Datasheet

ManualsBrandsLINEAR TECHNOLOGY ManualsPMICsPMIC - DC/DC voltage regulator Holder DFN 14

LT3681

3681f

36V, 2A, 2.8MHz Step-Down

Switching Regulator with

Integrated Power Schottky Diode

The LT

3681 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. An undedicated power Schottky diode is

integrated into the LT3681 to minimize the solution size.

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 LT3681 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

90% of the programmed output voltage. The LT3681

is available in 14-Pin 4mm x 3mm DFN package 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 34V Operating,

36V Maximum

■

2A Maximum Output Current

■

Low Ripple Burst Mode

Operation

50µA I

at 12V

to 3.3V

OUT

Output Ripple < 15mV

P-P

■

Adjustable Switching Frequency: 300kHz to 2.8MHz

■

Low Shutdown Current: I

< 1µA

■

Integrated Boost Diode

■

Integrated Power Schottky 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 14-Pin Thermally Enhanced

(4mm x 3mm) DFN Package

5V Step-Down Converter

APPLICATIO S

FEATURES

DESCRIPTIO

TYPICAL APPLICATIO

BIAS

6.3V TO

34V

OUT

4.7 F

0.47 F

330pF

22 F

200k

20k

60.4k

6.8 H

590k

GND

OFF ON

LT3681

3681 TA01

RUN/SS BOOST

L: NEC PLC-0745-6R8

Efﬁ ciency

LOAD

(A)

0.0001

EFFICIENCY (%)

POWER LOSS (W)

