Datasheet

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LT3680

Rev C

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36V, 3.5A, 2.4MHz

Step-Down Switching Regulator

with 75µA Quiescent Current

The LT

3680 is an adjustable frequency (200kHz to

2.4MHz) monolithic buck switching regulator that accepts

input voltages up to 36V. A high efficiency 95m switch

is included on the die along with a boost Schottky diode

and the necessary oscillator, control, and logic circuitry.

Current mode topology is used for fast transient response

and good loop stability. Low ripple Burst Mode operation

maintains high efficiency at low output currents while

keeping output ripple below 15mV in a typical application.

In addition, the LT3680 can further enhance low output

current efficiency by drawing bias current from the output

when V

OUT

is above 3V. Shutdown reduces input supply

current to less than 1µA while a resistor and capacitor on

the RUN/SS pin provide a controlled output voltage ramp

(soft-start). A power good flag signals when V

OUT

reaches

91% of the programmed output voltage. The LT3680 is

available in 10-Pin MSOP and 3mm × 3mm DFN packages

with exposed pads for low thermal resistance.

■

Automotive Battery Regulation

■

Power for Portable Products

■

Distributed Supply Regulation

■

Industrial Supplies

■

Wall Transformer Regulation

■

Wide Input Voltage Range: 3.6V to 36V

■

3.5A Maximum Output Current

■

Low Ripple (<15mV

P-P

) Burst Mode

Operation:

= 75µA at 12V

to 3.3V

OUT

■

Adjustable Switching Frequency: 200kHz to 2.4MHz

■

Low Shutdown Current: I

< 0.5µA

■

Integrated Boost Diode

■

Synchronizable Between 250kHz to 2MHz

■

Power Good Flag

■

Saturating Switch Design: 95m On-Resistance

■

0.790V Feedback Reference Voltage

■

Output Voltage: 0.79V to 30V

■

Thermal Protection

■

Soft-Start Capability

■

Small 10-Pin Thermally Enhanced MSOP and

(3mm × 3mm) DFN Packages

5V Step-Down Converter

6.3V TO 36V

OUT

3.5A

10µF

0.47µF

680pF

47µF

100k

15k

63.4k

4.7µH

536k

GND

OFF ON

LT3680

3680 TA01a

RUN/SS BOOST

SYNC

Efficiency

OUTPUT CURRENT (A)

0 0.5

EFFICIENCY (%)

100

2.5

3680 G01

1.5

3.5

= 12V

= 34V

OUT

= 5V

L = 4.7µH

f = 600kHz

= 24V

TYPICAL APPLICATION

DESCRIPTION

FEATURES

APPLICATIONS

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Summary of content (24 pages)

