LTC3850/LTC3850-1 Dual, 2-Phase Synchronous Step-Down Switching Controller DESCRIPTION FEATURES 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 High Efficiency: Up to 95% RSENSE or DCR Current Sensing ±1% 0.
LTC3850/LTC3850-1 ABSOLUTE MAXIMUM RATINGS (Note 1) SENSE1+, SENSE2+, SENSE1–, SENSE2– Voltages...................................... 5.5V to –0.3V MODE/PLLIN, ILIM,TK/SS1,TK/SS2, FREQ/PLLFLTR Voltages................................................. INTVCC to –0.3V ITH1 , ITH2 , VFB1 , VFB2 Voltages................... 2.7V to –0.3V INTVCC Peak Output Current.................................100mA Operating Temperature Range (Note 2)....–40°C to 85°C Junction Temperature (Note 3)...........................
LTC3850/LTC3850-1 ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE LTC3850EGN#PBF LTC3850EGN#TRPBF LTC3850GN 28-Lead Narrow Plastic SSOP –40°C to 85°C LTC3850EGN-1#PBF LTC3850EGN-1#TRPBF LTC3850GN-1 28-Lead Narrow Plastic SSOP –40°C to 85°C LTC3850IGN#PBF LTC3850IGN#TRPBF LTC3850GN 28-Lead Narrow Plastic SSOP –40°C to 85°C LTC3850IGN-1#PBF LTC3850IGN-1#TRPBF LTC3850GN-1 28-Lead Narrow Plastic SSOP –40°C to 85°C LTC3850EUF#PBF LTC38
LTC3850/LTC3850-1 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C, VIN = 15V, VRUN1,2 = 5V, unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 0.792 0.800 0.808 V –10 –50 nA 0.002 0.02 %/V 0.01 –0.01 0.1 –0.1 % % Main Control Loops VFB1,2 Regulated Feedback Voltage ITH1,2 Voltage = 1.
LTC3850/LTC3850-1 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C, VIN = 15V, VRUN1,2 = 5V, unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Oscillator and Phase-Locked Loop fNOM Nominal Frequency VFREQ = 1.2V 450 500 550 kHz fLOW Lowest Frequency VFREQ = 0V 210 250 290 kHz fHIGH Highest Frequency VFREQ ≥ 2.
LTC3850/LTC3850-1 TYPICAL PERFORMANCE CHARACTERISTICS Load Step (Burst Mode Operation) Load Step (Forced Continuous Mode) ILOAD 2A/DIV 200mA TO 2.5A ILOAD 2A/DIV 200mA TO 2.5A IL 2A/DIV IL 2A/DIV VOUT 100mV/DIV AC COUPLED VOUT 100mV/DIV AC COUPLED 40µs/DIV 38501 G04 CIRCUIT OF FIGURE 14 VIN = 12V, VOUT = 1.8V Load Step (Pulse-Skipping Mode) Inductor Current at Light Load FORCED CONTINUOUS MODE 2A/DIV ILOAD 2A/DIV 200mA TO 2.
LTC3850/LTC3850-1 TYPICAL PERFORMANCE CHARACTERISTICS Tracking Up and Down with External Ramp 5 TK/SS1 TK/SS2 2V/DIV Quiescent Current vs Input Voltage without EXTVCC 5.25 5.00 VOUT2 1.8V 1.5Ω LOAD 1V/DIV 3 2 1 38501 G10 10ms/DIV INTVCC VOLTAGE (V) SUPPLY CURRENT (mA) 4 VOUT1 3.3V 3Ω LOAD 1V/DIV 0 INTVCC Line Regulation 4.75 4.50 4.25 4.00 3.75 15 10 5 20 3.50 25 0 5 10 INPUT VOLTAGE (V) 15 20 38501 G11 80 20 ILIM = GND 0 –20 0 0.5 1 VITH (V) 1.
LTC3850/LTC3850-1 TYPICAL PERFORMANCE CHARACTERISTICS Regulated Feedback Voltage vs Temperature Shutdown (RUN) Threshold vs Temperature RUN PIN VOLTAGE (V) 1.4 1.3 ON 1.2 OFF 1.1 1.
