Datasheet

ManualsBrandsLINEAR TECHNOLOGY ManualsPMICsPMIC - DC/DC voltage regulator Holder MSOP 10 EP

LT3684

3684f

36V, 2A, 2.8MHz Step-Down

Switching Regulator

The LT

3684 is an adjustable frequency (300kHz to 2.8MHz)

monolithic buck switching regulator that accepts input

voltages up to 34V (36V maximum). A high efﬁ ciency

0.18Ω 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. The LT3684’s

high operating frequency allows the use of small, low cost

inductors and ceramic capacitors resulting in low output

ripple while keeping total solution size to a minimum.

The low current shutdown mode 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

90% of the programmed output voltage. The LT3684 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 Range: 3.6V to 34V Operating,

36V Maximum

■

2A Maximum Output Current

■

Adjustable Switching Frequency: 300kHz to 2.8MHz

■

Low Shutdown Current: I

< 1µA

■

Integrated Boost Diode

■

Power Good Flag

■

Saturating Switch Design: 0.18Ω On-Resistance

■

1.265V Feedback Reference Voltage

■

Output Voltage: 1.265V to 20V

■

Soft-Start Capability

■

Small 10-Pin Thermally Enhanced MSOP and

(3mm × 3mm) DFN Packages

3.3V Step-Down Converter

Efﬁ ciency

FEATURES DESCRIPTION

APPLICATIONS

TYPICAL APPLICATION

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.

All other trademarks are the property of their respective owners.

BIAS

4.5V TO

34V

OUT

3.3V

4.7µF

0.47µF

330pF

22µF

200k

16.2k

60.4k

4.7µH

324k

GND

OFF ON

LT3684

3684 TA01

RUN/SS BOOST

LOAD CURRENT (A)

EFFICIENCY (%)

0.5 1 1.5 2

3684 TA01b

= 12V

OUT

= 3.3V

L = 4.7µH

f = 800kHz

Summary of content (24 pages)

