Datasheet

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LTC3860

3860fc

TYPICAL APPLICATION

DESCRIPTION

Dual, Multiphase Step-Down

Voltage Mode DC/DC Controller

with Current Sharing

The LTC

3860 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 ampliﬁ ers and

a remote output sense differential ampliﬁ er 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 con-

stant operating frequency can be synchronized to an exter-

nal clock or linearly programmed from 250kHz to 1.25MHz.

Up to six LTC3860 controllers can operate in parallel for

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

The LTC3860 is available in a 32-pin 5mm × 5mm QFN

package.

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

Differential Remote Output Voltage Sense Ampliﬁ er

Multiphase Capability—Up to 12-Phase Operation

Programmable Current Limit

Safely Powers a Prebiased Load

Programmable or PLL-Synchronizable Switching

Frequency Up to 1.25MHz

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 32- Pin 5mm × 5mm QFN Package

High Current Distributed Power Systems

Digital Signal Processor and ASIC Supplies

Telecom Systems

Industrial Power Supplies

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

trademarks and No R

SENSE

is a trademark of Linear Technology Corporation. All other

trademarks are the property of their respective owners. Protected by U.S. Patents, including

6144194, 5055767

VINSNS

FREQ

FB2

LIM2

FB1

COMP1,2

SS1,2

LOGIC

BOOST

GND

RUN1,2

LIM1

ISNS1P

ISNS1N

ISNS2N

ISNS2P

LTC3860

PWM1

LTC4449

SW1

0.3μH

SGND CLKIN

VSNSOUT

VSNSN

VSNSP

PWM2

PWM1

470μF

1μF

0.22μF

TG1

BG1

0.22μF

100pF

1nF

0.1μF

220pF

12.7k

220Ω

50k

2.32k

20k

33pF

OUT

LOGIC

BOOST

GND

LTC4449

SW2

0.3μH

0.22μF

TG2

BG2

3860 TA01

330μF

OUT

1.2V

50A

AVG

Summary of content (36 pages)

