Datasheet

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LTC3646/LTC3646-1

36461fa

For more information www.linear.com/LTC3646

Typical applicaTion

FeaTures DescripTion

40V, 1A Synchronous

Step-Down Converter

The LTC

3646 is a high efficiency, step-down DC/DC

converter with internal high side and synchronous power

FETs. It draws only 140µA typical DC supply current in

Burst Mode operation at no load while maintaining output

voltage regulation.

The LTC3646 can supply up to a 1A load, and its combina

tion of Burst Mode operation, integrated power switches

and low quiescent current provides high efficiency over a

broad range of load currents. Alternatively, it can be used

in forced continuous mode for ripple sensitive applications.

The LTC3646 has a wide 4.0V to 40V input range and its

patented controlled on-time architecture and 0.6V reference

voltage allow for large step-down ratios without the risk of

overvoltage. The frequency may be set between 200kHz

to 3.0MHz with a program resistor or synchronized to an

external clock.

The internal soft-start, short-circuit protection and volt

age rating

make the LTC3646 a robust part ideal for high

voltage applications.

Efficiency Curve

applicaTions

Wide Input Voltage Range: 4.0V to 40V

1A Guaranteed Output Current

High Efficiency: Up to 95%

Wide Output Voltage Range

LTC3646: 2.0V to 30V

LTC3646-1: 0.6V to 15V

±1% Accurate Reference Voltage

Internal or External Compensation

Switching Frequency Adjustable and Synchronizable:

200kHz to 3MHz

Selectable High Efficiency Burst Mode

Operation or

Forced Continuous Mode

14 Lead 3mm × 4mm DFN or Thermally Enhanced

16-Lead MSOP Packages

Point of Load Power Supply

Intermediate Bus Power

Automotive Applications

L, LT, LT C , LT M , Linear Technology, the Linear logo and Burst Mode are registered trademarks

and Hot Swap is a trademark of Linear Technology Corporation. All other trademarks are the

property of their respective owners. Protected by U.S. Patents, including 6580258, 5481178,

5994885, 6304066, 5847554, 6476589, 6774611.

LOAD CURRENT (A)

0.001

EFFICIENCY (%)

POWER LOSS (W)

3646 TA01b

0.1

0.01

100

0.4

0.2

0.8

1.0

0.6

OUT

= 3.3V

OUT

= 5V

OUT

= 3.3V

OUT

= 5V

= 12V

f = 1MHz

PVIN

SVIN

RUN

INTV

MODE/SYNC

ITH

EXTV

BOOST

–

SWITCH

CONTROL

LTC3646

3.3µH

SGND

4.7µF

PGND

0.1µF

412k

10µF

0.6V

56.2k

3646 TA01a

15µF

OUT

40V MAX

Summary of content (28 pages)