PAGE 1
LT3684 36V, 2A, 2.8MHz Step-Down Switching Regulator FEATURES DESCRIPTION ■ The LT®3684 is an adjustable frequency (300kHz to 2.8MHz) monolithic buck switching regulator that accepts input voltages up to 34V (36V maximum). A high efﬁciency 0.18Ω 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
LT3684 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 BIAS, PG, BD Voltage ................................................30V Maximum Junction Temperature ..........................
PAGE 3
LT3684 ELECTRICAL CHARACTERISTICS The ● denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 10V, VRUNS/SS = 10V VBOOST = 15V, VBIAS = 3.3V unless otherwise noted. (Note 2) PARAMETER CONDITIONS MIN Minimum Bias Voltage Feedback Voltage ● ● FB Pin Bias Current (Note 3) FB Voltage Line Regulation 1.25 1.24 4V < VIN < 34V TYP MAX 2.7 3 V 1.265 1.265 1.28 1.29 V V 30 100 nA 0.002 0.
PAGE 4
LT3684 TYPICAL PERFORMANCE CHARACTERISTICS 90 80 70 VIN = 24V 80 VIN = 12V 75 70 65 0 55 50 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 LOAD CURRENT (A) Maximum Load Current 0 LOAD CURRENT (A) 2.0 VOUT = 3.3V TA = 25°C L = 4.7µH f = 800kHz 10 25 20 15 INPUT VOLTAGE (V) 3.0 2.5 MINIMUM 2.0 VOUT = 5V TA = 25°C L = 4.7µH f = 800kHz 1.5 1.0 5 10 25 20 15 INPUT VOLTAGE (V) 3684 G04 1.0 0 1.5 1.0 0 25 50 75 TEMPERATURE (°C) 100 125 3684 G07 100 80 500 400 300 200 100 0.
PAGE 5
LT3684 TYPICAL PERFORMANCE CHARACTERISTICS Switching Frequency 1.20 1.285 1.15 1.280 1.10 1.275 1.270 1.265 0.95 1.255 0.85 0 25 50 75 TEMPERATURE (°C) 100 0.80 –50 125 RT = 45.3k 1.00 0.90 –25 RT = 45.3k 1.05 1.260 1.250 –50 Frequency Foldback 1200 400 200 0 25 50 75 TEMPERATURE (°C) 100 0 125 120 3.5 80 60 40 20 RUN/SS Pin Current 3.0 2.5 2.0 1.5 1.0 25 0 50 75 TEMPERATURE (˚C) 100 125 8 6 4 0 0 –25 10 2 0.5 0 –50 0 0.5 1 2 2.5 1.
PAGE 6
LT3684 TYPICAL PERFORMANCE CHARACTERISTICS 6.0 2.00 5.5 5.0 VOUT = 5V TA = 25°C L = 4.7µH f = 800kHz 4.0 0.001 0.1 0.01 1 LOAD CURRENT (A) 1.200 THRESHOLD VOLTAGE (V) 2.50 4.5 Power Good Threshold VC Voltages 6.5 THRESHOLD VOLTAGE (V) INPUT VOLTAGE (V) Minimum Input Voltage (TA = 25°C unless otherwise noted) CURRENT LIMIT CLAMP 1.50 1.00 SWITCHING THRESHOLD 0.50 10 PG RISING 1.180 1.160 1.140 1.120 0 –50 –25 0 50 25 75 TEMPERATURE (°C) 100 125 1.
PAGE 7
LT3684 PIN FUNCTIONS BD (Pin 1): This pin connects to the anode of the boost Schottky diode. 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. VIN (Pin 4): The VIN pin supplies current to the LT3684’s internal regulator and to the internal power switch. This pin must be locally bypassed.
PAGE 8
LT3684 BLOCK DIAGRAM VIN 4 VIN C1 7 5 10 BIAS – + INTERNAL 1.265V REF RUN/SS Σ SLOPE COMP BD SWITCH LATCH BOOST 2 C3 R RT OSCILLATOR 300kHz–2.8MHz RT Q S SW SOFT-START 6 1 L1 VOUT 3 VC CLAMP C2 D1 PG ERROR AMP + – + – 1.
PAGE 9
LT3684 OPERATION The LT3684 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 10
LT3684 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⎟ ⎝ 1.265 ⎠ Reference designators refer to the Block Diagram. Setting the Switching Frequency The LT3684 uses a constant frequency PWM architecture that can be programmed to switch from 300kHz to 2.8MHz by using a resistor tied from the RT pin to ground.
PAGE 11
LT3684 APPLICATIONS INFORMATION frequency will be necessary to achieve safe operation at high input voltages. If the output is in regulation and no short-circuit or start-up events are expected, then input voltage transients of up to 36V are acceptable regardless of the switching frequency. In this mode, the LT3684 may enter pulse skipping operation where some switching pulses are skipped to maintain output regulation.
PAGE 12
LT3684 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 2A, 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 13
LT3684 APPLICATIONS INFORMATION Table 2. Capacitor Vendors VENDOR PHONE URL Panasonic (714) 373-7366 www.panasonic.com PART SERIES COMMANDS Ceramic, 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.
PAGE 14
LT3684 APPLICATIONS INFORMATION VOUT = 12V, FRONT PAGE APPLICATION LT3684 CURRENT MODE POWER STAGE gm = 3.5mho SW ERROR AMPLIFIER OUTPUT R1 IL 1A/DIV CPL FB – gm = 330µmho ESR 1.265V C1 + + 3M C1 VC CF RC POLYMER OR TANTALUM GND VOUT 100mV/DIV CERAMIC 10µs/DIV 3684 F03 R2 Figure 3. Transient Load Response of the LT3684 Front Page Application as the Load Current is Stepped from 500mA to 1500mA. VOUT = 3.3V CC 3684 F02 Figure 2.
PAGE 15
