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LTC3866

3866fb

TYPICAL APPLICATION

FEATURES DESCRIPTION

Current Mode Synchronous

Controller for Sub Milliohm

DCR Sensing

The LT C

3866 is a single phase current mode synchronous

step-down switching regulator controller that drives all

N-channel power MOSFET switches. It employs a unique

architecture which enhances the signal-to-noise ratio of

the current sense signal, allowing the use of a very low

DC resistance power inductor to maximize the efficiency

high current applications. This feature also reduces the

switching jitter commonly found in low DCR applications.

The LTC3866 also includes a high speed remote sense dif-

ferential amplifier, a programmable current sense limit that

can be selected to 10mV, 15mV, 20mV, 25mV or 30mV,

and DCR temperature compensation to limit the maximum

output current precisely over temperature.

The LTC3866 also features a precise 0.6V reference with

guaranteed limit of ±0.5% that provides an accurate output

voltage from 0.6V to 3.5V. A 4.5V to 38V input voltage

range allows it to support a wide variety of bus voltages

and various types of batteries.

The LTC3866 is offered in a low profile 24-lead 4mm ×

4mm QFN and 24-lead exposed pad FE packages.

APPLICATIONS

Sub Milliohm DCR Current Sensing

High Efficiency: Up to 95%

Selectable Current Sensing Limit

Programmable DCR Temperature Compensation

Die Overtemperature Thermal Shutdown

± 0.5% 0.6V Output Voltage Accuracy

Programmable Fixed Frequency 250kHz to 770kHz

High Speed Differential Remote Sense Amplifier

Wide Input Voltage Range: 4.5V to 38V

Output Voltage Range: 0.6V to 3.5V with Diffamp

Adjustable Soft-Start or Output Voltage Tracking

Foldback Output Current Limit

Short-Circuit Soft Recovery

Output Overvoltage Protection

24-Lead (4mm × 4mm) QFN and 24-Lead FE Packages

Computer Systems

Telecom Systems

Industrial and Medical Instruments

DC Power Distribution Systems

L, LT , LT C , LT M , Burst Mode, OPTI-LOOP, 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

5481178, 5705919, 5929620, 6177787, 6580258, 6498466, 6611131, patent pending.

High Efficiency, 1.5V/30A Step-Down Converter with Very Low DCR Sensing

FREQ MODE/PLLIN

RUN PGOOD

TK/SS ITEMP

ITH

EXTV

DIFFOUT

INTV

DIFFP BOOST

DIFFN TG

SNSD

SNS

–

OUT

470µF

×2

0.1µF

931Ω

4.64k

3866 TA01a

30.1k

100k

0.1µF

20k

10k

OUT

1.5V

30A

4.7µF

220nF

1.5nF

220pF

220µF

4.5V TO 20V

0.33µH

DCR = 0.32mΩ

SNSA

PGND

ILIM CLKOUT

LTC3866

SGND

Efficiency vs Load Current

and Mode

LOAD CURRENT (A)

EFFICIENCY (%)

100

0.01 1 10 100

3866 TA01b

0.1

CCM

PULSE SKIPPING

Burst Mode

OPERATION

= 12V

OUT

= 1.5V

L = 0.33µH

(DCR = 0.32mΩ TYP)

Summary of content (36 pages)

