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LT3480

3480fd

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

36V, 2A, 2.4MHz Step-Down

Switching Regulator with

70µA Quiescent Current

The LT

3480 is an adjustable frequency (200kHz to 2.4MHz)

monolithic buck switching regulator that accepts input volt-

ages up to 36V (60V maximum). A high efﬁciency 0.25

switch is included on the die along with a boost Schottky

diode and the necessary oscillator, control, and logic cir-

cuitry. Current mode topology is used for fast transient

response and good loop stability. Low ripple Burst Mode

operation maintains high efﬁciency at low output currents

while keeping output ripple below 15mV in a typical ap-

plication. In addition, the LT3480 can further enhance low

output current efﬁciency 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 ﬂag signals when

OUT

reaches 86% of the programmed output voltage. The

LT3480 is available in 10-pin MSOP and 3mm × 3mm DFN

packages with exposed pads for low thermal resistance.

Wide Input Range:

Operation from 3.6V to 36V

Over-Voltage Lockout Protects Circuits

through 60V Transients

2A Maximum Output Current

Low Ripple Burst Mode

Operation

70µA I

at 12V

to 3.3V

OUT

Output Ripple < 15mV

Adjustable Switching Frequency: 200kHz to 2.4MHz

Low Shutdown Current: I

< 1µA

Integrated Boost Diode

Synchronizable Between 250kHz to 2MHz

Power Good Flag

Saturating Switch Design: 0.25 On-Resistance

0.790V Feedback Reference Voltage

Output Voltage: 0.79V to 20V

Soft-Start Capability

Small 10-Pin Thermally Enhanced MSOP and

(3mm × 3mm) DFN Packages

Automotive Battery Regulation

Power for Portable Products

Distributed Supply Regulation

Industrial Supplies

, 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.

3.3V Step-Down Converter

4.5V TO 36V

TRANSIENT

TO 60V

OUT

3.3V

4.7µF

0.47µF

470pF

22µF

100k

14k

40.2k

4.7µH

316k

GND

OFF ON

LT3480

3480 TA01

RUN/SS BOOST

SYNC

Efﬁciency

LOAD CURRENT (A)

EFFICIENCY (%)

0.5 1.0 1.5 2

3480 TA01b

100

= 12V

L = 5.6µH

F = 800 kHz

OUT

= 3.3V

OUT

= 5V

Summary of content (24 pages)

