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Table Of Contents

LT1912

1912fa

TYPICAL APPLICATION

APPLICATIONS

36V, 2A, 500kHz Step-Down

Switching Regulator

The LT

1912 is an adjustable frequency (200kHz to 500kHz)

monolithic step-down switching regulator that accepts

input voltages up to 36V. A high efficiency 0.25Ω 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. The LT1912 allows the use of

ceramic capacitors resulting in low output ripple while

keeping total solution size to a minimum. The low current

shutdown mode 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). The

LT1912 is available in 10-pin MSOP and 3mm × 3mm DFN

packages with exposed pads for low thermal resistance.

■

Automotive Battery Regulation

■

Set-Top Box

■

Distributed Supply Regulation

■

Industrial Supplies

■

Wall Transformer Regulation

3.3V Step-Down Converter

4.5V TO 36V

OUT

3.3V

4.7µF

0.47µF

470pF

47µF

100k

20k

68.1k

6.8µH

316k

GND

OFF ON

LT1912

1912 TA01

RUN/SS BOOST

SYNC

Efficiency

LOAD CURRENT (A)

EFFICIENCY (%)

0.5 1.0 1.5 2

1912 TA01b

100

= 12V

L = 6.8µF

F = 500kHz

OUT

= 3.3V

OUT

= 5V

■

Wide Input Range: Operation From 3.6V to 36V

■

2A Maximum Output Current

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Adjustable Switching Frequency: 200kHz to 500kHz

■

Low Shutdown Current: I

< 1µA

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Integrated Boost Diode

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Synchronizable Between 250kHz to 500kHz

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Saturating Switch Design: 0.25Ω On-Resistance

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0.790V Feedback Reference Voltage

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Output Voltage: 0.79V to 20V

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Soft-Start Capability

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Small 10-Pin Thermally Enhanced MSOP and

(3mm × 3mm) DFN Packages

FEATURES DESCRIPTION

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

and ThinSOT is a trademark of Linear Technology Corporation. All other trademarks are the

property of their respective owners.

Summary of content (24 pages)