LTC3850/LTC3850-1 PIN FUNCTIONS (GN/UF/UFD) RUN1, RUN2 (Pins 1, 13/Pins 26, 9/Pins 27, 10): Run Control Inputs. A voltage above 1.2V on either pin turns on the IC. However, forcing either of these pins below 1.2V causes the IC to shut down that particular channel. There are 0.5µA pull-up currents for these pins. Once the RUN pin rises above 1.2V, an additional 4.5µA pull-up current is added to the pin. SENSE1+, SENSE2+ (Pins 2, 12/Pins 27, 8/Pins 28, 9): Current Sense Comparator Inputs.
LTC3850/LTC3850-1 PIN FUNCTIONS MODE/PLLIN (Pin 27/Pin 24/Pin 25): Force Continuous Mode, Burst Mode, or Pulse-Skipping Mode Selection Pin and External Synchronization Input to Phase Detector Pin. Connect this pin to SGND to force both channels into the continuous mode of operation. Connect to INTVCC to enable pulse-skipping mode of operation. Leaving the pin floating will enable Burst Mode operation.
LTC3850/LTC3850-1 OPERATION Main Control Loop The LTC3850 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.
LTC3850/LTC3850-1 OPERATION When a controller is enabled for Burst Mode operation, the peak current in the inductor is set to approximately one-third of the maximum sense voltage even though the voltage on the ITH pin indicates a lower value. If the average inductor current is higher than the load current, the error amplifier EA will decrease the voltage on the ITH pin. When the ITH voltage drops below 0.
LTC3850/LTC3850-1 APPLICATIONS INFORMATION The Typical Application on the first page is a basic LTC3850 application circuit. LTC3850 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.
LTC3850/LTC3850-1 APPLICATIONS INFORMATION close to the switching node, to prevent noise from coupling into sensitive small-signal nodes. The capacitor C1 should be placed close to the IC pins. Low Value Resistors Current Sensing A typical sensing circuit using a discrete resistor is shown in Figure 2a. RSENSE is chosen based on the required output current. The current comparator has a maximum threshold VSENSE(MAX) determined by the ILIM setting.
LTC3850/LTC3850-1 APPLICATIONS INFORMATION VESL(STEP) VSENSE 20mV/DIV 500ns/DIV 38501 F03 Figure 3. Voltage Waveform Measured Directly Across the Sense Resistor.
LTC3850/LTC3850-1 APPLICATIONS INFORMATION Ensure that R1 has a power rating higher than this value. If high efficiency is necessary at light loads, consider this power loss when deciding whether to use DCR sensing or sense resistors. Light load power loss can be modestly higher with a DCR network than with a sense resistor, due to the extra switching losses incurred through R1. However, DCR sensing eliminates a sense resistor, reduces conduction losses and provides higher efficiency at heavy loads.
LTC3850/LTC3850-1 APPLICATIONS INFORMATION voltage and maximum output current. Miller capacitance, CMILLER, can be approximated from the gate charge curve usually provided on the MOSFET manufacturers’ data sheet. CMILLER is equal to the increase in gate charge along the horizontal axis while the curve is approximately flat divided by the specified change in VDS. This result is then multiplied by the ratio of the application applied VDS to the gate charge curve specified VDS.
LTC3850/LTC3850-1 APPLICATIONS INFORMATION selected mode once TK/SS > 0.74V. The output ripple is minimized during the 100mV forced continuous mode window ensuring a clean PGOOD signal. When the channel is configured to track another supply, the feedback voltage of the other supply is duplicated by a resistor divider and applied to the TK/SS pin. Therefore, the voltage ramp rate on this pin is determined by the ramp rate of the other supply’s voltage.
