LTC3890-3 60V Low IQ, Dual, 2-Phase Synchronous Step-Down DC/DC Controller Description Features n n n n n n n n n n n n n n Wide VIN Range: 4V to 60V (65V Abs Max) Low Operating IQ: 50µA (One Channel On) Wide Output Voltage Range: 0.
LTC3890-3 Absolute Maximum Ratings (Note 1) Pin Configuration Input Supply Voltage (VIN).......................... –0.3V to 65V Topside Driver Voltages BOOST1, BOOST2 ...................................–0.3V to 71V Switch Voltage (SW1, SW2) .......................... –5V to 65V (BOOST1-SW1), (BOOST2-SW2) ................. –0.3V to 6V RUN1, RUN2 ................................................ –0.3V to 8V Maximum Current Sourced into Pin from Source >8V....................................................
LTC3890-3 Electrical Characteristics The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VRUN1,2 = 5V, EXTVCC = 0V unless otherwise noted. (Note 2) SYMBOL PARAMETER VIN Input Supply Operating Voltage Range VOUT Regulated Output Voltage Range VFB1,2 Regulated Feedback Voltage CONDITIONS ITH1,2 Voltage = 1.
LTC3890-3 Electrical Characteristics The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VRUN1,2 = 5V, EXTVCC = 0V unless otherwise noted.
LTC3890-3 Typical Performance Characteristics Efficiency and Power Loss vs Output Current VIN = 12V 90 VOUT = 3.3V BURST EFFICIENCY 90 100 60 50 BURST LOSS PULSE-SKIPPING LOSS 40 10 30 FCM EFFICIENCY 20 0 0.0001 1 PULSE-SKIPPING EFFICIENCY 10 0.1 10 98 VOUT = 3.
LTC3890-3 Typical Performance Characteristics Total Input Supply Current vs Input Voltage 200 300µA LOAD 150 100 NO LOAD 50 0 5.6 5.0 5.4 INTVCC 5.2 5.0 EXTVCC RISING 4.8 EXTVCC FALLING 4.6 4.4 20 0 –20 0.2 0.4 0.6 0.8 VITH (V) 1.0 600 500 400 300 200 100 0 FORCED CONTINUOUS MODE 0 1.2 –100 1.
LTC3890-3 Typical Performance Characteristics TRACK/SS Pull-Up Current vs Temperature Regulated Feedback Voltage vs Temperature Shutdown (RUN) Threshold vs Temperature 1.40 808 1.05 RUN PIN VOLTAGE (V) TRACK/SS CURRENT (µA) 1.35 1.00 0.95 1.30 RUN1 RISING RUN2 RISING 1.25 1.20 1.15 RUN1 FALLING 1.10 RUN2 FALLING 1.05 0.90 –75 –50 –25 1.
LTC3890-3 Pin Functions ITH1, ITH2 (Pin 1, Pin 13): Error Amplifier Outputs and Switching Regulator Compensation Points. Each associated channel’s current comparator trip point increases with this control voltage. VFB1, VFB2 (Pin 2, Pin 12): Receives the remotely sensed feedback voltage for each controller from an external resistive divider across the output.
LTC3890-3 Pin Functions BG1, BG2 (Pin 23, Pin 18): High Current Gate Drives for Bottom (Synchronous) N-Channel MOSFETs. Voltage swing at these pins is from ground to INTVCC. PGOOD1 (Pin 27): Open-Drain Logic Output. PGOOD1 is pulled to ground when the voltage on the VFB1 pin is not within ±10% of its set point. BOOST1, BOOST2 (Pin 24, Pin 17): Bootstrapped Supplies to the Topside Floating Drivers.