PAGE 1
LTC3646/LTC3646-1 40V, 1A Synchronous Step-Down Converter Features n n n n n n n n n Description Wide Input Voltage Range: 4.0V to 40V 1A Guaranteed Output Current High Efficiency: Up to 95% Wide Output Voltage Range LTC3646: 2.0V to 30V LTC3646-1: 0.
PAGE 2
LTC3646/LTC3646-1 Absolute Maximum Ratings (Notes 1, 7) PVIN, SVIN Supply Voltage......................... –0.3V to 45V SW Voltage (DC)............................ –0.3V to PVIN + 0.3V BOOST – SW Voltage.................................... –0.3V to 6V VON Voltage................................................. –0.3V to 33V MODE/SYNC, RT, ITH, VFB Voltage.................................–0.3V to INTVCC + 0.3V INTVCC Voltage............................................. –0.3V to 6V EXTVCC Voltage..........
PAGE 3
LTC3646/LTC3646-1 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C (Note 2), PVIN = SVIN = 12V unless otherwise noted.
PAGE 4
LTC3646/LTC3646-1 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C (Note 2), PVIN = SVIN = 12V unless otherwise noted.
PAGE 5
LTC3646/LTC3646-1 Typical Performance Characteristics TA = 25°C, SVIN/PVIN = 12V, fO = 1MHz unless otherwise noted. Efficiency vs Load Current, Burst Mode Operation Efficiency vs Load Current, Forced Continuous Mode Operation 100 100 VOUT = 1.8V 90 fO = 1.5MHz 80 70 70 50 40 30 EFFICIENCY (%) 80 70 60 60 50 40 0 0.001 0.01 0.1 LOAD CURRENT (A) 0 0.001 1 0.01 0.1 LOAD CURRENT (A) 95 LOAD = 500mA LOAD = 100mA 0.35 606 0.
PAGE 6
LTC3646/LTC3646-1 Typical Performance Characteristics TA = 25°C, SVIN/PVIN = 12V, fO = 1MHz unless otherwise noted. No Load Quiescent Current vs Temperature, Burst Mode Operation Bottom Switch Current Limit vs Temperature 180 160 140 120 100 –40 –20 0 1.6 1.4 1.2 1.3 1.2 0.4 0 –0.
PAGE 7
LTC3646/LTC3646-1 Pin Functions (DFN/MSOP) SGND (Pin 1/Pin 1): Analog Ground Pin. This pin should have a low noise connection to the reference ground. VFB (Pin 2/Pin 2): Output Voltage Feedback Pin. Input to the error amplifier that compares the feedback voltage with the internal 0.6V reference. Connect this pin to a resistor divider network to program the desired output voltage. ITH (Pin 3/Pin 3): Error Amplifier Output and Switching Regulator Compensation Point.
PAGE 8
LTC3646/LTC3646-1 Functional Block Diagram CPVIN CSVIN SVIN RUN 1.21V + – EXTVCC 4.5V PVIN + – INTVCC 5V REG MODE/SYNC CVCC OSCILLATOR RT RRT BOOST TG VON CBOOST SW ON-TIME CONTROLLER CONTROL LOGIC R V tON = ON VIN • fO S + – ICMP ITH Q L1 VOUT COUT ON + – BG IREV PGND SENSE+ SENSE– RCOMP 0.555V CCOMP + – EA + – INTVCC – 0.3V 0.6V REF 0.645V + – + – UV PGOOD OV SOFT-START VFB SGND R1 R2 3646 BD 8 36461fa For more information www.linear.
PAGE 9
LTC3646/LTC3646-1 Operation The LTC3646 and LTC3646-1 are current mode, monolithic, synchronous, step-down regulators capable of 40V input operation, and extremely high step down ratios while maintaining constant frequency. (Both will be referred to as the LTC3646 except as specifically noted.) Part operation is enabled by raising the voltage of the RUN pin above 1.21V.
PAGE 10
LTC3646/LTC3646-1 Operation SVIN/PVIN Overvoltage Protection Short-Circuit Protection In order to protect the internal power MOSFET devices against transient voltage spikes, the LTC3646 continuously monitors the SVIN pin for an overvoltage condition. When SVIN rises above VIN(OV), the regulator suspends operation by shutting off both power MOSFETs and resetting the soft-start level. Once VIN drops to below the specified range of VIN(OV), the regulator immediately restarts normal operation.
PAGE 11
