Datasheet

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

LT3481

3481f

36V, 2A, 2.8MHz Step-Down

Switching Regulator with

50µA Quiescent Current

The LT

3481 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. 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 LT3481 can fur-

ther 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 LT3481 is available in 10-Pin MSOP

and 3mm x 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

■

Low Ripple Burst Mode

Operation

50µA I

at 12V

to 3.3V

OUT

Output Ripple < 15mV

■

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

■

Synchronizable Between 275kHz to 475kHz

■

Small 10-Pin Thermally Enhanced MSOP and

(3mm x 3mm) DFN Packages

3.3V Step-Down Converter

APPLICATIO S

FEATURES

DESCRIPTIO

TYPICAL APPLICATIO

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

LT3481

3481 TA01

RUN/SS BOOST

Efﬁ ciency

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

All other trademarks are the property of their respective owners.

LOAD CURRENT (A)

0.0001

EFFICIENCY (%)

POWER LOSS (mW)

0.001 0.01 0.1 1 10

3481 TA01b

0.1

0.01

10.0

1.0

10000.0

1000.0

100.0

= 12V

OUT

= 3.3V

L = 4.7µ

F = 800 kHz

Summary of content (24 pages)

PAGE 1
LT3481 36V, 2A, 2.8MHz Step-Down Switching Regulator with 50µA Quiescent Current U DESCRIPTIO FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Wide Input Range: 3.6V to 34V Operating, 36V Maximum 2A Maximum Output Current Low Ripple Burst Mode® Operation 50µA IQ at 12VIN to 3.3VOUT Output Ripple < 15mV Adjustable Switching Frequency: 300kHz to 2.8MHz Low Shutdown Current: IQ < 1µA Integrated Boost Diode Power Good Flag Saturating Switch Design: 0.18Ω On-Resistance 1.265V Feedback Reference Voltage Output Voltage: 1.
PAGE 2
LT3481 U W W W ABSOLUTE AXI U RATI GS (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
LT3481 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) VFB = 1.25V, VC = 0.4V FB Voltage Line Regulation 4V < VIN < 34V 1.25 1.24 ● TYP MAX UNITS 2.7 3 V 1.265 1.265 1.29 1.3 V V 30 100 nA 0.002 0.
PAGE 4
LT3481 U W TYPICAL PERFOR A CE CHARACTERISTICS Efﬁciency (VOUT = 3.3V) Efﬁciency (VOUT = 5.0V) 90 VIN = 12V 80 EFFICIENCY (%) 50 40 60 40 30 20 20 L: NEC PLC-0745-4R7 f: 800kHz 0 0.0001 0.001 0.01 0.1 LOAD CURRENT (A) 1 0 0.0001 0.01 0.1 LOAD CURRENT (A) 1 20 Maximum Load Current 3.5 VIN = 12V VOUT = 3.3V 250 INCREASED SUPPLY CURRENT DUE TO CATCH DIODE LEAKAGE AT HIGH TEMPERATURE 200 150 FRONT PAGE APPLICATION 20 25 15 INPUT VOLTAGE (V) 30 25 0 50 75 TEMPERATURE (°C) 100 3.
PAGE 5
LT3481 U W TYPICAL PERFOR A CE CHARACTERISTICS 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 4381 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
LT3481 U W TYPICAL PERFOR A CE CHARACTERISTICS 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 INPUT VOLTAGE (V) 80 3.5 3.0 VOUT = 3.3V TA = 25 °C L = 4.7µ f = 800kHz 2.5 –60 –80 1.065 1.165 1.365 1 .265 FB PIN VOLTAGE (V) 2.0 0.001 1.465 5.0 4.5 0.1 0.01 1 LOAD CURRENT (A) 4.0 0.001 10 1.200 2.00 1.180 THRESHOLD VOLTAGE (V) 2.50 CURRENT LIMIT CLAMP 1.00 SWITCHING THRESHOLD 0.
PAGE 7
LT3481 U U U PI FU CTIO S 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 LT3481’s internal regulator and to the internal power switch.
PAGE 8
LT3481 OPERATION The LT3481 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
LT3481 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 LT3481 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
LT3481 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 LT3481 may enter pulse skipping operation where some switching pulses are skipped to maintain output regulation.
PAGE 11
LT3481 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
LT3481 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.
PAGE 13
LT3481 APPLICATIONS INFORMATION A ﬁnal precaution regarding ceramic capacitors concerns the maximum input voltage rating of the LT3481. A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT3481 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3481’s rating. This situation is easily avoided (see the Hot Plugging Safely section).
PAGE 14
