Datasheet

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LT3970 Series

3970fc

Typical applicaTion

DescripTion

40V, 350mA Step-Down

Regulator with 2.5µA

Quiescent Current and

Integrated Diodes

The LT

3970 is an adjustable frequency monolithic buck

switching regulator that accepts a wide input voltage

range up to 40V, and consumes only 2.5µA of quiescent

current. A high efﬁciency switch is included on the die

along with the catch diode, boost diode, and the neces

sary oscillator, control and logic circuitry. Low ripple Burst

Mode operation maintains high efﬁciency at low output

currents while keeping the output ripple below 5mV in a

typical application. Current mode topology is used for fast

transient response and good loop stability

. A catch diode

current limit provides protection against shorted outputs

and over

voltage conditions. An enable pin with accurate

threshold is available, producing a low shutdown current

of 0.7µA. A power good ﬂag signals when V

OUT

reaches

90% of the programmed output voltage. The LT3970 is

available in small 10-pin MSOP and 3mm × 2mm DFN

packages.

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.

5V Step-Down Converter

FeaTures

applicaTions

Low Ripple Burst Mode

Operation

2.5µA I

at 12V

to 3.3V

OUT

Output Ripple < 5mV

P-P

Wide Input Voltage Range: 4.2V to 40V Operating

Adjustable Switching Frequency: 200kHz to 2.2MHz

Integrated Boost and Catch Diodes

350mA Output Current

Fixed Output Voltages: 3.3V, 3.42V, 5V

1.8µA I

at 12V

Accurate 1V Enable Pin Threshold

Low Shutdown Current: I

= 0.7µA

Internal Sense Limits Catch Diode Current

Power Good Flag

Output Voltage: 1.21V to 25V

Internal Compensation

Small 10-Pin MSOP and (3mm × 2mm) DFN Packages

Automotive Battery Regulation

Power for Portable Products

Industrial Supplies

BOOST

LT3970

SWEN

0.22µF

22pF

22µF

2.2µF

6V TO 40V

OUT

350mA

316k

226k

f = 600kHz

22µH

GND

OFF ON

3490 TA01a

Efﬁciency

LOAD CURRENT (mA)

EFFICIENCY (%)

POWER LOSS (mW)

0.01 1 10 100

3970 TA01b

100

1000

0.01

0.1

= 12V

Summary of content (22 pages)