PAGE 1
LT1912 36V, 2A, 500kHz Step-Down Switching Regulator FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTION Wide Input Range: Operation From 3.6V to 36V 2A Maximum Output Current Adjustable Switching Frequency: 200kHz to 500kHz Low Shutdown Current: IQ < 1µA Integrated Boost Diode Synchronizable Between 250kHz to 500kHz Saturating Switch Design: 0.25Ω On-Resistance 0.790V Feedback Reference Voltage Output Voltage: 0.
PAGE 2
LT1912 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 BD, SYNC Voltage......................................................30V Operating Junction Temperature Range (Note 2) LT1912E.............................................
PAGE 3
LT1912 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 Feedback Voltage l FB Pin Bias Current (Note 3) VFB = 0.8V, VC = 0.4V FB Voltage Line Regulation 4V < VIN < 36V MIN TYP MAX UNITS 780 775 790 790 800 805 mV mV 7 30 nA 0.002 0.
PAGE 4
LT1912 TYPICAL PERFORMANCE CHARACTERISTICS Efficiency VIN = 12V VIN = 12V 3.5 80 VIN = 34V 80 VIN = 24V 70 4.0 VIN = 7V 85 EFFICIENCY (%) EFFICIENCY (%) Maximum Load Current 90 VIN = 34V 75 VIN = 24V 70 65 60 60 0 55 L: NEC PLC-0745-5R6 f: 500kHz VOUT = 5V 50 VOUT = 3.3V 50 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 LOAD CURRENT (A) 0 SWITCH CURRENT LIMIT(A) LOAD CURRENT (A) 2.5 MINIMUM 2.0 VOUT = 5V L = 4.
PAGE 5
LT1912 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted. Switching Frequency Feedback Voltage Frequency Foldback 1.2 1.20 SWITCHING FREQUENCY (NORMALIZED) 840 FREQUENCY (NORMALIZED) FEEDBACK VOLTAGE (mV) 1.15 820 800 780 1.10 1.05 1.00 0.95 0.90 0.85 760 –50 –25 0 0.80 –50 –25 25 50 75 100 125 150 TEMPERATURE (°C) 0 0.2 100 80 60 40 20 100 200 300 400 500 600 700 800 900 FB PIN VOLTAGE (mV) RUN/SS Pin Current 12 3.0 2.5 2.0 1.5 1.0 0 10 8 6 4 2 0.
PAGE 6
LT1912 TYPICAL PERFORMANCE CHARACTERISTICS 2.50 Switching Waveforms; Continuous Operation Switching Waveforms; Discontinuous Operation VC Voltages VSW 5V/DIV VSW 5V/DIV 2.00 CURRENT LIMIT CLAMP VC VOLTAGE (V) TA = 25°C unless otherwise noted. 1.50 1.00 IL 1A/DIV VOUT 10mV/DIV VOUT 10mV/DIV SWITCHING THRESHOLD 0.50 0 –50 –25 IL 0.5A/DIV 0 25 50 75 100 125 150 TEMPERATURE (°C) 1912 G19 2µs/DIV VIN = 12V; VOUT = 3.3V ILOAD = 110mA 1912 G21 2µs/DIV VIN = 12V; VOUT = 3.
PAGE 7
LT1912 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
LT1912 OPERATION The LT1912 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
LT1912 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⎟ ⎝ 0.79V ⎠ DCMIN = fSW tON(MIN ) Reference designators refer to the Block Diagram. Setting the Switching Frequency The LT1912 uses a constant frequency PWM architecture that can be programmed to switch from 200kHz to 500kHz by using a resistor tied from the RT pin to ground.
PAGE 10
LT1912 APPLICATIONS INFORMATION pulse-skipping operation where some switching pulses are skipped to maintain output regulation. In this mode the output voltage ripple and inductor current ripple will be higher than in normal operation. The minimum input voltage is determined by either the LT1912’s minimum operating voltage of ~3.6V or by its maximum duty cycle (see equation in previous section).
PAGE 11
LT1912 APPLICATIONS INFORMATION with a lower DCR resulting in higher efficiency. There are several graphs in the Typical Performance Characteristics section of this data sheet that show the maximum load current as a function of input voltage and inductor value for several popular output voltages. Low inductance may result in discontinuous mode operation, which is okay but further reduces maximum load current.
PAGE 12
LT1912 APPLICATIONS INFORMATION Table 2. Capacitor Vendors VENDOR PHONE URL PART SERIES Panasonic (714) 373-7366 www.panasonic.com Ceramic, Polymer, COMMANDS 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