PAGE 1
LT3480 36V, 2A, 2.4MHz Step-Down Switching Regulator with 70µA Quiescent Current FEATURES n n n n n n n n n n n n n DESCRIPTION Wide Input Range: Operation from 3.6V to 36V Over-Voltage Lockout Protects Circuits through 60V Transients 2A Maximum Output Current Low Ripple Burst Mode® Operation 70µA IQ at 12VIN to 3.3VOUT Output Ripple < 15mV Adjustable Switching Frequency: 200kHz to 2.
PAGE 2
LT3480 ABSOLUTE MAXIMUM RATINGS (Note 1) VIN, RUN/SS Voltage (Note 5)....................................60V 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) LT3480E................................................
PAGE 3
LT3480 ELECTRICAL CHARACTERISTICS The l 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 CONDITIONS MIN Minimum Input Voltage l VIN Overvoltage Lockout l Quiescent Current from VIN Quiescent Current from BD 36 TYP MAX 3 3.6 UNITS V 38 40 V VRUN/SS = 0.2V VBD = 3V, Not Switching VBD = 0, Not Switching l 0.01 30 105 0.
PAGE 4
LT3480 ELECTRICAL CHARACTERISTICS guaranteed over the –40°C to 125°C temperature range. The LT3480H specifications are guaranteed over the –40°C to 150°C temperature range. The LT3480MP specifications are guaranteed over the –55°C to 150°C temperature range. Note 3: Bias current flows out of the FB pin. Note 4: This is the minimum voltage across the boost capacitor needed to guarantee full saturation of the switch.
PAGE 5
LT3480 TYPICAL PERFORMANCE CHARACTERISTICS Maximum Load Current Switch Current Limit SWITCH CURRENT LIMIT(A) LOAD CURRENT (A) 2.5 MINIMUM 2.0 VOUT = 5V TA = 25 °C L = 4.7µH f = 800kHz 1.5 10 5 20 15 INPUT VOLTAGE (V) 25 4.5 4.0 3.5 SWITCH CURRENT LIMIT (A) TYPICAL 3.0 1.0 Switch Current Limit 4.0 3.5 3.0 2.5 2.0 1.5 1.0 30 0 20 60 40 DUTY CYCLE (%) 80 1.5 1.
PAGE 6
LT3480 TYPICAL PERFORMANCE CHARACTERISTICS Soft-Start RUN/SS Pin Current RUN/SS PIN CURRENT (µA) SWITCH CURRENT LIMIT (A) 3.5 3.0 2.5 2.0 1.5 1.0 1.4 10 1.2 8 6 4 2 0.5 0 0 0.5 2.5 2 1.5 RUN/SS PIN VOLTAGE (V) 1 3 0 3.5 Error Amp Output Current 0 5 20 30 15 25 10 RUN/SS PIN VOLTAGE (V) 10 0 –10 –20 0.5 1.0 1.5 BOOST DIODE CURRENT (A) –30 Minimum Input Voltage 6.0 4.0 3.5 3.0 2.5 –40 2.0 VOUT = 3.3V TA = 25°C L = 4.7µH f = 800kHz 1 200 10 5.5 5.0 VOUT = 5V TA = 25 °C L = 4.
PAGE 7
LT3480 TYPICAL PERFORMANCE CHARACTERISTICS Switching Waveforms; Transition from Burst Mode to Full Frequency Switching Waveforms; Full Frequency Continuous Operation VSW 5V/DIV VSW 5V/DIV IL 0.2A/DIV IL 0.5A/DIV VOUT 10mV/DIV VOUT 10mV/DIV 1µs/DIV VIN = 12V; FRONT PAGE APPLICATION ILOAD = 110mA 3480 G25 1µs/DIV VIN = 12V; FRONT PAGE APPLICATION ILOAD = 1A 3480 G26 PIN FUNCTIONS BD (Pin 1): This pin connects to the anode of the boost Schottky diode.
PAGE 8
LT3480 BLOCK DIAGRAM VIN 4 VIN C1 – + INTERNAL 0.79V REF 5 10 RT 6 RUN/SS Σ RT SLOPE COMP OSCILLATOR 200kHz–2.4MHz SWITCH LATCH BOOST 1 2 C3 R S Q SW DISABLE SYNC L1 VOUT 3 C2 D1 Burst Mode DETECT SOFT-START 7 BD PG ERROR AMP + – + – 0.7V GND 11 FB VC CLAMP VC 9 CC RC CF 8 R2 R1 3480 BD OPERATION The LT3480 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.
PAGE 9
LT3480 OPERATION The LT3480 contains a power good comparator which trips when the FB pin is at 86% of its regulated value. The PG output is an open-collector transistor that is off when the output is in regulation, allowing an external resistor to pull the PG pin high. Power good is valid when the LT3480 is enabled and VIN is above 3.6V. The LT3480 has an overvoltage protection feature which disables switching action when the VIN goes above 38V typical (36V minimum).
PAGE 10
LT3480 APPLICATIONS INFORMATION A good choice of switching frequency should allow adequate input voltage range (see next section) and keep the inductor and capacitor values small. Input Voltage Range The maximum input voltage for LT3480 applications depends on switching frequency, the Absolute Maximum Ratings of the VIN and BOOST pins, and the operating mode. The LT3480 can operate from input voltages up to 38V, and safely withstand input voltages up 60V.
PAGE 11
LT3480 APPLICATIONS INFORMATION where VD is the voltage drop of the catch diode (~0.4V), VIN(MAX) is the maximum input voltage, VOUT is the output voltage, fSW is the switching frequency (set by RT), and L is in the inductor value. The inductor’s RMS current rating must be greater than the maximum load current and its saturation current should be about 30% higher. For robust operation in fault conditions (start-up or short circuit) and high input voltage (>30V), the saturation current should be above 3.5A.
PAGE 12
