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LT3972

3972fa

TYPICAL APPLICATION

33V, 3.5A, 2.4MHz

Step-Down Switching Regulator

with 75µA Quiescent Current

The LT

3972 is an adjustable frequency (200kHz to 2.4MHz)

monolithic buck switching regulator that accepts input

voltages up to 33V (62V maximum). A high efﬁ ciency

95mΩ 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 LT3972 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 91% of the programmed

output voltage. The LT3972 is available in 10-pin MSOP

and 3mm × 3mm DFN packages with exposed pads for

low thermal resistance.

■

Automotive Battery Regulation

■

Distributed Supply Regulation

■

Industrial Supplies

■

Wall Transformer Regulation

■

Wide Input Range:

Operation from 3.6V to 33V

Overvoltage Lockout Protects Circuits

Through 62V Transients

■

3.5A Maximum Output Current

■

Low Ripple (<15mV

P-P

) Burst Mode

Operation:

= 75μA at 12V

to 3.3V

OUT

■

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: 95mΩ On-Resistance

■

Output Voltage: 0.79V to 30V

■

Thermal Protection

■

Soft-Start Capability

■

Small 10-Pin Thermally Enhanced MSOP and

(3mm × 3mm) DFN Packages

5V Step-Down Converter

6.3V TO 33V

TRANSIENT

TO 62V

OUT

3.5A

10μF

0.47μF

680pF

47μF

100k

15k

63.4k

4.7μH

536k

GND

OFF ON

LT3972

3972 TA01

RUN/SS BOOST

SYNC

Efﬁ ciency

OUTPUT CURRENT (A)

0 0.5

EFFICIENCY (%)

100

2.5

3972 TA01a

1.5

3.5

= 12V

= 30V

OUT

= 5V

L = 4.7μH

f = 600kHz

= 24V

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.

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Summary of content (24 pages)

