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LT3971/LT3971-3.3/LT3971-5

3971fd

TYPICAL APPLICATION

DESCRIPTION

38V, 1.2A, 2MHz

Step-Down Regulator with

2.8µA Quiescent Current

The LT

3971 is an adjustable frequency monolithic buck

switching regulator that accepts a wide input voltage range

up to 38V. Low quiescent current design consumes only

2.8µA of supply current while regulating with no load. Low

ripple Burst Mode operation maintains high efficiency at

low output currents while keeping the output ripple below

15mV in a typical application. An internally compensated

current mode topology is used for fast transient response

and good loop stability. A high efficiency 0.33Ω switch

is included on the die along with a boost Schottky diode

and the necessary oscillator, control and logic circuitry.

An accurate 1V threshold enable pin can be used to shut

down the LT3971, reducing the input supply current to

700nA.

A capacitor on the SS pin provides a controlled

inrush current (soft-start). A power good flag signals

when V

OUT

reaches 91% of the programmed output volt-

age. The LT3971 is available in small 10-lead MSOP and

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

thermal resistance. A 16-lead MSOP is also offered which

has enhanced pin-to-pin fault tolerance.

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

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

owners.

3.3V Step Down Converter

FEATURES

APPLICATIONS

Ultralow Quiescent Current:

2.8µA I

Regulating 12V

to 3.3V

OUT

Fixed Output Voltages: 3.3V, 5V,

2.1µA I

Regulating 12V

to 3.3V

OUT

Low Ripple Burst Mode

Operation:

Output Ripple < 15mV

P-P

Wide Input Voltage Range: 4.3V to 38V

1.2A Maximum Output Current

Adjustable Switching Frequency: 200kHz to 2MHz

Synchronizable Between 250kHz to 2MHz

Fast Transient Response

Accurate 1V Enable Pin Threshold

Low Shutdown Current: I

= 700nA

Power Good Flag

Soft-Start Capability

Internal Compensation

Output Voltage: 1.19V to 30V

Small Thermally Enhanced 10-Lead MSOP, 16-Lead

MSOP and (3mm × 3mm) DFN Packages

Automotive Battery Regulation

Power for Portable Products

Industrial Supplies

No Load Supply Current

OUT

4.5V TO 38V

OUT

3.3V

1.2A

4.7µF

0.47µF

22µF

49.9k

f = 800kHz

4.7µH

GND

SYNC

OFF ON

LT3971-3.3

3971 TA01

EN BOOST

INPUT VOLTAGE (V)

INPUT CURRENT (µA)

1.0

10 20 3015 25 35

3971 TA01b

2.0

1.5

3.0

2.5

LT3971-5

LT3971-3.3

OUTPUT IN REGULATION

Summary of content (28 pages)

