Datasheet

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LTC3856

3856f

LOAD CURRENT (A)

0.1

EFFICIENCY (%)

POWER LOSS (W)

1 10 100

100

–1

100

3856 TA01b

= 12V

OUT

= 1.5V

Burst Mode

OPERATION

Typical applicaTion

DescripTion

2-Phase Synchronous

Step-Down DC/DC

Controller with Diffamp

The LTC

3856 is a single output, dual channel PolyPhase

synchronous step-down DC/DC controller that drives all

N-channel power MOSFET stages. This device includes a

high speed differential ampliﬁer for remote output volt-

age sense. Power loss and supply noise are minimized by

operating the two controller output stages out-of-phase

and up to 12-phase operation can be achieved.

The LTC3856 monitors the output current by sensing

the voltage drop across the output inductor (DCR) or by

using a sense resistor. DCR temperature compensation

maintains an accurate current sense threshold over a

broad temperature range. A constant-frequency, current

mode architecture allows a phase-lockable frequency of

up to 770kHz.

A wide 4.5V to 38V input supply range encompasses

most intermediate bus voltages and battery chemistries.

Burst Mode operation, continuous or Stage Shedding

modes are supported. A TK/SS pin shared by both chan-

nels ramps the output voltage during start-up.

FeaTures

applicaTions

PolyPhase

Controller Reduces Input and Output

Capacitance and Power Supply Induced Noise

Wide V

Range: 4.5V to 38V Operation

±0.75%, 0.6V Reference Voltage Accuracy

High Efﬁciency: Up to 95%

Programmable Burst Mode

Operation or

Stage Shedding™ for Highest Light Load Efﬁciency

Active Voltage Positioning (AVP)

SENSE

or DCR Current Sensing

Programmable DCR Temperature Compensation

Phase-Lockable Fixed Frequency: 250kHz to 770kHz

True Remote Sense Differential Ampliﬁer

Dual N-Channel MOSFET Synchronous Drive

OUT

Range: 0.6V to 5V without Differential Ampliﬁer

OUT

Range: 0.6V to 3.3V with Differential Ampliﬁer

Adjustable Soft-Start or V

OUT

Tracking

Stackable for Up to 12-Phase Operation

32-Pin (5mm × 5mm) QFN and 38-Pin TSSOP Packages

Telecom and Datacom Systems

Industrial and Medical Instruments

DC Power Distribution Systems

Computer Systems

L, LT, LTC, LTM, Linear Technology, the Linear logo, PolyPhase, Burst Mode and OPTI-LOOP

are registered trademarks and Stage Shedding is a trademark of Linear Technology Corporation.

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

including 5481178, 5705919, 5929620, 6100678, 6144194, 6177787, 6304066, 6498466,

6580258, 6611131, 6674274.

Efﬁciency and Power Loss

vs Output Current

High Efﬁciency 1.5V/50A Step-Down Converter

3856 TA01a

TG1

BOOST1

SW1

BG1

PLLIN

SENSE1

–

TG2

BOOST2

SW2

BG2

INTV

SENSE2

–

DIFFOUT

ILIM

RUN

PGOOD

CLKOUT

PHASMD

MODE

ITEMP

FREQ

EXTV

DIFFN

DIFFP

ISETAVPGNDTK/SS

LTC3856

30.1k

20k

122k

10µF

s4

100µF

s8

OUT

1.5V

50A

0.33µH

4.5V TO

20V

0.1µF

4.7µF

0.1µF

2200pF

1.5k

0.1µF

Summary of content (40 pages)

