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LT1938

1938fa

25V, 2.2A, 2.8MHz

Step-Down

Switching Regulator

The LT

1938 is an adjustable frequency (300kHz to

2.8MHz) monolithic buck switching regulator that accepts

input voltages up to 25V. A high efﬁciency 0.18Ω 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 LT1938’s high operating fre-

quency allows the use of small, low cost inductors and

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). A

power good ﬂag signals when V

OUT

reaches 90% of the

programmed output voltage. The LT1938 is available in

a 3mm × 3mm DFN package with Exposed Pad for low

thermal resistance.

■

Automotive Battery Regulation

■

Power for Portable Products

■

Distributed Supply Regulation

■

Industrial Supplies

■

Wall Transformer Regulation

■

Wide Input Voltage Range: 3.6V to 25V

■

2.2A Maximum Output Current

■

Adjustable Switching Frequency: 300kHz to 2.8MHz

■

Low Shutdown Current: I

< 1µA

■

Integrated Boost Diode

■

Power Good Flag

■

Saturating Switch Design: 0.18Ω On-Resistance

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1.265V Feedback Reference Voltage

■

Output Voltage: 1.265V to 20V

■

Soft-Start Capability

■

Small 10-Pin Thermally Enhanced (3mm × 3mm)

DFN Package

3.3V Step-Down Converter

Efﬁciency (V

OUT

= 3.3V)

FEATURES DESCRIPTION

APPLICATIONS

TYPICAL APPLICATION

BIAS

4.5V TO

25V

OUT

3.3V

2.2A

4.7µF

0.47µF

680pF

22µF

200k

16.2k

60.4k

4.7µH

324k

GND

OFF ON

LT1938

1938 TA01

RUN/SS BOOST

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.

All other trademarks are the property of their respective owners.

LOAD CURRENT (A)

EFFICIENCY (%)

1.6

1938 G02

0.2 0.4 0.6

0.8 1.0

1.2 1.4 1.8

2.22.0

= 12V

= 7V

= 24V

L: NEC PLC-0745-4R7

f = 800kHz

Summary of content (24 pages)