PAGE 1
LT3972 33V, 3.5A, 2.4MHz Step-Down Switching Regulator with 75µA Quiescent Current FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTION Wide Input Range: Operation from 3.6V to 33V Overvoltage Lockout Protects Circuits Through 62V Transients 3.5A Maximum Output Current Low Ripple (<15mVP-P) Burst Mode® Operation: IQ = 75μA at 12VIN to 3.3VOUT Adjustable Switching Frequency: 200kHz to 2.
PAGE 2
LT3972 ABSOLUTE MAXIMUM RATINGS (Note 1) VIN , RUN/SS Voltage (Note 5)...................................62V 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) LT3972E .............................................
PAGE 3
LT3972 ELECTRICAL CHARACTERISTICS The ● 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 Quiescent Current from BD VRUN/SS = 0.2V VBD = 3V, Not Switching MIN ● VBD = 0, Not Switching Minimum Bias Voltage (BD Pin) Feedback Voltage ● FB Pin Bias Current (Note 3) VFB = 0.8V, VC = 1.
PAGE 4
LT3972 TYPICAL PERFORMANCE CHARACTERISTICS Efﬁciency Efﬁciency 100 VIN = 12V VIN = 24V 70 60 50 0 0.5 VIN = 30V 80 VIN = 24V 70 60 VOUT = 5V L = 4.7μH f = 600kHz 2 1.5 1 2.5 OUTPUT CURRENT (A) 0 2 1.5 1 2.5 OUTPUT CURRENT (A) 0.5 3 3972 G01 80 2.0 70 1.5 30 10 30 3.5 0.5 Maximum Load Current VIN = 12V VOUT = 3.3V 300 250 INCREASED SUPPLY CURRENT DUE TO CATCH DIODE LEAKAGE AT HIGH TEMPERATURE 200 150 4.5 MINIMUM 4.0 3.5 100 VOUT = 3.3V TA = 25°C L = 4.7μH f = 600kHz 3.0 2.
PAGE 5
LT3972 TYPICAL PERFORMANCE CHARACTERISTICS BOOST Pin Current 120 600 105 500 400 300 200 100 840 90 75 60 45 30 0 3 2 4 SWITCH CURRENT (A) 1 0 5 0 1 2 3 SWITCH CURRENT (A) 800 780 Switching Frequency 1.10 1.05 1.00 0.95 0.90 140 1000 120 800 600 400 200 0 25 50 75 100 125 150 TEMPERATURE (°C) 0 Soft-Start 40 20 3 2 0 3 3.5 3972 G16 25 50 75 100 125 150 TEMPERATURE (°C) Boost Diode 12 1.4 10 1.2 8 6 4 2 1 0 3972 G15 BOOST DIODE VF (V) RUN/SS PIN CURRENT (μA) 4 2 2.
PAGE 6
LT3972 TYPICAL PERFORMANCE CHARACTERISTICS Error Amp Output Current TA = 25°C unless otherwise noted. Minimum Input Voltage 50 Minimum Input Voltage 5.0 6.5 40 4.5 20 10 0 –10 –20 –30 4.0 3.5 3.0 2.5 –40 –50 –200 2.0 –100 100 0 FB PIN ERROR VOLTAGE (mV) 6.0 INPUT VOLTAGE (V) INPUT VOLTAGE (V) VC PIN CURRENT (μA) 30 VOUT = 3.3V TA = 25°C L = 4.
PAGE 7
LT3972 PIN FUNCTIONS BD (Pin 1): This pin connects to the anode of the boost Schottky diode. BD also supplies current to the internal regulator. 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.
PAGE 8
LT3972 OPERATION The LT3972 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
LT3972 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⎟ ⎝ 0.79 V ⎠ Reference designators refer to the Block Diagram. Setting the Switching Frequency The LT3972 uses a constant frequency PWM architecture that can be programmed to switch from 200kHz to 2.4MHz by using a resistor tied from the RT pin to ground.
PAGE 10
LT3972 APPLICATIONS INFORMATION VIN(MAX ) = VOUT + VD –V +V fSW tON(MIN) D SW where VIN(MAX) is the maximum operating input voltage, VOUT is the output voltage, VD is the catch diode drop (~0.5V), VSW is the internal switch drop (~0.5V at max load), fSW is the switching frequency (set by RT), and tON(MIN) is the minimum switch on-time (~100ns). Note that a higher switching frequency will depress the maximum operating input voltage.
PAGE 11
LT3972 APPLICATIONS INFORMATION Table 1. Inductor Vendors VENDOR URL PART SERIES TYPE Murata www.murata.com LQH55D Open TDK www.componenttdk.com SLF10145 Shielded Toko www.toko.com D75C D75F Shielded Open Sumida www.sumida.com CDRH74 CR75 CDRH8D43 Shielded Open Shielded NEC www.nec.com MPLC073 MPBI0755 Shielded Shielded Of course, such a simple design guide will not always result in the optimum inductor for your application.
PAGE 12
LT3972 APPLICATIONS INFORMATION Table 2. Capacitor Vendors VENDOR PHONE URL PART SERIES COMMANDS Panasonic (714) 373-7366 www.panasonic.com Ceramic, Polymer, Tantalum EEF Series Kemet (864) 963-6300 www.kemet.com Ceramic, Tantalum Sanyo (408) 749-9714 www.sanyovideo.com Ceramic, Polymer, Tantalum Murata (408) 436-1300 AVX Taiyo Yuden (864) 963-6300 www.murata.com Ceramic www.avxcorp.com Ceramic, Tantalum www.taiyo-yuden.
PAGE 13
LT3972 APPLICATIONS INFORMATION LT3972 CURRENT MODE POWER STAGE gm = 5.3mho SW ERROR AMPLIFIER OUTPUT R1 CPL FB gm = 500μmho + The LT3972 uses current mode control to regulate the output. This simpliﬁes loop compensation. In particular, the LT3972 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.
PAGE 14
