LTC1703 Dual 550kHz Synchronous 2-Phase Switching Regulator Controller with 5-Bit VID U FEATURES DESCRIPTIO ■ The LTC®1703 is a dual switching regulator controller optimized for high efficiency with low input voltages. It includes two complete, on-chip, independent switching regulator controllers. Each is designed to drive a pair of external N-channel MOSFET devices in a voltage mode feedback, synchronous buck configuration.
LTC1703 W W W AXI U U ABSOLUTE RATI GS U U W PACKAGE/ORDER I FOR ATIO (Note 1) Supply Voltage VCC ........................................................................................... 7V BOOSTn ............................................................... 15V BOOSTn – SWn .................................................... 7V Input Voltage SWn .......................................................... – 1V to 8V VIDn ....................................................... – 0.
LTC1703 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. VCC = 5V unless otherwise specified.
LTC1703 U W TYPICAL PERFOR A CE CHARACTERISTICS Efficiency vs Load Current VIN = 5V VOUT = 2.5V 90 35 VIN = 5V VOUT = 1.8V ILOAD = 0A-10A-0A ±2.2% MAX DEVIATION VOUT = 3.3V VOUT = 1.
LTC1703 U U U PI FU CTIO S PVCC (Pin 1): Driver Power Supply Input. PVCC provides power to the two BGn output drivers. PVCC must be connected to a voltage high enough to fully turn on the external MOSFETs QB1 and QB2. PVCC should generally be connected directly to VIN. PVCC requires at least a 1µF bypass capacitor directly to PGND. BOOST1 (Pin 2): Controller 1 Top Gate Driver Supply. The BOOST1 pin supplies power to the floating TG1 driver. BOOST1 should be bypassed to SW1 with a 1µF capacitor.
LTC1703 U U U PI FU CTIO S VIDn pin includes an on-chip 40kΩ pull-up resistor in series with a diode (see Block Diagram). VCC (Pin 18): Power Supply Input. All internal circuits except the output drivers are powered from this pin. VCC should be connected to a low noise power supply voltage between 3V and 7V and should be bypassed to SGND with at least a 1µF capacitor in close proximity to the LTC1703. FB2 (Pin 19): Controller 2 Feedback Input.
LTC1703 W BLOCK DIAGRA PVCC FCB VCC BOOST1,2 BURST LOGIC TG1,2 DRIVE LOGIC SW1,2 BG1,2 PGND 90% DUTY CYCLE OSC 550kHz SGND DIS 4µA SOFT START RUN/SS1,2 10µA FAULT COMP1,2 0V ILIM 10µA + FB MIN – MAX 25µs DELAY FLT FROM OTHER CONTROLLER IMAX1,2 800mV 760mV 840mV 920mV SHUTDOWN TO THIS CONTROLLER FB1,2 40k VCC SHUTDOWN TO ENTIRE CHIP 500mV VID0 40k SENSE VCC R11 10k VID1 FROM OTHER CONTROLLER 40k VCC VID2 40k TO FB1 SWITCH CONTROL LOGIC RB1 VCC VID3 40k VCC VID4 1703 BD
LTC1703 U W U U APPLICATIO S I FOR ATIO Table 1. VID Inputs and Corresponding Output Voltage for Channel 1 CODE VID4 VID3 VID2 VID1 VID0 VOUT1 00000 GND GND GND 00001 GND GND 00010 GND GND CODE VID4 VID3 VID2 VID1 VID0 VOUT1 GND GND 2.00V 10000 Float GND GND GND GND 1.275V GND GND Float 1.95V 10001 Float GND GND GND Float 1.250V GND Float GND 1.90V 10010 Float GND GND Float GND 1.225V 00011 GND GND GND Float Float 1.
LTC1703 U W U U APPLICATIO S I FOR ATIO converter, even in cases where the 1-step converter has higher total efficiency than the 2-step system. In a typical microprocessor core supply regulator, for example, the regulator is usually located right next to the CPU. In a 1-step design, all of the power dissipated by the core regulator is right there next to the hot CPU, aggravating thermal management.
LTC1703 U W U U APPLICATIO S I FOR ATIO of the equation—with a typical value on the order of 1µH, the inductor allows very fast di/dt slew rates. The result is superior transient response compared with conventional solutions. High Efficiency The LTC1703 uses a synchronous step-down (buck) architecture, with two external N-channel MOSFETs per output. A floating topside driver and a simple external charge pump provide full gate drive to the upper MOSFET.