PAGE 1
LT3680 36V, 3.5A, 2.4MHz Step-Down Switching Regulator with 75µA Quiescent Current FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTION Wide Input Voltage Range: 3.6V to 36V 3.5A Maximum Output Current Low Ripple (<15mVP-P) Burst Mode® Operation: IQ = 75µA at 12VIN to 3.3VOUT Adjustable Switching Frequency: 200kHz to 2.4MHz Low Shutdown Current: IQ < 0.5µA Integrated Boost Diode Synchronizable Between 250kHz to 2MHz Power Good Flag Saturating Switch Design: 95m On-Resistance 0.
PAGE 2
LT3680 ABSOLUTE MAXIMUM RATINGS (Note 1) VIN, RUN/SS Voltage..................................................36V BOOST Pin Voltage....................................................56V BOOST Pin Above SW Pin..........................................30V FB, RT, VC Voltage........................................................5V PG, BD, SYNC Voltage...............................................30V Operating Junction Temperature Range (Note 2) LT3680E.............................................
PAGE 3
LT3680 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, VRUN/SS = 10V VBOOST = 15V, VBD = 3.3V unless otherwise noted. (Note 2) PARAMETER Minimum Input Voltage Quiescent Current from VIN Quiescent Current from BD CONDITIONS MIN l VRUN/SS = 0.2V VBD = 3V, Not Switching VBD = 0, Not Switching VRUN/SS = 0.
PAGE 4
LT3680 TYPICAL PERFORMANCE CHARACTERISTICS Efficiency Efficiency 100 VIN = 34V 70 60 0.5 VIN = 24V 70 60 VOUT = 5V L = 4.7µH f = 600kHz 0 VIN = 34V 80 2 1 2.5 1.5 OUTPUT CURRENT (A) 3 50 3.5 0 0.5 2 1 2.5 1.5 OUTPUT CURRENT (A) SUPPLY CURRENT (µA) SUPPLY CURRENT (µA) 90 70 50 30 10 20 15 25 INPUT VOLTAGE (V) 30 300 INCREASED SUPPLY CURRENT DUE TO CATCH DIODE LEAKAGE AT HIGH TEMPERATURE 250 200 150 0 5 10 3.5 30 3680 G07 VOUT = 3.3V TA = 25°C L = 4.
PAGE 5
LT3680 TYPICAL PERFORMANCE CHARACTERISTICS Boost Pin Current 120 600 105 500 400 300 200 90 75 60 45 30 100 0 3 2 4 SWITCH CURRENT (A) 1 0 0 5 Switching Frequency 0 1 2 3 SWITCH CURRENT (A) 4 780 760 –50 –25 5 140 1.00 0.95 0.90 1000 0.85 0.80 –50 –25 0 800 600 400 200 0 25 50 75 100 125 150 TEMPERATURE (°C) MINIMUM SWITCH ON TIME (ns) SWITCHING FREQUENCY (kHz) 1.05 0 4 3 2 40 20 3 3.5 0 25 50 75 100 125 150 TEMPERATURE (°C) Boost Diode 12 1.4 10 1.2 8 6 4 2 1 2.
PAGE 6
LT3680 TYPICAL PERFORMANCE CHARACTERISTICS Error Amp Output Current TA = 25°C unless otherwise noted. Minimum Input Voltage 50 Minimum Input Voltage 5.0 6.5 40 4.5 10 0 –10 –20 –30 4.0 3.5 3.0 2.5 –40 –50 –200 2.0 –100 100 0 FB PIN ERROR VOLTAGE (mV) 6.0 INPUT VOLTAGE (V) 20 INPUT VOLTAGE (V) VC PIN CURRENT (µA) 30 VOUT = 3.3V TA = 25°C L = 4.7µH f = 800kHz 1 10 100 1000 LOAD CURRENT (mA) 3680 G20 VC Voltages 10000 3680 G21 Power Good Threshold 2.
PAGE 7
LT3680 PIN FUNCTIONS BD (Pin 1): This pin connects to the anode of the boost Schottky diode. BD also supplies current to the internal regulator. BOOST (Pin 2): This pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar NPN power switch. SW (Pin 3): The SW pin is the output of the internal power switch. Connect this pin to the inductor, catch diode and boost capacitor.
PAGE 8
LT3680 OPERATION The LT3680 is a constant frequency, current mode stepdown regulator. An oscillator, with frequency set by RT, enables an RS flip-flop, turning on the internal power switch. An amplifier and comparator monitor the current flowing between the VIN and SW pins, turning the switch off when this current reaches a level determined by the voltage at VC. An error amplifier measures the output voltage through an external resistor divider tied to the FB pin and servos the VC pin.