PAGE 1
LTC3866 Current Mode Synchronous Controller for Sub Milliohm DCR Sensing FEATURES n n n n n n n n n n n n n n n DESCRIPTION Sub Milliohm DCR Current Sensing High Efficiency: Up to 95% Selectable Current Sensing Limit Programmable DCR Temperature Compensation Die Overtemperature Thermal Shutdown ± 0.5% 0.6V Output Voltage Accuracy Programmable Fixed Frequency 250kHz to 770kHz High Speed Differential Remote Sense Amplifier Wide Input Voltage Range: 4.5V to 38V Output Voltage Range: 0.6V to 3.
PAGE 2
LTC3866 ABSOLUTE MAXIMUM RATINGS (Note 1) Input Supply Voltage................................... –0.3V to 40V Topside Driver Voltage (BOOST)................. –0.3V to 46V Switch Voltage(SW) ...................................... –5V to 40V INTVCC, EXTVCC, RUN, PGOOD, BOOST-SW Voltages..................................... –0.3V to 6V SNSD+, SNSA+, SNS– Voltages.............. –0.3V to INTVCC MODE/PLLIN, ILIM, TK/SS, FREQ, DIFFOUT Voltages.................................. –0.3V to INTVCC DIFFP, DIFFN.....
PAGE 3
LTC3866 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 15V, VRUN = 5V unless otherwise specified. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Main Control Loops VIN Input Voltage Range VOUT Output Voltage Range with Diffamp in Loop VFB Regulated Feedback Voltage Current ITH Voltage = 1.
PAGE 4
LTC3866 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 15V, VRUN = 5V unless otherwise specified.
PAGE 5
LTC3866 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 15V, VRUN = 5V unless otherwise specified. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS On-Chip Driver TG RUP TG Pull-Up RDS(ON) TG High 2.6 Ω TG RDOWN TG Pull-Down RDS(ON) TG Low 1.5 Ω BG RUP BG Pull-Up RDS(ON) BG High 2.4 Ω BG RDOWN BG Pull-Down RDS(ON) BG Low 1.
PAGE 6
LTC3866 TYPICAL PERFORMANCE CHARACTERISTICS Load Step (Burst Mode® Operation) TA = 25°C, unless otherwise noted. Load Step (Continuous Conduction Mode) Load Step (Pulse-Skipping Mode) ILOAD 10A/DIV 1.5A TO 15A 0A ILOAD 10A/DIV 1.5A TO 15A 0A ILOAD 10A/DIV 1.5A TO 15A 0A IL 10A/DIV 0A VOUT 100mV/DIV AC-COUPLED IL 10A/DIV 0A VOUT 100mV/DIV AC-COUPLED IL 10A/DIV 0A VOUT 100mV/DIV AC-COUPLED 3866 G04 20µs/DIV VIN = 12V VOUT = 1.
PAGE 7
LTC3866 TYPICAL PERFORMANCE CHARACTERISTICS ILIM = INTVCC 30 ILIM = 3/4 INTVCC 25 ILIM = 1/4 INTVCC 15 1.35 1.4 1.30 1.0 0.8 0.4 5 0 1.6 0.6 ILIM = 0V 10 1.40 1.2 ILIM = 1/2 INTVCC 20 1.8 RUN THRESHOLD (V) 40 35 Shutdown (RUN) Threshold vs Temperature TK/SS Pull-Up Current vs Temperature TK/SS (µA) MAXIMUM CURRENT SENSE THRESHOLD (mV) Maximum Current Sense Threshold Voltage vs Feedback Voltage (Current Foldback) 0 0.1 0.2 0.4 0.5 0.3 FEEDBACK VOLTAGE (V) 0 –50 –25 0.
PAGE 8
LTC3866 TYPICAL PERFORMANCE CHARACTERISTICS Input Quiescent Current vs Input Voltage without EXTVCC Shutdown Current vs Temperature 40 35 30 25 20 4.0 3.8 3.75 QUIESCENT CURRENT (mA) QUIESCENT CURRENT (mA) 45 SHUTDOWN CURRENT (µA) Quiescent Current vs Temperature without EXTVCC 4.00 50 3.50 3.25 3.00 2.75 15 10 –50 –25 TA = 25°C, unless otherwise noted. 0 25 50 75 100 125 150 TEMPERATURE (°C) 2.50 3.4 3.2 3.0 2.8 2.6 5 10 25 20 30 15 INPUT VOLTAGE (V) 3866 G22 PIN FUNCTIONS 3.
PAGE 9
LTC3866 PIN FUNCTIONS (FE/UF) PGND (Pin 14/Pin 11): Power Ground. Connect to the source of the bottom N-channel MOSFET and the negative terminals of the VIN and INTVCC decoupling capacitors close to this pin. BG (Pin 15/Pin 12): Bottom Gate Driver Output. This pin drives the gate of the bottom N-channel MOSFET and swings between INTVCC or EXTVCC and PGND. SW (Pin 16/Pin 13): Switch Node Connection.
PAGE 10
