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LTC3786

3786fc

For more information www.linear.com/LTC3786

TYPICAL APPLICATION

FEATURES DESCRIPTION

Low I

Synchronous

Boost Controller

The LT C

3786 is a high performance synchronous boost

converter controller that drives all N-channel power

MOSFETs. Synchronous rectification increases efficiency,

reduces power losses and eases thermal requirements,

allowing the LTC3786 to be used in high power boost

applications.

A 4.5V to 38V input supply range encompasses a wide

range of system architectures and battery chemistries.

When biased from the output of the boost converter or

another auxiliary supply, the LTC3786 can operate from

an input supply as low as 2.5V after start-up. The 55µA

no-load quiescent current extends operating run time in

battery-powered systems.

The operating frequency can be set for a 50kHz to 900kHz

range or synchronized to an external clock using the

internal PLL. The LTC3786 also features a precision 1.2V

reference and a power good output indicator. The SS pin

ramps the output voltage during start-up. The PLLIN/MODE

pin selects among Burst Mode

operation, pulse-skipping

mode or continuous inductor current mode at light loads.

12V to 24V/5A Synchronous Boost Converter

APPLICATIONS

Synchronous Operation For Highest Efficiency and

Reduced Heat Dissipation

Wide V

Range: 4.5V to 38V (40V Abs Max) and

Operates Down to 2.5V After Start-Up

Output Voltages Up to 60V

±1% 1.2V Reference Voltage

SENSE

or Inductor DCR Current Sensing

100% Duty Cycle Capability for Synchronous MOSFET

Low Quiescent Current: 55µA

Phase-Lockable Frequency (75kHz to 850kHz)

Programmable Fixed Frequency (50kHz to 900kHz)

Adjustable Output Voltage Soft-Start

Power Good Output Voltage Monitor

Low Shutdown Current I

: <8µA

Internal 5.4V LDO for Gate Drive Supply

Thermally Enhanced 16-Pin 3mm × 3mm QFN and

MSOP Packages

Industrial and Automotive Power Supplies

Automotive Start-Stop Systems

Medical Devices

High Voltage Battery-Powered Systems

All registered trademarks and trademarks are the property of their respective owners.

SENSE

VBIAS

SENSE

–

0.1µF

220pF

LTC3786

0.1µF

15nF

220µF

3786 TA01a

OUT

24V

4.7µF

4.5V TO 24V

4mΩ

3.3µH

BOOST

GND

INTV

VFB

ITH

12.1k

8.66k

232k

FREQ

RUN

PLLIN/MODE

PGOOD

Efficiency and Power Loss

vs Load Current

OUTPUT CURRENT (A)

0.010.0010.00010.00001

EFFICIENCY (%)

POWER LOSS (mW)

0.1 1 10

3786 TA01b

100

1000

0.1

10000

= 12V

OUT

= 24V

Burst Mode OPERATION

FIGURE 8 CIRCUIT

BURST

EFFICIENCY

BURST

LOSS

Downloaded from Arrow.com.

Summary of content (34 pages)