PAGE 1
LTC3860 Dual, Multiphase Step-Down Voltage Mode DC/DC Controller with 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
LTC3860 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION PWM1 PWMEN1 PGOOD1 IAVG SGND SGND TOP VIEW VINSNS VCC Voltage .................................................. –0.3V to 6V VINSNS Voltage ......................................... –0.3V to 30V VSNSN Voltage ............................................ –0.3V to 2V RUN Voltage ................................................ –0.3V to 6V ISNS1P , ISNS1N, ISNS2P , ISNS2N ...........................–0.3V to (VCC + 0.1V) All Other Voltages .............
PAGE 3
LTC3860 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating junction temperature range, otherwise speciﬁcations are at TA = 25°C (Note 3). VCC = 5V, VRUN1,2 = 5V, VFREQ = VCLKIN = 0V, VFB = 0.6V, fOSC = 0.6MHz unless otherwise speciﬁed.
PAGE 4
LTC3860 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating junction temperature range, otherwise speciﬁcations are at TA = 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 TYP MAX 21 23 UNITS IFREQ FREQ Pin Output Current VFREQ = 0.
PAGE 5
LTC3860 TYPICAL PERFORMANCE CHARACTERISTICS Load Step Transient Response (2-Phase) Load Step Transient Response (Single Phase) ILOAD 10A/DIV ILOAD 10A/DIV IL 10A/DIV IL1 5A/DIV IL2 5A/DIV VOUT 50mV/DIV VOUT 50mV/DIV 3860 G01 Load Step Transient Response (2-Phase with 50% Inductor Mismatch) Load Step Transient Response (2-Phase) VOUT(AC) 100mV/DIV ILOAD 10A/DIV ILOAD 20A/DIV IL1 5A/DIV IL1 = 320nH 10A/DIV IL2 5A/DIV IL2 = 220nH 10A/DIV VOUT 50mV/DIV Efﬁciency and Power Loss vs Supply Voltage
PAGE 6
LTC3860 TYPICAL PERFORMANCE CHARACTERISTICS Short-Circuit Protection Regulated VFB vs Supply Voltage Oscillator Frequency vs RFREQ 1.7 0.604 OSCILLATOR FREQUENCY (MHz) SW NODE 5V/DIV VOUT 1V/DIV 3860 G07 VIN = 12V 10ms/DIV VOUT = 1.2V ILOAD = SHORTED REGULATED VFB (V) 0.602 0.600 0.598 1.1 0.9 0.7 0.5 0.1 3 6 4 5 SUPPLY VOLTAGE (V) Oscillator Frequency vs Temperature 0 20 40 3860 G08 80 60 RFREQ (kΩ) 100 120 3860 G09 Oscillator Frequency vs Supply Voltage 610 1.300 605 1.
PAGE 7
LTC3860 TYPICAL PERFORMANCE CHARACTERISTICS Soft-Start Start-Up Ratiometric Tracking Start-Up TRACK/SS1 500mV/DIV VOUT1 200mV/DIV VOUT1 500mV/DIV VIN = 12V 5ms/DIV VOUT1 = 1.2V 0.
PAGE 8
LTC3860 PIN FUNCTIONS VCC (Pin 1): Chip Supply Voltage. Bypass this pin to GND with a capacitor (0.1μF to 1μF ceramic) in close proximity to the chip. ISNS1N (Pin 21), ISNS2N (Pin 20): Current Sense Ampliﬁer (–) Input. The (–) input to the current ampliﬁer is normally connected to the respective VOUT. FB1 (Pin 2), FB2 (Pin 8): Error Ampliﬁer Inverting Inputs. FB1 or FB2 can be connected to VSNSOUT via a resistor divider for remote VOUT sensing.
PAGE 9
LTC3860 PIN FUNCTIONS A change in VIN immediately modulates the input to the PWM comparator and changes the pulse width in an inversely proportional manner, thus bypassing the feedback loop and providing excellent transient line regulation. An external lowpass ﬁlter can be added to this pin to prevent noisy signals from affecting the loop gain. TRACK/SS1 (Pin 32), TRACK/SS2 (Pin 9): Soft-Start. The voltage ramp rate at these pins sets the voltage ramp rate of the outputs.
PAGE 10
LTC3860 OPERATION (Refer to Functional Diagram) Main Control Architecture The LTC3860 is a dual-channel/dual-phase, constant frequency, 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 external drivers and N-channel MOSFETs using single wire three-state PWM interfaces.
PAGE 11