LTC3866 FUNCTIONAL BLOCK DIAGRAM EXTVCC ITEMP MODE/PLLIN 4.7V FREQ + – TEMPSNS F 0.6V MODE/SYNC DETECT VIN + – + 5.5V REG CIN INTVCC F PLL-SYNC VIN BOOST BURST EN CLKOUT S R Q ICOMP + – M1 VOUT SNSA+ SWITCH LOGIC AND ANTISHOOTTHROUGH IREV CB SW ON – + TG FCNT OSC DB SNS– + BG RUN M2 OV PGND COUT CVCC ILIM PGOOD SLOPE COMPENSATION + INTVCC UVLO UV 0.54V VFB R2 – 1 R SNSD+ + ACTIVE CLAMP ITHB AMP SLEEP – – + – – + + 0.5V SS RUN 1.
PAGE 11
LTC3866 OPERATION Main Control Loop The LTC3866 uses LTC proprietary current sensing, current mode step-down architecture. During normal operation, the top MOSFET is turned on every cycle when the oscillator sets the RS latch, and turned off when the main current comparator, ICMP , resets the 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 12
LTC3866 OPERATION Shutdown and Start-Up (RUN and TK/SS Pins) The LTC3866 can be shut down using the RUN pin. Pulling the RUN pin below 1.14V shuts down the main control loop for the controller and most internal circuits, including the INTVCC regulator. Releasing the RUN pin allows an internal 1.0µA current to pull up the pin and enable the controller. Alternatively, the RUN pin may be externally pulled up or driven directly by logic. Be careful not to exceed the absolute maximum rating of 6V on this pin.
PAGE 13
LTC3866 OPERATION If the MODE/PLLIN pin is not being driven by an external clock source, the FREQ pin can be used to program the controller’s operating frequency from 250kHz to 770kHz. There is a precision 10µA current flowing out of the FREQ pin so that the user can 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 14
LTC3866 OPERATION Another way to detect an undervoltage condition is to monitor the VIN supply. Because the RUN pin has a precision turn-on reference of 1.22V, one can use a resistor divider to VIN to turn on the IC when VIN is high enough. An extra 4.5µA of current flows out of the RUN pin once the RUN pin voltage passes 1.22V. The RUN comparator itself has about 80mV of hysteresis. One can program additional hysteresis for the RUN comparator by adjusting the values of the resistive divider.
PAGE 15
LTC3866 APPLICATIONS INFORMATION VIN INTVCC VIN BOOST INDUCTOR LTC3866 RITEMP ITEMP TG SNSD+ SNS– RP 90.9k DCR VOUT BG PGND RS 22.6k RNTC 100k L SW SNSA+ R1 R2 C1 C2 SGND PLACE C1, C2 NEXT TO IC PLACE R1, R2 NEXT TO INDUCTOR R1C1 = 5 • R2C2 3866 F03 Figure 3. Inductor DCR Current Sensing The LTC3866 could also be used like any typical current mode controller by disabling the SNSD+ pin, shorting it to ground.
PAGE 16
LTC3866 APPLICATIONS INFORMATION There will be some power loss in R1 and R2 that relates to the duty cycle, and will be the most in continuous mode at the maximum input voltage: PLOSS ( VIN(MAX) – VOUT ) • VOUT (R) = R Ensure that R1 and R2 have a power rating higher than this value. However, DCR sensing eliminates the conduction loss of a sense resistor; it will provide a better efficiency at heavy loads.
PAGE 17
LTC3866 APPLICATIONS INFORMATION 50 45 40 IMAX (A) 35 CORRECTED IMAX 30 25 RITEMP: RS = 22.6k UNCORRECTED RP = 90.9k IMAX 15 NTC THERMISTOR: RO = 100k 10 TO = 25°C 5 B = 4334 NOMINAL IMAX = 30A 0 25 50 75 100 125 150 –50 –25 0 INDUCTOR TEMPERATURE (°C) 20 3866 F05 Figure 5. Worst-Case IMAX Versus Inductor Temperature Curve with and without NTC Temperature Compensation VOUT RNTC L1 SW1 3867 F08 Figure 6. Thermistor Location.
PAGE 18
LTC3866 APPLICATIONS INFORMATION Inductor Value Calculation Given the desired input and output voltages, the inductor value and operating frequency, fOSC, directly determine the inductor’s peak-to-peak ripple current: IRIPPLE = VOUT  VIN – VOUT    VIN  fOSC • L  Lower ripple current reduces core losses in the inductor, ESR losses in the output capacitors, and output voltage ripple. Thus, highest efficiency operation is obtained at low frequency with a small ripple current.