PAGE 1
LTC3786 Low IQ Synchronous Boost Controller DESCRIPTION FEATURES n n n n n n n n n n n n n n Synchronous Operation For Highest Efficiency and Reduced Heat Dissipation Wide VIN Range: 4.5V to 38V (40V Abs Max) and Operates Down to 2.5V After Start-Up Output Voltages Up to 60V ±1% 1.
PAGE 2
LTC3786 ABSOLUTE MAXIMUM RATINGS (Notes 1, 3) VBIAS ........................................................ –0.3V to 40V BOOST ........................................................–0.3V to 71V SW ............................................................. –0.3V to 65V RUN ............................................................. –0.3V to 8V Maximum Current Sourced into Pin from Source >8V ..............................................100µA PGOOD, PLLIN/MODE .................................. –0.
PAGE 3
LTC3786 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C. VBIAS = 12V, unless otherwise noted (Note 2). SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Main Control Loop VBIAS Chip Bias Voltage Operating Range 4.5 VFB Regulated Feedback Voltage ITH = 1.
PAGE 4
LTC3786 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C. VBIAS = 12V, unless otherwise noted (Note 2). SYMBOL PARAMETER CONDITIONS MIN 5.2 TYP MAX UNITS INTVCC Linear Regulator VINTVCC(VIN) Internal VCC Voltage 6V < VBIAS < 38V VLDO INT INTVCC Load Regulation ICC = 0mA to 50mA 5.4 5.6 V 0.
PAGE 5
LTC3786 TYPICAL PERFORMANCE CHARACTERISTICS Efficiency and Power Loss vs Output Current Efficiency and Power Loss vs Output Current 100 100 10000 80 80 50 40 30 20 10 0 0.01 0.1 1 OUTPUT CURRENT (A) 100 10 1 0.1 10 BURST EFFICIENCY 1000 BURST LOSS 70 60 100 50 40 10 30 VIN = 12V 1 VOUT = 24V Burst Mode OPERATION 10 FIGURE 8 CIRCUIT 0 0.1 0.1 1 10 0.00001 0.0001 0.001 0.
PAGE 6
LTC3786 TYPICAL PERFORMANCE CHARACTERISTICS Regulated Feedback Voltage vs Temperature Soft-Start Pull-Up Current vs Temperature Shutdown Current vs Temperature 11.0 1.206 1.203 1.200 1.197 1.194 SHUTDOWN CURRENT (µA) 1.209 SOFT-START CURRENT (µA) REGULATED FEEDBACK VOLTAGE (V) 1.212 10.5 10.0 9.5 1.191 1.188 –45 –25 –5 15 35 55 75 95 115 135 155 TEMPERATURE (°C) 9.
PAGE 7
LTC3786 TYPICAL PERFORMANCE CHARACTERISTICS INTVCC vs Load Current 5.50 5.2 VIN = 12V 600 VIN = 5V FREQ = INTVCC 5.0 5.35 5.30 5.25 550 FREQUENCY (kHz) 5.40 INTVCC VOLTAGE (V) INTVCC VOLTAGE (V) 5.45 4.8 4.6 4.4 4.0 0 20 40 500 450 400 FREQ = GND 4.2 5.20 5.
PAGE 8
LTC3786 TYPICAL PERFORMANCE CHARACTERISTICS Charge Pump Charging Current vs Operating Frequency 100 90 VBOOST = 16.
PAGE 9
LTC3786 PIN FUNCTIONS (MSOP/QFN) FREQ (Pin 7/Pin 9): The Frequency Control Pin for the Internal VCO. Connecting the pin to GND forces the VCO to a fixed low frequency of 350kHz. Connecting the pin to INTVCC forces the VCO to a fixed high frequency of 535kHz. The frequency can be programmed from 50kHz to 900kHz by connecting a resistor from the FREQ pin to GND. The resistor and an internal 20µA source current create a voltage used by the internal oscillator to set the frequency.
PAGE 10
LTC3786 BLOCK DIAGRAM INTVCC PGOOD 1.32V VFB 1.08V DB BOOST + – S R Q CB TG SHDN + – SWITCHING LOGIC AND CHARGE PUMP 20µA FREQ VOUT SW COUT INTVCC BG CLK + – 0.425V + – VCO PFD ICMP –+ SLEEP +– + IREV – L SENSE – 2mV 2.8V 0.7V PLLIN/ MODE SENSE+ SLOPE COMP SYNC DET 100k SENS LO VIN + – CIN VFB 2.5V + – 1.2V SS + – 1.32V EA – VBIAS OV SHDN 5.4V LDO RSENSE 0.5µA/ 4.5µA ITH CC CC2 – + 11V RC 10µA 3786 BD 3.