LTC3890-3 FUNCTIONAL Diagram INTVCC DUPLICATE FOR SECOND CONTROLLER CHANNEL PGOOD1 BOOST DROP OUT DET 0.88V VFB1 + – + 0.72V S Q R Q D BOT SWITCH LOGIC BOT INTVCC BG – VCO VOUT CLK2 0.425V CLK1 + SLEEP – ICMP PFD + – CLP –+ +– + SYNC DET PLLIN/MODE IR – SENSE+ 2.7V 0.65V 100k SENSE– SLOPE COMP VFB VIN + EA – OV – 5.1V LDO EN LDO EN 7µA (RUN1) 0.5µA (RUN2) SHDN RST 2(VFB) + – FOLDBACK 11V SGND INTVCC 0.80V TRACK/SS RB RA + EXTVCC 5.
LTC3890-3 Operation (Refer to the Functional Diagram) Main Control Loop The LTC3890-3 uses a constant frequency, current mode step-down architecture with the two controller channels operating 180 degrees out-of-phase. During normal operation, each external top MOSFET is turned on when the clock for that channel sets the RS latch, and is turned off when the main current comparator, ICMP, resets the RS latch.
LTC3890-3 Operation (Refer to the Functional Diagram) Light Load Current Operation (Burst Mode Operation, Pulse-Skipping, or Forced Continuous Mode) (PLLIN/MODE Pin) just before the inductor current reaches zero, preventing it from reversing and going negative. Thus, the controller operates in discontinuous operation. The LTC3890-3 can be enabled to enter high efficiency Burst Mode operation, constant frequency pulse-skipping mode, or forced continuous conduction mode at low load currents.
LTC3890-3 Operation (Refer to the Functional Diagram) If the PLLIN/MODE pin is not being driven by an external clock source, the FREQ pin can be tied to SGND, tied to INTVCC or programmed through an external resistor. Tying FREQ to SGND selects 350kHz while tying FREQ to INTVCC selects 535kHz. Placing a resistor between FREQ and SGND allows the frequency to be programmed between 50kHz and 900kHz, as shown in Figure 10.
LTC3890-3 Operation (Refer to the Functional Diagram) 5V SWITCH 20V/DIV 3.3V SWITCH 20V/DIV INPUT CURRENT 5A/DIV INPUT VOLTAGE 500mV/DIV IIN(MEAS) = 2.53ARMS IIN(MEAS) = 1.55ARMS 38903 F01 Figure 1. Input Waveforms Comparing Single-Phase (a) and 2-Phase (b) Operation for Dual Switching Regulators Converting 12V to 5V and 3.3V at 3A Each.
LTC3890-3 Applications Information The Typical Application on the first page is a basic LTC3890‑3 application circuit. LTC3890-3 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.
LTC3890-3 Applications Information Low Value Resistor Current Sensing A typical sensing circuit using a discrete resistor is shown in Figure 4a. RSENSE is chosen based on the required output current. The current comparator has a maximum threshold VSENSE(MAX). The current comparator threshold voltage sets the peak of the inductor current, yielding a maximum average output current, IMAX, equal to the peak value less half the peak-to-peak ripple current, ∆IL.
LTC3890-3 Applications Information The equivalent resistance R1|| R2 is scaled to the room temperature inductance and maximum DCR: R1|| R2 = L (DCR at 20°C) • C1 ∆IL = The sense resistor values are: R1= R1|| R2 R1• RD ; R2 = RD 1– RD The maximum power loss in R1 is related to duty cycle, and will occur in continuous mode at the maximum input voltage: PLOSS R1= The inductor value has a direct effect on ripple current.
LTC3890-3 Applications Information Power MOSFET and Schottky Diode (Optional) Selection The MOSFET power dissipations at maximum output current are given by: Two external power MOSFETs must be selected for each controller in the LTC3890-3: one N-channel MOSFET for the top (main) switch, and one N-channel MOSFET for the bottom (synchronous) switch. The peak-to-peak drive levels are set by the INTVCC voltage. This voltage is typically 5.1V during start-up (see EXTVCC Pin Connection).
LTC3890-3 Applications Information The optional Schottky diodes D3 and D4 shown in Figure 11 conduct during the dead-time between the conduction of the two power MOSFETs. This prevents the body diode of the bottom MOSFET 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.