LT3684 APPLICATIONS INFORMATION 6.0 VOUT TO START BD 5.5 VIN LT3684 GND 4.7µF INPUT VOLTAGE (V) BOOST VIN C3 SW 5.0 4.5 4.0 TO RUN 3.5 3.0 (4a) For VOUT > 2.8V 2.5 2.0 0.001 VOUT D2 BD BOOST VIN VIN LT3684 VOUT = 3.3V TA = 25°C L = 4.7µH f = 800kHz 0.1 0.01 1 LOAD CURRENT (A) 10 8.0 TO START C3 7.0 SW INPUT VOLTAGE (V) GND 4.7µF (4b) For 2.5V < VOUT < 2.8V VOUT BD VIN LT3684 5.0 TO RUN 4.0 3.0 2.0 0.001 BOOST VIN 6.0 C3 VOUT = 5V TA = 25°C L = 4.7µH f = 800kHz 0.1 0.
PAGE 16
LT3684 APPLICATIONS INFORMATION Soft-Start D4 MBRS140 The RUN/SS pin can be used to soft-start the LT3684, reducing the maximum input current during start-up. The RUN/SS pin is driven through an external RC ﬁlter to create a voltage ramp at this pin. Figure 7 shows the startup and shut-down waveforms with the soft-start circuit. By choosing a large RC time constant, the peak start-up current can be reduced to the current that is required to regulate the output, with no overshoot.
PAGE 17
LT3684 APPLICATIONS INFORMATION L1 C2 VOUT RRT CC RC R2 R1 D1 C1 RPG GND 3684 F08 VIAS TO LOCAL GROUND PLANE VIAS TO RUN/SS VIAS TO VOUT VIAS TO PG VIAS TO VIN OUTLINE OF LOCAL GROUND PLANE Figure 8. A Good PCB Layout Ensures Proper, Low EMI Operation Hot Plugging Safely The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LT3684 circuits.
PAGE 18
LT3684 APPLICATIONS INFORMATION CLOSING SWITCH SIMULATES HOT PLUG IIN VIN DANGER VIN 20V/DIV RINGING VIN MAY EXCEED ABSOLUTE MAXIMUM RATING LT3684 + 4.7µF LOW IMPEDANCE ENERGIZED 24V SUPPLY IIN 10A/DIV STRAY INDUCTANCE DUE TO 6 FEET (2 METERS) OF TWISTED PAIR 20µs/DIV (9a) 0.7Ω LT3684 VIN 20V/DIV + 0.1µF 4.7µF IIN 10A/DIV (9b) LT3684 + 22µF 35V AI.EI. 20µs/DIV VIN 20V/DIV + 4.7µF IIN 10A/DIV (9c) 20µs/DIV 3684 F09 Figure 9.
PAGE 19
LT3684 TYPICAL APPLICATIONS 5V Step-Down Converter VOUT 5V 2A VIN 6.3V TO 34V BD VIN RUN/SS ON OFF BOOST 0.47µF VC 4.7µF SW LT3684 D RT 20k L 6.8µH BIAS PG 590k 60.4k FB GND 330pF 22µF 200k f = 800kHz 3684 TA02 D: DIODES INC. DFLS240L L: TAIYO YUDEN NP06DZB6R8M 3.3V Step-Down Converter VOUT 3.3V 2A VIN 4.4V TO 34V BD VIN RUN/SS ON OFF BOOST 0.47µF VC 4.7µF LT3684 SW D RT 16.2k L 4.7µH BIAS PG 324k 60.4k 330pF f = 800kHz GND FB 22µF 200k 3684 TA03 D: DIODES INC.
PAGE 20
LT3684 TYPICAL APPLICATIONS 2.5V Step-Down Converter VOUT 2.5V 2A VIN 4V TO 34V BD VIN RUN/SS ON OFF D2 BOOST L 4.7mH 1mF VC 4.7mF LT3684 SW D1 RT 22.1k BIAS PG 196k 84.5k GND FB 220pF 47mF 200k f = 600kHz 3684 TA04 D1: DIODES INC. DFLS240L D2: MBR0540 L: TAIYO YUDEN NP06DZB4R7M 5V, 2MHz Step-Down Converter VIN 8.6V TO 22V TRANSIENT TO 36V VOUT 5V 2A BD VIN RUN/SS ON OFF BOOST 0.47mF VC 2.2mF LT3684 SW D RT 20k L 2.2mH BIAS PG 590k 16.
PAGE 21
LT3684 TYPICAL APPLICATIONS 12V Step-Down Converter VOUT 12V 2A VIN 15V TO 34V BD VIN RUN/SS ON OFF BOOST L 10µH 0.47µF VC 10µF SW LT3684 D RT 30k BIAS PG 845k 60.4k FB GND 330pF 22µF 100k f = 800kHz 3684 TA06 D: DIODES INC. DFLS240L L: NEC/TOKIN PLC-0755-100 1.8V Step-Down Converter VOUT 1.8V 2A VIN 3.5V TO 27V BD VIN RUN/SS ON OFF BOOST L 3.3µH 0.47µF VC 4.7µF LT3684 SW D RT 15.4k BIAS PG 84.5k 105k 330pF f = 500kHz GND FB 47µF 200k 3684 TA07 D: DIODES INC.
PAGE 22
LT3684 PACKAGE DESCRIPTION DD Package 10-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1699) 0.675 ±0.05 3.50 ±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 = 0.115 TYP 6 0.38 ± 0.10 10 1.65 ± 0.10 (2 SIDES) PIN 1 TOP MARK (SEE NOTE 6) (DD) DFN 1103 5 0.200 REF 1 0.75 ±0.05 0.00 – 0.05 0.25 ± 0.05 0.50 BSC 2.38 ±0.10 (2 SIDES) BOTTOM VIEW—EXPOSED PAD NOTE: 1.
PAGE 23
LT3684 PACKAGE DESCRIPTION MSE Package 10-Lead Plastic MSOP (Reference LTC DWG # 05-08-1664) BOTTOM VIEW OF EXPOSED PAD OPTION 2.794 ± 0.102 (.110 ± .004) 5.23 (.206) MIN 0.889 ± 0.127 (.035 ± .005) 1 2.06 ± 0.102 (.081 ± .004) 1.83 ± 0.102 (.072 ± .004) 2.083 ± 0.102 3.20 – 3.45 (.082 ± .004) (.126 – .136) 10 0.50 0.305 ± 0.038 (.0197) (.0120 ± .0015) BSC TYP RECOMMENDED SOLDER PAD LAYOUT 3.00 ± 0.102 (.118 ± .004) (NOTE 3) 3.00 ± 0.102 (.118 ± .004) (NOTE 4) 4.90 ± 0.152 (.193 ± .006) 0.254 (.
PAGE 24
LT3684 TYPICAL APPLICATION 1.265V Step-Down Converter VOUT 1.265V 2A VIN 3.6V TO 27V BD VIN RUN/SS ON OFF BOOST 0.47mF VC 4.7mF LT3648 SW D RT 13k BIAS PG 105k L 3.3mH GND FB 47mF 330pF f = 500kHz 3648 TA08 D: DIODES INC. DFLS240L L: TAIYO YUDEN NP06DZB3R3M RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1933 500mA (IOUT), 500kHz Step-Down Switching Regulator in SOT-23 VIN: 3.6V to 36V, VOUT(MIN) = 12V, IQ = 1.