LTC3850/LTC3850-1 APPLICATIONS INFORMATION VOUT1 OUTPUT VOLTAGE OUTPUT VOLTAGE VOUT1 VOUT2 TIME VOUT2 TIME 38501 F03a (5a) Coincident Tracking 38501 F03b (5b) Ratiometric Tracking Figure 5. Two Different Modes of Output Voltage Tracking VOUT1 TO TK/SS2 PIN VOUT1 VOUT2 R3 R4 R1 R2 TO VFB1 PIN TO VFB2 PIN R3 TO TK/SS2 PIN R4 VOUT2 R1 R2 TO VFB1 PIN TO VFB2 PIN R3 R4 38501 F06 (6a) Coincident Tracking Setup (6b) Ratiometric Tracking Setup Figure 6.
LTC3850/LTC3850-1 APPLICATIONS INFORMATION exceeded. The INTVCC current, which is dominated by the gate charge current, may be supplied by either the 5V linear regulator or EXTVCC. When the voltage on the EXTVCC pin is less than 4.7V, the linear regulator is enabled. Power dissipation for the IC in this case is highest and is equal to VIN • IINTVCC. The gate charge current is dependent on operating frequency as discussed in the Efficiency Considerations section.
LTC3850/LTC3850-1 APPLICATIONS INFORMATION turns on the topside switch. The switch node voltage, SW, rises to VIN and the BOOST pin follows. With the topside MOSFET on, the boost voltage is above the input supply: VBOOST = VIN + VINTVCC. The value of the boost capacitor CB needs to be 100 times that of the total input capacitance of the topside MOSFET(s). The reverse breakdown of the external Schottky diode must be greater than VIN(MAX).
LTC3850/LTC3850-1 APPLICATIONS INFORMATION suggested. A 2.2Ω – 10Ω resistor placed between CIN (C1) and the VIN pin provides further isolation between the two channels. The selection of COUT is driven by the effective series resistance (ESR). Typically, once the ESR requirement is satisfied, the capacitance is adequate for filtering.
LTC3850/LTC3850-1 APPLICATIONS INFORMATION Typically, the external clock (on MODE/PLLIN pin) input high threshold is 1.6V, while the input low threshold is 1V. 900 800 FREQUENCY (kHz) 700 600 Minimum On-Time Considerations 500 Minimum on-time tON(MIN) is the smallest time duration that the LTC3850 is capable of turning on the top MOSFET. It is determined by internal timing delays and the gate charge required to turn on the top MOSFET.
LTC3850/LTC3850-1 APPLICATIONS INFORMATION Although all dissipative elements in the circuit produce losses, four main sources usually account for most of the losses in LTC3850 circuits: 1) IC VIN current, 2) INTVCC regulator current, 3) I2R losses, 4) Topside MOSFET transition losses. 1. The VIN current is the DC supply current given in the Electrical Characteristics table, which excludes MOSFET driver and control currents. VIN current typically results in a small (<0.1%) loss. 2.
LTC3850/LTC3850-1 APPLICATIONS INFORMATION pin. The bandwidth can also be estimated by examining the rise time at the pin. The ITH external components shown in the Typical Application circuit will provide an adequate starting point for most applications. The ITH series RC-CC filter sets the dominant pole-zero loop compensation. The values can be modified slightly (from 0.
LTC3850/LTC3850-1 APPLICATIONS INFORMATION ITH1 TK/SS1 RPU2 PGOOD PGOOD VPULL-UP VFB1 L1 SENSE1+ TG1 SENSE1– SW1 CB1 ILIM BG1 MODE/PLLIN 1µF CERAMIC VIN INTVCC SENSE2+ BG2 TK/SS2 + SENSE2– CIN CINTVCC COUT2 1µF CERAMIC M3 BOOST2 GND + EXTVCC ITH2 COUT1 PGND SGND VFB2 RIN CVIN D1 + VIN RUN1 RUN2 M2 VOUT1 + fIN M1 BOOST1 PLLLPF RSENSE M4 D2 CB2 SW2 RSENSE TG2 VOUT2 L2 38501 F12 Figure 12.