LTC3890-3 Applications Information 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. The output ripple (∆VOUT) is approximated by: 1 ∆VOUT ≈ ∆IL ESR + 8 • f • COUT where f is the operating frequency, COUT is the output capacitance and ∆IL is the ripple current in the inductor. The output ripple is highest at maximum input voltage since ∆IL increases with input voltage.
LTC3890-3 Applications Information OUTPUT VOLTAGE VX(MASTER) For coincident tracking (VOUT = VX during start-up): RA = RTRACKA RB = RTRACKB VOUT(SLAVE) 38903 F07a TIME (7a) Coincident Tracking OUTPUT VOLTAGE VX(MASTER) VOUT(SLAVE) 38903 F07b TIME (7b) Ratiometric Tracking Figure 7. Two Different Modes of Output Voltage Tracking Vx VOUT RB 1/2 LTC3890-3 VFB RA RTRACKB TRACK/SS RTRACKA 38903 F08 Figure 8.
LTC3890-3 Applications Information To prevent the maximum junction temperature from being exceeded, the input supply current must be checked while operating in forced continuous mode (PLLIN/MODE = INTVCC) at maximum VIN. When the voltage applied to EXTVCC rises above 4.7V, the VIN LDO is turned off and the EXTVCC LDO is enabled. The EXTVCC LDO remains on as long as the voltage applied to EXTVCC remains above 4.5V. The EXTVCC LDO attempts to regulate the INTVCC voltage to 5.
LTC3890-3 Applications Information Topside MOSFET Driver Supply (CB, DB) 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. This enhances the top MOSFET switch and turns it on.
LTC3890-3 Applications Information The LTC3890-3 has an internal phase-locked loop (PLL) comprised of a phase frequency detector, a lowpass filter, and a voltage-controlled oscillator (VCO). 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 PLLIN/MODE pin. The turn-on of controller 2’s top MOSFET is thus 180 degrees out of phase with the external clock.
LTC3890-3 Applications Information Minimum On-Time Considerations Minimum on-time, tON(MIN), is the smallest time duration that the LTC3890-3 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.
LTC3890-3 Applications Information 3. I2R losses are predicted from the DC resistances of the fuse (if used), MOSFET, inductor, current sense resistor, and input and output capacitor ESR. In continuous mode the average output current flows through L and RSENSE, but is chopped between the topside MOSFET and the synchronous MOSFET. If the two MOSFETs have approximately the same RDS(ON), then the resistance of one MOSFET can simply be summed with the resistances of L, RSENSE and ESR to obtain I2R losses.
LTC3890-3 Applications Information The ITH series RC-CC filter sets the dominant pole-zero loop compensation. The values can be modified slightly (from 0.5 to 2 times their suggested values) to optimize transient response once the final PC layout is done and the particular output capacitor type and value have been determined. The output capacitors need to be selected because the various types and values determine the loop gain and phase.
LTC3890-3 Applications Information The equivalent RSENSE resistor value can be calculated by using the minimum value for the maximum current sense threshold (43mV): RSENSE ≤ 64mV ≈ 0.01Ω 5.73A VORIPPLE = RESR (∆IL) = 0.02Ω(1.45A) = 29mVP-P Choosing 1% resistors: RA = 25k and RB = 78.7k yields an output voltage of 3.32V. The power dissipation on the topside MOSFET can be easily estimated. Choosing a Fairchild FDS6982S dual MOSFET results in: RDS(ON) = 0.035Ω/0.022Ω, CMILLER = 215pF.
LTC3890-3 Applications Information 4. Are the SENSE– and SENSE+ leads routed together with minimum PC trace spacing? The filter capacitor between SENSE+ and SENSE– should be as close as possible to the IC. Ensure accurate current sensing with Kelvin connections at the SENSE resistor. 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.
LTC3890-3 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.