LTC1703 U W U U APPLICATIO S I FOR ATIO This constant frequency operation brings with it a couple of benefits. Inductor and capacitor values can be chosen with a precise operating frequency in mind and the feedback loop components can be similarly tightly specified. Noise generated by the circuit will always be in a known frequency band with the 550kHz frequency designed to leave the 455kHz IF band free of interference.
LTC1703 U W U U APPLICATIO S I FOR ATIO current limits or QB dies trying to save the load. This behavior provides maximum protection against overvoltage faults at the output, while allowing the circuit to resume normal operation when the fault is removed. The overvoltage protection circuit can optionally be set to latch the output off permanently (see the Overvoltage Fault section).
LTC1703 U W U U APPLICATIO S I FOR ATIO is limited to 10%. The maximum duty cycle limit increases linearly between 1V and 2.5V, reaching its final value of 90% when RUN/SS is above 2.5V. Somewhere before this point, the feedback amplifier will assume control of the loop and the output will come into regulation. When RUN/ SS rises to 0.5V below VCC, the MIN feedback comparator is enabled, and the LTC1703 is in full operation.
LTC1703 U W U U APPLICATIO S I FOR ATIO Discontinuous Mode To minimize the efficiency loss due to reverse current flow at light loads, the LTC1703 switches to a second mode of operation: discontinuous mode (Figure 5b). In discontinuous mode, the LTC1703 detects when the inductor current approaches zero and turns off QB for the remainder of the switch cycle.
LTC1703 U W U U APPLICATIO S I FOR ATIO Burst Mode Operation Discontinuous mode removes a loss term due to resistive drop in QB, but the LTC1703 is still switching QT and QB on and off once a cycle. Each time an external MOSFET is turned on, the internal driver must charge its gate to VCC. Each time it is turned off, that charge is lost to ground. At the high switching frequencies that the LTC1703 operates at, the charge lost to the gates can add up to tens of milliamps from VCC.
LTC1703 U W U U APPLICATIO S I FOR ATIO internal latch. This latch releases the pull-down at FAULT, allowing the 10µA pull-up to take it high. When FAULT goes high, the LTC1703 stops all switching, turns both QB (bottom synchronous) MOSFETs on continuously and remains in this state until both RUN/SS pins are pulled low simultaneously, the power supply is recycled, or the FAULT pin is pulled low externally.
LTC1703 U W U U APPLICATIO S I FOR ATIO use in LTC1703 circuits running from a 5V supply. As current flows through this resistance while the MOSFET is on, it generates I2R watts of heat, where I is the current flowing (usually equal to the output current) and R is the MOSFET RDS(ON). This heat is only generated when the MOSFET is on. When it is off, the current is zero and the power lost is also zero (and the other MOSFET is busy losing power).
LTC1703 U W U U APPLICATIO S I FOR ATIO At the same time, the input supply needs to supply several amps of current without excessive voltage drop. The input supply must have regulation adequate to prevent sudden load changes from causing the LTC1703 input voltage to dip. In most typical applications where the LTC1703 is generating a secondary low voltage logic supply, all of these input conditions are met by the main system logic supply when fortified with an input bypass capacitor.
LTC1703 U W U U APPLICATIO S I FOR ATIO OUTPUT BYPASS CAPACITOR The output bypass capacitor has quite different requirements from the input capacitor. The ripple current at the output of a buck regulator like the LTC1703 is much lower than at the input, due to the fact that the inductor current is constantly flowing at the output whenever the LTC1703 is operating in continuous mode. The primary concern at the output is capacitor ESR.
LTC1703 U W U U APPLICATIO S I FOR ATIO from CIN (time point A). 50% of the way through, TG2 turns on and the total current is 13A (time point B). Shortly thereafter, TG1 turns off and the current drops to 10A (time point C). Finally, TG2 turns off and the current spends a short time at 0 before TG1 turns on again (time point D). ( ) ( ) (10A • 0.16) + (0A • 0.18) = 5.18A IAVG = 3A • 0.5 + 13A • 0.16 + Now we can calculate the RMS current.
