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LTC3861

3861fa

For more information www.linear.com/LTC3861

Typical applicaTion

DescripTion

Dual, Multiphase Step-Down

Voltage Mode DC/DC Controller

with Accurate Current Sharing

The LTC

3861 is a dual PolyPhase

synchronous step-

down switching regulator controller for high current

distributed power systems, digital signal processors, and

other telecom and industrial DC/DC power supplies. It uses

a constant-frequency voltage mode architecture combined

with very low offset, high bandwidth error amplifiers and a

remote output sense differential amplifier per channel for

excellent transient response and output regulation.

The controller incorporates lossless inductor DCR current

sensing to maintain current balance between phases and to

provide overcurrent protection. The chip operates from a

supply between 3V and 5.5V and is designed for step-

down conversion from V

between 3V and 24V to output

voltages between 0.6V and V

– 0.5V.

The TRACK/SS pins provide programmable soft-start or

tracking functions. Inductor current reversal is disabled

during soft-start to safely power prebiased loads. The

constant operating frequency

can be synchronized to an

external clock or

linearly programmed from 250kHz to

2.25MHz. Up to six LTC3861 controllers can operate in

parallel for 1-, 2-, 3-, 4-, 6- or 12-phase operation.

The LTC3861 is available in a 36-pin 5mm × 6mm QFN

pack-

age. LTC3861-1 is a 32-pin QFN version of the LTC3861,

which

has a single differential remote output voltage sense

amplifier.

FeaTures

applicaTions

Operates with Power Blocks, DrMOS or External

Gate Drivers and MOSFETs

Constant-Frequency Voltage Mode Control with

Accurate Current Sharing

±0.75% 0.6V Voltage Reference

Dual Differential Output Voltage Sense Amplifiers

Multiphase Capability—Up to 12-Phase Operation

Programmable Current Limit

Safely Powers a Prebiased Load

Programmable or PLL-Synchronizable Switching

Frequency Up to 2.25MHz

Lossless Current Sensing Using Inductor DCR or

Precision Current Sensing with Sense Resistor

Range: 3V to 5.5V

Range: 3V to 24V

Power Good Output Voltage Monitor

Output Voltage Tracking Capability

Programmable Soft-Start

Available in a 36-Pin 5mm × 6mm QFN Package

High Current Distributed Power Systems

DSP, FPGA and ASIC Supplies

Datacom and Telecom Systems

Industrial Power Supplies

L, LT, LTC, LTM, PolyPhase, Linear Technology and the Linear logo are registered trademarks

of Linear Technology Corporation. All other trademarks are the property of their respective

owners. Protected by U.S. Patents, including 6144194, 5055767

VSNSOUT1

VSNSP1

VSNSN1

VSNSOUT2

VSNSP2

VSNSN2

CONFIG

FREQ

FB2

LIM2

FB1

COMP1,2

SS1,2

VINSNS

LOGIC

BOOST

GND

RUN1,2

LIM1

ISNS1P

ISNS1N

ISNS2N

ISNS2P

LTC3861

PWM1

LTC4449

, 7V TO 14V

SW1

0.47µH

SGND CLKIN

PWM2

180µF

1µF

10k

0.22µF

TG1

BG1

0.22µF

100pF

3.3nF

0.1µF

4.7nF

5.9k

280Ω

59k

2.87k

10k

100pF

OUT

LOGIC

BOOST

GND

LTC4449

SW2

0.47µH

0.22µF

TG2

BG2

3861 TA01

330µF

× 6

100µF

× 4

OUT

1.2V

60A

AVG

28k

Summary of content (38 pages)