PAGE 9
LT3680 APPLICATIONS INFORMATION FB Resistor Network The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the 1% resistors according to: R1= R2 VOUT –1 0.79 V Reference designators refer to the Block Diagram. Setting the Switching Frequency The LT3680 uses a constant frequency PWM architecture that can be programmed to switch from 200kHz to 2.4MHz by using a resistor tied from the RT pin to ground.
PAGE 10
LT3680 APPLICATIONS INFORMATION maximum operating input voltage. Conversely, a lower switching frequency will be necessary to achieve safe operation at high input voltages. If the output is in regulation and no short-circuit, startup, or overload events are expected, then input voltage transients of up to 36V are acceptable regardless of the switching frequency. In this mode, the LT3680 may enter pulse skipping operation where some switching pulses are skipped to maintain output regulation.
PAGE 11
LT3680 APPLICATIONS INFORMATION Of course, such a simple design guide will not always result in the optimum inductor for your application. A larger value inductor provides a slightly higher maximum load current and will reduce the output voltage ripple. If your load is lower than 3.5A, then you can decrease the value of the inductor and operate with higher ripple current. This allows you to use a physically smaller inductor, or one with a lower DCR resulting in higher efficiency.
PAGE 12
LT3680 APPLICATIONS INFORMATION Table 2. Capacitor Vendors VENDOR PHONE URL PART SERIES Panasonic (714) 373-7366 www.panasonic.com Ceramic, COMMANDS Polymer, EEF Series Tantalum Kemet (864) 963-6300 www.kemet.com Sanyo (408) 749-9714 www.sanyovideo.com Ceramic, Tantalum T494, T495 Ceramic, Polymer, POSCAP Tantalum Murata (408) 436-1300 AVX www.murata.com Ceramic www.avxcorp.com Ceramic, Tantalum Taiyo Yuden (864) 963-6300 www.taiyo-yuden.com capacitor.
PAGE 13
LT3680 APPLICATIONS INFORMATION Loop compensation determines the stability and transient performance. Designing the compensation network is a bit complicated and the best values depend on the application and in particular the type of output capacitor. A practical approach is to start with one of the circuits in this data sheet that is similar to your application and tune the compensation network to optimize the performance.
PAGE 14
LT3680 APPLICATIONS INFORMATION Low-Ripple Burst Mode and Pulse-Skip Mode The LT3680 is capable of operating in either Low-Ripple Burst Mode or pulse-skipping mode which are selected using the SYNC pin. See the Synchronization section for details. To enhance efficiency at light loads, the LT3680 can be operated in Low-Ripple Burst Mode operation which keeps the output capacitor charged to the proper voltage while minimizing the input quiescent current.
PAGE 15
LT3680 APPLICATIONS INFORMATION 6.0 VOUT BD 5.5 BOOST VIN LT3680 GND 4.7µF C3 INPUT VOLTAGE (V) VIN SW TO START (WORST CASE) 5.0 4.5 4.0 TO RUN 3.5 3.0 (5a) For VOUT > 2.8V VOUT = 3.3V TA = 25°C L = 8.2µH f = 700kHz 2.5 2.0 VOUT BD BOOST VIN LT3680 GND 4.7µF VOUT 5.0 TO RUN 4.0 VOUT = 5V TA = 25°C L = 8.2µH f = 700kHz 2.0 BOOST LT3680 6.0 3.0 BD VIN 10000 TO START (WORST CASE) 7.0 SW (5b) For 2.5V < VOUT < 2.8V VIN 100 1000 LOAD CURRENT (mA) 8.
PAGE 16
LT3680 APPLICATIONS INFORMATION IL 1A/DIV RUN 15k RUN/SS 0.22µF VRUN/SS 2V/DIV GND VOUT 2V/DIV 2ms/DIV 3680 F07 Figure 7. To Soft-Start the LT3680, Add a Resisitor and Capacitor to the RUN/SS Pin Synchronizing the LT3680 oscillator to an external frequency can be done by connecting a square wave (with 20% to 80% duty cycle) to the SYNC pin. The square wave amplitude should have valleys that are below 0.3V and peaks that are above 0.8V (up to 6V).