PAGE 19
LTC3866 APPLICATIONS INFORMATION VIN MILLER EFFECT VGS a V b + QIN VGS – CMILLER = (QB – QA)/VDS +V DS – 3766 F07 Figure 7. Gate Charge Characteristic due to the drain-to-gate accumulation capacitance and the gate-to-source capacitance.
PAGE 20
LTC3866 APPLICATIONS INFORMATION This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Note that capacitor manufacturers’ ripple current ratings are often based on only 2000 hours of life. This makes it advisable to further derate the capacitor, or to choose a capacitor rated at a higher temperature than required.
PAGE 21
LTC3866 APPLICATIONS INFORMATION available in case heights ranging from 2mm to 4mm. Other capacitor types include Sanyo POSCAP, Sanyo OS-CON, Nichicon PL series and Sprague 595D series. Consult the manufacturers for other specific recommendations. Differential Amplifier The LTC3866 has true remote voltage sense capability. The sense connections should be returned from the load, back to the differential amplifier’s inputs through a common, tightly coupled pair of PC traces.
PAGE 22
LTC3866 APPLICATIONS INFORMATION VOUT1 OUTPUT VOLTAGE OUTPUT VOLTAGE VOUT1 VOUT2 VOUT2 TIME TIME (8a) Coincident Tracking 3866 F08 (8b) Ratiometric Tracking Figure 8. Two Different Modes of Output Voltage Tracking VOUT1 VOUT2 TO TK/SS2 PIN R3 R1 R4 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 3866 F09 (9a) Coincident Tracking Setup (9b) Ratiometric Tracking Setup Figure 9.
PAGE 23
LTC3866 APPLICATIONS INFORMATION To prevent the maximum junction temperature from being exceeded, the input supply current must be checked while operating in continuous conduction mode (MODE/PLLIN = SGND) at maximum VIN. When the voltage applied to EXTVCC rises above 4.7V, the INTVCC LDO is turned off and the EXTVCC is connected to the INTVCC. The EXTVCC remains on as long as the voltage applied to EXTVCC remains above 4.5V.
PAGE 24
LTC3866 APPLICATIONS INFORMATION as shown in Figure 11. The regulated output voltage is determined by:  R  VOUT = 0.6V • 1+ B   RA  To improve the frequency response, a feedforward capacitor, CFF , may be used. Great care should be taken to route the VFB line away from noise sources, such as the inductor or the SW line.
PAGE 25
LTC3866 APPLICATIONS INFORMATION 900 Minimum On-Time Considerations 800 Minimum on-time, tON(MIN), is the smallest time duration that the LTC3866 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. Low duty cycle applications may approach this minimum on-time limit and care should be taken to ensure that: FREQUENCY (kHz) 700 600 500 400 300 200 tON(MIN) < 100 0 0 0.5 1 1.5 FREQ PIN VOLTAGE (V) 2 2.
PAGE 26
LTC3866 APPLICATIONS INFORMATION 1. The VIN current is the DC supply current given in the Electrical Characteristics table, which excludes MOSFET driver and control currents. VIN current typically results in a small (<0.1%) loss. 4. Transition losses apply only to the topside MOSFET(s), and become significant only when operating at high input voltages (typically 15V or greater). Transition losses can be estimated from: 2. INTVCC current is the sum of the MOSFET driver and control currents.
PAGE 27
LTC3866 APPLICATIONS INFORMATION adequate starting point for most applications. 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.
PAGE 28
LTC3866 APPLICATIONS INFORMATION 2. Place the feedback divider between the + and – terminals of COUT. Route DIFFP and DIFFN with minimum PC trace spacing from the IC to the feedback divider. 3. Are the SNSD+, SNSA+ and SNS– printed circuit traces routed together with minimum PC trace spacing? The filter capacitors between SNSD+, SNSA+ and SNS– should be as close as possible to the pins of the IC. Connect the SNSD+ and SNSA+ pins to the filter resistors as illustrated in Figure 3. 4.
