LTC3853 Triple Output, Multiphase Synchronous Step-Down Controller DESCRIPTION FEATURES n n n n n n n n n n n n n n n Triple, 120° Phased Controllers Reduce Required Input Capacitance and Power Supply Induced Noise Configurable as a 180° Dual Phase Controller Plus a Single Phase Controller The Third Phase Can Regulate Up to a 13.5V Output High Efficiency: Up to 92% RSENSE or DCR Current Sensing ±0.75% 0.
LTC3853 SW1 TG1 BOOST1 RUN3 RUN2 RUN1 MODE/PLLIN FREQ/PLLFLTR ILIM TOP VIEW 40 39 38 37 36 35 34 33 32 31 TK/SS2 1 30 BG1 TK/SS3 2 29 DRVCC12 SNSP1 3 28 BG2 SNSN1 4 27 SW2 SNSP2 5 26 TG2 41 SNSN2 6 25 BOOST2 SNSP3 7 24 VIN SNSN3 8 23 EXTVCC VFB1 9 22 INTVCC ITH1 10 21 BG3 SW3 TG3 BOOST3 PGOOD12 PGOOD3 ITH3 VFB3 ITH2 11 12 13 14 15 16 17 18 19 20 VFB2 Input Supply Voltage (VIN).......................... 28V to –0.3V Topside Driver Voltages BOOST1, BOOST2, BOOST3.......
LTC3853 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C (Note 3), VIN = 15V, VRUN1,2,3 = 5V, unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 0.792 0.794 0.800 0.800 0.808 0.806 V V –10 –50 nA 0.002 0.02 %/V Main Control Loops VFB1,2,3 Regulated Feedback Voltage ITH1,2,3 Voltage = 1.2V (Note 4) ITH1,2,3 Voltage = 1.
LTC3853 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C (Note 3), VIN = 15V, VRUN1,2,3 = 5V, unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 4.8 5 5.2 V 0.
LTC3853 TYPICAL PERFORMANCE CHARACTERISTICS 100 90 90 80 80 70 70 40 DCM EFFICIENCY (%) 50 BURST CCM 30 20 10 0 0.01 VIN = 12V VOUT = 1.8V FIGURE 15 MODIFIED WITH DCR SENSING 0.1 1 LOAD CURRENT (A) 10 CCM DCM 30 10 0.1 1 LOAD CURRENT (A) 1.4 85 1.2 1.0 80 60 0.8 POWER LOSS 75 0.6 VOUT = 3.3V IOUT = 2A FIGURE 15 MODIFIED WITH DCR SENSING 65 10 3853 G02 EFFICIENCY 70 VIN = 12V VOUT = 3.3V FIGURE 15 MODIFIED WITH DCR SENSING 20 3853 G01 1.6 90 BURST 50 0 0.01 1.
LTC3853 TYPICAL PERFORMANCE CHARACTERISTICS Tracking Up and Down with External Ramp Coincident Tracking Prebiased Output at 2V VOUT 1V/DIV VTK/SS 500mV/DIV VFB 500mV/DIV RUN1 2V/DIV TK/SS1 TK/SS2 TK/SS3 2V/DIV VOUT1,2,3 1V/DIV VOUT1,2,3 1V/DIV 3853 G08 VIN = 12V VOUT1 = 3.3V VOUT2 = 2.5V VOUT3 = 1.8V Quiescent Current vs Input Voltage Without EXTVCC VIN = 12V VOUT1 = 3.3V VOUT2 = 2.5V VOUT3 = 1.8V INTERNAL VCC (V) 5.5 5.0 4.5 5.25 80 5.00 60 4.75 4.50 4.25 5 15 10 20 4.00 3.
LTC3853 TYPICAL PERFORMANCE CHARACTERISTICS Shutdown (RUN) Threshold vs Temperature TK/SS Pull-Up Current vs Temperature 1.75 1.55 1.45 1.35 1.25 –50 50 0 75 25 TEMPERATURE (°C) –25 100 1.3 ON 1.2 OFF 1.1 1.0 –50 125 –25 75 0 25 50 TEMPERATURE (°C) Oscillator Frequency vs Temperature 4.25 900 802 800 798 796 794 –50 125 50 25 0 TEMPERATURE (°C) –25 100 75 3853 G19 Oscillator Frequency vs Input Voltage Undervoltage Lockout Threshold (INTVCC) vs Temperature 500 VFREQ = INTVCC 800 4.