LT1912 APPLICATIONS INFORMATION Capacitor C3 and the internal boost Schottky diode (see the Block Diagram) are used to generate a boost voltage that is higher than the input voltage. In most cases a 0.22µF capacitor will work well. Figure 2 shows three ways to arrange the boost circuit. The BOOST pin must be more than 2.3V above the SW pin for best efficiency. For outputs of 3V and above, the standard circuit (Figure 4a) is best. For outputs between 2.8V and 3V, use a 1µF boost capacitor. A 2.
PAGE 14
LT1912 APPLICATIONS INFORMATION 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 5 shows a plot of minimum load to start and to run as a function of input voltage.
PAGE 15
LT1912 APPLICATIONS INFORMATION 6.0 INPUT VOLTAGE (V) 5.5 TO START (WORST CASE) 5.0 4.5 15k 4.0 TO RUN 3.5 3.0 0.22µF RUN/SS VRUN/SS 2V/DIV GND VOUT = 3.3V TA = 25°C L = 8.2µH f = 500kHz 2.5 2.0 10 1 VOUT 2V/DIV 100 1000 LOAD CURRENT (A) 2ms/DIV 10000 TO START (WORST CASE) 7.0 D4 MBRS140 6.0 VIN 5.0 1912 F06 Figure 6. To Soft-Start the LT1912, Add a Resisitor and Capacitor to the RUN/SS Pin 8.
PAGE 16
LT1912 APPLICATIONS INFORMATION The SW and BOOST nodes should be as small as possible. Finally, keep the FB and VC nodes small so that the ground traces will shield them from the SW and BOOST nodes. The exposed pad on the bottom of the package must be soldered to ground so that the pad acts as a heat sink. To keep thermal resistance low, extend the ground plane as much as possible, and add thermal vias under and near the LT1912 to additional ground planes within the circuit board and on the bottom side.
PAGE 17
LT1912 APPLICATIONS INFORMATION 125°C. When operating at high ambient temperatures, the maximum load current should be derated as the ambient temperature approaches 125°C. Power dissipation within the LT1912 can be estimated by calculating the total power loss from an efficiency measurement and subtracting the catch diode loss and inductor loss. The die temperature is calculated by multiplying the LT1912 power dissipation by the thermal resistance from junction to ambient.
PAGE 18
LT1912 TYPICAL APPLICATIONS 5V Step-Down Converter VIN 6.8V TO 36V VIN ON OFF VOUT 5V 2A BD RUN/SS BOOST L 6.8µH 0.47µF VC 4.7µF LT1912 SW D RT 16.2k SYNC 68.1k 470pF 536k FB GND 47µF 100k f = 500kHz 1912 TA02 D: DIODES INC. DFLS240L L: TAIYO YUDEN NP06DZB6R8M 3.3V Step-Down Converter VIN 4.4V TO 36V VIN ON OFF VOUT 3.3V 2A BD RUN/SS BOOST 0.47µF VC 4.7µF LT1912 L 6.8µH SW D RT 20k 68.1k 470pF SYNC f = 500kHz GND 316k FB 47µF 100k 1912 TA03 D: DIODES INC.
PAGE 19
LT1912 TYPICAL APPLICATIONS 2.5V Step-Down Converter VIN 4V TO 36V VIN ON OFF BD RUN/SS D2 BOOST 1µF VC 4.7µF VOUT 2.5V 2A LT1912 L 6.8µH SW D1 RT 20k 68.1k 330pF SYNC f = 500kHz GND 215k FB 47µF 100k 1912 TA04 D1: DIODES INC.
PAGE 20
LT1912 TYPICAL APPLICATIONS 12V Step-Down Converter VIN 15V TO 36V VIN ON OFF VOUT 12V 2A BD RUN/SS BOOST 0.47µF VC 10µF L 10µH SW LT1912 D RT 26.1k SYNC 68.1kHz 330pF 715k FB GND 22µF 50k f = 500kHz 1912 TA06 D: DIODES INC. DFLS240L L: NEC/TOKIN PLC-0755-100 1.8V Step-Down Converter VOUT 1.8V 2A VIN 3.5V TO 27V VIN ON OFF BD RUN/SS BOOST 0.47µF VC 4.7µF LT1912 L 3.3µH SW D RT 18.2k 68.1k 330pF SYNC f = 500kHz GND 127k FB 47µF 100k 1912 TA08 D: DIODES INC.
PAGE 21
LT1912 PACKAGE DESCRIPTION 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 0.35 × 45° CHAMFER PIN 1 TOP MARK (SEE NOTE 6) 0.200 REF 0.75 ±0.05 0.00 – 0.05 5 1 (DD) DFN REV C 0310 0.25 ± 0.05 0.50 BSC 2.
PAGE 22
LT1912 PACKAGE DESCRIPTION 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
LT1912 REVISION HISTORY REV DATE DESCRIPTION PAGE NUMBER A 11/10 Changed units to mA for Quiescent Current from VIN for VBD = 0, Not Switching in Electrical Characteristics 2 1912fa Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
PAGE 24
LT1912 TYPICAL APPLICATION 1.2V Step-Down Converter VOUT 1.2V 2A VIN 3.6V TO 27V VIN BD RUN/SS ON OFF BOOST 0.47µF VC 4.7µF LT1912 L 3.3µH SW D RT 16.2k SYNC 68.1k 330pF GND 52.3k FB 100k f = 500kHz 47µF 1912 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.