PAGE 29
LTC3866 APPLICATIONS INFORMATION This design will require 0.33µH. The Würth 744301033, 0.32µH inductor is chosen. At the nominal input voltage (12V), the ripple current will be: ∆IL(NOM) =  VOUT  V 1− OUT  f • L  VIN(NOM)  It will have 10A (33%) ripple. The peak inductor current will be the maximum DC value plus one-half the ripple current, or 35A. The minimum on-time occurs at the maximum VIN, and should not be less than 90ns: VOUT 1.
PAGE 30
LTC3866 TYPICAL APPLICATIONS Very Low Output Ripple Converter The LTC3866 can work with very low DCR inductors because it can operate with only a small peak-to-peak sense voltage. Two inductor characteristics can diminish this signal: lower DC resistance and higher inductance. While lower DCR improves efficiency, higher inductance reduces output ripple.
PAGE 31
LTC3866 TYPICAL APPLICATIONS VOUT TYPICAL FRONT PAGE 10mV/DIV AC-COUPLED IL 10A/DIV 0A VOUT LOW RIPPLE FIGURE 15 10mV/DIV AC-COUPLED VOUT 100mV/DIV AC-COUPLED VIN = 12V VOUT = 1.5V 3866 F16 50µs/DIV Figure 16. Load Step Transient Response 100 3866 F17 2µs/DIV VIN = 12V VOUT = 1.5V Figure 17. Very Low Output Voltage Ripple VIN = 12V VOUT = 1.5V 90 EFFICIENCY (%) 80 70 60 50 40 30 20 10 0 0.01 1 0.1 10 LOAD CURRENT (A) 100 3866 F18 Figure 18.
PAGE 32
LTC3866 TYPICAL APPLICATIONS High Efficiency, Dual Phase Very Low DCR Sensing 1.5V/60A Step-Down Supply 100k 0.1µF FREQ PGOOD RUN 3.57k 120k ITH EXTVCC VFB VIN DIFFOUT 10µF ×2 2.2Ω ITEMP TK/SS 30.1k VIN 7V TO 20V MODE/PLLIN CMDSH-3 INTVCC LTC3866 DIFFP BOOST 20k 0.1µF DIFFN 202nF + 220nF 220nF BSC050NE2LS TG SNSD SW SNS– BG SNSA+ BSC010NE2LS 931Ω 4.64k PGND 4.7µF ILIM 0.33µH DCR = 0.32mΩ VOUT 1.5V 60A 330µF ×2 100µF ×2 1µF CLKOUT SGND GND 30.
PAGE 33
LTC3866 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. FE Package 24-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1771 Rev B) Exposed Pad Variation AA 7.70 – 7.90* (.303 – .311) 3.25 (.128) 3.25 (.128) 24 23 22 21 20 19 18 17 16 15 14 13 6.60 ±0.10 2.74 (.108) 4.50 ±0.10 6.40 2.74 (.252) (.108) BSC SEE NOTE 4 0.45 ±0.05 1.05 ±0.10 0.65 BSC 1 2 3 4 5 6 7 8 9 10 11 12 RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.50* (.
PAGE 34
LTC3866 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. UF Package 24-Lead Plastic QFN (4mm × 4mm) (Reference LTC DWG # 05-08-1697) 0.70 ±0.05 4.50 ±0.05 2.45 ±0.05 3.10 ±0.05 (4 SIDES) PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 4.00 ±0.10 (4 SIDES) BOTTOM VIEW—EXPOSED PAD R = 0.115 TYP 0.75 ±0.05 PIN 1 NOTCH R = 0.20 TYP OR 0.35 × 45 CHAMFER 23 24 PIN 1 TOP MARK (NOTE 6) 0.40 ±0.10 1 2 2.
PAGE 35
LTC3866 REVISION HISTORY REV DATE DESCRIPTION PAGE NUMBER A 08/12 Clarified operating temperatures. Modified the PD equation thermal resistance value. 5 Modified the Block Diagram sense amplifier. 10 Clarified the SNS section values. 14 Modified the ripple value in the Soft-Start section. 21 Modified values in the INTVCC and EXTVCC section. 22-23 Modified the 5V/25A Step-Down Converter circuit schematic. B 10/12 2-5 Added "junction” to clarify operating temperature range.
PAGE 36
LTC3866 TYPICAL APPLICATION High Efficiency Step-Down Converter with Power Block 100k INTVCC 100k 0.1µF 6 ITEMP PGOOD RUN MODE/PLLIN VIN INTVCC LTC3866EUF DIFFN BOOST DIFFP TG SNSD+ SW 47nF 25 7 8 9 PGND 5 DIFFOUT SGND 20k 4 470µF 2.2Ω EXTVCC CLKOUT 30.1k VFB ILIM 10k ITH SNSA+ 3 220pF FREQ TK/SS 2 1500pF SNS– 1 VIN 1µF 24 23 22 21 20 19 BG 18 VIN INTVCC 4.7µF 15 CMDSH-3 0.