PAGE 1
LT3971/LT3971-3.3/LT3971-5 38V, 1.2A, 2MHz Step-Down Regulator with 2.8µA Quiescent Current FEATURES n n n n n n n n n n n n n n n DESCRIPTION Ultralow Quiescent Current: 2.8µA IQ Regulating 12VIN to 3.3VOUT Fixed Output Voltages: 3.3V, 5V, 2.1µA IQ Regulating 12VIN to 3.3VOUT Low Ripple Burst Mode® Operation: Output Ripple < 15mVP-P Wide Input Voltage Range: 4.3V to 38V 1.
PAGE 2
LT3971/LT3971-3.3/LT3971-5 ABSOLUTE MAXIMUM RATINGS (Note 1) VIN, EN Voltage..........................................................38V BOOST Pin Voltage....................................................55V BOOST Pin Above SW Pin..........................................30V FB, VOUT, RT, SYNC, SS Voltage..................................6V PG, BD Voltage..........................................................30V Boost Diode Current.....................................................
PAGE 3
LT3971/LT3971-3.
PAGE 4
LT3971/LT3971-3.3/LT3971-5 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VEN = 12V, VBD = 3.3V unless otherwise noted. (Note 2) PG Leakage PG Sink Current SYNC Threshold SYNC Pin Current SS Source Current VPG = 3V VPG = 0.4V VSS = 1V 0.6 TYPICAL PERFORMANCE CHARACTERISTICS Efficiency, VOUT = 5V Efficiency, VOUT = 3.
PAGE 5
LT3971/LT3971-3.3/LT3971-5 TYPICAL PERFORMANCE CHARACTERISTICS LT3971-3.3 Output Voltage LT3971 Feedback Voltage LT3971-5 Output Voltage 3.345 5.06 1.200 3.330 5.04 1.195 1.190 1.185 1.180 OUTPUT VOLTAGE (V) 1.205 OUTPUT VOLTAGE (V) FEEDBACK VOLTAGE (V) TA = 25°C, unless otherwise noted. 3.315 3.300 3.285 3.270 1.175 –55 –25 5 35 65 95 TEMPERATURE (°C) 125 155 3.255 –55 –25 5 35 65 95 TEMPERATURE (°C) 125 LOAD CURRENT (A) MINIMUM 1.0 0.20 TYPICAL 1.5 MINIMUM 1.
PAGE 6
LT3971/LT3971-3.3/LT3971-5 TYPICAL PERFORMANCE CHARACTERISTICS Boost Pin Current 500 25 BOOST PIN CURRENT (mA) 30 300 200 100 800 20 15 10 5 0 250 500 750 1000 1250 SWITCH CURRENT (mA) 0 1500 0 250 500 750 1000 1250 SWITCH CURRENT (mA) 3971 G14 400 800 350 SWITCH ON/OFF TIME (ns) SWITCHING FREQUENCY (kHz) 900 500 400 300 200 0 20 60 40 80 VOUT (% OF REGULATION VOLTAGE) MIN TOFF 1A LOAD 250 200 MIN TOFF 0.
PAGE 7
LT3971/LT3971-3.3/LT3971-5 TYPICAL PERFORMANCE CHARACTERISTICS 95 1.4 94 1.2 1.0 0.8 0.6 0.4 0.2 0 Transient Load Response, Load Current Stepped from 25mA (Burst Mode Operation) to 525mA Power Good Threshold 1.6 THRESHOLD VOLTAGE (%) BOOST DIODE VF (V) Boost Diode Forward Voltage TA = 25°C, unless otherwise noted.
PAGE 8
LT3971/LT3971-3.3/LT3971-5 TYPICAL PERFORMANCE CHARACTERISTICS 5V Start-Up and Dropout PIN FUNCTIONS 3971 G34 1.6 5 1.2 4 INPUT VOLTAGE (V) FEEDBACK VOLTAGE (V) 1V/DIV VOUT 800kHz 10Ω LOAD Minimum Input Voltage to Switch Feedback Regulation Voltage VIN 0.5s/DIV TA = 25°C, unless otherwise noted. 0.8 0.4 0 2 2.5 3 3.5 4 INPUT VOLTAGE (V) 4.
PAGE 9
LT3971/LT3971-3.3/LT3971-5 BLOCK DIAGRAM VIN C1 INTERNAL 1.19V REF 1V EN RT – + VIN + – Σ SHDN BD SWITCH LATCH SLOPE COMP OSCILLATOR 200kHz TO 2MHz RT BOOST R S C3 Q L1 SYNC PG ERROR AMP + – + – 1.09V D1 1µA SS C5 SHDN GND R3 C4 R1 VOUT FB R2 C2 VC CLAMP VC R2 VOUT SW Burst Mode DETECT R1 LT3971-3.3 LT3971-5 ONLY 3991 BD LT3971 ONLY C5 LT3971-3.3: R1 = 6.39M, R2 = 3.61M LT3971-5: R1 = 7.62M, R2 = 2.
PAGE 10
LT3971/LT3971-3.3/LT3971-5 OPERATION The LT3971 is a constant frequency, current mode stepdown regulator. An oscillator, with frequency set by RT, sets 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 (see Block Diagram).
PAGE 11