LTC3646/LTC3646-1 Applications Information A general LTC3646 application circuit is shown on the first page of this data sheet. External component selection is largely driven by the load requirement and begins with the selection of the inductor L. Once the inductor is chosen, the input capacitor, CIN, the output capacitor, COUT, the internal regulator capacitor, CVCC, and the boost capacitor, CBOOST, can be selected. Next, the feedback resistors are selected to set the desired output voltage.
PAGE 12
LTC3646/LTC3646-1 Applications Information Ferrite designs exhibit very low core loss and are preferred at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Ferrite core materials saturate “hard,” meaning the inductance collapses abruptly when the peak design current is exceeded. This collapse will result in an abrupt increase in inductor ripple current, so it is important to ensure the core will not saturate.
PAGE 13
LTC3646/LTC3646-1 Applications Information Thus, a good place to start is with the output capacitor size of approximately: COUT ≈ 3 • ΔIOUT fO • VDROOP Though this equation provides a good approximation, more capacitance may be required depending on the duty cycle and load step requirements. The actual VDROOP should be verified by applying a load step to the output. Using Ceramic Input and Output Capacitors Higher value, lower cost ceramic capacitors are now available in small case sizes.
PAGE 14
LTC3646/LTC3646-1 Applications Information The design example on the back page uses a DFLS1200 based on its low reverse leakage over the voltage and temperature ratings of the LTC3646. Output Voltage Programming The LTC3646 will adjust the output voltage such that VFB equals the reference voltage of 0.6V according to: ⎛ R1 ⎞ VOUT = 0.
PAGE 15
LTC3646/LTC3646-1 Applications Information Minimum on-time can be affected by the output load current and the trough current level. During the transition between the top switch turn-off and the synchronous switch turn-on, the inductor current discharges the SW pin capacitance. When the inductor trough current level is low, or reversing in forced continuous operation, the minimum on-time can increase by approximately 20nS. ITH RCOMP LTC3646 CBYP CCOMP SGND 3634 F03 Figure 3.
PAGE 16
LTC3646/LTC3646-1 Applications Information Once RCOMP is determined, CCOMP can be chosen to set the zero frequency (fZ): 1 fZ = 2π • CCOMP • RCOMP For maximum phase margin, fZ should be chosen to be less than one-tenth of fC. Since the ITH node is sensitive to noise coupling, a small bypass capacitor (CBYP) may be used to filter out board noise.
PAGE 17
LTC3646/LTC3646-1 Applications Information operation is selected creating the lowest fixed output ripple at the expense of light load efficiency. The LTC3646 will detect the presence of the external clock signal on the MODE/SYNC pin and synchronize the internal oscillator to the phase and frequency of the incoming clock. The presence of an external clock will place the LTC3646 into forced continuous mode operation.
PAGE 18
LTC3646/LTC3646-1 Applications Information Efficiency Considerations The percent efficiency of a switching regulator is equal to the output power divided by the input power times 100%. 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: % Efficiency = 100% – (L1 + L2 + L3 +…) where L1, L2, etc. are the individual loss terms as a percentage of input power.
PAGE 19
LTC3646/LTC3646-1 Applications Information Thermal Considerations The LTC3646 requires the exposed package backplane metal (PGND) to be well soldered to the PC board to provide good thermal contact. This gives the QFN and MSOP packages exceptional thermal properties, compared to other packages of similar size, making it difficult in normal operation to exceed the maximum junction temperature of the part. In many applications, the LTC3646 does not generate much heat due to its high efficiency.