PAGE 1
LTC3861 Dual, Multiphase Step-Down Voltage Mode DC/DC Controller with Accurate Current Sharing Description Features n n n n n n n n n n n n n n n Operates with Power Blocks, DrMOS or External Gate Drivers and MOSFETs Constant-Frequency Voltage Mode Control with Accurate Current Sharing ±0.75% 0.
PAGE 2
LTC3861 PWM1 PWMEN1 PGOOD1 IAVG CONFIG TOP VIEW VINSNS VCC Voltage................................................... –0.3V to 6V VINSNS Voltage.......................................... –0.3V to 30V RUN1, RUN2 Voltage.........................–0.3V to VCC + 0.3V ISNS1P , ISNS1N, ISNS2P , ISNS2N............................ –0.3V to (VCC + 0.1V) All Other Pins.................................–0.3V to (VCC + 0.3V) Operating Junction Temperature Range (Notes 2, 3)........................................
PAGE 3
LTC3861 Electrical Characteristics The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TJ = 25°C (Note 3). VCC = 5V, VRUN1,2 = 5V, VFREQ = VCLKIN = 0V, VFB = 0.6V, fOSC = 0.6MHz, unless otherwise specified.
PAGE 4
LTC3861 Electrical Characteristics The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TJ = 25°C (Note 3). VCC = 5V, VRUN1,2 = 5V, VFREQ = VCLKIN = 0V, VFB = 0.6V, fOSC = 0.6MHz, unless otherwise specified. SYMBOL PARAMETER CONDITIONS MIN VCM(ISENSE) Input Common Mode Range IISENSE SENSE Pin Input Current VCM = 1.5V VOS Current Sense Input Referred Offset –40°C to 125°C l –1.
PAGE 5
LTC3861 Typical Performance Characteristics Load Step Transient Response (2-Phase Using D12S1R845A Power Block) Load Step Transient Response (Single Phase Using LTC4449) ILOAD 20A/DIV Load Step Transient Response (3-Phase Using FDMF6707B DrMOS) IL1 10A/DIV ILOAD 20A/DIV IL1 20A/DIV IL 10A/DIV IL2 10A/DIV IL2 20A/DIV VOUT 50mV/DIV AC-COUPLED IL3 10A/DIV VOUT 50mV/DIV AC-COUPLED VIN = 12V VOUT = 1.2V 50µs/DIV ILOAD STEP = 0A TO 18A fSW = 300kHz 3861 G01 VIN = 12V VOUT = 1.
PAGE 6
LTC3861 Typical Performance Characteristics Start-Up Transient Response (4-Phase Using TDA21220 DrMOS) Start-Up Response (2-Phase Using LTC4449) VRUN 5V/DIV Soft-Start Start-Up Response (2-Phase Using D12S1R845A Power Block) RUN 5V/DIV IL1 10A/DIV ILOAD 50A/DIV IL2 10A/DIV VOUT 1V/DIV INTERNAL SOFT-START VIN = 12V VOUT = 1.
PAGE 7
LTC3861 Typical Performance Characteristics Overcurrent Threshold vs Temperature Oscillator Frequency vs RFREQ 40 2.3 CURRENT SENSE VOLTAGE (mV) 1.9 1.7 1.5 1.3 1.1 0.9 0.7 0.5 0.3 0 40 20 80 60 RFREQ (kΩ) 100 30 25 20 595 OSCILLATOR FREQUENCY (kHz) FREQ PIN CURRENT (µA) 20.4 19.8 19.6 19.
PAGE 8
LTC3861 Typical Performance Characteristics RUN Threshold vs Temperature RUN Pull-Up Current vs Temperature 2.25 2.2 RISING 2.0 RUN PIN CURRENT (µA) RUN PIN VOLTAGE (V) 2.20 2.15 2.10 2.05 2.00 FALLING 0 25 50 1.6 1.4 1.2 1.95 1.90 –50 –25 1.8 1.0 –50 75 100 125 150 TEMPERATURE (°C) 3861 G28 100 150 3861 G29 TRACK/SS Pull-Up Current vs Temperature 0.5 3.0 0 2.9 TRACK/SS PIN CURRENT (µA) TRACK/SS PIN CURRENT (µA) 50 TEMPERATURE (°C) TRACK/SS Current vs TRACK/SS Voltage –0.5 –1.
PAGE 9
LTC3861 Pin Functions VSNSOUT1 (Pin 5), VSNSOUT2 (Pin 6): Differential Amplifier Output. Connect to FB1, FB2 with a compensation network for remote VOUT sensing. FREQ (Pin 12): Frequency Set/Select Pin. This pin sources 20µA current. If CLKIN is high or floating, then a resistor between this pin and SGND sets the switching frequency. If CLKIN is low, the logic state of this pin selects an internal 600kHz or 1MHz preset frequency. CLKIN (Pin 13): External Clock Synchronization Input.
PAGE 10
LTC3861 Pin Functions CONFIG (Pin 33): Line Feedforward Configuration Pin. This pin allows the user to configure the multiplier to achieve accurate modulator gain over varying VIN and switching frequencies. This pin can be connected to VCC or SGND. An internal resistor will pull this pin to SGND when it is floated. TRACK/SS1 (Pin 35), TRACK/SS2 (Pin 11): Combined Soft-Start and Tracking Inputs.