LT3971/LT3971-3.3/LT3971-5 APPLICATIONS INFORMATION feedback resistors and a low leakage Schottky catch diode in applications utilizing the ultralow quiescent current performance of the LT3971. The feedback resistors should preferably be on the order of MΩ and the Schottky catch diode should have less than 1µA of typical reverse leakage at room temperature. These two considerations are reiterated in the FB Resistor Network and Catch Diode Selection sections.
PAGE 12
LT3971/LT3971-3.3/LT3971-5 APPLICATIONS INFORMATION Table 1. Switching Frequency vs RT Value SWITCHING FREQUENCY (MHz) RT VALUE (kΩ) 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 255 118 71.5 49.9 35.7 28.0 22.1 17.4 14.0 11.0 Operating Frequency Tradeoffs Selection of the operating frequency is a tradeoff between efficiency, component size, minimum dropout voltage, and maximum input voltage. The advantage of high frequency operation is that smaller inductor and capacitor values may be used.
PAGE 13
LT3971/LT3971-3.3/LT3971-5 APPLICATIONS INFORMATION Ratings of the VIN and BOOST pins, regardless of chosen switching frequency. However, during such transients where VIN is higher than VIN(OP-MAX), the LT3971 will enter pulse-skipping operation where some switching pulses are skipped to maintain output regulation. The output voltage ripple and inductor current ripple will be higher than in typical operation. Do not overload when VIN is greater than VIN(OP-MAX).
PAGE 14
LT3971/LT3971-3.3/LT3971-5 APPLICATIONS INFORMATION maximum load current. For details of maximum output current and discontinuous operation, see Linear Technology’s Application Note 44. Finally, for duty cycles greater than 50% (VOUT/VIN>0.5), a minimum inductance is required to avoid sub-harmonic oscillations. See Application Note 19. One approach to choosing the inductor is to start with the simple rule given above, look at the available inductors, and choose one to meet cost or space goals.
PAGE 15
LT3971/LT3971-3.3/LT3971-5 APPLICATIONS INFORMATION for nominal operation is for the worst-case condition of shorted output. The diode current will then increase to the typical peak switch current. Peak reverse voltage is equal to the regulator input voltage. Use a diode with a reverse voltage rating greater than the input voltage. Table 4. Schottky Diodes. The Reverse Current Values Listed Are Estimates Based Off of Typical Curves for Reverse Current vs Reverse Voltage at 25°C. IAVE (A) MBR0520L 20 0.
PAGE 16
LT3971/LT3971-3.3/LT3971-5 APPLICATIONS INFORMATION For outputs of 3V and above, the standard circuit (Figure 3a) is best. For outputs between 2.8V and 3V, use a 1μF boost capacitor. A 2.5V output presents a special case because it is marginally adequate to support the boosted drive stage while using the internal boost diode. For reliable BOOST pin operation with 2.5V outputs use a good external Schottky diode (such as the ON Semi MBR0540), and a 1μF boost capacitor (Figure 3b). For output voltages below 2.
PAGE 17
LT3971/LT3971-3.3/LT3971-5 APPLICATIONS INFORMATION For lower start-up voltage, the boost diode can be tied to VIN; however, this restricts the input range to one-half of the absolute maximum rating of the BOOST pin. At light loads, the inductor current becomes discontinuous and this reduces the minimum input voltage to approximately 400mV above VOUT.
PAGE 18
LT3971/LT3971-3.3/LT3971-5 APPLICATIONS INFORMATION Soft-Start The SS pin can be used to soft-start the LT3971 by throttling the maximum input current during start-up. An internal 1μA current source charges an external capacitor generating a voltage ramp on the SS pin. The SS pin clamps the internal VC node, which slowly ramps up the current limit. Maximum current limit is reached when the SS pin is about 1.5V or higher.
PAGE 19