PAGE 11
LTC3786 OPERATION (Refer to the Block Diagram) Main Control Loop The LTC3786 uses a constant-frequency, current mode step-up control architecture. During normal operation, the external bottom MOSFET is turned on when the clock sets the RS latch, and is turned off when the main current comparator, ICMP , resets the RS latch. The peak inductor current at which ICMP trips and resets the latch is controlled by the voltage on the ITH pin, which is the output of the error amplifier, EA.
PAGE 12
LTC3786 OPERATION (Refer to the Block Diagram) In sleep mode, much of the internal circuitry is turned off and the LTC3786 draws only 55µA of quiescent current. In sleep mode, the load current is supplied by the output capacitor. As the output voltage decreases, the EA’s output begins to rise.
PAGE 13
LTC3786 OPERATION (Refer to the Block Diagram) The typical capture range of the LTC3786’s PLL is from approximately 55kHz to 1MHz, and is guaranteed to lock to an external clock source whose frequency is between 75kHz and 850kHz. The typical input clock thresholds on the PLLIN/MODE pin are 1.6V (rising) and 1.2V (falling). Operation When VIN > Regulated VOUT When VIN rises above the regulated VOUT voltage, the boost controller can behave differently depending on the mode, inductor current and VIN voltage.
PAGE 14
LTC3786 APPLICATIONS INFORMATION The Typical Application on the first page is a basic LTC3786 application circuit. LTC3786 can be configured to use either inductor DCR (DC resistance) sensing or a discrete sense resistor (RSENSE) for current sensing. The choice between the two current sensing schemes is largely a design tradeoff between cost, power consumption and accuracy.
PAGE 15
LTC3786 APPLICATIONS INFORMATION less half the peak-to-peak ripple current, ∆IL. To calculate the sense resistor value, use the equation: R SENSE = VSENSE(MAX) I MAX + ∆IL 2 When using the controller in low VIN and very high voltage output applications, the maximum inductor current and correspondingly the maximum output current level will be reduced due to the internal compensation required to meet stability criterion for boost regulators operating at greater than 50% duty factor.
PAGE 16
LTC3786 APPLICATIONS INFORMATION Inductor Value Calculation The operating frequency and inductor selection are interrelated in that higher operating frequencies allow the use of smaller inductor and capacitor values. Why would anyone ever choose to operate at lower frequencies with larger components? The answer is efficiency. A higher frequency generally results in lower efficiency because of MOSFET gate charge and switching losses.
PAGE 17
LTC3786 APPLICATIONS INFORMATION accounts for the loss caused by reverse recovery current, is inversely proportional to the gate drive current and has an empirical value of 1.7. Both MOSFETs have I2R losses while the bottom N-channel equation includes an additional term for transition losses, which are highest at low input voltages.
PAGE 18
LTC3786 APPLICATIONS INFORMATION Soft-Start (SS Pin) The start-up of the VOUT is controlled by the voltage on the SS pin. When the voltage on the SS pin is less than the internal 1.2V reference, the LTC3786 regulates the VFB pin voltage to the voltage on the SS pin instead of 1.2V. Soft-start is enabled by simply connecting a capacitor from the SS pin to ground, as shown in Figure 4. An internal 10µA current source charges the capacitor, providing a linear ramping voltage at the SS pin.