LTC1703 U W U U APPLICATIO S I FOR ATIO FEEDBACK LOOP/COMPENSATION1 Feedback Loop Types In a typical LTC1703 circuit, the feedback loop consists of the modulator, the external inductor and output capacitor, and the feedback amplifier and its compensation network. All of these components affect loop behavior and need to be accounted for in the loop compensation. The modulator consists of the internal PWM generator, the output MOSFET drivers and the external MOSFETs themselves.
LTC1703 U W U U APPLICATIO S I FOR ATIO C2 C2 C3 R2 R1 C1 R2 R1 – IN + RB VREF VREF 1703 F10a Figure 10a. Type 2 Amplifier Schematic Diagram PHASE (DEG) GAIN (dB) C1 – IN OUT + RB R3 OUT 1703 F11a Figure 11a. Type 3 Amplifier Schematic Diagram GAIN (dB) PHASE (DEG) –6dB/OCT –6dB/OCT +6dB/OCT GAIN GAIN 0 0 –6dB/OCT 0 0 –6dB/OCT –90 –90 –180 –180 PHASE PHASE –270 –270 1703 F11b 1703 F10b Figure 10b. Type 2 Amplifier Transfer Function Figure 11b.
LTC1703 U W U U APPLICATIO S I FOR ATIO accurate results, but simulation can often get close enough to give a working system. To measure the modulator gain and phase directly, wire up a breadboard with an LTC1703 and the actual MOSFETs, inductor, and input and output capacitors that the final design will use.
LTC1703 U W U U APPLICATIO S I FOR ATIO Now calculate the remaining values: CURRENT LIMIT PROGRAMMING (K is a constant used in the calculations) Programming the current limit on the LTC1703 is straightforward. The IMAX pin sets the current limit by setting the maximum allowable voltage drop across QB (the bottom MOSFET) before the current limit circuit engages. The voltage across QB is set by its on-resistance and the current flowing in the inductor, which is the same as the output current.
LTC1703 U W U U APPLICATIO S I FOR ATIO Accuracy Trade-Offs The VDS sensing scheme used in the LTC1703 is not particularly accurate, primarily due to uncertainty in the RDS(ON) from MOSFET to MOSFET. A second error term arises from the ringing present at the SW pin, which causes the VDS to look larger than (ILOAD)(RDS(ON)) at the beginning of QB’s on-time.
LTC1703 U W U U APPLICATIO S I FOR ATIO OPTIMIZING PERFORMANCE 2-Step Conversion The LTC1703 is ideally suited for use in 2-step conversion systems. 2-step systems use a primary regulator to convert the input power source (batteries or AC line voltage) to an intermediate supply voltage, often 5V. The LTC1703 then converts the intermediate voltage to the low voltage, high current supplies required by the system.
LTC1703 U W U U APPLICATIO S I FOR ATIO Efficiency = TotalOutputPower (100%) TotalOutputPower + TotalPowerLost In our example 2-step system, the total output power is: Total output power = 15W + 16.5W + 1.25W + 3W + 13W = 48.75W corresponding to 5V, 3.3V, 2.5V, 1.5V and 1.3V output voltages. Assuming the LTC1703 provides 90% efficiency at each output, the additional load on the 5V and 3.3V supplies is: 1.3V: 13W/90% = 14.4W/3.3V = 4.4A from 3.3V 1.5V: 3W/90% = 3.3W/5V = 0.67A from 5V 2.5V: 1.
LTC1703 U W U U APPLICATIO S I FOR ATIO discontinuous mode happens when Burst Mode operation is invoked. At typical power levels, when Burst Mode operation is activated, gate drive is the dominant loss term. Burst Mode operation turns off all output switching for several clock cycles in a row, significantly cutting gate drive losses. As the load current in Burst Mode operation falls toward zero, the current drawn by the circuit falls to the LTC1703’s background quiescent level—about 3mA per channel.
LTC1703 U W U U APPLICATIO S I FOR ATIO Very quickly, the feedback loop will realize that something has changed and will move at the bandwidth allowed by the external compensation network towards a new duty cycle. If the bandwidth is set to 50kHz, the COMP pin will get to 60% of the way to 90% duty cycle in 3µs. Now the inductor is seeing 3.5V across itself for a large portion of the cycle, and its current will increase from 1A at a rate set by di/dt = V/L. If the inductor value is 0.