PAGE 1
LT3970 Series 40V, 350mA Step-Down Regulator with 2.5µA Quiescent Current and Integrated Diodes Features Description Low Ripple Burst Mode® Operation 2.5µA IQ at 12VIN to 3.3VOUT Output Ripple < 5mVP-P n Wide Input Voltage Range: 4.2V to 40V Operating n Adjustable Switching Frequency: 200kHz to 2.2MHz n Integrated Boost and Catch Diodes n 350mA Output Current n Fixed Output Voltages: 3.3V, 3.42V, 5V 1.8µA IQ at 12VIN n Accurate 1V Enable Pin Threshold n Low Shutdown Current: I = 0.
PAGE 2
LT3970 Series Absolute Maximum Ratings (Note 1) VIN, EN Voltage..........................................................40V BOOST Pin Voltage....................................................55V BOOST Pin Above SW Pin..........................................30V FB/VOUT, RT Voltage.....................................................6V PG, BD Voltage..........................................................30V Operating Junction Temperature Range (Note 2) E-, I-grade.....................................
PAGE 3
LT3970 Series Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VBD = 3.3V unless otherwise noted. (Note 2) PARAMETER CONDITIONS Minimum Input Voltage MIN l TYP MAX UNITS 4 4.2 V 0.7 1.7 1.2 2.7 3.5 4 µA µA µA µA VEN Low VEN High VEN High, –40°C to 125°C VEN High, –40°C to 150°C l l –40°C to 125°C –40°C to 150°C l l 1.195 1.185 1.18 1.21 1.21 1.21 1.225 1.235 1.
PAGE 4
LT3970 Series Electrical Characteristics Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LT3970E is guaranteed to meet performance specifications from 0°C to 125°C junction temperature.
PAGE 5
LT3970 Series Typical Performance Characteristics TA = 25°C, unless otherwise noted. No-Load Supply Current 3.0 2.5 12 FRONT PAGE APPLICATION VIN = 12V VOUT = 3.3V R1 = 1M R2 = 576k 9 6 3 2.
PAGE 6
LT3970 Series Typical Performance Characteristics TA = 25°C, unless otherwise noted. Switch VCESAT (ISW = 200mA) vs Temperature Minimum Switch On-Time/Switch Off-Time LOAD CURRENT = 175mA 180 Switch VCESAT 400 250 MINIMUM OFF-TIME 120 100 80 MINIMUM ON-TIME 60 SWITCH VCESAT (mV) 160 140 SWITCH VCESAT (mV) 200 150 40 0 –50 –25 0 100 –50 –25 25 50 75 100 125 150 TEMPERATURE (°C) 0 200 100 16 5.0 Minimum Input Voltage, VOUT = 3.3V 6.5 FRONT PAGE APPLICATION VOUT = 3.
PAGE 7
LT3970 Series Typical Performance Characteristics TA = 25°C, unless otherwise noted. EN Threshold 92 1.050 91 1.025 VOUT 100mV/DIV 1.000 IL 100mA/DIV THRESHOLD VOLTAGE (V) THRESHOLD (%) Power Good Threshold Transient Load Response; Load Current is Stepped from 10mA (Burst Mode Operation) to 110mA 90 89 88 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 100µs/DIV FRONT PAGE APPLICATION VIN = 12V VOUT = 5V 0.975 0.
PAGE 8
LT3970 Series Pin Functions FB (Pin 1, LT3970 Only): The LT3970 Regulates the FB Pin to 1.21V. Connect the feedback resistor divider tap to this pin. GND (Pins 4, 5, Exposed Pad (Pin 11, DFN Only)): Ground. Must be soldered to PCB. SW (Pin 6): The SW pin is the output of an internal power switch. Connect this pin to the inductor. VOUT (Pin 1, LT3970-X Only): The LT3970-3.3, LT3970-3.42 and LT3970-5 regulate the VOUT Pin to 3.3V, 3.42V and 5V respectively.
PAGE 9
LT3970 Series Operation The LT3970 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 10
LT3970 Series 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  1.21  Reference designators refer to the Block Diagram. Note that choosing larger resistors will decrease the quiescent current of the application circuit. Setting the Switching Frequency The LT3970 uses a constant frequency PWM architecture that can be programmed to switch from 200kHz to 2.
PAGE 11
LT3970 Series Applications Information The highest allowed VIN during normal operation (VIN(OP‑MAX)) is limited by minimum duty cycle and can be calculated by the following equation: VIN(OP-MAX) = VOUT + VD – VD + VSW fSW • tON(MIN) where tON(MIN) is the minimum switch on-time (~150ns). However, the circuit will tolerate inputs up to the absolute maximum ratings of the VIN and BOOST pins, regardless of chosen switching frequency.
PAGE 12
LT3970 Series Applications Information Step-down regulators draw current from the input supply in pulses with very fast rise and fall times. The input capacitor is required to reduce the resulting voltage ripple at the LT3970 and to force this very high frequency switching current into a tight local loop, minimizing EMI. A 1µF capacitor is capable of this task, but only if it is placed close to the LT3970 (see the PCB Layout section).