0.001 0.01 0.1 1 10

3681 TA01b

100

0.0001

0.0100

0.0010

10.0000

1.0000

0.1000

= 12V

OUT

= 3.3V

L = 4.7 H

F = 800 kHz

, LT, LTC and LTM 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
LT3681 36V, 2A, 2.8MHz Step-Down Switching Regulator with Integrated Power Schottky Diode U FEATURES DESCRIPTIO ■ The LT®3681 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. An undedicated power Schottky diode is integrated into the LT3681 to minimize the solution size.
PAGE 2
LT3681 U W W W ABSOLUTE AXI U RATI GS U W U PACKAGE/ORDER I FOR ATIO (Note 1) TOP VIEW 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
LT3681 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 Power Schottky Diode Forward Voltage IDA = 1A IDA = 2A Power Schottky Diode Leakage Current VDC-DA = 40V Switching Frequency RT = 8.66k RT = 29.4k RT = 187k MIN MAX 0.50 0.56 2.5 1.25 250 ● Minimum Switch Off-Time TYP 3.
PAGE 4
LT3681 U W TYPICAL PERFOR A CE CHARACTERISTICS TA = 25°C unless otherwise noted. Efﬁciency (VOUT = 3.3V) Efﬁciency (VOUT = 5.0V) 90 VIN = 12V 80 EFFICIENCY (%) 50 40 40 20 20 0.001 0.01 0.1 LOAD CURRENT (A) 1 VIN = 24V 50 30 L: NEC PLC-0745-4R7 f: 800kHz VIN = 12V 60 30 0 0.0001 80 70 60 70 65 0 0.0001 50 0.001 0.01 0.1 LOAD CURRENT (A) 1 0 10 100 90 Maximum Load Current 90 30 20 3.
PAGE 5
LT3681 U W TYPICAL PERFOR A CE CHARACTERISTICS TA = 25°C unless otherwise noted. Switch Voltage Drop 400 300 200 1.290 80 1.285 FEEDBACK VOLTAGE (V) 500 90 70 BOOST PIN CURRENT (mA) VOLTAGE DROP (mV) 600 60 50 40 30 100 0 500 0 –25 0 25 50 75 TEMPERATURE (°C) Frequency Foldback RT = 45.3kΩ 100 125 4681 G12 Minimum Switch On-Time 140 1200 SWITCHING FREQUENCY (kHz) 1.05 1.00 0.95 0.90 0.85 100 MINIMUM SWITCH ON TIME (ns) RT = 45.3kΩ 1.10 FREQUENCY (MHz) 1.
PAGE 6
LT3681 U W TYPICAL PERFOR A CE CHARACTERISTICS TA = 25°C unless otherwise noted. Error Amp Output Current Minimum Input Voltage Minimum Input Voltage 100 4.5 6.5 4.0 6.0 INPUT VOLTAGE (V) VC PIN CURRENT (µA) 60 40 20 0 –20 –40 3.5 3.0 2.5 –60 –80 1.065 1.165 1.365 1 .265 FB PIN VOLTAGE (V) INPUT VOLTAGE (V) 80 0.1 0.01 1 LOAD CURRENT (A) 1.200 2.00 1.180 THRESHOLD VOLTAGE (V) 2.50 1.
PAGE 7
LT3681 U U U PI FU CTIO S PG (Pin 1): The PG pin is the open collector output of an internal comparator. PG remains low until the FB pin is within 10% of the ﬁnal regulation voltage. PG output is valid when VIN is above 3.5V and RUN/SS is high. BIAS (Pin 2): The BIAS pin supplies the current to the LT3681’s internal regulator. Tie this pin to the lowest available voltage source above 3V (typically VOUT).
PAGE 8
LT3681 W BLOCK DIAGRA VIN 13 VIN C1 2 14 6 BIAS – + INTERNAL 1.265V REF RUN/SS 5 SLOPE COMP BD SWITCH LATCH BOOST 11 C3 R RT OSCILLATOR 300kHz–2.8MHz Q S SW RT DISABLE DC SOFT-START 1 10 BurstMode DETECT DC L1 VOUT 12 C2 9 16 PG ERROR AMP + – GND GND 4 7 + – 1.12V FB GND 15 VC CLAMP DA VC 8 5 CC RC CF 3 R2 R1 3681 BD OPERATION The LT3681 is a constant frequency, current mode stepdown regulator.
PAGE 9
LT3681 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 LT3681 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 10
LT3681 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 LT3681 may enter pulse skipping operation where some switching pulses are skipped to maintain output regulation.
PAGE 11
LT3681 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 12
LT3681 APPLICATIONS INFORMATION Table 2. Capacitor Vendors VENDOR PHONE URL PART SERIES Panasonic (714) 373-7366 www.panasonic.com Ceramic, Polymer, COMMENTS 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.
PAGE 13
LT3681 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
LT3681 APPLICATIONS INFORMATION BOOST and BIAS Pin Considerations Capacitor C3 and the internal boost Schottky diode (see the Block Diagram) are used to generate a boost voltage that is higher than the input voltage. In most cases a 0.22µF capacitor will work well. Figure 5 shows three ways to arrange the boost circuit. The BOOST pin must be more than 2.3V above the SW pin for best efﬁciency. For outputs of 2.8V and above, the standard circuit (Figure 5a) is best. For outputs between 2.