PAGE 11
LTC3861 Functional Diagram 5 3 4 6 8 7 2 35 1 VSNSOUT1 36 21 VCC SGND VCC VSNSP1 26 19 RUN1 RUN2 31 1.5µA 100k PGOOD2 PGOOD 1.
PAGE 12
LTC3861 Operation (Refer to Functional Diagram) Main Control Architecture The LTC3861 is a dual-channel/dual-phase, constantfrequency, voltage mode controller for DC/DC step-down applications. It is designed to be used in a synchronous switching architecture with external integrated-driver MOSFETs or power blocks, or external drivers and N-channel MOSFETs using single wire three-state PWM interfaces.
PAGE 13
LTC3861 Operation (Refer to Functional Diagram) turned on for 20ns every eight cycles to allow the driver IC to recharge its topside gate drive capacitor. The negative OC threshold is equal to –3/4 the positive OC threshold. See the Applications Information section for guidelines on setting these thresholds. Excellent Transient Response The LTC3861 error amplifiers are true operational amplifiers, meaning that they have high bandwidth, high DC gain, low offset and low output impedance.
PAGE 14
LTC3861 Operation (Refer to Functional Diagram) a result, the bottom switch turns on until the amount of reverse current trips the NOC comparator threshold. Nonsynchronous Start-Up and Prebiased Output Load The LTC3861 will start up with seven cycles of nonsynchronous operation before switching over to a forced continuous mode of operation. The PWM output will be in a three-state condition until start-up.
PAGE 15
LTC3861 Operation (Refer to Functional Diagram) Frequency Selection and the Phase-Locked Loop (PLL) The selection of the switching frequency is a trade-off between efficiency, transient response and component size. High frequency operation reduces the size of the inductor and output capacitor as well as increasing the maximum practical control loop bandwidth. However, efficiency is generally lower due to increased transition and switching losses.
PAGE 16
LTC3861 Operation (Refer to Functional Diagram) reduced thermal stress on the inductors and MOSFETs due to current sharing between phases. These advantages allow for the use of a smaller size or a smaller number of components. Power Good Indicator Pins (PGOOD1, PGOOD2) Each PGOOD pin is connected to the open drain of an internal pull-down device which pulls the PGOOD pin low when the corresponding FB pin voltage is outside the PGOOD regulation window (±7.5% entering regulation, ±10% leaving regulation).
PAGE 17
LTC3861 Applications Information Output Voltage Programming and Differential Output Sensing reference, and thus, is a differential quantity. The minimum differential output voltage is limited to the internal reference, 0.6V, and the maximum differential output voltage is VCC – 0.5V. The LTC3861 integrates differential output sensing with output voltage programming, allowing for a simple and seamless design.
PAGE 18
LTC3861 Applications Information amplifier of the LTC3861 has a gain bandwidth of 40MHz, high enough not to affect main loop compensation and transient behavior. To avoid noise coupling into VSNSP, the resistor divider should be placed near the VSNSP and VSNSN pins and physically close to the LTC3861. The remote output and ground traces should be routed parallel to each other as a differential pair to the remote output.
PAGE 19
LTC3861 Applications Information The programmed current limit must be low enough to ensure that the inductor never saturates and high enough to allow increased current during transient conditions and allow margin for DCR variation. For example, if: ISAT = 2.2 • IOUT and IMAX = 1.6 • IOUT A reasonable ILIMIT would be: ILIMIT = 1.
PAGE 20