LTC3860 OPERATION (Refer to Functional Diagram) Excellent Transient Response Shutdown Control Using the RUN Pins The LTC3860 error ampliﬁers are true operational ampliﬁers, meaning that they have high bandwidth, high DC gain, low offset and low output impedance. Their bandwidth, when combined with high switching frequencies and lowvalue inductors, allows the compensation network to be optimized for very high control loop crossover frequencies and excellent transient response.
PAGE 12
LTC3860 OPERATION (Refer to Functional Diagram) Certain applications can result in the start-up of the converter into a non-zero load voltage, where residual charge is stored on the output capacitor at the onset of converter switching. In order to prevent the output from discharging under these conditions, the bottom MOSFET is disabled until soft-start is complete. However, the bottom MOSFET will be turned on for 20ns every 8 cycles to allow the driver IC to recharge its topside gate drive capacitor.
PAGE 13
LTC3860 OPERATION (Refer to Functional Diagram) relationship between channel 1 and CLKOUT, as summarized in Table 2. The phases are calculated relative to zero degrees, deﬁned as the rising edge of PWM1. Refer to Applications Information for more details on how to create multiphase applications. Table 2.
PAGE 14
LTC3860 APPLICATIONS INFORMATION Setting the Output Voltage Programming the Operating Frequency The LTC3860 regulates the FB pins to 0.6V. FB is connected to VOUT or VSNSOUT (for remote output sensing) via an external resistive divider as shown in Figure 3. The divider sets the output voltage according to the following equation: The LTC3860 can be hard wired to one of two ﬁxed frequencies, linearly programmed to any frequency between 250kHz and 1.25MHz or synchronized to an external clock.
PAGE 15
LTC3860 APPLICATIONS INFORMATION Choosing the Inductor and Setting the Current Limit The inductor value is related to the switching frequency, which is chosen based on the trade-offs discussed in the Operation section. The inductor can be sized using the following equation: ⎛ V ⎞ L = ⎜ OUT ⎟ ⎝ f • ΔIL ⎠ ⎛ V ⎞ • ⎜1− OUT ⎟ VIN ⎠ ⎝ Choosing a larger value of ΔIL leads to smaller L, but results in greater core loss (and higher output voltage ripple for a given output capacitance and/or ESR).
PAGE 16
LTC3860 APPLICATIONS INFORMATION supply can make up the difference. Generally, a capacitor (particularly a non-ceramic type) that meets the ﬁrst two parameters will have far more capacitance than is required to keep capacitance-based droop under control. The input capacitor’s voltage rating should be at least 1.4 times the maximum input voltage. Power loss due to ESR occurs not only as I2R dissipation in the capacitor itself, but also in overall battery efﬁciency.
PAGE 17
LTC3860 APPLICATIONS INFORMATION Manufacturers such as Sanyo, Panasonic and Cornell Dubilier should be considered for high performance throughhole capacitors. The OS-CON semiconductor electrolyte capacitor available from Sanyo has a good (ESR)(size) product. An additional ceramic capacitor in parallel with OS-CON capacitors is recommended to offset the effect of lead inductance.
PAGE 18
LTC3860 APPLICATIONS INFORMATION VIN 12V VINSNS 5V VCC SENSE RESISTOR PLUS PARASITIC INDUCTANCE VLOGIC BOOST VCC TG LTC4449 IN TS LTC3860 PWM L RS VOUT BG GND ISNSN ISNSP CF ESL CF • 2RF ≤ ESL/RS POLE-ZERO CANCELLATION GND RF RF 3860 F06a FILTER COMPONENTS PLACED NEAR SENSE PINS (6a) Using a Resistor to Sense Current VIN 12V VINSNS 5V LTC3860 VCC PWM VLOGIC BOOST VCC TG LTC4449 TS IN INDUCTOR L DCR VOUT BG GND ISNSN ISNSP GND R1* C1* 3860 F06b R1 • C1 = L *PLACE R1 NEAR INDUCTOR DC
PAGE 19