LT3971/LT3971-3.3/LT3971-5 APPLICATIONS INFORMATION D4 MBRS140 VIN VIN BOOST EN SW L1 C2 VOUT VOUT LT3971 GND BD FB + BACKUP RPG C4 3971 F07 Figure 8. Diode D4 Prevents a Shorted Input from Discharging a Backup Battery Tied to the Output. It Also Protects the Circuit from a Reversed Input. The LT3971 Runs Only When the Input is Present PCB Layout For proper operation and minimum EMI, care must be taken during printed circuit board layout.
PAGE 20
LT3971/LT3971-3.3/LT3971-5 APPLICATIONS INFORMATION loss. The die temperature is calculated by multiplying the LT3971 power dissipation by the thermal resistance from junction to ambient. There are four items which require consideration in terms of the application circuit to achieve fault tolerance: VIN-EN pin short, SYNC-GND pin short, SYNC-PG pin short, and PG-RT pin short.
PAGE 21
LT3971/LT3971-3.3/LT3971-5 APPLICATIONS INFORMATION Table 5: Effects of Pin Shorts PINS EFFECT VIN-EN No effect. In most applications, EN is tied to VIN. If EN is driven with a logic signal, a series resistor is recommended to protect the circuit generating the logic signal from the full VIN voltage. SS-RT VOUT may fall below regulation voltage. The switching frequency will be increased and the current limit will be reduced. RT-PG No effect if PG is floated.
PAGE 22
LT3971/LT3971-3.3/LT3971-5 TYPICAL APPLICATIONS 5V Step-Down Converter VIN 7V TO 38V VIN EN OFF ON BOOST 0.47µF PG 4.7µF SS LT3971 SW 4.7µH RT BD 49.9k GND SYNC 10pF 22µF 309k f = 800kHz VOUT 5V 1.2A 1M FB 3971 TA02 3.3V Step Down Converter No Load Supply Current VIN 4.5V TO 38V 4.0 0.47µF PG SS 4.7µF 3.5 BOOST LT3971 4.7µH SW INPUT CURRENT (µA) VIN EN OFF ON VOUT 3.3V 1.2A RT BD 49.9k SYNC GND 10pF 1.78M FB 3.0 2.5 2.0 1.5 22µF 1M 1.
PAGE 23
LT3971/LT3971-3.3/LT3971-5 TYPICAL APPLICATIONS 1.8V Step-Down Converter 12V Step-Down Converter VIN 4.3V TO 27V VIN 15V TO 38V VIN BD EN OFF ON VIN BOOST 0.47µF PG SS 4.7µF BOOST 0.47µF PG 4.7µH SW LT3971 EN OFF ON 10µF SS RT SW LT3971 10µH RT 10pF 118k SYNC GND BD VOUT 1.8V 1.2A 511k FB 100µF 1M f = 400kHz 49.9k GND SYNC 10pF 110k f = 800kHz 3971 TA05 VOUT 12V 1.2A 1M FB 10µF 3971 TA06 3.
PAGE 24
LT3971/LT3971-3.3/LT3971-5 TYPICAL APPLICATIONS 4V Step-Down Converter with a High Impedance Input Source + – 11M 24V + VIN EN CBULK 100µF 1M BOOST 0.47µF PG SS 4.7µF LT3971 SW * AVERAGE OUTPUT POWER CANNOT EXCEED THAT WHICH CAN BE PROVIDED BY HIGH IMPEDANCE SOURCE. NAMELY, V2 •η POUT(MAX) = 4R 4.7µH RT BD 1nF 10pF 49.9k SYNC GND VOUT 4V 1.
PAGE 25
LT3971/LT3971-3.3/LT3971-5 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 H) 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 ± .
PAGE 26
LT3971/LT3971-3.3/LT3971-5 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. MSE Package 16-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1667 Rev E) BOTTOM VIEW OF EXPOSED PAD OPTION 2.845 ±0.102 (.112 ±.004) 5.23 (.206) MIN 2.845 ±0.102 (.112 ±.004) 0.889 ±0.127 (.035 ±.005) 8 1 1.651 ±0.102 (.065 ±.004) 1.651 ±0.102 3.20 – 3.45 (.065 ±.004) (.126 – .136) 0.305 ±0.038 (.0120 ±.0015) TYP 16 0.50 (.0197) BSC 4.
PAGE 27
LT3971/LT3971-3.3/LT3971-5 REVISION HISTORY REV DATE DESCRIPTION A 2/11 Added fixed voltage options LT3971-3.3 and LT3971-5 reflected throughout data sheet B 8/11 Added fixed voltage options LT3971-3.
PAGE 28
LT3971/LT3971-3.3/LT3971-5 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT3970 40V, 350mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter with IQ = 2.5µA VIN: 4.2V to 40V, VOUT(MIN) = 1.21V, IQ = 2.5µA, ISD <1µA, 3mm × 2mm DFN-10 and MSOP-10 Packages LT3990 62V, 350mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter with IQ = 2.5µA VIN: 4.2V to 62V, VOUT(MIN) = 1.21V, IQ = 2.5µA, ISD <1µA, 3mm × 2mm DFN-10 and MSOP-10 Packages LT3991 55V, 1.2A, 2.