LT3480 APPLICATIONS INFORMATION energy in order to satisfy transient loads and stabilize the LT3480’s control loop. Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance. A good starting value is: COUT = 100 VOUT fSW Catch Diode The catch diode conducts current only during switch off time.
PAGE 13
LT3480 APPLICATIONS INFORMATION A final precaution regarding ceramic capacitors concerns the maximum input voltage rating of the LT3480. A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT3480 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3480’s rating. This situation is easily avoided (see the Hot Plugging Safely section).
PAGE 14
LT3480 APPLICATIONS INFORMATION VOUT 100mV/DIV VSW 5V/DIV IL 0.2A/DIV IL 0.5A/DIV VOUT 10mV/DIV VIN = 12V; FRONT PAGE APPLICATION 10µs/DIV 3480 F03 Figure 3. Transient Load Response of the LT3480 Front Page Application as the Load Current Is Stepped from 500mA to 1500mA. VOUT = 3.3V Low-Ripple Burst Mode and Pulse-Skip Mode The LT3480 is capable of operating in either Low-Ripple Burst Mode or Pulse-Skip Mode which are selected using the SYNC pin. See the Synchronization section for details.
PAGE 15
LT3480 APPLICATIONS INFORMATION 6.0 VOUT BD 5.5 BOOST VIN LT3480 GND 4.7µF C3 INPUT VOLTAGE (V) VIN SW TO START (WORST CASE) 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 D2 BD BOOST VIN LT3480 GND 4.7µF VOUT BD 4.7µF LT3480 GND 6.0 5.0 TO RUN 4.0 VOUT = 5V TA = 25°C L = 8.2µH f = 700kHz 3.0 2.0 BOOST VIN 10000 TO START (WORST CASE) 7.0 SW (5b) For 2.5V < VOUT < 2.8V VIN 100 1000 LOAD CURRENT (A) 8.
PAGE 16
LT3480 APPLICATIONS INFORMATION Soft-Start The RUN/SS pin can be used to soft-start the LT3480, 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
LT3480 APPLICATIONS INFORMATION PCB Layout Hot Plugging Safely 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 LT3480’s VIN and SW pins, the catch diode (D1) and the input capacitor (C1). The loop formed by these components should be as small as possible.
PAGE 18
LT3480 APPLICATIONS INFORMATION CLOSING SWITCH SIMULATES HOT PLUG IIN VIN + LOW IMPEDANCE ENERGIZED 24V SUPPLY DANGER RINGING VIN MAY EXCEED ABSOLUTE MAXIMUM RATING LT3480 4.7µF IIN 10A/DIV STRAY INDUCTANCE DUE TO 6 FEET (2 METERS) OF TWISTED PAIR 0.7Ω + VIN 20V/DIV 0.1µF 20µs/DIV (10a) VIN 20V/DIV LT3480 4.7µF IIN 10A/DIV (10b) + 22µF 35V AI.EI. + 20µs/DIV VIN 20V/DIV LT3480 4.7µF IIN 10A/DIV (10c) 20µs/DIV 3480 F10 Figure 10.
PAGE 19
LT3480 TYPICAL APPLICATIONS 5V Step-Down Converter VIN 6.8V TO 36V TRANSIENT TO 60V* BD VIN RUN/SS ON OFF VOUT 5V 2A BOOST 0.47µF VC 4.7µF SW LT3480 D RT 16.2k PG SYNC 40.2k 470pF L 6.8µH 536k FB GND 22µF 100k f = 800kHz 3480 TA02 D: DIODES INC. DFLS240L L: TAIYO YUDEN NP06DZB6R8M 3.3V Step-Down Converter VIN 4.4V TO 36V TRANSIENT TO 60V* BD VIN RUN/SS ON OFF VOUT 3.3V 2A BOOST L 4.7µH 0.47µF VC 4.7µF SW LT3480 D RT 14k PG SYNC 40.
PAGE 20
LT3480 TYPICAL APPLICATIONS 5V, 2MHz Step-Down Converter VIN 8.6V TO 22V TRANSIENT TO 38V BD VIN RUN/SS ON OFF VOUT 5V 2A BOOST L 2.2µH 0.47µF VC 2.2µF SW LT3480 D RT 14k PG SYNC 11.5k 470pF 536k FB GND 22µF 100k f = 2MHz 3480 TA05 D: DIODES INC. DFLS240L L: SUMIDA CDRH4D22/HP-2R2 12V Step-Down Converter VIN 15V TO 36V TRANSIENT TO 60V* BD VIN ON OFF VOUT 12V 2A RUN/SS BOOST 0.47µF VC 10µF SW LT3480 D RT 26.1k PG SYNC 40.
PAGE 21
LT3480 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. DD Package 10-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1699 Rev C) 0.70 ±0.05 3.55 ±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 = 0.125 TYP 6 0.40 ± 0.10 10 1.65 ± 0.10 (2 SIDES) PIN 1 NOTCH R = 0.20 OR 0.
PAGE 22
LT3480 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. MSE Package 10-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1664 Rev G) BOTTOM VIEW OF EXPOSED PAD OPTION 1.88 ± 0.102 (.074 ± .004) 5.23 (.206) MIN 1 0.889 ± 0.127 (.035 ± .005) 0.05 REF 10 0.305 ± 0.038 (.0120 ± .0015) TYP RECOMMENDED SOLDER PAD LAYOUT 3.00 ± 0.102 (.118 ± .004) (NOTE 3) DETAIL “B” CORNER TAIL IS PART OF DETAIL “B” THE LEADFRAME FEATURE.
PAGE 23
LT3480 REVISION HISTORY (Revision history begins at Rev D) REV DATE DESCRIPTION D 10/11 Added H- and MP-grades for the MSE package PAGE NUMBER 2, 3 Revised BD pin description 7 Revised Figure 5 to add capacitors 15 3480fd 23
PAGE 24
LT3480 TYPICAL APPLICATION 1.2V Step-Down Converter VOUT 1.2V 2A VIN 3.6V TO 27V BD VIN RUN/SS ON OFF BOOST 0.47µF VC 4.7µF LT3480 SW D RT 16.2k PG SYNC 68.1k 330pF L 3.3µH GND 52.3k FB 100k f = 500kHz 47µF 3480 TA09 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.