LT3481 APPLICATIONS INFORMATION Burst Mode Operation To enhance efﬁciency at light loads, the LT3481 automatically switches to Burst Mode operation which keeps the output capacitor charged to the proper voltage while minimizing the input quiescent current. During Burst Mode operation, the LT3481 delivers single cycle bursts of current to the output capacitor followed by sleep periods where the output power is delivered to the load by the output capacitor.
PAGE 15
LT3481 APPLICATIONS INFORMATION may not be fully charged. Because the boost capacitor is charged with the energy stored in the inductor, the circuit will rely on some minimum load current to get the boost circuit running properly. This minimum load will depend on input and output voltages, and on the arrangement of the boost circuit. The minimum load generally goes to zero once the circuit has started. Figure 6 shows a plot of minimum load to start and to run as a function of input voltage.
PAGE 16
LT3481 APPLICATIONS INFORMATION from 5% up to a maximum value given by the following equation: ⎛ VOUT + VD ⎞ – f • 600ns DCSYNC(MAX ) = ⎜ 1 – VIN – VSW + VD ⎟⎠ SW ⎝ where VOUT is the programmed output voltage, VD is the diode forward drop, VIN is the typical input voltage, VSW is the switch drop, and fSW is the desired switching frequency. For example, a 24V input to 5V output at 300kHz can be synchronized to a square wave with a maximum duty cycle of 60%.
PAGE 17
LT3481 APPLICATIONS INFORMATION Hot Plugging Safely L1 The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LT3481 circuits. However, these capacitors can cause problems if the LT3481 is plugged into a live supply (see Linear Technology Application Note 88 for a complete discussion).
PAGE 18
LT3481 APPLICATIONS INFORMATION ﬁrst plot is the response with a 4.7µF ceramic capacitor at the input. The input voltage rings as high as 50V and the input current peaks at 26A. A good solution is shown in Figure 10b. A 0.7Ω resistor is added in series with the input to eliminate the voltage overshoot (it also reduces the peak input current). A 0.1µF capacitor improves high frequency ﬁltering. For high input voltages its impact on efﬁciency is minor, reducing efﬁciency by 1.
PAGE 19
LT3481 TYPICAL APPLICATIONS 3.3V Step-Down Converter VOUT 3.3V 2A VIN 4.4V TO 34V BD VIN ON OFF RUN/SS BOOST L 4.7µH 0.47µF VC 4.7µF SW LT3481 D RT 16.2k BIAS PG 324k 60.4k FB GND 330pF 22µF 200k f = 800kHz 3481 TA03 D: DIODES INC. DFLS240L L: TAIYO YUDEN NP06DZB4R7M 2.5V Step-Down Converter VOUT 2.5V 2A VIN 4V TO 34V BD VIN ON OFF RUN/SS D2 BOOST L 4.7µH 1µF VC 4.7µF SW LT3481 D1 RT 22.1k BIAS PG 196k 84.
PAGE 20
LT3481 TYPICAL APPLICATIONS 12V Step-Down Converter VOUT 12V 2A VIN 15V TO 34V BD VIN ON OFF RUN/SS BOOST L 10µH 0.47µF VC 10µF LT3481 SW D RT 30k BIAS PG 845k 60.4k GND FB 330pF 22µF 100k f = 800kHz 3481 TA06 D: DIODES INC. DFLS240L L: NEC/TOKIN PLC-0755-100 5V Step-Down Converter with Sync Input VIN 20V TO 34V VIN 4.7µF NOTE: DO NOT APPLY SYNC SIGNAL UNTIL PGOOD GOES HIGH BD RUN/SS ON OFF VOUT 5V 2A BOOST 0.47µF VC 8.2pF RT SYNC IN L 8.2µH SW D LT3481 3.
PAGE 21
LT3481 TYPICAL APPLICATIONS 5V Step-Down Converter with Sync and One-Shot VIN 8V TO 34V VIN 4.7µF BD RUN/SS ON OFF VOUT 5V 2A BOOST L 15µH 0.47µF VC 15pF 1k SYNC IN RT 3.3V SQ WAVE 360kHz TO 450kHz SW D LT3481 AND BIAS PG 25k 11.8k 75pF 100k Q1 29.4k 133k 25k 50pF 1000pF FB GND VOUT 47µF 10k f = 300kHz 3481 TA08 D: DIODES INC. DFLS240L L: NEC/TOKIN PLC-0755-150 Q1: ON SEMI MMBT3904 1.8V Step-Down Converter VOUT 1.8V 2A VIN 3.5V TO 27V BD VIN ON OFF RUN/SS BOOST L 3.
PAGE 22
LT3481 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 R = 0.115 TYP 6 3.00 ±0.10 (4 SIDES) 0.38 ± 0.10 10 1.65 ± 0.10 (2 SIDES) PIN 1 TOP MARK (SEE NOTE 6) (DD10) 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
LT3481 PACKAGE DESCRIPTION MSE Package 10-Lead Plastic MSOP (Reference LTC DWG # 05-08-1663) 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
LT3481 U TYPICAL APPLICATIO 1.265V Step-Down Converter VIN 3.6V TO 27V BD VIN RUN/SS ON OFF VOUT 1.265V 2A BOOST 0.47µF VC 4.7µF LT3481 SW D RT 13k BIAS PG 105k L 3.3µH GND FB 330pF 47µF f = 500kHz 3481 TA10 D: DIODES INC. DFLS240L L: TAIYO YUDEN NP06DZB3R3M RELATED PARTS VOUT MIN (V) IQ ISD PACKAGE 36 1.2 1.6mA <1µA ThinSOT 3.3 80 1.25 100µA <1µA DFN 36V, 1.4A (IOUT), 500kHz High Efﬁciency Step-Down DC/DC Converter 3.6 36 1.2 1.9mA <1µA MS8E LT3493 36V, 1.