LTC3861 Applications Information drawn from the other out-of-phase controller will actually decrease the input RMS ripple current from this maximum value. The out-of-phase technique typically reduces the input capacitor’s RMS ripple current by a factor of 30% to 70% when compared to a single phase power supply solution. In continuous mode, the source current of the top N‑channel MOSFET is approximately a square wave of duty cycle VOUT/ VIN.
PAGE 21
LTC3861 Applications Information Current Sensing To maximize efficiency the LTC3861 is designed to sense current through the inductor’s DCR, as shown in Figure 6. The DCR of the inductor represents the small amount of DC winding resistance of the copper, which for most inductors applicable to this application, is between 0.3mΩ and 1mΩ.
PAGE 22
LTC3861 Applications Information Since the temperature coefficient of the inductor’s DCR is 3900ppm/°C, first order compensation of the filter time constant is possible by using filter resistors with an equal but opposite (negative) TC, assuming a low TC capacitor is used. That is, as the inductor’s DCR rises with increasing temperature, the L/DCR time constant drops.
PAGE 23
LTC3861 Applications Information VCC FB1 0, 120 CLKIN CLKOUT PHSMD FB1 FB2 LTC3861 ILIM2 COMP1 ILIM1 COMP2 IAVG TRACK/SS1,2 +240 VCC VCC 240, 60 CLKIN CLKOUT PHSMD TRACK/SS2 FB1 FB2 LTC3861 ILIM2 COMP1 ILIM1 COMP2 IAVG TRACK/SS1 0, 180 CLKIN CLKOUT PHSMD FB1 FB2 LTC3861 ILIM2 COMP1 ILIM1 COMP2 IAVG TRACK/SS1,2 FB1 FB2 +90 VCC 90, 270 CLKIN CLKOUT PHSMD FB1 FB2 LTC3861 ILIM2 COMP1 ILIM1 COMP2 IAVG TRACK/SS1,2 3961 F08b 3961 F08a Figure 8a. 3-Phase Operation VCC FB1 Figure 8b.
PAGE 24
LTC3861 Applications Information A multiphase power supply significantly reduces the amount of ripple current in both the input and output capacitors. The RMS input ripple current is divided by, and the effective ripple frequency is multiplied by, the number of phases used (assuming that the input voltage is greater than the number of phases used times the output voltage). The output ripple amplitude is also reduced by the number of phases used. Figure 9 graphically illustrates the principle.
PAGE 25
LTC3861 Applications Information Output Current Sharing When multiple LTC3861s are daisychained to drive a common load, accurate output current sharing is essential to achieve optimal performance and efficiency. Otherwise, if one stage is delivering more current than another, then the temperature between the two stages will be different, and that could translate into higher switch RDS(ON), lower efficiency, and higher RMS ripple.
PAGE 26
LTC3861 Applications Information For example, Figure 12a shows the start-up of VOUT2 controlled by the voltage on the TRACK/SS2 pin. Normally this pin is used to allow the start-up of VOUT2 to track that of VOUT1 as shown qualitatively in Figures 12a and 12b. When the voltage on the TRACK/SS2 pin is less than the internal 0.6V reference, the LTC3861 regulates the FB2 voltage to the TRACK/SS2 pin voltage instead of 0.6V.
PAGE 27
LTC3861 Applications Information Feedback Loop Compensation The LTC3861 is a voltage mode controller with a second dedicated current sharing loop to provide excellent phaseto-phase current sharing in multiphase applications. The current sharing loop is internally compensated.
PAGE 28
LTC3861 Applications Information The RC network across the error amplifier and the feedforward components R3 and C3 introduce two pole-zero pairs to obtain a phase boost at the system unity-gain frequency, fC. In theory, the zeros and poles are placed symmetrically around fC, and the spread between the zeros and the poles is adjusted to give the desired phase boost at fC.
PAGE 29
LTC3861 Applications Information Inductor The inductor in a typical LTC3861 circuit is chosen for a specific ripple current and saturation current. Given an input voltage range and an output voltage, the inductor value and operating frequency directly determine the ripple current. The inductor ripple current in the buck mode is: ΔIL = VOUT ⎛ VOUT ⎞ 1– ( f)(L) ⎜⎝ VIN ⎟⎠ Lower ripple current reduces core losses in the inductor, ESR losses in the output capacitors and output voltage ripple.