PAGE 17
LT3680 APPLICATIONS INFORMATION L1 that the ground traces will shield them from the SW and BOOST nodes. The Exposed Pad on the bottom of the package must be soldered to ground so that the pad acts as a heat sink. To keep thermal resistance low, extend the ground plane as much as possible, and add thermal vias under and near the LT3680 to additional ground planes within the circuit board and on the bottom side.
PAGE 18
LT3680 APPLICATIONS INFORMATION and the voltage at the VIN pin of the LT3680 can ring to twice the nominal input voltage, possibly exceeding the LT3680’s rating and damaging the part. If the input supply is poorly controlled or the user will be plugging the LT3680 into an energized supply, the input network should be designed to prevent this overshoot. Figure 10 shows the waveforms that result when an LT3680 circuit is connected to a 24V supply through six feet of 24-gauge twisted pair.
PAGE 19
LT3680 TYPICAL APPLICATIONS 3.3V Step-Down Converter VOUT 3.3V 3.5A VIN 4.4V TO 36V BD VIN RUN/SS ON OFF BOOST L 3.3µH 0.47µF VC 4.7µF SW LT3680 D RT 19k PG SYNC 63.4k 680pF 316k FB GND 22µF 100k f = 600kHz 3680 TA03 D: ON SEMI MBRA340 L: NEC MPLC0730L3R3 2.5V Step-Down Converter VOUT 2.5V 3.5A VIN 4V TO 36V BD VIN RUN/SS ON OFF D2 BOOST 1µF VC 4.7µF LT3680 SW D1 RT 15.4k PG SYNC 63.4k 680pF GND L 3.
PAGE 20
LT3680 TYPICAL APPLICATIONS 5V, 2MHz Step-Down Converter VIN 8.6V TO 22V TRANSIENT TO 36V BD VIN RUN/SS ON OFF VOUT 5V 2.5A BOOST 0.47µF VC 4.7µF SW LT3680 D RT 15k PG SYNC 12.7k 680pF L 2.2µH 536k FB GND 22µF 100k f = 2MHz 3680 TA05 D: ON SEMI MBRA340 L: NEC MPLC0730L2R2 12V Step-Down Converter VOUT 12V 3.5A VIN 15V TO 36V BD VIN RUN/SS ON OFF BOOST 0.47µF VC 10µF SW LT3680 D RT 17.4k PG SYNC 63.4k 680pF L 8.
PAGE 21
LT3680 PACKAGE DESCRIPTION Please refer to http://www.linear.com/product/LT3680#packaging for the most recent package drawings. DD Package 10-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1699 Rev C) 0.70 ±0.05 3.55 ±0.05 1.65 ±0.05 2.15 ±0.05 (2 SIDES) PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC 2.38 ±0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 ±0.10 (4 SIDES) R = 0.125 TYP 6 0.40 ±0.10 10 1.65 ±0.10 (2 SIDES) PIN 1 NOTCH R = 0.20 OR 0.
PAGE 22
LT3680 PACKAGE DESCRIPTION Please refer to http://www.linear.com/product/LT3680#packaging for the most recent package drawings. MSE Package 10-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1664 Rev I) BOTTOM VIEW OF EXPOSED PAD OPTION 1.88 ±0.102 (.074 ±.004) 5.10 (.201) MIN 1 0.889 ±0.127 (.035 ±.005) 1.68 ±0.102 (.066 ±.004) 0.05 REF 10 0.305 ± 0.038 (.0120 ±.0015) TYP RECOMMENDED SOLDER PAD LAYOUT 3.00 ±0.102 (.118 ±.
PAGE 23
LT3680 REVISION HISTORY (Revision history begins at Rev C) REV DATE DESCRIPTION C 04/18 Clarified Switch Current Limit Max to 6.6A. PAGE NUMBER 3 Rev C Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice.
PAGE 24
LT3680 TYPICAL APPLICATION 1.2V Step-Down Converter VOUT 1.2V 3.5A VIN 3.6V TO 27V BD VIN RUN/SS ON OFF BOOST 0.47µF VC 4.7µF LT3680 SW D RT 17k PG SYNC 78.7k 470pF L 3.3µH GND 52.3k FB 100k f = 500kHz 100µF 3680 TA09 D: ON SEMI MBRA340 L: NEC MPLC0730L3R3 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1766 60V, 1.2A (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter VIN: 5.5V to 60V, VOUT(MIN) = 1.2V, IQ = 2.5mA, ISD = 25µA, TSSOP16/E Package LT1767 25V, 1.2A (IOUT), 1.