LTC3890-3 Applications Information ITH1 TRACK/SS1 VFB1 PGOOD1 R1* SENSE1+ C1* fIN PLLIN/MODE RUN1 VPULL-UP PGOOD1 L1 TG1 SW1 SENSE1– LTC3890-3 BOOST1 FREQ RPU1 CB1 M1 C2* RIN VIN D1* CVIN 1µF CERAMIC COUT1 + PGND EXTVCC SENSE2– INTVCC SENSE2+ BG2 + CINTVCC VFB2 BOOST2 ITH2 SW2 VIN GND M3 + CIN COUT2 1µF CERAMIC R2* TRACK/SS2 VOUT1 BG1 RUN2 SGND M2 RSENSE M4 D2* CB2 RSENSE TG2 VOUT2 L2 38903 F11 *R1, R2, C1, C2, D1, D2 ARE OPTIONAL. Figure 11.
LTC3890-3 Applications Information SW1 L1 D1 RSENSE1 VOUT1 COUT1 RL1 VIN RIN CIN SW2 BOLD LINES INDICATE HIGH SWITCHING CURRENT. KEEP LINES TO A MINIMUM LENGTH. D2 L2 RSENSE2 VOUT2 COUT2 RL2 38903 F12 Figure 12. Branch Current Waveforms 32 38903f For more information www.linear.
LTC3890-3 Typical Applications C1 1nF RB1 100k RA1 31.6k SENSE1+ INTVCC – SENSE1 PGOOD1 100k VFB1 CITH1A 100pF BG1 RITH1 34.8k CITH1 1000pF MBOT1 SW1 BOOST1 ITH1 LTC3890-3 CSS1 0.01µF L1 4.7µH TRACK/SS1 TG1 CB1 0.1µF RSENSE1 8mΩ VOUT1 3.3V 5A COUT1 470µF MTOP1 D1 VIN PLLIN/MODE SGND EXTVCC RUN1 RUN2 FREQ VOUT2 RFREQ 41.2k INTVCC L2 8µH SW2 VFB2 RB2 100k 2.2µF ×3 MTOP2 CB2 0.1µF BOOST2 ITH2 RA2 10.5k VIN 9V TO 60V CIN 100µF D2 TG2 TRACK/SS2 RITH2 34.8k CINT 4.
LTC3890-3 Typical Applications High Efficiency 8.5V Dual-Phase Step-Down Converter RB1 100k RA1 10.5k C1 1nF SENSE1+ SENSE1– INTVCC PGOOD1 100k VFB1 MBOT1 CITH1A 100pF BG1 L1 8µH SW1 RITH1 34.8k BOOST1 ITH1 CITH1 C SS1 0.01µF 470pF LTC3890-3 TRACK/SS1 TG1 CB1 0.1µF VIN VIN PLLIN/MODE SGND RRUN V 1000k OUT VIN 9V TO 60V 2.2µF ×3 D2 TG2 TRACK/SS2 CITH2 100pF CIN 100µF CINT 4.7µF PGND EXTVCC RUN1 RUN2 FREQ RFREQ 41.2k + INTVCC COUT1 330µF MTOP1 D1 INTVCC RMODE 100k VOUT1 8.
LTC3890-3 Typical Applications High Efficiency Dual 12V/5V Step-Down Converter RB1 100k RA1 6.98k C1 1nF SENSE1+ SENSE1– INTVCC PGOOD1 100k VFB1 CITH1A 100pF CITH1 470pF BG1 RITH1 34.8k MBOT1 SW1 BOOST1 ITH1 LTC3890-3 CSS1 0.01µF L1 8µH TRACK/SS1 TG1 CB1 0.47µF RSENSE1 9mΩ COUT1 180µF MTOP1 D1 VIN PLLIN/MODE SGND EXTVCC RUN1 RUN2 FREQ RFREQ 41.2k CITH2 470pF RITH2 20k CINT 4.7µF PGND CIN 100µF VIN 12.5V TO 60V 2.2µF ×3 D2 TG2 CSS2 0.