LTC3860 APPLICATIONS INFORMATION VSNSOUT2 VSNSOUT1 VCC VCC 0, 120 +240 CLKIN CLKOUT PHSMD FB1 FB2 LTC3860 ILIM2 COMP1 ILIM1 COMP2 IAVG TRACK/SS1,2 VSNSOUT1 240, 60 VCC VCC 0, 180 CLKIN CLKOUT PHSMD FB1 FB2 LTC3860 ILIM2 COMP1 ILIM1 COMP2 IAVG TRACK/SS1,2 CLKIN CLKOUT PHSMD TRACK/SS2 FB1 FB2 LTC3860 ILIM2 COMP1 ILIM1 COMP2 IAVG TRACK/SS1 +90 90, 270 CLKIN CLKOUT PHSMD FB1 FB2 LTC3860 ILIM2 COMP1 ILIM1 COMP2 IAVG TRACK/SS1,2 3960 F07b 3960 F07a Figure 7a. 3-Phase Operation Figure 7b.
PAGE 20
LTC3860 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 8 graphically illustrates the principle.
PAGE 21
LTC3860 APPLICATIONS INFORMATION Dual-Channel Operation The LTC3860 can control two independent power supply outputs with no channel-to-channel interaction or jitter. The following recommendations will ensure maximum performance in this mode of operation: n The output of channel 1 should be sensed using the remote sense differential ampliﬁer. The SGND pins and exposed pad and all local small-signal GND should then be a Kelvin connection to the negative terminal of the channel 2 output.
PAGE 22
LTC3860 APPLICATIONS INFORMATION Feedback Loop Compensation the TRACK/SS2 pin voltage instead of 0.6V. The start-up of VOUT2 may ratiometrically track that of VOUT1, according to a ratio set by a resistor divider (Figure 11c): The LTC3860 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 23
LTC3860 APPLICATIONS INFORMATION In a typical LTC3860 circuit, the feedback loop consists of the line feedforward circuit, the modulator, the external inductor, the output capacitor and the feedback ampliﬁer with its compensation network. All these components affect loop behavior and need to be accounted for in the loop compensation. The modulator consists of the PWM generator, the output MOSFET drivers and the external MOSFETs themselves. The modulator gain varies linearily with the input voltage.
PAGE 24
LTC3860 APPLICATIONS INFORMATION Once the value of resistor R1, poles and zeros location have been decided, the value of R2, C1, C2, R3 and C3 can be obtained from the previous equations. Compensating a switching power supply feedback loop is a complex task. The applications shown in this data sheet show typical values, optimized for the power components shown. Though similar power components should sufﬁce, substantially changing even one major power component may degrade performance signiﬁcantly.
PAGE 25
LTC3860 APPLICATIONS INFORMATION The curve is generated by forcing a constant input current into the gate of a common source, current source loaded stage and then plotting the gate voltage versus time. The initial slope is the effect of the gate-to-source and the gate-to-drain capacitance. The ﬂat portion of the curve is the result of the Miller multiplication effect of the drain-to-gate capacitance as the drain drops the voltage across the current source load.
PAGE 26
LTC3860 APPLICATIONS INFORMATION Efﬁciency Considerations Design Example The efﬁciency of a switching regulator is equal to the output power divided by the input power. It is often useful to analyze individual losses to determine what is limiting the efﬁciency and which change would produce the most improvement. Percent efﬁciency can be expressed as: As a design example, consider a 2-phase application where VIN = 12V, VOUT = 1.2V, ILOAD = 50A and fSWITCH = 600kHz.
PAGE 27
LTC3860 APPLICATIONS INFORMATION To set the output voltage equal to 1.2V: RB = 10k The Renesas R2J20602NP integrated-driver MOSFET is chosen for the power stages because of its high efﬁciency and high level of integration. Printed Circuit Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the converter. 1.