PAGE 19
LTC3786 APPLICATIONS INFORMATION A leaky diode DB in the boost converter can not only prevent the top MOSFET from fully turning on but it can also completely discharge the bootstrap capacitor CB and create a current path from the input voltage to the BOOST pin to INTVCC. This can cause INTVCC to rise if the diode leakage exceeds the current consumption on INTVCC. This is particularly a concern in Burst Mode operation where the load on INTVCC can be very small.
PAGE 20
LTC3786 APPLICATIONS INFORMATION Table 1 summarizes the different states in which the FREQ pin can be used. Table 1 FREQ PIN PLLIN/MODE PIN FREQUENCY 0V DC Voltage 350kHz INTVCC DC Voltage 535kHz Resistor DC Voltage 50kHz to 900kHz Any of the Above External Clock Phase Locked to External Clock Minimum On-Time Considerations Minimum on-time, tON(MIN), is the smallest time duration that the LTC3786 is capable of turning on the bottom MOSFET.
PAGE 21
LTC3786 APPLICATIONS INFORMATION Checking Transient Response The regulator loop response can be checked by looking at the load current transient response. Switching regulators take several cycles to respond to a step in load current. When a load step occurs, VOUT shifts by an amount equal to ∆ILOAD • ESR, where ESR is the effective series resistance of COUT .
PAGE 22
LTC3786 APPLICATIONS INFORMATION The RSENSE resistor value can be calculated by using the maximum current sense voltage specification with some accommodation for tolerances: R SENSE ≤ 75mV 9.25A = 0.008Ω Choosing 1% resistors: RA = 5k and RB = 95.3k yields an output voltage of 24.072V. The power dissipation on the topside MOSFET in each channel can be easily estimated. Choosing a Vishay Si7848BDP MOSFET results in: RDS(ON) = 0.012Ω, CMILLER = 150pF.
PAGE 23
LTC3786 APPLICATIONS INFORMATION PC Board Layout Debugging It is helpful to use a DC-50MHz current probe to monitor the current in the inductor while testing the circuit. Monitor the output switching node (SW pin) to synchronize the oscilloscope to the internal oscillator and probe the actual output voltage. Check for proper performance over the operating voltage and current range expected in the application.
PAGE 24
LTC3786 APPLICATIONS INFORMATION SENSE+ PGOOD SENSE– SW LTC3786 TG CB + M1 M2 BG PLLIN/MODE RSENSE L1 BOOST FREQ fIN VPULLUP RUN VFB VBIAS SS + GND ITH VIN GND INTVCC VOUT 3786 F06 Figure 6. Recommended Printed Circuit Layout Diagram RSENSE VIN L1 VOUT SW RIN COUT CIN RL 3786 F07 BOLD LINES INDICATE HIGH SWITCHING CURRENT. KEEP LINES TO A MINIMUM LENGTH Figure 7. Branch Current Waveforms 3786fc 24 Downloaded from Arrow.com. For more information www.linear.
PAGE 25
LTC3786 APPLICATIONS INFORMATION VBIAS SENSE+ RSENSE 4mΩ LTC3786 SENSE– L 3.3µH PLLIN/MODE RUN FREQ TG CSS 0.1µF SW CB 0.1µF SS CITH 15nF VIN 5V TO 24V CIN 22µF MTOP BOOST RITH 8.66k COUTA 22µF ×4 + COUTB 220µF VOUT 24V 5A* MBOT BG ITH D CITHA 220pF INTVCC CINT 4.7µF GND RA 12.1k 100k VFB PGOOD RS 232k 3786 F08 CIN, COUTA: TDK C4532X5R1E226M COUTB: SANYO 50CE220LX D: BAS140W L: PULSE PA1494.
PAGE 26
LTC3786 APPLICATIONS INFORMATION VBIAS SENSE+ RSENSE 5mΩ LTC3786 SENSE– L 10.2µH PLLIN/MODE RUN FREQ TG CSS 0.1µF SW SS CITH 15nF VIN 5V TO 36V CIN 6.8µF ×4 CB 0.1µF COUTA 6.8µF ×4 MTOP BOOST RITH 8.66k BG + COUTB 220µF VOUT 36V 3A* MBOT ITH D CITHA 220pF INTVCC CINT 4.7µF GND RA 12.1k 100k VFB PGOOD RS 357k 3786 F10 CIN, COUTA: TDK C4532X7R1H685K COUTB: SANYO 63CE220KX D: BAS170W L: PULSE PA2050.