PAGE 1
LT1938 25V, 2.2A, 2.8MHz Step-Down Switching Regulator DESCRIPTION FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Wide Input Voltage Range: 3.6V to 25V 2.2A Maximum Output Current Adjustable Switching Frequency: 300kHz to 2.8MHz Low Shutdown Current: IQ < 1µA Integrated Boost Diode Power Good Flag Saturating Switch Design: 0.18Ω On-Resistance 1.265V Feedback Reference Voltage Output Voltage: 1.
PAGE 2
LT1938 Absolute Maximum Ratings (Note 1) pIN CONFIGURATION VIN, RUN/SS Voltage..................................................25V BOOST Pin Voltage....................................................50V BOOST Pin Above SW Pin..........................................25V FB, RT, VC Voltage........................................................5V BIAS, PG, BD Voltage.................................................25V Operating Junction Temperature Range (Note 2) LT1938E..................................
PAGE 3
LT1938 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, VBIAS = 3.3V unless otherwise noted. (Note 2) PARAMETER CONDITIONS MIN TYP Error Amp gm 330 Error Amp Gain 1000 MAX UNITS µMho VC Source Current 75 µA VC Sink Current 100 µA VC Pin to Switch Current Gain 3.5 A/V VC Clamp Voltage Switching Frequency 2 RT = 8.66k RT = 29.
PAGE 4
LT1938 typical performance characteristics 90 VIN = 24V 75 70 65 55 L: NEC PLC-0745-4R7 f = 800kHz 50 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 LOAD CURRENT (A) Maximum Load Current 2.0 VOUT = 3.3V TA = 25°C L = 4.7µH f = 800kHz 10 5 3.0 MINIMUM 2.5 2.0 VOUT = 5V TA = 25°C L = 4.7µH f = 800kHz 1.0 10 5 20 15 INPUT VOLTAGE (V) 4.5 1.5 1.0 0 25 50 75 TEMPERATURE (°C) 1.5 100 125 1938 G07 0 20 60 40 DUTY CYCLE (%) 100 80 500 400 300 200 0 80 90 100 0.5 –25 2.
PAGE 5
LT1938 typical performance characteristics Switching Frequency 1.20 1.285 1.15 1.280 1.10 1.275 1.270 1.265 1.00 0.95 0.90 1.255 0.85 –25 0 25 50 75 TEMPERATURE (°C) 100 RT = 45.3k 1.05 1.260 1.250 –50 0.80 –50 125 Frequency Foldback 1200 SWITCHING FREQUENCY (kHz) 1.290 FREQUENCY (MHz) FEEDBACK VOLTAGE (V) Feedback Voltage (TA = 25°C unless otherwise noted) –25 0 25 50 75 TEMPERATURE (°C) 100 1938 G10 400 200 0 125 3.
PAGE 6
LT1938 typical performance characteristics 6.0 2.00 5.5 5.0 VOUT = 5V TA = 25°C L = 4.7µH f = 800kHz 4.5 4.0 0.001 0.1 0.01 1 LOAD CURRENT (A) Power Good Threshold VC Voltages 1.200 THRESHOLD VOLTAGE (V) 2.50 THRESHOLD VOLTAGE (V) INPUT VOLTAGE (V) Minimum Input Voltage 6.5 (TA = 25°C unless otherwise noted) CURRENT LIMIT CLAMP 1.50 1.00 SWITCHING THRESHOLD 0.50 10 0 –50 –25 25 75 0 50 TEMPERATURE (°C) 100 125 PG RISING 1.180 1.160 1.140 1.120 1.
PAGE 7
LT1938 PIN FUNCTIONS BD (Pin 1): This pin connects to the anode of the boost Schottky diode. 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. VIN (Pin 4): The VIN pin supplies current to the LT1938’s internal regulator and to the internal power switch. This pin must be locally bypassed.
PAGE 8
LT1938 BLOCK DIAGRAM VIN 4 C1 7 5 10 VIN BIAS RUN/SS ∑ RT SLOPE COMP OSCILLATOR 300kHz–2.8MHz RT SOFT-START 6 – + INTERNAL 1.265V REF BD SWITCH LATCH BOOST 1 2 C3 R S Q SW L1 VOUT 3 VC CLAMP C2 D1 PG ERROR AMP + – + – 1.
PAGE 9
LT1938 Operation The LT1938 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 10
LT1938 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⎟ ⎝ 1.265 ⎠ Reference designators refer to the Block Diagram. Setting the Switching Frequency The LT1938 uses a constant frequency PWM architecture that can be programmed to switch from 300kHz to 2.8MHz by using a resistor tied from the RT pin to ground.
PAGE 11
LT1938 Applications Information frequency will be necessary to achieve safe operation at high input voltages. If the output is in regulation and no short-circuit or start-up events are expected, then input voltage transients of up to 25V are acceptable regardless of the switching frequency. In this mode, the LT1938 may enter pulse skipping operation where some switching pulses are skipped to maintain output regulation.
PAGE 12
LT1938 Applications Information Of course, such a simple design guide will not always result in the optimum inductor for your application. A larger value inductor provides a slightly higher maximum load current and will reduce the output voltage ripple. If your load is lower than 2A, then you can decrease the value of the inductor and operate with higher ripple current. This allows you to use a physically smaller inductor, or one with a lower DCR resulting in higher efficiency.