LTC3853 PIN FUNCTIONS SENSE1+, SENSE2+, SENSE3+ (Pins 3, 5, 7): Current Sense Comparator Inputs. The (+) inputs to the current comparators are normally connected to DCR sensing networks or current sensing resistors. SENSE3+ common modes up to 13.5V, allowing higher VOUT voltages on channel 3. PGOOD12 (Pin 17): Power Good Indicator Output for Phases 1 and 2. Open-drain logic out that is pulled to ground when any channel output exceeds the ±7.
LTC3853 PIN FUNCTIONS TG1, TG2, TG3 (Pins 32, 26, 19): Top Gate Driver Outputs. These are the outputs of floating drivers with a voltage swing equal to INTVCC superimposed on the switch nodes voltages. FREQ/PLLFLTR (Pin 38): The phase-locked loop’s lowpass filter is tied to this pin. Alternatively, this pin can be driven with a DC voltage to vary the frequency of the internal oscillator. BOOST1, BOOST2, BOOST3 (Pins 33, 25, 18): Boosted Floating Driver Supplies.
LTC3853 FUNCTIONAL DIAGRAM FREQ/PLLFLTR MODE/PLLIN EXTVCC VIN + 4.7V – + F 0.8V MODE/SYNC DETECT – PLL-SYNC F VIN CIN 5V REG + INTVCC INTVCC BOOST OSC BURSTEN S R ON 3k + – ICMP + – IREV CB TG FCNT Q M1 SW SWITCH LOGIC AND ANTISHOOT THROUGH SENSE+ DB L1 VOUT SENSE– + RUN OV M2 CVCC SLOPE COMPENSATION ILIM PGND PGOOD INTVCC UVLO + SLOPE RECOVERY ACTIVE CLAMP 1 51k ITHB – 0.74V VFB + – – + SS + – RUN – + R2 R1 OV 0.86V SGND 1.3µA EA – + + 0.
LTC3853 OPERATION Main Control Loop The LTC3853 is a constant-frequency, current mode step-down controller with three channels operating 120 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.
LTC3853 OPERATION voltage below 0.8V (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. When the 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.
LTC3853 OPERATION Triple vs Dual (2 + 1) Operation The LTC3853 can be used to regulate three different outputs. It can also be used as a dual output controller with a high current 2-phase output and a single phase output. Tying VFB2 to VIN through a 200k resistor switches the controller from triple to dual (2 + 1) operation. Do not exceed the absolute maximum current rating for the VFB2 pin.
LTC3853 APPLICATIONS INFORMATION the information at the sense terminals and making the programmed current limit unpredictable. If DCR sensing is used (Figure 2b), sense resistor R1 should be placed 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.
LTC3853 APPLICATIONS INFORMATION scope probes and waveform math to obtain a differential measurement. Based on additional measurements of the inductor ripple current and the on-time and off-time of the top switch, the value of the parasitic inductance was determined to be 0.
LTC3853 APPLICATIONS INFORMATION To ensure that the application will deliver full load current over the full operating temperature range, choose the minimum value for the Maximum Current Sense Threshold (VSENSE(MAX)) in the Electrical Characteristics table (22mV, 42mV, or 65mV, depending on the state of the ILIM pin). Next, determine the DCR of the inductor. Where provided, use the manufacturer’s maximum value, usually given at 20°C.
LTC3853 APPLICATIONS INFORMATION 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. Unfortunately, increased inductance requires more turns of wire and therefore copper losses will increase.
LTC3853 APPLICATIONS INFORMATION The optional Schottky diodes conduct during the dead time between the conduction of the two power MOSFETs. These prevent the body diodes of the bottom MOSFETs from turning on, storing charge during the dead time and requiring a reverse recovery period that could cost as much as 3% in efficiency at high VIN. A 1A to 3A Schottky is generally a good compromise for both regions of operation due to the relatively small average current.
LTC3853 APPLICATIONS INFORMATION VOUT1 OUTPUT VOLTAGE OUTPUT VOLTAGE VOUT1 VOUT2 TIME VOUT2 TIME 3853 F05a (5a) Coincident Tracking 3853 F05b (5b) Ratiometric Tracking Figure 5. Two Different Modes of Output Voltage Tracking VOUT1 VOUT2 TO TK/SS2 PIN R3 R1 R4 R2 TO VFB1 PIN R3 TO VFB2 PIN R4 VOUT1 VOUT2 TO TK/SS2 PIN R1 TO VFB1 PIN R2 TO VFB2 PIN R3 R4 3853 F06 (6a) Coincident Tracking Setup (6b) Ratiometric Tracking Setup Figure 6.
