/LTC3869-2 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 ripple voltage and current will increase. The minimum on-time for the LTC3869 is approximately 90ns, with reasonably good PCB layout, minimum 40% inductor current ripple and at least 10mV – 15mV ripple on the current sense signal.
LTC3869/LTC3869-2 Applications Information losses can be minimized by making sure that CIN has adequate charge storage and very low ESR at the switching frequency. A 25W supply will typically require a minimum of 20µF to 40µF of capacitance having a maximum of 20mΩ to 50mΩ of ESR. The LTC3869 2-phase architecture typically halves this input capacitance requirement over competing solutions.
LTC3869/LTC3869-2 Applications Information PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the IC. These items are also illustrated graphically in the layout diagram of Figure 11. Figure 12 illustrates the current waveforms present in the various branches of the 2-phase synchronous regulators operating in the continuous mode. Check the following in your layout: 1.
LTC3869/LTC3869-2 Applications Information ITH1 TK/SS1 RPU2 PGOOD PGOOD VPULL-UP LTC3869 VFB1 L1 SENSE1+ SENSE1– CB1 ILIM M1 BG1 MODE/PLLIN RIN VIN M2 1µF CERAMIC COUT1 + CVIN RUN1 D1 PGND RUN2 VIN – SENSE2 SENSE2+ INTVCC BG2 SW2 ITH2 TK/SS2 COUT2 1µF CERAMIC M3 BOOST2 VFB2 CIN CINTVCC + SGND GND + EXTVCC + fIN VOUT1 SW1 BOOST1 FREQ RSENSE TG1 M4 D2 CB2 RSENSE TG2 VOUT2 L2 3869 F11 Figure 11.
LTC3869/LTC3869-2 Applications Information SW1 L1 RSENSE1 D1 VOUT1 COUT1 RL1 VIN RIN CIN SW2 BOLD LINES INDICATE HIGH SWITCHING CURRENT. KEEP LINES TO A MINIMUM LENGTH. L2 RSENSE2 D2 VOUT2 COUT2 RL2 3869 F12 Figure 12. Branch Current Waveforms 38692fa For more information www.linear.
LTC3869/LTC3869-2 Applications Information Reduce VIN from its nominal level to verify operation of the regulator in dropout. Check the operation of the undervoltage lockout circuit by further lowering VIN while monitoring the outputs to verify operation. Investigate whether any problems exist only at higher output currents or only at higher input voltages.
LTC3869/LTC3869-2 Applications Information 4.7µF M1 L1 0.56µH D3 TG1 0.1µF M2 L2 0.56µH BOOST2 SW2 M4 BG2 MODE/PLLIN PGND ILIM FREQ SENSE1+ SENSE2+ SENSE1– SENSE2– 20k 1% 12.1k 1% 20k 1% VFB1 ITH1 VFB2 ITH2 TK/SS1 150pF 3.09k 1% 0.1µF RUN1 1nF COUT1 330µF ×2 M3 0.1µF RUN2 40.2k 1% SGND 1nF TK/SS2 0.1µF 4.99k 1% 100k 1% 0.1µF 150pF VOUT2 1.2V 15A + 20k 1% COUT2 330µF ×2 3869 F13 L1, L2: VISHAY IHLP4040DZ-01, 0.
LTC3869/LTC3869-2 Applications Information For each channel, 0.1µF is selected for C1. R1= (DCRMAX L 0.56µH = = 3.11k at 25°C) • C1 1.8mΩ • 0.1µF Choose R1 = 3.09k The power loss in R1 at the maximum input voltage is: PLOSS R1= (VIN(MAX) − VOUT ) • VOUT For a 2mΩ sense resistor, a short-circuit to ground will result in a folded back current of: ISC = (1/ 3) 50mV – 1 ⎛ 90ns(20V) ⎞ = 6.7A 0.002Ω ⎜ ⎟ 2 ⎝ 0.56µH ⎠ A Renesas RJK0330DPB, RDS(ON) = 3.9mΩ, is chosen for the bottom FET.
1nF 0.1µF 15k 100pF 100pF For more information www.linear.com/LTC3869 SENSE2– SENSE2+ TK/SS2 ITH2 VFB2 SGND VFB1 ITH1 TK/SS1 PGOOD LTC3869 RUN1 100k BOOST2 PGND BG2 EXTVCC INTVCC VIN BG1 PGND BOOST1 TG1 0.1µF 4.7µF 0.1µF CMDSH-3 0.1µF 2.2Ω CMDSH-3 M4 RJK0330DPB M3 RJK0305DPB M2 RJK0330DPB M1 RJK0305DPB 10µF ×2 Figure 15. 2.5V, 15A and 1.8V, 15A Supply with DCR Sensing, fSW = 350kHz L1, L2: VISHAY IHLP5050CE-01, 0.
1nF 100Ω 1.5nF 0.1µF 15k 150pF 150pF 18k For more information www.linear.com/LTC3869 20k SENSE2– SENSE2+ TK/SS2 ITH2 VFB2 SGND VFB1 ITH1 TK/SS1 PGOOD 86.6k LTC3869 RUN1 L1, L2: VITEC 59PR9875 COUT1, COUT3: MURATA GRM31CR60J107ME39L COUT2, COUT4: SANYO 2R5TPE330M9 100Ω 0.1µF 20k 20k 63.4k SENSE1+ ILIM 0.1µF SW1 100k BOOST2 PGND BG2 EXTVCC INTVCC VIN BG1 PGND BOOST1 TG1 0.1µF 4.7µF 0.1µF CMDSH-3 0.1µF 2.2Ω CMDSH-3 10µF ×2 M4 RJK0330DPB L2 0.