PAGE 27
LTC3786 APPLICATIONS INFORMATION VBIAS SENSE+ RSENSE 5mΩ LTC3786 VIN 4.5V TO 24V START-UP VOLTAGE OPERATES THROUGH TRANSIENTS DOWN TO 2.5V CIN 22µF SENSE– 60.4k fSW = 400kHz L 3.2µH RUN FREQ TG CSS 0.1µF SW CB 0.1µF SS CITH 10nF BOOST RITH 4.64k + COUTB 150µF MBOT BG ITH CITHA 100pF COUTA 22µF ×3 MTOP VOUT* 10V 5A D INTVCC RA 12.1k CINT 4.7µF GND VFB 100k PLLIN/MODE RB 88.
PAGE 28
LTC3786 APPLICATIONS INFORMATION SENSE+ CIN 47µF ×2 RSENSE 6mΩ LTC3786 VIN 2.7V TO 4.2V SENSE– L 0.67µH PLLIN/MODE CSS 0.1µF RUN TG FREQ SW RITH 5.11k MBOT D1 ITH VBIAS INTVCC CITHA 100pF GND RA 150k CINT 4.7µF D2 Q 100k 1M C2 10µF PGOOD VFB COUT 47µF ×4 MTOP BOOST BG SS CITH 6.8nF CB 0.
PAGE 29
LTC3786 APPLICATIONS INFORMATION VBIAS SENSE+ LTC3786 RUN FREQ CSS 0.1µF SENSE– TG SW SS CITH 15nF C1 0.1µF RITH 8.87k RS2 26.1k RS1 1% 53.6k 1% L 10.2µH CB 0.1µF MTOP BOOST CITHA 220pF COUTA 22µF ×4 + COUTB 220µF VOUT 24V 4A MBOT BG ITH VIN 5V TO 24V CIN 22µF D INTVCC GND PLLIN/MODE RA 12.1k VFB PGOOD CINT 4.7µF 100k RB 232k 3786 F14a C1: TDK C1005X7R1C104K CIN, COUTA: TDK C4532X5R1E226M COUTB: SANYO, 50CE220AX L: PULSE PA2050.
PAGE 30
LTC3786 APPLICATIONS INFORMATION DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY VBIAS SENSE+ RSENSE 15mΩ T D 1:10 10nF LTC3786 SENSE– PLLIN/MODE 25k fSW = 105kHz CSS 0.1µF • RUN FREQ TG VIN 5V TO 12V VOUT 350V 10mA 220pF SW BOOST BG RITH 8.66k ITH CITHA 100pF COUT 68nF ×2 • 22Ω SS CITH 22nF CIN 22µF ×2 MBOT INTVCC 16.2k 1% CINT 4.7µF GND 100k VFB 1M 1% PGOOD 1M 1% 1.
PAGE 31
LTC3786 PACKAGE DESCRIPTION Please refer to http://www.linear.com/product/LTC3786#packaging for the most recent package drawings. MSE Package 16-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1667 Rev F) BOTTOM VIEW OF EXPOSED PAD OPTION 2.845 0.102 (.112 .004) 5.10 (.201) MIN 2.845 0.102 (.112 .004) 0.889 0.127 (.035 .005) 8 1 1.651 0.102 (.065 .004) 1.651 0.102 3.20 – 3.45 (.065 .004) (.126 – .136) 0.305 0.038 (.0120 .0015) TYP 16 0.50 (.0197) BSC 4.039 0.102 (.159 .
PAGE 32
LTC3786 PACKAGE DESCRIPTION Please refer to http://www.linear.com/product/LTC3786#packaging for the most recent package drawings. UD Package 16-Lead Plastic QFN (3mm × 3mm) (Reference LTC DWG # 05-08-1691) 0.70 0.05 3.50 0.05 2.10 1.45 0.05 0.05 (4 SIDES) PACKAGE OUTLINE 0.25 0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 0.10 (4 SIDES) 0.75 BOTTOM VIEW—EXPOSED PAD PIN 1 NOTCH R = 0.20 TYP OR 0.25 × 45 CHAMFER R = 0.115 TYP 0.05 15 PIN 1 TOP MARK (NOTE 6) 16 0.40 0.10 1 1.
PAGE 33
LTC3786 REVISION HISTORY REV DATE DESCRIPTION A 9/11 Updated the Topside MOSFET Driver Supply (CB, DB) section. PAGE NUMBER Updated Figure 12. 27 B 9/16 Added H-Grade 2, 4 C 11/17 18 Clarified graphs G01 and G08 5 Changed INTVCC to GND on G20 7 Changed Pin 9 to Pin 8 for PLLIN/Mode pin function 8 Changed from (VOUT • VIN) to (VOUT – VIN) 15 3786fc Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
PAGE 34
LTC3786 TYPICAL APPLICATION High Efficiency 48V Boost Converter VBIAS SENSE+ RSENSE 8mΩ LTC3786 SENSE– PLLIN/MODE RUN FREQ CSS 0.1µF L 16µH TG SW SS CITH 15nF CB 0.1µF MTOP BOOST RITH 8.66k BG COUTA 6.8µF ×4 + VOUT 48V 2A* COUTB 220µF MBOT ITH CITHA 220pF VIN 5V TO 38V CIN 6.8µF ×4 D INTVCC CINT 4.7µF GND RA 12.1k VFB RS 475k 100k PGOOD 3786 TA02 CIN, COUTA: TDK C4532X7R1H685K COUTB: SANYO 63CE220KX D: BAS170W L: PULSE PA2050.