PAGE 28
VDIFF1 RUN2 CLKOUT VOUT2 20k 4.7k 4.7k SGND PWMEN1,2 FB1 COMP1 VSNSOUT VSNSN VSNSP COMP2 FB2 20k 20k VOUT2 0.1μF 45.3k FREQ SET FOR 600kHz 470pF 47pF VDIFF1 VOS1N VOS1P 1μF 470μF 100k VCC LTC3860 ILIM1 ISNS1P ISNS1N ISNS2N ISNS2P ILIM2 RUN2 PWM1 PWM2 100k VCC 7V TO 14V IN AND 1.2V/1.
PAGE 29
LTC3860 TYPICAL APPLICATIONS Quad-Phase Single Output with DRMOS 2.2Ω VIN 7V TO 14V 470μF TRACK/SS1 VCC I AVG1 0.01μF 100k 1μF COMP1 VDIFF 220Ω 47pF 20k 4.7k 470pF VDIFF VOSN VOSP VCC LTC3860 VLDRV VSWH 0.4μH SW1 2k PGND 4.7μF 5V 0.22μF 0.22μF VCC RUN1 5V 4.7μF VCC TRACK/SS1 VCC CGND 49.
PAGE 30
VDIFF1 20k RUN2 SGND PWMEN1,2 FB1 COMP1 VSNSOUT VSNSN VSNSP COMP2 FB2 45.3k SS1 FREQ SET FOR 600kHz VCC VDIFF1 VOS1N VOS1P 1μF 0.1μF TRACK/SS1 100k VCC LTC3860 ILIM1 ISNS1P ISNS1N ISNS2N ISNS2P ILIM2 RUN2 PWM1 100pF PWM2 7V TO 14V IN AND 1.2V OUT AT 50A fSW = 600kHz, DCR SENSING NOTE: PLEASE REFER TO THE PIN CONFIGURATION OF THIS DATA SHEET FOR THE LTC3860 PINOUT 220pF 12.7k 20k 1.33k CLKOUT 33pF 1.
PAGE 31
LTC3860 TYPICAL APPLICATIONS Dual Output—3-Channel + Single Channel, Synchronized to External Clock 2.2Ω VIN 7V TO 14V 470μF VCC TRACK/SS1 0.01μF IAVG1 100k COMP1 VDIFF1 20k 220Ω 20k 4.7k 47pF 470pF VDIFF VOSN VOSP VCC LTC3860 VLDRV 0.3μH SW1 2.3k PGND 4.7μF 5V 0.22μF VCC RUN1 5V 4.7μF VCC OPEN (OPT) PWM2 VCC CGND VSWH 0.22μF TRACK/SS1 CLOCKIN 600kHz SYNC INPUT REG5V BOOT 49.
PAGE 32
13.3k 20k CLOCKIN 400kHz SYNC INPUT VDIFF 2200pF VCC 5V RUN1 6.8k 470pF 523Ω 100pF COMP1 1μF 470μF TRACK/SS1 VCC VDIFF VOSN VOSP VCC FB1 COMP1 VSNSOUT VSNSN VSNSP COMP2 FB2 0.1μF TRACK/SS1 100k VCC IAVG1 LTC3860 34.8k 100pF RUN1 ILIM1 ISNS1P ISNS1N ISNS2N ISNS2P ILIM2 RUN2 TRACK/SS1 VINSNS SGND SGND IAVG PGOOD1 PWMEN1 PWM1 32 10k TRACK/SS2 FREQ CLKIN CLKOUT PHSMD PGOOD2 PWMEN2 PWM2 VIN 7V TO 14V VCC RUN1 59k 0.22μF 0.
PAGE 33
RUN2 VOUT2 VDIFF1 6.8k 523Ω 6.8k VDIFF1 VOS1N VOS1P 470pF 100pF 470pF 523Ω 100pF 1μF 0.1μF 0.1μF SGND PWMEN1,2 FB1 COMP1 VSNSOUT VSNSN VSNSP COMP2 FB2 8V TO 14V IN AND 1.0V/1.8V OUT AT 25A fSW = 400kHz CH1 USES DIFFAMP 10k 20k 2200pF 29.4k 20k 2200pF RUN1 VCC 5V 470μF 100k VCC LTC3860 VCC TRACK/SS1 VINSNS IAVG PGOOD1 PWM1 RUN1 34.
PAGE 34
LTC3860 PACKAGE DESCRIPTION UH Package 32-Lead Plastic QFN (5mm × 5mm) (Reference LTC DWG # 05-08-1693 Rev D) 0.70 p0.05 5.50 p0.05 4.10 p0.05 3.45 p 0.05 3.50 REF (4 SIDES) 3.45 p 0.05 PACKAGE OUTLINE 0.25 p 0.05 0.50 BSC RECOMMENDED SOLDER PAD LAYOUT APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 5.00 p 0.10 (4 SIDES) BOTTOM VIEW—EXPOSED PAD 0.75 p 0.05 R = 0.05 TYP 0.00 – 0.05 PIN 1 NOTCH R = 0.30 TYP OR 0.35 s 45o CHAMFER R = 0.115 TYP 31 32 0.40 p 0.10 PIN 1 TOP MARK (NOTE 6) 1 2 3.
PAGE 35
LTC3860 REVISION HISTORY REV DATE DESCRIPTION A 04/10 Updates to Features PAGE NUMBER Updates to Absolute Maximum Ratings, Order Information sections, Electrical Characteristics heading and Note 2 Update to Design Example section Updates to Typical Applications B 10/10 Updated Electrical Characteristics section 27 30, 31, 33, 35, 38 2, 3, 4 8 Updated Frequency Selection and the Phase Locked Loop (PLL) section 12 Updated Theory and Beneﬁts of Multiphase Operation section 13 15, 27 Updated Typi
PAGE 36
LTC3860 TYPICAL APPLICATION Dual Phase Single Output with DRMOS and RSENSE 2.2Ω VIN 7V TO 14V VCC TRACK/SS1 0.01μF IAVG1 100k COMP1 20k 220Ω 20k 4.7k 47pF 470pF VDIFF VOSN VOSP VCC LTC3860 RUN1 ILIM1 ISNS1P ISNS1N ISNS2N ISNS2P ILIM2 RUN2 TRACK/SS2 FREQ CLKIN CLKOUT PHSMD PGOOD2 PWMEN2 PWM2 1000pF VDIFF1 VCC FB1 COMP1 VSNSOUT VSNSN VSNSP COMP2 FB2 REG5V BOOT CGND VLDRV VSWH SW1 0.3μH 1mΩ PGND 4.7μF 59.0k 100Ω 5V 0.01μF 0.01μF VCC RUN1 5V 4.