PAGE 1
LTC3856 2-Phase Synchronous Step-Down DC/DC Controller with Diffamp Description Features PolyPhase® Controller Reduces Input and Output Capacitance and Power Supply Induced Noise Wide VIN Range: 4.5V to 38V Operation ±0.75%, 0.
PAGE 2
LTC3856 Absolute Maximum Ratings (Note 1) Input Supply Voltage (VIN).......................... 40V to –0.3V Topside Driver Voltages (BOOSTn)............. 46V to –0.3V Switch Voltage (SWn).................................... 40V to –5V INTVCC, RUN, PGOOD, EXTVCC, (BOOSTn – SWn).......................................... 6V to –0.3V SENSEn Voltages....................................... 5.5V to –0.3V MODE, PLLIN, ILIM, TK/SS, AVP, FREQ, ISET Voltages.............................. INTVCC to –0.
PAGE 3
LTC3856 Order Information LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LTC3856EFE#PBF LTC3856EFE#TRPBF LTC3856FE 38-Lead Plastic TSSOP –40°C to 125°C LTC3856IFE#PBF LTC3856IFE#TRPBF LTC3856FE 38-Lead Plastic TSSOP –40°C to 125°C LTC3856EUH#PBF LTC3856EUH#TRPBF 3856 32-Lead (5mm × 5mm) Plastic QFN –40°C to 125°C LTC3856IUH#PBF LTC3856IUH#TRPBF 3856 32-Lead (5mm × 5mm) Plastic QFN –40°C to 125°C Consult LTC Marketing for parts specified with wider
PAGE 4
LTC3856 Electrical Characteristics The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 15V, VRUN = 5V, unless otherwise noted.
PAGE 5
LTC3856 Electrical Characteristics The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 15V, VRUN = 5V, unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 0.998 0.997 1 1 1.002 1.003 V/V V/V Differential Amplifier ADA Gain E-Grade I-Grade RIN Input Resistance Measured at DIFFP Input VOS Input Offset Voltage VDIFFP = VDIFFOUT = 1.
PAGE 6
LTC3856 Typical Performance Characteristics Load Step: Burst Mode Operation Load Step: Forced Continuous Mode VBAT = 3.6V ILOAD ICPO = 200µA 40A/DIV CCPO = 2.2ΩF VBAT = 3.6V ILOAD ICPO = 200µA 40A/DIV CCPO = 2.2ΩF IL1 20A/DIV IL1 20A/DIV IL2 20A/DIV IL2 20A/DIV VOUT 200mV/DIV VOUT 200mV/DIV 3856 G01 100µs/DIV VIN = 12V VOUT = 1.5V ILOAD = 1A TO 40A VIN = 12V VOUT = 1.
PAGE 7
LTC3856 Typical Performance Characteristics Load Step without AVP Load Step with AVP 4.5 4.3 VOUT 50mV/DIV 54mV 50A IL 20A/DIV 50A IL 20A/DIV 25A 3856 G07 100µs/DIV VIN = 12V VOUT = 1.5V VIN = 12V VOUT = 1.5V 4.50 VSENSE (mV) 4.25 4.00 20 ILIM = GND 0 3.75 3.50 –20 3.25 20 25 INPUT VOLTAGE (V) 30 35 –40 40 ILIM = INTVCC 70 60 ILIM = FLOAT 40 ILIM = GND 30 20 10 0 3.1 2.
PAGE 8
LTC3856 Typical Performance Characteristics Shutdown (RUN) Threshold vs Temperature 1.25 1.15 OFF 1.10 –50 –25 0 25 50 75 TEMPERATURE (°C) 700 0.603 0.602 0.601 0.600 –25 0 25 50 75 100 TEMPERATURE (°C) FREQUENCY (kHz) 3.
PAGE 9
LTC3856 Pin Functions (TSSOP/QFN) FREQ (Pin 1/Pin 29): Frequency Setting Pin. A resistor to ground sets the operating frequency of the controller. This pin can also be driven with a DC voltage to vary the frequency of the internal oscillator. RUN (Pin 2/Pin 30): Run Control Input. A voltage above 1.22V on this pin turns on the IC. There is a 1µA pull-up current for this pin. Once the RUN pin rises above 1.22V, an additional 4.5µA pull-up current is added to the pin.
PAGE 10
LTC3856 Pin Functions (TSSOP/QFN) INTVCC (Pin 29/Pin 21): Internal 5V Regulator Output. The control circuits are powered from this voltage. Decouple this pin to PGND with a minimum of 4.7µF low ESR tantalum or ceramic capacitor. the sources of the bottom N-channel MOSFETs, the (–) terminal of CVCC and the (–) terminal of CIN. All small-signal components and compensation components should also connect to this ground. VIN (Pin 30/Pin 22): Main Input Supply. Decouple this pin to PGND with a capacitor (0.
PAGE 11
LTC3856 FUNCTIONAL Diagram EXTVCC ITEMP MODE PLLIN PHASMD 4.7V FREQ + – TEMPSNS 0.6V MODE/SYNC DETECT VIN F + 5V REG + – INTVCC BURSTEN CLKOUT FCNT OSC S + ICMP + – IREV ISET CB SENSE– L1 VOUT + BG CVCC PGND ILIM RAVP PGOOD SLOPE COMPENSATION DIFFOUT + INTVCC 1 51k ITHB UVLO UV + 0.54V VFB SLEEP – – + – – + + 0.5V RUN 1.25µA + EA SS 40k DIFFN 0.66V SGND RPRE-AVP AVP 1.22V + – 0.