PAGE 13
LT1938 Applications Information Table 2. Capacitor Vendors VENDOR PHONE URL PART SERIES Panasonic (714) 373-7366 www.panasonic.com Ceramic, COMMANDS Polymer, 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 14
LT1938 Applications Information VOUT = 12V, FRONT PAGE APPLICATION LT1938 CURRENT MODE POWER STAGE gm = 3.5mho SW ERROR AMPLIFIER OUTPUT R1 – + CF ESR 1.265V C1 RC C1 + POLYMER OR TANTALUM GND VC IL 1A/DIV FB gm = 330µmho 3M CPL VOUT 100mV/DIV CERAMIC 10µs/DIV R2 1938 F03 Figure 3. Transient Load Response of the LT1938 Front Page Application as the Load Current is Stepped from 500mA to 1500mA. VOUT = 3.3V CC 1938 F02 Figure 2.
PAGE 15
LT1938 Applications Information 6.0 VOUT BD BOOST VIN LT1938 GND 4.7µF C3 INPUT VOLTAGE (V) VIN TO START 5.5 SW 5.0 4.5 4.0 3.5 TO RUN 3.0 (4a) For VOUT > 2.8V 2.5 2.0 0.001 VOUT D2 BD BOOST VIN LT1938 GND 4.7µF 8.0 C3 VOUT BD 4.7µF LT1938 GND TO START 6.0 5.0 TO RUN 4.0 3.0 2.0 0.001 BOOST VIN 10 7.0 (4b) For 2.5V < VOUT < 2.8V VIN 0.1 0.01 1 LOAD CURRENT (A) SW INPUT VOLTAGE (V) VIN VOUT = 3.3V TA = 25°C L = 4.7µH f = 800kHz C3 VOUT = 5V TA = 25°C L = 4.7µH f = 800kHz 0.
PAGE 16
LT1938 Applications Information Soft-Start The RUN/SS pin can be used to soft-start the LT1938, 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
LT1938 Applications Information L1 C2 VOUT RRT CC RC R2 R1 D1 C1 RPG GND 1938 F08 VIAS TO LOCAL GROUND PLANE VIAS TO RUN/SS VIAS TO VOUT VIAS TO PG VIAS TO VIN OUTLINE OF LOCAL GROUND PLANE Figure 8. A Good PCB Layout Ensures Proper, Low EMI Operation Hot Plugging Safely The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LT1938 circuits.
PAGE 18
LT1938 Applications Information CLOSING SWITCH SIMULATES HOT PLUG IIN VIN + LOW IMPEDANCE ENERGIZED 24V SUPPLY DANGER RINGING VIN MAY EXCEED ABSOLUTE MAXIMUM RATING LT1938 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 (9a) LT1938 VIN 20V/DIV 4.7µF IIN 10A/DIV (9b) + 22µF 35V AI.EI. + LT1938 20µs/DIV VIN 20V/DIV 4.7µF IIN 10A/DIV (9c) 20µs/DIV 1938 F09 Figure 9.
PAGE 19
LT1938 Typical Applications 5V Step-Down Converter VOUT 5V 2.2A VIN 6.3V TO 25V BD VIN RUN/SS ON OFF BOOST 0.47µF VC 4.7µF SW LT1938 D RT 20k BIAS PG 60.4k 680pF L 6.8µH 590k FB GND 22µF 200k f = 800kHz 1938 TA02 D: DIODES INC. DFLS230L L: TAIYO YUDEN NP06DZB6R8M 3.3V Step-Down Converter VOUT 3.3V 2.2A VIN 4.4V TO 25V BD VIN RUN/SS ON OFF BOOST 0.47µF VC 4.7µF LT1938 SW D RT 16.2k 680pF BIAS PG 60.4k f = 800kHz L 4.7µH GND 324k FB 22µF 200k 1938 TA03 D: DIODES INC.
PAGE 20
LT1938 Typical Applications 2.5V Step-Down Converter VOUT 2.5V 2.2A VIN 4V TO 25V BD VIN RUN/SS ON OFF D2 BOOST L 4.7µH 1µF VC 4.7µF SW LT1938 D1 RT 22.1k BIAS PG 84.5k 680pF 196k FB GND 47µF 200k f = 600kHz 3684 TA04 D1: DIODES INC. DFLS230L D2: MBR0540 L: TAIYO YUDEN NP06DZB4R7M 5V, 2MHz Step-Down Converter VOUT 5V 2A VIN 8.6V TO 22V BD VIN RUN/SS ON OFF BOOST 0.47µF VC 2.2µF LT1938 SW D RT 20k 680pF BIAS PG 16.9k f = 2MHz L 2.
PAGE 21
LT1938 Typical Applications 12V Step-Down Converter VOUT 12V 2.2A VIN 15V TO 25V BD VIN RUN/SS ON OFF BOOST 0.47µF VC 10µF LT1938 SW D RT 30k 680pF BIAS PG 60.4k f = 800kHz L 10µH GND 845k FB 22µF 100k 3684 TA06 D: DIODES INC.
PAGE 22
LT1938 Typical Applications 1.8V Step-Down Converter VOUT 1.8V 2.2A VIN 3.5V TO 25V BD VIN RUN/SS ON OFF BOOST 0.47µF VC 4.7µF LT1938 SW D RT 15.4k 680pF BIAS PG 105k f = 500kHz L 3.3µH GND 84.5k FB 47µF 200k 1938 TA07 D: DIODES INC.
PAGE 23
LT1938 Package Description DD Package 10-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1699) 0.675 ±0.05 3.50 ±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 = 0.115 TYP 6 0.38 ± 0.10 10 1.65 ± 0.10 (2 SIDES) PIN 1 TOP MARK (SEE NOTE 6) 0.200 REF 0.75 ±0.05 0.00 – 0.05 5 1 (DD) DFN 1103 0.25 ± 0.05 0.50 BSC 2.38 ±0.10 (2 SIDES) BOTTOM VIEW—EXPOSED PAD NOTE: 1.
PAGE 24
LT1938 Typical Application 1.265V Step-Down Converter VOUT 1.265V 2.2A VIN 3.6V TO 25V BD VIN RUN/SS ON OFF BOOST 0.47µF VC 4.7µF LT1938 SW D RT 13k BIAS PG 105k 680pF L 3.3µH GND FB 47µF f = 500kHz 1938 TA08 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) = 12V, IQ = 1.