PAGE 15
LT3681 APPLICATIONS INFORMATION 6.0 INPUT VOLTAGE (V) 5.5 5.0 Synchronization VOUT = 3.3V TA = 25°C L = 4.7m f = 800 kHz The internal oscillator of the LT3681 can be synchronized to an external 275kHz to 475kHz clock by using a 5pF to 20pF capacitor to connect the clock signal to the RT pin. The resistor tying the RT pin to ground should be chosen such that the LT3681 oscillates 20% lower than the intended synchronization frequency (see Setting the Switching Frequency section). 4.5 4.0 3.5 3.0 2.5 2.
PAGE 16
LT3681 APPLICATIONS INFORMATION Caution should be used when synchronizing more than 50% above the initial switching frequency (as set by the RT resistor) because at higher clock frequencies the amplitude of the internal slope compensation used to prevent subharmonic switching is reduced. This type of subharmonic switching only occurs at input voltages less than twice output voltage. Higher inductor values will tend to reduce this problem.
PAGE 17
LT3681 APPLICATIONS INFORMATION PCB Layout For proper operation and minimum EMI, care must be taken during printed circuit board layout. Figure 9 shows the recommended component placement with trace, ground plane and via locations. Note that large, switched currents ﬂow in the LT3681’s VIN and SW pins, the integrated Schottky diode the input capacitor (CIN) and the output capacitor (COUT). The loop formed by these components should be as small as possible.
PAGE 18
LT3681 APPLICATIONS INFORMATION The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LT3681 circuits. However, these capacitors can cause problems if the LT3681 is plugged into a live supply (see Linear Technology Application Note 88 for a complete discussion).
PAGE 19
LT3681 APPLICATIONS INFORMATION CLOSING SWITCH SIMULATES HOT PLUG IIN VIN DANGER VIN 20V/DIV RINGING VIN MAY EXCEED ABSOLUTE MAXIMUM RATING LT3681 + LOW IMPEDANCE ENERGIZED 24V SUPPLY 4.7mF IIN 10A/DIV STRAY INDUCTANCE DUE TO 6 FEET (2 METERS) OF TWISTED PAIR 20ms/DIV (10a) 0.7W + 0.1mF LT3681 VIN 20V/DIV 4.7mF IIN 10A/DIV (10b) LT3681 + 22mF 35V AI.EI. + 20ms/DIV VIN 20V/DIV 4.7mF IIN 10A/DIV (10c) 20ms/DIV 3681 F10 Figure 10.
PAGE 20
LT3681 TYPICAL APPLICATIONS 5V Step-Down Converter VOUT 5V 2A VIN 6.3V TO 34V VIN ON OFF BD RUN/SS BOOST 0.47mF VC 4.7mF L 6.8mH SW LT3681 DC RT DA 20k PG BIAS 590k 60.4k FB GND 330pF 22mF 200k f = 800kHz 3681 TA02 L: NEC PLC-0745-6R8 3.3V Step-Down Converter VOUT 3.3V 2A VIN 4.4V TO 34V BD VIN ON OFF RUN/SS BOOST 0.47mF VC 4.7mF LT3681 L 4.7mH SW DC RT DA 20k BIAS PG 324k 60.
PAGE 21
LT3681 TYPICAL APPLICATIONS 2.5V Step-Down Converter VOUT 2.5V 2A VIN 4V TO 34V VIN BD RUN/SS ON OFF D2 BOOST L 4.7mH 1mF VC 4.7mF SW LT3681 DC RT 22k DA BIAS PG 196k 84.5k FB GND 680pF 47mF 200k f = 600kHz 3681 TA04 D2: MBR0540 L: SUMIDA CDRH8D43-4R7 5V, 2MHz Step-Down Converter VIN 8.6V TO 22V TRANSIENT TO 36V VIN ON OFF VOUT 5V 2A BD RUN/SS BOOST 0.47mF VC 2.2mF LT3681 L 2.2mH SW DC RT 6.8k DA BIAS PG 590k 16.
PAGE 22
LT3681 TYPICAL APPLICATIONS 12V Step-Down Converter VIN 15V TO 34V BD VIN RUN/SS ON OFF VOUT 12V 2A BOOST 0.47mF VC 10mF L 10mH SW LT3681 DC RT DA 24k BIAS PG 845k 60.4k FB GND 470pF 22mF 100k f = 800kHz 3681 TA06 L: SUMIDA CDRH8D43-100 1.8V Step-Down Converter VOUT 1.8V 2A VIN 3.6V TO 27V BD VIN RUN/SS ON OFF BOOST 0.47µF VC 10µF LT3681 L 3.3µH SW DC RT DA 15k BIAS PG 84.
PAGE 23
LT3681 PACKAGE DESCRIPTION DE14MA Package 14-Lead Plastic DFN, Multichip (4mm × 3mm) (Reference LTC DWG # 05-08-1731 Rev 0) 1.78 ±0.05 0.70 ±0.05 0.10 TYP 0.51 TYP 3.50 ±0.05 1.65 ± 0.05 2.10 ±0.05 1.07 ±0.05 1.65 ± 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 1.78 ±0.10 14 1.07 ±0.10 1.65 ± 0.10 0.10 TYP 0.51 TYP 1.65 ± 0.
PAGE 24
LT3681 U TYPICAL APPLICATIO 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 L 3.3mH SW LT3681 DC RT DA 13k BIAS PG 105k GND FB 47mF 330pF f = 500kHz 3681 TA10 L: NEC PLC-0745-3R3 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 = 1.20V, IQ = 2.5mA, ISD 25μA TSSOP16E Package LT1767 25V, 1.2A (IOUT), 1.