PAGE 12
LTC3856 Operation (Refer to Functional Diagram) Main Control Loop The LTC3856 uses a constant-frequency, current mode step-down architecture. During normal operation, each top MOSFET is turned on each cycle when the oscillator sets the RS latch, and turned off when the main current comparator, ICMP , resets each 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 the error amplifier, EA.
PAGE 13
LTC3856 Operation (Refer to Functional Diagram) Light Load Current Operation (Burst Mode Operation, Stage Shedding or Continuous Conduction) The LTC3856 can be enabled to enter high efficiency Burst Mode operation, Stage Shedding mode or forced continuous conduction mode. To select forced continuous operation, tie the MODE pin to a DC voltage below 0.6V (e.g., SGND). To select Stage Shedding mode of operation, tie the MODE pin to INTVCC. To select Burst Mode operation, float the MODE pin.
PAGE 14
LTC3856 Operation (Refer to Functional Diagram) operating frequency from 250kHz to 770kHz. There is a precision 10µA current flowing out of the FREQ pin enabling the user to program the controller’s switching frequency with a single resistor to SGND. A curve is provided later in the Applications Information section showing the relationship between the voltage on the FREQ pin and switching frequency.
PAGE 15
LTC3856 Applications Information The Typical Application on the first page of this data sheet is a basic LTC3856 application circuit. LTC3856 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.
PAGE 16
LTC3856 Applications Information Low Value Resistors Current Sensing A typical sensing circuit using a discrete resistor is shown in Figure 2a. RSENSE is chosen based on the required output current. The current comparator has a maximum threshold, VSENSE(MAX), determined by the ILIM setting. The input common mode range of the current comparator is 0V to 5V.
PAGE 17
LTC3856 Applications Information sheet for information about parasitic inductance. In the absence of data, measure the voltage drop directly across the sense resistor to extract the magnitude of the ESL step and use Equation 1 to determine the ESL. However, do not overfilter. Keep the RC time constant, less than or equal to the inductor time constant to maintain a high enough ripple voltage of ∆VSENSE.
PAGE 18
LTC3856 Applications Information The LTC3856 also features a DCR temperature compensation circuit by using a NTC temperature sensor. See the Inductor DCR Sensing Temperature Compensation and the ITEMP Pin section for details. The maximum power loss in R1 is related to duty cycle, and will occur in continuous mode at the maximum input voltage: PLOSS R1= (V IN(MAX ) − VOUT R1 )•V OUT Ensure that R1 has a power rating higher than this value.
PAGE 19
LTC3856 Applications Information NTC Compensated DCR Sensing For DCR sensing applications where a more accurate current limit is required, a network consisting of an NTC thermistor placed from the ITEMP pin to ground will provide correction of the current limit over temperature. Figure 2b shows this network. Resistors RS and RP will linearize the impedance the ITEMP pin sees.
PAGE 20
LTC3856 Applications Information After determining the components for the temperature compensation network, check the results by plotting IMAX versus inductor temperature using the following equations: Typical values for the NTC compensation network are: • NTC RO = 100k, B-constant = 3000 to 4000 • RS ≈ 20k • RP ≈ 50k IMAX = VSENSEMAX( ADJ) − ∆ VSENSE 2  0.4  DCRMAX AT 25 ° C • 1 + (TL(MAX ) − 25 °C) • 100   where VSENSEMAX( ADJ) = VSENSE(MAX ) • 1. 8 V − VITMP −A 1.
PAGE 21
LTC3856 Applications Information Inductor Value Calculation and Output Ripple Current The operating frequency and inductor selection are interrelated in that higher operating frequencies allow the use of smaller inductor and capacitor values. A higher frequency generally results in lower efficiency because of MOSFET gate charge and transition losses. In addition to this basic trade-off, the effect of inductor value on ripple current and low current operation must also be considered.
