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LT3680

3680fb

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 efﬁ ciency 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 efﬁ ciency at low output currents while

keeping output ripple below 15mV in a typical application.

In addition, the LT3680 can further enhance low output

current efﬁ ciency 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 ﬂ ag 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

Efﬁ ciency

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

L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of

Linear Technology Corporation. Burst Mode is a registered trademark of Linear Technology

Corporation. All other trademarks are the property of their respective owners.

Summary of content (24 pages)

PAGE 1
LT3680 36V, 3.5A, 2.4MHz Step-Down Switching Regulator with 75µA Quiescent Current FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTION 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.
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 ● 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 ● VRUN/SS = 0.2V VBD = 3V, Not Switching VBD = 0, Not Switching VRUN/SS = 0.
PAGE 4
LT3680 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C unless otherwise noted. Efﬁciency Efﬁciency Efﬁciency 100 100 100 3.0 90 2.5 80 2.0 70 1.5 VIN = 12V EFFICIENCY (%) VIN = 24V 70 60 50 0.5 VIN = 24V 70 60 VOUT = 5V L = 4.7μH f = 600kHz 0 VIN = 34V 80 2 1.5 1 2.5 OUTPUT CURRENT (A) 3 50 3.5 0 0.5 50 2 1.5 1 2.5 OUTPUT CURRENT (A) 3 3680 G01 No Load Supply Current VOUT = 3.
PAGE 5
LT3680 TYPICAL PERFORMANCE CHARACTERISTICS Boost Pin Current 120 600 105 500 400 300 200 90 75 60 45 30 100 0 1 0 2 4 3 SWITCH CURRENT (A) 5 0 1 2 3 SWITCH CURRENT (A) 800 780 4 760 –50 –25 5 Switching Frequency Minimum Switch On-Time Frequency Foldback 140 1.00 0.95 0.90 1000 0.85 0.80 –50 –25 MINIMUM SWITCH ON TIME (ns) SWITCHING FREQUENCY (kHz) 1.15 1.
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 20 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) INPUT VOLTAGE (V) VC PIN CURRENT (μA) 30 VOUT = 3.3V TA = 25°C L = 4.7μH f = 800kHz 1 200 VC Voltages 4.
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: ⎛ V ⎞ R1= R2 ⎜ OUT – 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 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 Ceramic, Tantalum Sanyo (408) 749-9714 www.sanyovideo.com 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 LT3680 CURRENT MODE POWER STAGE gm = 5.3mho SW ERROR AMPLIFIER OUTPUT R1 CPL FB gm = 500μmho + The LT3680 uses current mode control to regulate the output. This simpliﬁes loop compensation. In particular, the LT3680 does not require the ESR of the output capacitor for stability, so you are free to use ceramic capacitors to achieve low output ripple and small circuit size. Frequency compensation is provided by the components tied to the VC pin, as shown in Figure 2.
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 TO START (WORST CASE) VIN VIN LT3680 GND 4.7μF INPUT VOLTAGE (V) BOOST C3 SW 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 VIN LT3680 TO START (WORST CASE) SW (5b) For 2.5V < VOUT < 2.8V VOUT LT3680 5.0 TO RUN 4.0 VOUT = 5V TA = 25°C L = 8.2μH f = 700kHz 2.0 BOOST VIN 6.0 3.0 BD VIN 10000 8.0 INPUT VOLTAGE (V) GND 100 1000 LOAD CURRENT (mA) C3 7.0 4.
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 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 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 VIN BD RUN/SS ON OFF BOOST L 3.3μH 0.47μF VC 4.7μF SW LT3680 D RT 19k PG SYNC 63.4k 316k FB GND 680pF 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 VIN BD RUN/SS ON OFF D2 BOOST 1μF VC 4.7μF LT3680 SW D1 RT 15.4k L 3.3μH PG 215k SYNC 63.
PAGE 20
LT3680 TYPICAL APPLICATIONS 5V, 2MHz Step-Down Converter VOUT 5V 2.5A VIN 8.6V TO 22V TRANSIENT TO 36V VIN BD RUN/SS ON OFF BOOST 0.47μF VC 4.7μF SW LT3680 D RT 15k L 2.2μH PG 536k SYNC 12.7k FB GND 680pF 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 VIN BD RUN/SS ON OFF BOOST 0.47μF VC 10μF LT3680 SW D RT 17.4k L 8.2μH PG 715k SYNC 63.
PAGE 21
LT3680 TYPICAL APPLICATIONS 1.8V Step-Down Converter VOUT 1.8V 3.5A VIN 3.5V TO 27V VIN BD RUN/SS ON OFF BOOST 0.47μF VC 4.7μF LT3680 SW D RT 16.9k L 3.3μH PG SYNC 78.
PAGE 22
LT3680 PACKAGE DESCRIPTION DD Package 10-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1699) 0.675 p 0.05 3.50 p 0.05 1.65 p 0.05 2.15 p 0.05 (2 SIDES) PACKAGE OUTLINE 0.25 p 0.05 0.50 BSC 2.38 p 0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 p 0.10 (4 SIDES) R = 0.115 TYP 6 0.38 p 0.10 10 1.65 p 0.10 (2 SIDES) PIN 1 TOP MARK (SEE NOTE 6) (DD) DFN 1103 5 0.200 REF 1 0.25 p 0.05 0.50 BSC 0.75 p 0.05 0.00 – 0.05 2.38 p 0.10 (2 SIDES) BOTTOM VIEW—EXPOSED PAD NOTE: 1.
PAGE 23
LT3680 PACKAGE DESCRIPTION MSE Package 10-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1664 Rev B) BOTTOM VIEW OF EXPOSED PAD OPTION 2.794 p 0.102 (.110 p .004) 5.23 (.206) MIN 0.889 p 0.127 (.035 p .005) 1 2.06 p 0.102 (.081 p .004) 1.83 p 0.102 (.072 p .004) 2.083 p 0.102 3.20 – 3.45 (.082 p .004) (.126 – .136) 10 0.50 0.305 p 0.038 (.0197) (.0120 p .0015) BSC TYP RECOMMENDED SOLDER PAD LAYOUT 3.00 p 0.102 (.118 p .004) (NOTE 3) 3.00 p 0.102 (.118 p .004) (NOTE 4) 4.90 p 0.
PAGE 24
LT3680 TYPICAL APPLICATION 1.2V Step-Down Converter VOUT 1.2V 3.5A VIN 3.6V TO 27V VIN BD RUN/SS ON OFF BOOST 0.47μF VC 4.7μF LT3680 SW D RT 17k L 3.3μH PG 52.3k SYNC 78.7k GND 470pF FB 100k 100μF f = 500kHz 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.