LTC1703 U W U U APPLICATIO S I FOR ATIO capacitors and/or paralleling multiple capacitors at the output. The capacitance value accounts for the rest of the voltage drop until the inductor current rises. With most output capacitors, several devices paralleled to get the ESR down will have so much capacitance that this drop term is negligible.
LTC1703 U W U U APPLICATIO S I FOR ATIO specified tolerance, the output voltage will ride high when ILOAD is low and will ride low when ILOAD is high. Compared to a traditional regulator, a voltage positioning regulator can theoretically stand as much as twice the ESR drop across the output capacitor while maintaining output voltage regulation. This means smaller, cheaper output capacitors can be used while keeping the output voltage within acceptable limits. load which can dissipate 2.
33k 330pF 330pF 56pF 0.1µF 33k 0.1µF 0.1µF 0.01µF D1 TO D7: MOTOROLA (800) 441-2447 Q1 TO Q5, QT1A/1B, QB1A/1B: INTERNATIONAL RECTIFIER (310) 22-3331 QT2, QB2: FAIRCHILD (207) 775-4503 L1, L2, L3: PANASONIC (201) 348-7522 L4, L5: COILTRONICS (561) 241-7876 3.3VENABLE STDBY5V STDBY3.3V STBYMD 5VENABLE 56pF 14 13 12 11 10 9 8 7 6 5 4 3 2 1 SENSE2+ SENSE2– VOSENSE2 ITH2 3.
1µF VOUT4 0.9V TO 2V 12A 180µF, 4V ×6 D1 TO D7: MOTOROLA (800) 441-2447 Q1 TO Q5, QT1A/1B, QB1A/1B: INTERNATIONAL RECTIFIER (310) 22-3331 QT2, QB2: FAIRCHILD (207) 775-4503 L1, L2, L3: PANASONIC (201) 348-7522 L4, L5: COILTRONICS (561) 241-7876 COREVENABLE 1.5V ENABLE FAULT + L3, 0.8µH ETQP6F0R8L 0.22µF 220pF 100k D5 MBRD- QB1A IRF7811 835L QT1A IRF7811 10k 200pF QB1B IRF7811 QT1B IRF7811 VID0 VID1 VID2 VID3 VID4 15pF R18, 1M 18.
LTC1703 U TYPICAL APPLICATIO S Single Output, 2-Phase, 25A VID Converter (VIN = 5V, VOUT = 0.9V to 2.0V) VIN 5V ±10% 10Ω 0.1µF 10Ω 0.003Ω 0.5W 0.003Ω 0.5W 0.1µF 10k – + LT®1006 470µF* 10k + + 0.1µF + 470µF* + 10µF MBR0530T 470µF* MBR 0530T MBR0530T VCC 11k BOOST1 FB1 120pF 220pF 330pF 0.1µF TG1 RUN/SS1 SW1 RUN/SS2 BG1 10k COMP2 SW2 IMAX1 BG2 IMAX2 FAULT 47k 22k 1Ω Q2 1Ω MBR 330T Q3 VOUT 1.3V TO 3.
LTC1703 U PACKAGE DESCRIPTIO G Package 28-Lead Plastic SSOP (0.209) (LTC DWG # 05-08-1640) 9.90 – 10.50* (.390 – .413) 28 27 26 25 24 23 22 21 20 19 18 17 16 15 1.25 ±0.12 7.8 – 8.2 5.3 – 5.7 7.40 – 8.20 (.291 – .323) 0.42 ±0.03 0.65 BSC 1 RECOMMENDED SOLDER PAD LAYOUT 2 3 4 5 6 7 8 9 10 11 12 13 14 2.0 (.079) MAX 5.00 – 5.60** (.197 – .221) 0° – 8° 0.09 – 0.25 (.0035 – .010) 0.55 – 0.95 (.022 – .037) NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS 2.
LTC1703 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC1530 High Power Synchronous Step-Down Controller SO-8 with Current Limit. No RSENSE Required LTC1625 No RSENSETM Current Mode Synchronous Step-Down Controller Above 95% Efficiency, Needs No RSENSE, 16-Lead SSOP Package Fits SO-8 Footprint LTC1628 Dual High Efficiency 2-Phase Synchronous Step-Down Controller Constant Frequency, Standby 5V and 3.3V LDOs, 3.