PAGE 22
LTC3856 Applications Information The peak-to-peak MOSFET gate drive levels are set by the voltage, VCC, requiring the use of logic-level threshold MOSFETs in most applications. Pay close attention to the BVDSS specification for the MOSFETs as well; many of the logic-level MOSFETs are limited to 30V or less. Selection criteria for the power MOSFETs include the on-resistance, RDS(ON), input capacitance, input voltage and maximum output current.
PAGE 23
LTC3856 Applications Information The term (1 + δ ) is generally given for a MOSFET in the form of a normalized RDS(ON) vs temperature curve, but δ = 0.005/°C can be used as an approximation for low voltage MOSFETs. The optional Schottky diodes conduct during the dead time between the conduction of the two large power MOSFETs.
PAGE 24
LTC3856 Applications Information reduced by the reduction of the input ripple current in a PolyPhase system. The required amount of input capacitance is further reduced by the factor N, due to the effective increase in the frequency of the current pulses. Ceramic capacitors are becoming very popular for small designs but several cautions should be observed.
PAGE 25
LTC3856 Applications Information error amplifier. The amplifier has an output swing range of 0V to 3.6V. The output uses an NPN emitter follower with 80k feedback resistance. Active Voltage Positioning (AVP) In an application, the AVP scheme modifies the regulated output voltage depending on its current loading. AVP can improve overall transient response and save power consumption.
PAGE 26
LTC3856 Applications Information Soft-Start and Tracking The LTC3856 has the ability to either soft-start by itself with a capacitor or track the output of another external supply. When the controller is configured to soft-start by itself, a capacitor should be connected to its TK/SS pin. The controller is in the shutdown state if its RUN pin voltage is below 1.22V and its TK/SS pin is actively pulled to ground in this shutdown state. Once the RUN pin voltage is above 1.22V, the controller powers up.
PAGE 27
LTC3856 Applications Information VOUT1 OUTPUT VOLTAGE OUTPUT VOLTAGE VOUT1 VOUT2 TIME VOUT2 TIME 3856 F11a 3856 F11b (11b) Ratiometric Tracking (11a) Coincident Tracking Figure 11. Two Different Modes of Output Voltage Tracking VOUT1 VOUT2 TO TK/SS2 PIN R3 R4 R1 R2 TO VFB1 PIN TO VFB2 PIN R3 R4 VOUT1 TO TK/SS2 PIN VOUT2 R1 R2 TO VFB1 PIN TO VFB2 PIN R3 R4 3856 F12 (12a) Coincident Tracking Set-Up (12b) Ratiometric Tracking Set-Up Figure 12.
PAGE 28
LTC3856 Applications Information short circuit). If more current is required through the EXTVCC than is specified, an external Schottky diode can be added between the EXTVCC and INTVCC pins. Do not apply more than 6V to the EXTVCC pin and make sure that EXTVCC < VIN. Significant efficiency and thermal gains can be realized by powering INTVCC from the output, since the VIN current resulting from the driver and control currents will be scaled by a factor of (duty cycle)/(switcher efficiency).
PAGE 29
LTC3856 Applications Information Setting Output Voltage Fault Conditions: Current Limit and Current Foldback If the DIFFAMP is not used, the LTC3856 output voltage is set by an external feedback resistive divider carefully placed across the output, as shown in Figure 14. The regulated output voltage is determined by: The LTC3856 includes current foldback to help limit load current when the output is shorted to ground.
PAGE 30
LTC3856 Applications Information Phase-Locked Loop and Frequency Synchronization The LTC3856 has a phase-locked loop (PLL) comprised of an internal voltage-controlled oscillator (VCO) and a phase detector. 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 pin. The turn-on of the second phase’s top MOSFETs is thus 180° out-of-phase with the external clock, and so on.
PAGE 31