LTC3850/LTC3850-1 APPLICATIONS INFORMATION 5. Is the INTVCC decoupling capacitor connected close to the IC, between the INTVCC and the power ground pins? This capacitor carries the MOSFET drivers current peaks. An additional 1µF ceramic capacitor placed immediately next to the INTVCC and PGND pins can help improve noise performance substantially. 6.
LTC3850/LTC3850-1 APPLICATIONS INFORMATION 4.7µF D3 M1 0.1µF L1 3.3µH VIN PGOOD EXTVCC INTVCC TG1 TG2 BOOST1 SW1 BG1 6.19k 1% LTC3850 MODE/PLLIN ILIM 1.33k 1% COUT1 100µF X2 1800pF 20k 1% M2 0.1µF 4.75k 1% L2 2.2µH BOOST2 SW2 BG2 4.12k 1% 10k, 1% PGND FREQ/PLLFLTR SENSE1+ SENSE2+ SENSE1– SENSE2– 0.1µF RUN1 63.4k 1% D4 0.1µF 33pF VOUT1 3.3V 5A 22µF 50V 1µF 2.2Ω 100pF 1.5k 1% 33pF RUN2 VFB1 ITH1 TK/SS1 VIN 7V TO 20V VFB2 ITH2 SGND 0.1µF TK/SS2 0.1µF 2200pF 3.16k 1% 5.
LTC3850/LTC3850-1 APPLICATIONS INFORMATION The Coiltronics (Cooper) HCP0703-2R2 (20mΩ DCRMAX at 20°C) and HCP0703-3R3 (30mΩ DCRMAX at 20°C) are chosen. At 100°C, the estimated maximum DCR values are 26.4mΩ and 39.6mΩ. The divider ratios are: RD = RSENSE(EQUIV) DCRMAX at TL(MAX) = 7mΩ = 0.26; 26.4mΩ 7mΩ ≅ 0.18 39.6mΩ and For each channel, 0.1µF is selected for C1. L 2.2µH = (DCRMAX at 20°C) • C1 20mΩ • 0.1µF 3.3µH = 1.1k ; and = 1.1k 30mΩ • 0.
LTC3850/LTC3850-1 TYPICAL APPLICATIONS VIN 7V TO 24V 22µF 50V 2.2Ω 1µF 4.7µF D3 M1 0.1µF L2 2.2µH BOOST1 SW1 BG1 10Ω 15pF + 10Ω 63.4k 1% COUT1 220µF TG2 LTC3850 M2 0.1µF 20k 1% 1000pF 100pF 10k 1% L2 3.3µH BOOST2 SW2 BG2 MODE/PLLIN PGND ILIM FREQ/PLLFLTR SENSE1+ SENSE2+ SENSE1– SENSE2– 10k 1% 10Ω 1000pF 8mΩ VOUT1 3.3V 5A D4 VIN PGOOD INTVCC TG1 1000pF RUN1 TK/SS1 EXTVCC VFB2 ITH2 0.1µF SGND 105k 1% 1000pF TK/SS2 0.1µF 10pF 10Ω RUN2 VFB1 ITH1 8mΩ 3.
25.5k 20k C12 100pF C11 1000pF C7 1000pF C6 100pF CSS 0.1µF R18 4.99k R12 7.5k 2.10k C15 47pF C10 33pF BOOST2 PGND BG2 INTVCC C5 0.1µF PGOOD CVCC 4.7µF CB2 0.1µF D4 CMDSH-3 D3 CMDSH-3 CB1 0.1µF CVIN 1µF L2 0.68µH M4 RJK0301DPB R30 4.02k M3 HAT2168H PGND GND M2 RJK0301DPB R27 L1 4.02k 0.68µH M1 HAT2168H Figure 17. 2.5V/15A, 1.
CSS2 0.1µF C12 100pF C11 1000pF C7 1000pF C6 100pF CSS1 0.1µF R18 5.9k R12 5.9k R20 100Ω R10 100Ω R5 10k PLLIN 400kHz BOOST2 PGND BG2 INTVCC C5 1000pF RPG 100k PGOOD CVCC 4.7µF CB2 0.1µF D4 CMDSH-3 D3 CMDSH-3 CB1 0.1µF RVIN 2.2Ω CVIN 1µF M4 RJK0301DPB M3 RJK0305DPB PGND GND M2 RJK0301DPB M1 RJK0305DPB L2 0.4µH L1 0.4µH Figure 18. 1.5V/15A, 1.