LTC3853 APPLICATIONS INFORMATION ceramic capacitor placed directly adjacent to the INTVCC and PGND pins is highly recommended. Good bypassing is needed to supply the high transient currents required by the MOSFET gate drivers and to prevent interaction between the channels. High input voltage applications in which large MOSFETs are being driven at high frequencies may cause the maximum junction temperature rating for the LTC3853 to be exceeded.
LTC3853 APPLICATIONS INFORMATION Topside MOSFET Driver Supply (CB, DB) CIN and COUT Selection External bootstrap capacitors, CB, connected to the BOOST pins supply the gate drive voltages for the topside MOSFETs. Capacitor CB in the Functional Diagram is charged though external diode, DB, from INTVCC when the SW pin is low. When one of the topside MOSFETs is to be turned on, the driver places the CB voltage across the gate source of the desired MOSFET.
LTC3853 APPLICATIONS INFORMATION time. The total RMS power lost is lower when more than one controller is operating due to the reduced overlap of current pulses required through the input capacitor’s ESR. This is why the input capacitor’s requirement calculated above for the worst-case controller is adequate for the dual or triple controller design.
LTC3853 APPLICATIONS INFORMATION controller 1 to be locked to the rising edge of an external clock signal applied to the MODE/PLLIN pin. The phase detector is an edge sensitive digital type that provides zero degrees phase shift between the external and internal oscillators. This type of phase detector does not exhibit false lock to harmonics of the external clock.
LTC3853 APPLICATIONS INFORMATION on the current sense signal. The minimum on-time can be affected by PCB switching noise in the voltage and current loop. However, as the peak sense voltage decreases the minimum on-time gradually increases to 130ns. This is of particular concern in forced continuous applications with low ripple current at light loads.
LTC3853 APPLICATIONS INFORMATION minimum of 20µF to 40µF of capacitance having a maximum of 20mΩ to 50mΩ of ESR. The LTC3853 3-phase architecture reduces this input capacitance requirement up to 66% over competing solutions. Other losses including Schottky conduction losses during dead time and inductor core losses generally account for less than 2% total additional loss. Checking Transient Response The regulator loop response can be checked by looking at the load current transient response.
LTC3853 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. Figure 12 illustrates the current waveforms present in the various branches of the 3-phase synchronous regulators operating in the continuous mode. Check the following in your layout: 1.
LTC3853 APPLICATIONS INFORMATION SW1 L1 VOUT1 RSENSE1 COUT1 D1 SW2 VIN RIN + L2 BOLD LINES INDICATE HIGH SWITCHING CURRENTS. KEEP LINES TO A MINIMUM LENGTH. COUT2 D2 SW3 L3 + RL2 VOUT3 RSENSE3 D3 RL1 VOUT2 RSENSE2 CIN + COUT3 + RL3 3853 F12 Figure 12.
LTC3853 APPLICATIONS INFORMATION PC Board Layout Debugging Start with one controller at a time. It is helpful to use a DC-50MHz current probe to monitor the current in the inductor while testing the circuit. Monitor the output switching node (SW pin) to synchronize the oscilloscope to the internal oscillator and probe the actual output voltage as well. Check for proper performance over the operating voltage and current range expected in the application.
LTC3853 APPLICATIONS INFORMATION 4.7µF M1 VIN DRVCC12 INTVCC TG1 PGOOD12 PGOOD3 SW1 3.3µH SENSE1 0.1µF 1430Ω SENSE1– VFB1 105k 1% 10pF + 330µF 10V ITH1,2,3 RITH1,2,3 CP1,2,3 20k 1% M2 M3 3.3µH SW1,2,3 SW2 1.5µH 4.75k 0.1µF BG2 10k SENSE2+ SENSE2– INTVCC VFB2 1430Ω 63.4k 1% RUN1,2,3 3.16k 1nF TG3 SW3 BG3 SENSE3+ EXTVCC SENSE3– TK/SS1,2,3 2.43k 20k 1% 0.068µF 1.82k VOUT2 3.3V 5A 10pF + VFB3 VOUT3 1.2V 5A 10k 1% 330µF 6V SGND CITH1,2,3 VIN 7V TO 20V CB1,2,3 0.
LTC3853 APPLICATIONS INFORMATION For channel 1, the DCRSENSE filter/divider values are: R1|| R2 1100Ω = @ 4.75k; RD 0.23 R1 • RD 4.75k • 0.23 R2 = = @ 1430Ω 1− RD 1− 0.23 R1= (VIN(MAX) − VOUT ) • VOUT 1 1 5 – 2.3 + 2.3 ( 500kHz ) = 243mW R1 (20V − 5V) • 5V = = 15.8mW 4.75k The respective values for Channel 2 are R1 = 4.75k, R2 = 1430Ω; and PLOSSR1 = 11.6mW. And for Channel 3 are R1 = 2.21k, R2 = 1.82k; and PLOSSR1 = 10.2mW.