LT3972 APPLICATIONS INFORMATION Low Ripple Burst Mode Operation and Pulse-Skipping Mode The LT3972 is capable of operating in either low ripple Burst Mode operation or pulse-skipping mode which are selected using the SYNC pin. See the Synchronization section for details. To enhance efﬁciency at light loads, the LT3972 can be operated in low ripple Burst Mode operation which keeps the output capacitor charged to the proper voltage while minimizing the input quiescent current.
PAGE 15
LT3972 APPLICATIONS INFORMATION 6.0 VOUT BD 5.5 TO START (WORST CASE) VIN VIN LT3972 GND 4.7μF INPUT VOLTAGE (V) BOOST C3 SW 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 VOUT BD BOOST VIN VIN LT3972 TO START (WORST CASE) SW (5b) For 2.5V < VOUT < 2.8V VOUT LT3972 5.0 TO RUN 4.0 VOUT = 5V TA = 25°C L = 8.2μH f = 700kHz 2.0 BOOST VIN 6.0 3.0 BD VIN 10000 8.0 INPUT VOLTAGE (V) GND 100 1000 LOAD CURRENT (mA) C3 7.0 4.
PAGE 16
LT3972 APPLICATIONS INFORMATION Shorted and Reversed-Input Protection IL 1A/DIV RUN 15k RUN/SS VRUN/SS 2V/DIV GND 0.22μF VOUT 2V/DIV 2ms/DIV 3972 F07 Figure 7. To Soft-Start the LT3972, add a Resisitor and Capacitor to the RUN/SS Pin Synchronization To select low ripple Burst Mode operation, tie the SYNC pin below 0.5V (this can be ground or a logic output).
PAGE 17
LT3972 APPLICATIONS INFORMATION diode (D1) and the input capacitor (C1). The loop formed by these components should be as small as possible. These components, along with the inductor and output capacitor, should be placed on the same side of the circuit board, and their connections should be made on that layer. Place a local, unbroken ground plane below these components. The SW and BOOST nodes should be as small as possible.
PAGE 18
LT3972 APPLICATIONS INFORMATION to ambient can be reduced to JA = 35°C/W or less. With 100 LFPM airﬂow, this resistance can fall by another 25%. Further increases in airﬂow will lead to lower thermal resistance. Because of the large output current capability of the LT3972, it is possible to dissipate enough heat to raise the junction temperature beyond the absolute maximum of 125°C.
PAGE 19
LT3972 TYPICAL APPLICATIONS 5V Step-Down Converter VOUT 5V 3.5A VIN 6.3V TO 33V TRANSIENT TO 62V BD VIN RUN/SS ON OFF BOOST 0.47μF VC 10μF SW LT3972 D RT 15k L 4.7μH PG SYNC 63.4k 536k FB GND 680pF 47μF 100k f = 600kHz 3972 TA02 D: ON SEMI MBRA340 L: NEC MPLC0730L4R7 3.3V Step-Down Converter VOUT 3.3V 3.5A VIN 4.4V TO 33V TRANSIENT TO 62V BD VIN RUN/SS ON OFF BOOST 0.47μF VC 4.7μF LT3972 SW D RT 19k L 3.3μH PG SYNC 63.
PAGE 20
LT3972 TYPICAL APPLICATIONS 2.5V Step-Down Converter VOUT 2.5V 3.5A VIN 4V TO 33V TRANSIENT TO 62V BD VIN RUN/SS ON OFF D2 BOOST 1μF VC 4.7μF SW LT3972 D1 RT 15.4k L 3.3μH PG 215k SYNC 63.4k FB GND 680pF 47μF 100k f = 600kHz 3972 TA04 D1: ON SEMI MBRA340 D2: MBR0540 L: NEC MPLC0730L3R3 5V, 2MHz Step-Down Converter VOUT 5V 2.5A VIN 8.6V TO 22V TRANSIENT TO 62V BD VIN RUN/SS ON OFF BOOST 0.47μF VC 4.7μF LT3972 SW D RT 15k L 2.2μH PG 536k SYNC 12.
PAGE 21
LT3972 TYPICAL APPLICATIONS 12V Step-Down Converter VOUT 12V 3.5A VIN 15V TO 33V TRANSIENT TO 62V BD VIN RUN/SS ON OFF BOOST 0.47μF VC 10μF SW LT3972 D RT 17.4k L 8.2μH PG 715k SYNC 63.4k FB GND 680pF 47μF 50k f = 600kHz 3972 TA06 D: ON SEMI MBRA340 L: NEC MBP107558R2P 1.8V Step-Down Converter VOUT 1.8V 3.5A VIN 3.5V TO 27V BD VIN RUN/SS ON OFF BOOST 0.47μF VC 4.7μF LT3972 SW D RT 16.9k L 3.3μH PG SYNC 78.
PAGE 22
LT3972 PACKAGE DESCRIPTION DD Package 10-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1699) 0.675 p 0.05 3.50 p 0.05 1.65 p 0.05 2.15 p 0.05 (2 SIDES) PACKAGE OUTLINE 0.25 p 0.05 0.50 BSC 2.38 p 0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 p 0.10 (4 SIDES) R = 0.115 TYP 6 0.38 p 0.10 10 1.65 p 0.10 (2 SIDES) PIN 1 TOP MARK (SEE NOTE 6) (DD) DFN 1103 5 0.200 REF 1 0.25 p 0.05 0.50 BSC 0.75 p 0.05 0.00 – 0.05 2.38 p 0.10 (2 SIDES) BOTTOM VIEW—EXPOSED PAD NOTE: 1.
PAGE 23
LT3972 PACKAGE DESCRIPTION MSE Package 10-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1664 Rev B) 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 2.06 p 0.102 (.081 p .004) 1.83 p 0.102 (.072 p .004) 2.083 p 0.102 3.20 – 3.45 (.082 p .004) (.126 – .136) 10 0.50 0.305 p 0.038 (.0197) (.0120 p .0015) BSC TYP RECOMMENDED SOLDER PAD LAYOUT 3.00 p 0.102 (.118 p .004) (NOTE 3) 3.00 p 0.102 (.118 p .004) (NOTE 4) 4.90 p 0.
PAGE 24
LT3972 TYPICAL APPLICATION 1.2V Step-Down Converter VOUT 1.2V 3.5A VIN 3.6V TO 27V BD VIN RUN/SS ON OFF BOOST 0.47μF VC 4.7μF LT3972 SW D RT 17k L 3.3μH PG 52.3k SYNC 78.7k GND 470pF FB 100k 100μF f = 500kHz 3972 TA09 D: ON SEMI MBRA340 L: NEC MPLC0730L3R3 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1933 500mA (IOUT), 500kHz Step-Down Switching Regulator in SOT-23 LT1936 36V, 1.4A (IOUT), 500kHz, High Efﬁciency Step-Down DC/DC Converter Dual 25V, 1.4A (IOUT), 1.