PAGE 30
LTC3861 Applications Information When the controller is operating in continuous mode the duty cycles for the top and bottom MOSFETs are given by: Main Switch Duty Cycle = VOUT VIN V –V Synchronous Switch Duty Cycle = IN OUT VIN The power dissipation for the main and synchronous MOSFETs at maximum output current are given by: VOUT 2 IMAX ) (1+ δ)RDS(ON) + ( VIN I VIN2 MAX (RDR )(CMILLER ) • 2 ⎡ 1 1 ⎤ + ⎢ ⎥ ( f) ⎢⎣ VCC – VTH(IL) VTH(IL) ⎥⎦ V −V PSYNC = IN OUT (IMAX )2(1+ δ)RDS(0N) VIN PMAIN = where δ is th
PAGE 31
LTC3861 Applications Information Other sources of loss include body or Schottky diode conduction during the driver dependent non-overlap time and inductor core losses. Design Example As a design example, consider a 2-phase application where VIN = 12V, VOUT = 1.2V, ILOAD = 60A and fSWITCH = 300kHz. Assume that a secondary 5V supply is available for the LTC3861 VCC supply. The inductance value is chosen based on a 25% ripple assumption. Each channel supplies an average 30A to the load resulting in 7.
PAGE 32
LTC3861 Applications Information minimum spacing between them. If DCR sensing is used, place the top resistor (Figure 6b, R1) close to the switching node. 4. The input capacitor should be kept as close as possible to the power MOSFETs. The loop from the input capacitor’s positive terminal, through the MOSFETs and back to the input capacitor’s negative terminal should also be as small as possible. 5.
PAGE 33
LTC3861 Typical Applications 1.5V/30A and 1.2V/30A Converter with Discrete Gate Drivers and MOSFETs, fSW = 300kHz VIN VCC VCC 5V 178Ω 10k 15k 4.75k 1µF 10k VOUT2 10k 10k 6.8nF 10k 100pF 348Ω 4.64k 4.7nF 100k D1 RUN1 3.3nF 100pF 4.7µF VCC SS1 VINSNS CONFIG IAVG PGOOD1 PWMEN1 PWM1 VOUT1 3.9nF VCC 0.1µF CIN1 180µF FB1 COMP1 VSNSP1 VSNSN1 VSNSOUT1 VSNSOUT2 VSNSN2 VSNSP2 COMP2 FB2 LTC3861 0.047µF M1 BSC050NE2LS ×2 M2 BSC010NE2LS ×2 L1 0.47µH 2.87k VOUT1 1.
PAGE 34
LTC3861 Typical Applications 4-Phase 1V/100A Converter with DrMOS, fSW = 500kHz SS1 VIN 7V TO 14V 280Ω 10k 10k 10.7k 100k LTC3861 RUN1 ILIM1 SGND ISNS1P ISNS1N ISNS2N ISNS2P SGND ILIM2 RUN2 SS2 FREQ CLKIN CLKOUT PHSMD PGOOD2 PWMEN2 PWM2 15k VCC SS1 CLKIN 500kHz EXTERNAL SYNC INPUT VCC RUN1 FB1 COMP1 VSNSP1 VSNSN1 VSNSOUT1 VSNSOUT2 VSNSN2 VSNSP2 COMP2 FB2 470pF CIN2 22µF × 2 10k 16V VCC 1µF 2.2nF 0.22µF VIN VCC SS1 VINSNS CONFIG IAVG PGOOD1 PWMEN1 PWM1 3.3nF 100pF 0.
PAGE 35
LTC3861 Typical Applications Dual-Output Converter: Triple Phase + Single Phase with DrMOS, Synchronized to an External 500kHz Clock SS1 VIN 7V TO 14V CIN1 180µF VCC 5V 3.3nF 280Ω 10k 10k 5.62k 100pF 0.1µF 3.
PAGE 36
LTC3861 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. UHE Package UHE Package 36-Lead Plastic QFN (5mm × 6mm) 36-Lead LTC Plastic QFN (5mm × 6mm) (Reference DWG # 05-08-1876 Rev Ø) (Reference LTC DWG # 05-08-1876 Rev Ø) 0.70 ± 0.05 5.50 ± 0.05 4.10 ± 0.05 3.50 REF PACKAGE OUTLINE 3.60 ± 0.05 4.60 ± 0.05 0.25 ± 0.05 0.50 BSC 4.50 REF 5.10 ± 0.05 6.50 ± 0.05 PIN 1 NOTCH R = 0.30 TYP OR 0.
PAGE 37
LTC3861 Revision History REV DATE DESCRIPTION A 06/13 Fixed schematics, graphs, tables, and clarified text PAGE NUMBER 1, 9, 11, 31, 32, 33, 34 3861fa 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.
PAGE 38
LTC3861 TYPICAL APPLICATION Dual Phase 1.2V/60A Converter with Discrete Gate Drivers and MOSFETs, fSW = 300kHz 10k 5.9k 1µF SS1 4.7nF 100pF 10k VCC 4.7µF 100k D1 RUN1 FB1 COMP1 VSNSP1 VSNSN1 VSNSOUT1 VSNSOUT2 VSNSN2 VSNSP2 COMP2 FB2 LTC3861 CIN2 22µF × 2 M1 BSC050NE2LS ×2 M2 BSC010NE2LS ×2 L1 0.47µH VOUT 1.2V/ 60A COUT1 COUT2 100µF × 4 330µF × 6 6.3V 2.5V 2.87k 59k RUN1 ILIM1 SGND ISNS1P ISNS1N ISNS2N ISNS2P SGND ILIM2 RUN2 0.22µF 0.