LTC3853 APPLICATIONS INFORMATION 4.7µF M1 VIN DRVCC12 INTVCC TG1 PGOOD12 PGOOD3 SW1 1.5µH VOUT1 2.5V 5A SENSE1 1000pF* SENSE1– VFB1 43.2k 1% 10pF ITH1,2,3 RITH1,2,3 CP1,2,3 + 220µF 4V 20k 1% + CITH1,2,3 M2 M3 CB1,2,3 0.1µF 1.5µH SW1,2,3 2.2µH SW2 BG2 PGND MODE/PLLIN ILIM FREQ/PLLFLTR 10Ω* DB1,2,3 BOOST1,2,3 LTC3853 BG1 0.008Ω 5% 10Ω* TG2 VIN 7V TO 24V 33µF 35V 10k SENSE2+ 10Ω* INTVCC 10Ω* 1000pF* SENSE2– 24.9k 1% VFB2 RUN1,2,3 3.
LTC3853 TYPICAL APPLICATIONS Triple 3.3V/2.5V/12V, 5A Step-Down Converter with RSENSE Synchronized at 400kHz 4.7µF M1 VIN DRVCC12 INTVCC TG1 PGOOD12 PGOOD3 SW1 3.3µH TG2 ILIM 0.008Ω 5% 10Ω* 1000pF* BG2 PLLIN 400kHz SENSE1+ SENSE2+ MODE/PLLIN SENSE2– SENSE1– 6.8µH 0.008Ω 5% 10Ω* 10Ω* RITH1,2,3 43.2k 1% RUN1,2,3 10pF + 20k 1% 1000pF 10k TG3 SW3 BG3 SGND SENSE3+ EXTVCC SENSE3– TK/SS1,2,3 CITH1,2,3 CSS1,2,3 0.1µF 1000pF* 10Ω* VOUT2 2.
LTC3853 TYPICAL APPLICATIONS Dual 1.2V/2.5V High Current Step-Down Converter with RSENSE 4.7µF VIN DRVCC12 INTVCC TG1 PGOOD12 PGOOD3 SW1 0.47µH VOUT1 1.2V 30A + 1000pF* + 660µF 2.5V ×2 SENSE1 EXTVCC SENSE2– SENSE1– RITH1,3 20k 1% CITH1,3 100Ω* 10k 0.78µH 1000pF* VFB2 RUN2 0.002Ω 5% 100Ω* INTVCC 100Ω* 1000pF* 100Ω* VOUT1 200k RUN1,3 ITH1,2,3 CP1,3 0.47µH SW1,2,3 BG2 SENSE2+ VFB1 10k 1% 47pF CB1,2,3 0.
LTC3853 PACKAGE DESCRIPTION UJ Package 40-Lead Plastic QFN (6mm × 6mm) (Reference LTC DWG # 05-08-1728 Rev Ø) 0.70 ±0.05 6.50 ±0.05 5.10 ±0.05 4.42 ±0.05 4.50 ±0.05 (4 SIDES) 4.42 ±0.05 PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 6.00 ± 0.10 (4 SIDES) 0.75 ± 0.05 R = 0.10 TYP R = 0.115 TYP 39 40 0.40 ± 0.10 PIN 1 TOP MARK (SEE NOTE 6) 1 4.50 REF (4-SIDES) 4.42 ±0.10 2 PIN 1 NOTCH R = 0.45 OR 0.
LTC3853 REVISION HISTORY REV DATE DESCRIPTION PAGE NUMBER A 12/10 Change to Operating Temperature Range 2 Updated Order Information Part Marking 2 Edits made to Note 2 and 3 4 Changes to graphs G01 and G02 5 Updated Related Parts table 36 3853fa Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use.
LTC3853 TYPICAL APPLICATION Three Phase 2.5V Output High Current Step-Down Converter with RSENSE 4.7µF 0.78µH 1000pF* CP1 RITH1 20k 1% CITH1 0.78µH SW1,2,3 BG2 100Ω* 10k SENSE2+ EXTVCC 0.78µH SENSE2 VFB1 VFB2 VFB3 RUN1 RUN2 RUN3 ITH1 ITH2 ITH3 TG3 SW3 BG3 SENSE3+ SENSE3– SGND 2.55k 0.002Ω 5% 100Ω* INTVCC 1000pF* – SENSE1– 43.2k 1% 47pF CB1,2,3 0.1µF VIN 6.5V TO 14V SW2 SENSE1+ 0.