LTC3856 Applications Information less than fOSC, current is sunk continuously, pulling down the filter network. If the external and internal frequencies are the same but exhibit a phase difference, the current sources turn on for an amount of time corresponding to the phase difference. The voltage on the filter network is adjusted until the phase and frequency of the internal and external oscillators are identical.
PAGE 32
LTC3856 Applications Information 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 and RSENSE to obtain I2R losses. For example, if each RDS(ON) = 10mΩ, RL = 10mΩ, RSENSE = 5mΩ, then the total resistance is 25mΩ. This results in losses ranging from 2% to 8% as the output current increases from 3A to 15A for a 5V output, or a 3% to 12% loss for a 3.3V output.
PAGE 33
LTC3856 Applications Information A second, more severe transient is caused by switching in loads with large (>1µF) supply bypass capacitors. The discharged bypass capacitors are effectively put in parallel with COUT , causing a rapid drop in VOUT . No regulator can alter its delivery of current quickly enough to prevent this sudden step change in output voltage if the load switch resistance is low and it is driven quickly.
PAGE 34
LTC3856 Applications Information A short-circuit to ground will result in a folded back current of: I SC 25mV 1  90ns 5.5V 3 = −  0.002Ω 2  2µH ( )   = 4.04A  The worst-case power dissipated by the synchronous MOSFET under short-circuit conditions at elevated ambient temperature and estimated 50°C junction temperature rise is: 5 . 5V − 1 . 8 V 2 4 . 04A ) (1 . 25) ( 0 . 013Ω ) ( 5 . 5V = 0 . 18W PSYNC = which is much less than normal, full-load conditions.
PAGE 35
LTC3856 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. These items are also illustrated graphically in the layout diagram of Figure 17. Check the following in the PC layout: 1. Are the signal and power ground paths Kelvin connected? Keep the SGND at one end of a printed circuit path thus preventing MOSFET currents from traveling under the IC.
PAGE 36
LTC3856 Applications Information SW1 L1 RSENSE1 D1 VIN VOUT RIN CIN + + SW2 L2 COUT RL RSENSE2 D2 BOLD LINES INDICATE HIGH, SWITCHING CURRENT LINES. KEEP LINES TO A MINIMUM LENGTH. 3856 F17 Figure 17.
PAGE 37
LTC3856 Typical Application VIN + VIN 1nF 0.1µF 2.2Ω S 5.6k 100pF 100Ω VIN 100Ω Q1 RJK0305DPB S S CLKOUT 100k, 1% 0.1µF VIN PLLIN TG1 RUN SW1 AVP PHASMD DIFFP DIFFN 100k LTC3856 BOOST1 BG1 INTVCC D1, CMDSH-3 INTVCC 4.7µF TG2 PGOOD SW2 EXTVCC BG2 330µF 2.5V s4 4.7µF 6.3V 22µF Q7 RJK0305DPB Q4 RJK0330DPB L2 0.22µH 0.001Ω Q8 RJK0330DPB SENSE2+ 1nF MODE S 100µF 6.3V + s4 22µF INTVCC 0.1µF VOUT 1.5V/ 50A 0.
PAGE 38
LTC3856 Package Description FE Package 38-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1772 Rev A) Exposed Pad Variation AA 4.75 REF 38 9.60 – 9.80* (.378 – .386) 4.75 REF (.187) 20 6.60 ±0.10 4.50 REF 2.74 REF SEE NOTE 4 6.40 2.74 REF (.252) (.108) BSC 0.315 ±0.05 1.05 ±0.10 0.50 BSC RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.50* (.169 – .177) 0.09 – 0.20 (.0035 – .0079) 0.50 – 0.75 (.020 – .030) NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS 2. DIMENSIONS ARE IN MILLIMETERS (INCHES) 3.
PAGE 39
LTC3856 Package Description UH Package 32-Lead Plastic QFN (5mm × 5mm) (Reference LTC DWG # 05-08-1693 Rev D) 0.57 p 0.05 5.35 p 0.05 4.20 p 0.05 3.45 p 0.05 (4 SIDES) PACKAGE OUTLINE 0.23 p 0.05 0.50 BSC RECOMMENDED SOLDER PAD LAYOUT 5.00 p 0.10 (4 SIDES) BOTTOM VIEW—EXPOSED PAD 0.75 p 0.05 0.00 – 0.05 R = 0.115 TYP 0.40 p 0.10 31 32 PIN 1 TOP MARK 1 2 3.45 p 0.10 (4-SIDES) (UH) QFN 0102 0.200 REF NOTE: 1.
PAGE 40
LTC3856 Typical Application VIN + VIN 330pF 0.1µF 2.2Ω S 2.68k 47pF 200Ω VIN 200Ω Q1 RJK0305DPB S S CLKOUT PLLIN 100k 0.1µF 20k 49.9Ω 100pF 30.1k VIN S TG1 RUN SW1 TK/SS ITH VFB AVP S SENSE1+ FREQ 0.1µF LTC3856 INTVCC ITEMP DIFFP DIFFN 100Ω 100k PGOOD CMDSH-3 INTVCC 100µF 6.3V + s2 TG2 ISET SW2 EXTVCC BG2 ILIM 22µF L2 0.22µH INTVCC 0.1µF 4.7µF 6.3V 22µF Q7 RJK0305DPB CMDSH-3 BOOST2 330µF 2.5V s4 VIN Q3 RJK0305DPB 4.7µF 0.