Datasheet

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

LT3481

3481fc

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

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

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

TYPICAL APPLICATION

DESCRIPTION

FEATURES

APPLICATIONS

L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered

trademarks of Linear Technology Corporation. All other trademarks are the property of their

respective owners.

Summary of content (24 pages)

PAGE 1
LT3481 36V, 2A, 2.8MHz Step-Down Switching Regulator with 50µA Quiescent Current FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTION 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 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........................... 125°C LT3481E, LT3481I ...........................
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 Input Voltage Quiescent Current from VIN VRUN/SS = 0.2V VBIAS = 3V, Not Switching ● VBIAS = 0, Not Switching Quiescent Current from BIAS VRUN/SS = 0.
PAGE 4
LT3481 TYPICAL PERFORMANCE CHARACTERISTICS Efﬁciency (VOUT = 3.3V) Efﬁciency (VOUT = 5.0V) 90 VIN = 12V 80 EFFICIENCY (%) 60 50 40 60 20 0 0.0001 0.001 0.01 0.1 LOAD CURRENT (A) 1 VIN = 24V 40 20 L: NEC PLC-0745-4R7 f: 800kHz 0 0.0001 0.001 0.01 0.1 LOAD CURRENT (A) 1 30 20 10 VIN = 12V VOUT = 3.3V 3.5 250 200 INCREASED SUPPLY CURRENT DUE TO CATCH DIODE LEAKAGE AT HIGH TEMPERATURE 150 100 25 20 15 INPUT VOLTAGE (V) 30 0 25 50 75 100 TEMPERATURE (°C) 3481 G04 3.
PAGE 5
LT3481 TYPICAL PERFORMANCE CHARACTERISTICS Switch Voltage Drop 500 400 300 200 100 90 1.290 80 1.285 70 FEEDBACK VOLTAGE (V) BOOST PIN CURRENT (mA) VOLTAGE DROP (mV) 600 60 50 40 30 20 500 0 1000 1500 2000 2500 3000 3500 SWITCH CURRENT (mA) 500 1000 1500 2000 2500 3000 3500 SWITCH CURRENT (mA) RT = 45.3k 1.00 0.95 0.90 125 150 Minimum Switch On-Time 140 1000 800 600 400 200 0.85 0 200 Soft-Start 400 600 800 1000 1200 1400 FB PIN VOLTAGE (mV) 80 60 40 20 RUN/SS Pin Current 2.5 2.
PAGE 6
LT3481 TYPICAL PERFORMANCE 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 = 4.7μH f = 800kHz 2.5 –60 –80 1.065 1?.265 1.165 1.365 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 2.50 THRESHOLD VOLTAGE (V) 1.50 1.00 SWITCHING THRESHOLD 0.
PAGE 7
LT3481 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 LT3481’s internal regulator and to the internal power switch. This pin must be locally bypassed.
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 The LT3481 uses current mode control to regulate the output. This simpliﬁes loop compensation. In particular, the LT3481 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. Generally a capacitor (CC) and a resistor (RC) in series to ground are used.
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 ⎞ DCSYNC(MAX ) = ⎜ 1 – – f • 600ns 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 C2 VOUT CC RRT RC R2 R1 D1 C1 RPG GND 3481 F09 VIAS TO LOCAL GROUND PLANE VIAS TO RUN/SS VIAS TO VOUT VIAS TO PG VIAS TO VIN OUTLINE OF LOCAL GROUND PLANE Figure 9. A Good PCB Layout Ensures Proper, Low EMI Operation CLOSING SWITCH SIMULATES HOT PLUG IIN VIN The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LT3481 circuits.
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 VIN BD RUN/SS ON OFF BOOST L 4.7μH 0.47μF VC 4.7μF SW LT3481 D RT 16.2k PG BIAS 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 VIN BD RUN/SS ON OFF 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 VIN BD RUN/SS ON OFF BOOST 0.47μF VC 10μF SW LT3481 D RT 30k L 10μH PG BIAS 845k 60.4k FB GND 330pF 22μF 100k f = 800kHz 3481 TA06 D: DIODES INC. DFLS240L L: NEC/TOKIN PLC-0755-100 5V Step-Down Converter with Sync Input VOUT 5V 2A VIN 20V TO 34V BD VIN 4.7μF NOTE: DO NOT APPLY SYNC SIGNAL UNTIL PGOOD GOES HIGH RUN/SS ON OFF BOOST L 8.2μH 0.47μF VC 8.2pF RT SYNC IN SW D LT3481 3.
PAGE 21
LT3481 TYPICAL APPLICATIONS 5V Step-Down Converter with Sync and One-Shot VOUT 5V 2A VIN 8V TO 34V BD VIN 4.7μF RUN/SS ON OFF BOOST L 15μH 0.47μF VC 15pF 1k SYNC IN SW RT 3V SQ WAVE Hz TO 450kHz LT3481 AND BIAS PG 25k 11.8k 75pF 100k Q1 29.4k 133k 25k D 50pF VOUT 1000pF FB GND 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 VIN 3.5V TO 27V VIN ON OFF BD RUN/SS BOOST 0.47μF VC 4.
PAGE 22
LT3481 PACKAGE DESCRIPTION DD Package 10-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1699 Rev B) 0.70 p0.05 3.55 p0.05 1.65 p0.05 2.15 p0.05 (2 SIDES) PACKAGE OUTLINE 0.25 p 0.05 0.50 BSC 2.38 p0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 p0.10 (4 SIDES) R = 0.125 TYP 0.40 p 0.10 6 10 5 1 1.65 p 0.10 (2 SIDES) PIN 1 TOP MARK (SEE NOTE 6) 0.200 REF (DD) DFN REV B 0309 0.25 p 0.05 0.50 BSC 0.75 p0.05 0.00 – 0.05 2.38 p0.
PAGE 23
LT3481 PACKAGE DESCRIPTION MSE Package 10-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1664 Rev C) 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 0.05 REF 10 DETAIL “B” CORNER TAIL IS PART OF DETAIL “B” THE LEADFRAME FEATURE. FOR REFERENCE ONLY NO MEASUREMENT PURPOSE 3.00 p 0.102 (.118 p .004) (NOTE 3) 10 9 8 7 6 DETAIL “A” 0o – 6o TYP 1 2 3 4 5 GAUGE PLANE 0.53 p 0.152 (.021 p .006) DETAIL “A” 0.18 (.007) 0.497 p 0.
PAGE 24
LT3481 TYPICAL APPLICATION 1.265V Step-Down Converter VIN 3.6V TO 27V BD VIN RUN/SS ON OFF BOOST 0.47μF VC 4.7μF LT3481 SW D RT 13k VOUT 1.265V 2A BIAS PG 105k L 3.3μH GND FB 330pF 47μF f = 500kHz 3481 TA10 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) = 1.2V, IQ = 1.