PAGE 20
LTC3646/LTC3646-1 Applications Information The following list summarizes the three possible connections for EXTVCC: 1. EXTVCC grounded. This will cause INTVCC to be powered from the internal 5.0V regulator. 2. EXTVCC connected directly to VOUT. This is the normal connection for a 4.5V to 6V regulated output and provides the highest efficiency. 3. EXTVCC connected to an external supply. If an external supply is available in the 4.5V to 6V range, it may be used to power EXTVCC. Ensure that EXTVCC < VIN.
PAGE 21
LTC3646/LTC3646-1 Applications Information RSVIN R2 CSVIN R1 CF CCOMP CVCC RCOMP RRT CBOOST RPGOOD SW GND CPVIN PVIN VIA TO VOUT VIA TO INTVCC VIA TO PVIN VIA TO GND COUT L1 VOUT 3646 F07 Figure 7. Example of Power Component Layout for DFN Package. Because of the Similar Pinout, MSE Package Layout is Similar PVIN BOOST PVIN RSVIN SVIN SW INTVCC RPGOOD CIN 10µF CBOOST LTC3646 RUN L1 VOUT PGOOD ITH RT CSVIN MODE/SYNC RCOMP EXTVCC CVCC RRT CCOMP 0.
PAGE 22
LTC3646/LTC3646-1 Applications Information Design Example As a design example, consider using the LTC3646-1 in an application with the following specifications: VIN = 12V, VOUT = 1.8V, IOUT(MAX) = 1A, IOUT(MIN) = 10mA. Because efficiency is important at both high and low load currents, Burst Mode operation and 1MHz operation is chosen. First the correct RRT resistor value for 1MHz switching frequency must be chosen.
PAGE 23
LTC3646/LTC3646-1 Typical Applications 12V to 1.8V Output with 400kHz External Sync VIN 12V BOOST PVIN 2Ω 0.1µF SVIN LTC3646 RUN SW INTVCC L1 10µH VOUT 1.8V 22µF ITH RT RCOMP 49.9k 2.2µF MODE/SYNC 4.7µF 226k 400kHz EXTERNAL CLOCK EXTVCC 0.6V 100k CCOMP 220pF SWITCH VON CONTROL SGND 274k + – PGND CF 4.
PAGE 24
LTC3646/LTC3646-1 Typical Applications 24V Input to 5V Output at 1MHz Frequency and EXTVCC VIN 24V BOOST PVIN 2Ω SVIN LTC3646 RUN SW INTVCC 100k 22µF 0.1µF L1 10µH VOUT 5V PGOOD ITH RT 2.2µF RCOMP 100k MODE/SYNC EXTVCC 4.7µF 90.9k CCOMP 33pF 0.6V SGND COUT 30µF SWITCH VON CONTROL 487k + – 20pF VFB PGND 66.5k 3646 TA03a Efficiency Curve 50mA to 1A Load Step 100 90 ILOAD 1A/DIV EFFICIENCY (%) 80 70 IL1 1A/DIV 60 50 40 VOUT 200mV/DIV AC-COUPLED 30 20 40µs/DIV 10 0 0.001 0.
PAGE 25
LTC3646/LTC3646-1 Package Description DEPackage Package DE 14-LeadPlastic PlasticDFN DFN (4mm (4mm × × 3mm) 14-Lead 3mm) (ReferenceLTC LTCDWG DWG ## 05-08-1708 05-08-1708 Rev (Reference RevB) B) 0.70 ±0.05 3.30 ±0.05 3.60 ±0.05 2.20 ±0.05 1.70 ±0.05 PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC 3.00 REF RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 4.00 ±0.10 (2 SIDES) R = 0.05 TYP 3.00 ±0.10 (2 SIDES) R = 0.115 TYP 8 0.40 ±0.10 14 3.30 ±0.10 1.70 ±0.
PAGE 26
LTC3646/LTC3646-1 Package Description MSE Package 16-Lead Plastic , Exposed Die Pad MSEMSOP Package (Reference LTC DWG # Rev Pad F) 16-Lead Plastic MSOP,05-08-1667 Exposed Die (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 27
LTC3646/LTC3646-1 Revision History REV DATE DESCRIPTION A 07/13 Changed '%' to '%/V’ regarding VREF Line Regulation in the Electrical Characteristics Table. PAGE NUMBER 3 36461fa 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.
PAGE 28
LTC3646/LTC3646-1 Typical Application Efficiency D1: DFLS 1200 100 LTC3646 RUN SW INTVCC 100k 80 0.1µF SVIN 22µF 90 BOOST PVIN 2Ω L1 47µH VOUT 28V PGOOD ITH RT 2.2µF RCOMP 402k MODE/SYNC SWITCH VON CONTROL EXTVCC 4.7µF 180k 0.6V CCOMP 220pF COUT 33µF SGND 60 50 40 30 10 0 0.001 VFB PGND 70 20 487k + – EFFICIENCY (%) VIN 40V 10.7k Burst Mode OPERATION FORCED CONTINUOUS 0.1 0.