LTC3890-3 Typical Applications High Efficiency Dual 24V/5V Step-Down Converter RB1 487k C1 1nF CF1 33pF RA1 16.9k SENSE1– INTVCC PGOOD1 100k VFB1 CITH1A 100pF CITH1 680pF SENSE1+ BG1 RITH1 46k MBOT1 L1 22µH SW1 RSENSE1 25mΩ BOOST1 ITH1 CSS1 0.01µF TRACK/SS1 COUT1 22µF ×2 CERAMIC CB1 0.47µF LTC3890-3 TG1 MTOP1 D1 VIN PLLIN/MODE SGND EXTVCC RUN1 RUN2 FREQ RFREQ 60k CSS2 0.01µF CITH2 470pF TRACK/SS2 RITH2 20k + INTVCC TG2 CB2 0.47µF MTOP2 L2 4.7µH BOOST2 BG2 RA2 18.7k 2.
LTC3890-3 Typical Applications 12V SEPIC and 3.3V Step-Down Converter L1 6.8µF VOUT1 RB1 100k C1 100pF RA1 6.98k SENSE1– M1 TG1 D2 COUT 68µF 6.8µF 100Ω VOUT1 12V 2A RSNS1 6mΩ SW1 BOOST1 ITH1 CSS1 0.01µF 10µH 6.8µF BG1 RITH1 12.1k 10µH PGOOD1 VFB1 CITH1A 47pF CITH1 10nF SENSE1+ • • LTC3890-3 TRACK/SS1 VIN RMODE 100k INTVCC PLLIN/MODE SGND EXTVCC RUN1 RUN2 FREQ VOUT1 RFREQ 41.2k CSS2 0.01µF TRACK/SS2 CITH2 4.7nF RITH2 7.15k + INTVCC CINT 4.
LTC3890-3 Typical Applications High Efficiency 12V at 25A Dual-Phase Step-Down Converter RB1 499k 10pF C1 1nF RA1 35.7k SENSE1– INTVCC PGOOD1 100k VFB1 CITH1A 100pF BG1 RITH1 9.76k CITH1 4.7nF SENSE1+ MBOT1 L1 10µH SW1 BOOST1 ITH1 CSS1 0.1µF TRACK/SS1 TG1 MTOP1 D1 PLLIN/MODE VIN SGND FREQ RRUN1 1000k + INTVCC CIN 100µF CINT 4.7µF PGND VOUT 12V 25A COUT 150µF ×2 CB1 0.1µF LTC3890-3 VIN RFREQ 30.1k RSENSE1 3mΩ VIN 16V TO 60V 2.2µF x2 D2 RUN1 RUN2 RRUN2 57.
LTC3890-3 Package Description 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) .0075 – .0098 (0.19 – 0.25) 2 3 4 5 6 7 8 9 10 11 12 13 14 .0532 – .0688 (1.35 – 1.75) .004 – .0098 (0.
LTC3890-3 typical application High Efficiency Dual 12V/3.3V Step-Down Converter C1 1nF RB1 100k RA1 6.98k SENSE1+ SENSE1– INTVCC PGOOD1 100k VFB1 CITH1A 100pF BG1 RITH1 34.8k CITH1 470pF MBOT1 SW1 L1 8µH BOOST1 ITH1 CB1 0.47µF LTC3890-3 CSS1 0.01µF TRACK/SS1 TG1 RSENSE1 9mΩ COUT1 180µF MTOP1 D1 VIN PLLIN/MODE SGND VOUT1 RFREQ 41.2k EXTVCC RUN1 RUN2 FREQ TRACK/SS2 RITH2 34.8k VIN 12.5V TO 60V D2 TG2 CB2 0.47µF BOOST2 MTOP2 L2 4.7µH SW2 RSENSE2 10mΩ ITH2 CITH2A 100pF RA2 31.