20k 20k 100pF For more information www.linear.com/LTC3869 100Ω 5.9k 100Ω 0.1µF SENSE2– SENSE2+ TK/SS2 ITH2 VFB2 SGND VFB1 ITH1 PGOOD LTC3869 RUN1 100k BOOST2 PGND BG2 EXTVCC INTVCC VIN BG1 PGND BOOST1 TG1 0.1µF 4.7µF 0.1µF CMDSH-3 0.1µF 2.2Ω CMDSH-3 M4 RJK0330DPB L2 0.44µH M3 RJK0305DPB M2 RJK0330DPB ×2 L1 0.44µH M1 RJK0305DPB 10µF ×4 Figure 17. High Efficiency Dual Phase 1.2V, 40A Supply, fSW = 250kHz L1, L2: PULSE PA0513.
20k 20k 330pF 10k For more information www.linear.com/LTC3869 0.1µF SENSE2– SENSE2+ TK/SS2 ITH2 VFB2 SGND VFB1 ITH1 SENSE1+ ILIM PGOOD RUN1 LTC3869 SW1 100k BOOST2 PGND BG2 EXTVCC INTVCC VIN BG1 PGND BOOST1 TG1 0.1µF 4.7µF 0.1µF CMDSH-3 1µF 2.2Ω CMDSH-3 M4 RJK0330DPB ×2 M3 RJK0305DPB M2 RJK0330DPB ×2 M1 RJK0305DPB 10µF ×4 3.92k 3869 F18 L2 0.47µH L1 0.47µH 3.92k Figure 18. High Efficiency Dual Phase 1.
10k 20k 220pF 100Ω 5.1k For more information www.linear.com/LTC3869 100Ω 1nF SENSE2– SENSE2+ TK/SS2 ITH2 VFB2 SGND VFB1 PGOOD LTC3869 RUN1 400kHz 100k BOOST2 PGND BG2 EXTVCC INTVCC VIN BG1 PGND1 BOOST1 TG1 0.1µF 4.7µF 0.1µF CMDSH-3 1µF 2.2Ω CMDSH-3 10µF ×4 M4 RJK0330DPB ×2 L2 0.23µH M3 RJK0305DPB ×2 M2 RJK0330DPB ×2 L1 0.23µH M1 RJK0305DPB ×2 Figure 19. Small Size, Dual Phase 0.9V, 50A Supply, fSW = 400kHz L1, L2: PULSE PA0513.
0.1µF 4.99k 5.6nF 47pF 47pF 5.6nF For more information www.linear.com/LTC3869 147k SENSE2– SENSE2+ TK/SS2 ITH2 VFB2 SGND VFB1 ITH1 TK/SS1 PGOOD RUN1 LTC3869 SW1 100k BOOST2 PGND BG2 EXTVCC INTVCC VIN BG1 PGND1 BOOST TG1 0.1µF 4.7µF 0.1µF SDM10K45 0.1µF 2.2Ω SDM10K45 4.7µF ×6 M4 BSC093N040LS M3 BSC093N040LS M2 BSC093N040LS M1 BSC093N040LS Figure 20.
LTC3869/LTC3869-2 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. UFD Package 28-Lead Plastic QFN (4mm × 5mm) (Reference LTC DWG # 05-08-1712 Rev B) 0.70 ±0.05 4.50 ±0.05 3.10 ±0.05 2.50 REF 2.65 ±0.05 3.65 ±0.05 PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC 3.50 REF 4.10 ±0.05 5.50 ±0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 4.00 ±0.10 (2 SIDES) 0.75 ±0.05 R = 0.
LTC3869/LTC3869-2 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. GN Package 28-Lead Plastic SSOP (Narrow .150 Inch) (Reference LTC DWG # 05-08-1641 Rev B) .386 – .393* (9.804 – 9.982) .045 ±.005 28 27 26 25 24 23 22 21 20 19 18 17 1615 .254 MIN .033 (0.838) REF .150 – .165 .229 – .244 (5.817 – 6.198) .0165 ±.0015 .150 – .157** (3.810 – 3.988) .0250 BSC 1 RECOMMENDED SOLDER PAD LAYOUT .015 ±.004 × 45° (0.38 ±0.10) .
LTC3869/LTC3869-2 Revision History REV DATE DESCRIPTION A 04/13 Revised schematics PAGE NUMBER 35-38 Updated package drawings 39-40 38692fa 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. For more information www.linear.
LTC3869/LTC3869-2 Typical Application 3.3V/5A, 5V/5A Converter Using Sense Resistors VIN 7V TO 24V 22µF 50V 2.2Ω 1µF Si4816BDY 4.7µF D3 M1 0.1µF L2 2.2µH TG1 BOOST1 SW1 BG1 10Ω 10Ω + LTC3869 MODE/PLLIN ILIM 0.1µF BG2 PGND FREQ SENSE2+ SENSE1– SENSE2– 10Ω 1000pF RUN1 90.9k 1% VFB1 ITH1 TK/SS1 20k 1% 1000pF 100pF 10k 1% L2 3.3µH BOOST2 SW2 SENSE1+ 15pF COUT1 220µF M2 TG2 1000pF 8mΩ VOUT1 3.3V 5A Si4816BDY D4 VIN PGOOD INTVCC 8mΩ 10Ω 10pF RUN2 EXTVCC VFB2 ITH2 SGND 0.