LTC3850/LTC3850-1 TYPICAL APPLICATIONS 5V ± 0.5V 4.7µF 6.3V 2x 1Ω 4.7µF D3 M1 TG1 0.1µF L1 0.75µH BOOST1 SW1 BG1 1.2k 1% PLLIN 750kHz 2.94k 1% LTC3850 ILIM COUT1 100µF X2 0.1µF L2 0.75µH BOOST2 SW2 BG2 1.2k 1% PGND SENSE1+ SENSE2+ SENSE1– SENSE2– 0.047µF TK/SS1 100pF 14k 1% 4.99k 1% 0.047µF 100pF RUN2 VFB1 ITH1 2200pF 20k 1% M2 FREQ/PLLFLTR RUN1 25.5k 1% D4 TG2 MODE/PLLIN 47pF VOUT1 1.8V 5A VIN PGOOD EXTVCC INTVCC VFB2 ITH2 SGND 0.1µF 1nF TK/SS2 10k 1% 0.
LTC3850/LTC3850-1 PACKAGE DESCRIPTION GN Package 28-Lead Plastic SSOP (Narrow .150 Inch) (Reference LTC DWG # 05-08-1641) .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) .0075 – .0098 (0.19 – 0.25) 2 3 4 5 6 7 8 .0532 – .0688 (1.35 – 1.75) 9 10 11 12 13 14 .004 – .0098 (0.
LTC3850/LTC3850-1 UF Package 28-Lead Plastic QFN (4mm × 4mm) (Reference LTC DWG # 05-08-1721 Rev A) 0.70 ±0.05 4.50 ± 0.05 3.10 ± 0.05 2.64 ± 0.05 (4 SIDES) PACKAGE OUTLINE 0.20 ±0.05 0.40 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 4.00 ± 0.10 (4 SIDES) 0.75 ± 0.05 R = 0.05 TYP PIN 1 TOP MARK (NOTE 6) PIN 1 NOTCH R = 0.20 TYP OR 0.35 × 45° CHAMFER BOTTOM VIEW—EXPOSED PAD R = 0.115 TYP 27 28 0.40 ± 0.05 1 2 2.64 ± 0.
LTC3850/LTC3850-1 PACKAGE DESCRIPTION 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 = 0.05 TYP PIN 1 NOTCH R = 0.20 OR 0.35 × 45° CHAMFER 2.50 REF R = 0.115 TYP 27 28 0.40 ± 0.
LTC3850/LTC3850-1 REVISION HISTORY (Revision history begins at Rev C) REV DATE DESCRIPTION PAGE NUMBER C 3/11 Updated Switch Voltage (SW1, SW2) LTC3850I only from 30V to –0.3V to 30V to –5V 2 38501fc Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use.
LTC3850/LTC3850-1 TYPICAL APPLICATION 20k 2.55k 1nF 0.1µF 7.5k 0.1µF 10k RUN RJK0305DPB SENSE1– SENSE1+ RUN1 FREQ MODE SW1 TG1 220pF TK/SS1 2.2nF 2.74k 20k 0.1µF 2.21k L1 0.56µH 10µF 2x + VIN 7V TO 14V 180µF RJK0301DPB CMDSH-3 BOOST1 ITH1 BG1 VFB1 VIN LTC3850 VFB2 2.2Ω INTVCC ITH2 BG2 4.7µF 1µF PGND TK/SS2 100µF 2x CMDSH-3 SENSE2– + BOOST2 SENSE2+ SGND RUN2 ILIM EXTVCC PGOOD SW2 TG2 RJK0305DPB 0.1µF 0.1µF L2 0.56µH VOUT 1.1V/30A COUT1 330µF 2.