PAGE 13
LT3970 Series Applications Information A final precaution regarding ceramic capacitors concerns the maximum input voltage rating of the LT3970. As previously mentioned, a ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT3970 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3970’s rating. This situation is easily avoided (see the Hot Plugging Safely section).
PAGE 14
LT3970 Series Applications Information 5.0 VOUT BD VIN 4.5 BOOST C3 LT3970 INPUT VOLTAGE (V) VIN SW GND (4a) For VOUT ≥ 2.2V VIN TO START/RUN 3.5 2.5 0 50 BOOST C3 LT3970 SW 6.5 (4b) For VOUT < 2.2V; VIN < 27V Figure 4. Two Circuits for Generating the Boost Voltage If the input voltage is ramped slowly, the boost capacitor may not be fully charged.
PAGE 15
LT3970 Series Applications Information VIN LT3970 VIN R3 EN INPUT CURRENT (µA) 160 1V + – SHDN R4 VIN(EN) = 6V R3 = 5M R4 = 1M 120 80 40 0 3970 F06 OUTPUT VOLTAGE (V) 4 Figure 6. Enable Be aware that while VIN is below 4.2V, the input current may rise up to several hundred µA and the part may begin to switch while the internal circuitry starts up. Figure 7 shows the startup behavior of a typical application with different programmed VIN(EN).
PAGE 16
LT3970 Series Applications Information PCB Layout 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 LT3970’s VIN and SW pins, the internal catch diode and the input capacitor. The loop formed by these components should be as small as possible.
PAGE 17
LT3970 Series Typical Applications 3.3V Step-Down Converter VIN 4.2V TO 40V VIN BOOST C1 2.2µF EN SW PG BD VIN 6V TO 40V C3 0.22µF VIN L1 22µH LT3970 OFF ON 5V Step-Down Converter R1 1M RT GND 226k FB C2 22µF R2 576k f = 600kHz OFF ON C1 2.2µF RT RT 22µH VOUT OFF ON OFF ON C1 2.2µF BOOST EN PG 226k f = 600kHz FB VOUT 5V 350mA BD GND VOUT 22µF 3490 TA03b 1.8V Step-Down Converter VIN 4.2V TO 27V VIN L1 15µH R1 1M R2 931k 3490 TA04 C2 47µF BOOST LT3970 VOUT 2.
PAGE 18
LT3970 Series TYPICAL APPLICATIONS 12V Step-Down Converter VIN 14V TO 40V VIN BOOST EN PG VIN 8.5V TO 16V TRANSIENTS TO 40V C3 0.1µF L1 33µH LT3970 OFF ON 5V, 2MHz Step-Down Converter RT 226k SW BD GND VIN VOUT 12V 350mA R1 1M 22pF C1 2.2µF C3 0.1µF FB f = 600kHz OFF ON C2 22µF R2 113k BOOST LT3970 EN SW PG BD L1 10µH 22pF C1 1µF RT 49.
PAGE 19
LT3970 Series Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. DDB Package 10-Lead Plastic DFN (3mm × 2mm) (Reference LTC DWG # 05-08-1722 Rev Ø) 0.64 ±0.05 (2 SIDES) 3.00 ±0.10 (2 SIDES) R = 0.05 TYP 0.70 ±0.05 2.55 ±0.05 1.15 ±0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 2.39 ±0.05 (2 SIDES) PIN 1 BAR TOP MARK (SEE NOTE 6) 0.200 REF RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS R = 0.115 TYP 6 0.40 ± 0.10 10 2.00 ±0.
PAGE 20
LT3970 Series Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. MS Package 10-Lead Plastic MSOP (Reference LTC DWG # 05-08-1661 Rev E) 0.889 ± 0.127 (.035 ± .005) 5.23 (.206) MIN 3.20 – 3.45 (.126 – .136) 3.00 ± 0.102 (.118 ± .004) (NOTE 3) 0.50 0.305 ± 0.038 (.0197) (.0120 ± .0015) BSC TYP RECOMMENDED SOLDER PAD LAYOUT 0.254 (.010) 10 9 8 7 6 3.00 ± 0.102 (.118 ± .004) (NOTE 4) 4.90 ± 0.152 (.193 ± .006) DETAIL “A” 0.497 ± 0.
PAGE 21
LT3970 Series Revision History REV DATE DESCRIPTION A 5/10 Added LT3970-3.3 and LT3970-5 B 3/12 C 09/13 PAGE NUMBER 1 - 22 Title and Features clarified to add 3.42V fixed output version. 1 Clarified the Absolute Maximum Ratings section, added 3.42V output option in the Order Information section. 2 Added 3.42V output option in the Electrical Characteristics table. 3 Added 3.42V Output Voltage vs Temperature graph. 4 Clarified VOUT Pin Function and Block Diagram.
PAGE 22
LT3970 Series Related Parts PART NUMBER DESCRIPTION COMMENTS LT3971 38V, 1.2A (IOUT), IQ = 2.8µA High Efficiency Step-Down DC/DC Converter VIN = 4.3V to 38V, VOUT(MIN) = 1.2V, IQ = 2.8µA, ISD < 1µA, MSOP-10E, 3mm × 3mm DFN-10 LT3991 55V, 1.2A (IOUT), IQ = 2.8µA High Efficiency Step-Down DC/DC Converter VIN = 4.3V to 55V, VOUT(MIN) = 1.2V, IQ = 2.8µA, ISD < 1µA, MSOP-10E, 3mm × 3mm DFN-10 LT3689 36V, 60V Transient Protection, 800mA, 2.