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LT3502/LT3502A

3502fd

Features

applications

Description

1.1MHz/2.2MHz, 500mA

Step-Down Regulators in

2mm ×

2mm DFN and MS10

The LT

3502/LT3502A are current mode PWM step-down

DC/DC converters with an internal 500mA power switch,

in tiny 8-lead 2mm × 2mm DFN and 10-lead MS10

packages. The wide input voltage range of 3V to 40V makes

the LT3502/LT3502A suitable for regulating power from a

wide variety of sources, including 24V industrial supplies

and automotive batteries. Its high operating frequency

allows the use of tiny, low cost inductors and capacitors,

resulting in a very small solution. Constant frequency

above the AM band avoids interfering with radio reception,

making the LT3502A particularly suitable for automotive

applications.

Cycle-by-cycle current limit and frequency foldback

provide protection against shorted outputs. Soft-start

and frequency foldback eliminates input current surge

during start-up. DA current sense provides further protec

tion in fault conditions. An internal boost diode reduces

component count.

3.3V Step-Down Converter

3V to 40V Input Voltage Range

500mA Output Current

Switching Frequency: 2.2MHz (LT3502A),

1.1MHz (LT3502)

800mV Feedback Voltage

Short-Circuit Robust

Soft-Start

Low Shutdown Current: <2µA

Internally Compensated

Internal Boost Diode

Thermally Enhanced 2mm × 2mm 8-Lead DFN

and 10-Lead MS10 Package

Automotive Systems

Battery-Powered Equipment

Wall Transformer Regulation

Distributed Supply Regulation

LT3502A 12V

Efﬁciency

0.1µF

3502 TA01a

31.6k

10k

6.8µH

SHDN

BOOST

LT3502A

GND

OFF ON

10µF

1µF

4.7V TO 40V

OUT

3.3V

500mA

LOAD CURRENT (A)

EFFICIENCY (%)

0.2

0.4

0.5

3502 TA01b

0.1 0.3

3.3V

OUT

typical application

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

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

Summary of content (24 pages)

PAGE 1
LT3502/LT3502A 1.1MHz/2.2MHz, 500mA Step-Down Regulators in 2mm × 2mm DFN and MS10 Description Features n n n n n n n n n n 3V to 40V Input Voltage Range 500mA Output Current Switching Frequency: 2.2MHz (LT3502A), 1.
PAGE 2
LT3502/LT3502A Absolute Maximum Ratings (Note 1) Input Voltage (VIN).....................................................40V BOOST Voltage..........................................................50V BOOST Pin Above SW Pin............................................7V FB Voltage....................................................................6V SHDN Voltage............................................................40V BD Voltage...................................................................
PAGE 3
LT3502/LT3502A Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, VSHDN = 5V, VBOOST = 15V. PARAMETER CONDITIONS MIN Undervoltage Lockout 2.6 TYP MAX UNITS 2.8 3 V Quiescent Current at Shutdown VSHDN = 0V 0.5 2 µA Quiescent Current Not Switching 1.5 2 mA Feedback Voltage 2mm × 2mm DFN 2mm × 2mm DFN MS10 MS10 l 0.8 0.8 0.8 0.8 0.813 0.81 0.816 0.
PAGE 4
LT3502/LT3502A Typical Performance Characteristics LT3502A 3.3VOUT Efficiency 70 80 12VIN 70 24VIN EFFICIENCY (%) 60 LT3502 3.3VOUT Efficiency 100 90 12VIN 80 EFFICIENCY (%) LT3502A 5VOUT Efficiency 50 40 30 80 60 50 40 30 40 30 20 10 10 0.1 0.3 0.2 LOAD CURRENT (A) 0 0.5 0.4 0 0.1 0.3 0.2 LOAD CURRENT (A) 0 0.5 0.4 40 30 MINIMUM 0.7 0.6 0.5 0.4 0.3 0.7 0.5 0.4 0.3 20 0.2 10 0.1 0.1 0 0 0.1 0.3 0.4 0.2 LOAD CURRENT (A) 0.
PAGE 5
LT3502/LT3502A Typical Performance Characteristics UVLO (TA = 25°C unless otherwise noted) Switching Frequency 3.5 Soft-Start (SHDN) 0.9 2.5 LT3502A 3.0 2.0 1.5 1.0 1.5 LT3502 1.0 0.5 0.5 0 50 0 –50 150 100 TEMPERATURE (°C) 0 50 100 TEMPERATURE (°C) 3502 G10 0.5 0.4 0.3 0.2 0.1 –0.1 150 200 400 600 800 1000 1200 1400 1600 SHDN PIN VOLTAGE (mV) 3502 G12 Switch Current Limit SHDN Pin Current Switch Current Limit 1.0 1.2 0.9 CURRENT LIMIT (A) 0.8 200 150 100 1.
PAGE 6
LT3502/LT3502A Typical Performance Characteristics LT3502A Typical Minimum Input Voltage (VOUT = 3.3V) LT3502A Typical Minimum Input Voltage (VOUT = 5V) 8 7 6 7 6 6 5 4 3 VIN (V) 5 VIN (V) VIN (V) LT3502 Typical Minimum Input Voltage (VOUT = 3.3V) 7 5 4 3 2 0.1 0.01 LOAD CURRENT (A) 1 0 0.001 0.01 0.1 LOAD CURRENT (A) LT3502 Typical Minimum Input Voltage (VOUT = 5V) 5 3 3502 G21 IL 200mA/DIV VOUT 20mV/DIV VOUT 20mV/DIV 2 1 1 1 VSW 5V/DIV IL 200mA/DIV 4 0.1 0.
PAGE 7
LT3502/LT3502A Pin Functions (DFN/MS) VIN (Pin 1/Pin 10): The VIN pin supplies current to the LT3502/LT3502A’s internal regulator and to the internal power switch. This pin must be locally bypassed. BD (Pin 2/Pin 8): The BD pin is used to provide current to the internal boost Schottky diode. FB (Pin 3/Pin 7): The LT3502/LT3502A regulate their feedback pin to 0.8V. Connect the feedback resistor divider tap to this pin. Set the output voltage according to VOUT = 0.8 (1 + R1/R2). A good value for R2 is 10k.
PAGE 8
ON OFF VIN C4 R3 C2 4 1 SHDN VIN SLOPE COMP FREQUENCY FOLDBACK OSC INT REG AND UVLO ∑ VC gm S R R2 0.
PAGE 9
LT3502/LT3502A Operation The LT3502/LT3502A are constant frequency, current mode step-down regulators. An oscillator enables an RS flip-flop, turning on the internal 500mA power switch Q1. 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 node.
PAGE 10
LT3502/LT3502A 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⎟ ⎝ 0.8V ⎠ R2 should be 20k or less to avoid bias current errors. Reference designators refer to the Block Diagram. Input Voltage Range The input voltage range for the LT3502/LT3502A applications depends on the output voltage and on the absolute maximum ratings of the VIN and BOOST pins.
PAGE 11
LT3502/LT3502A Applications Information VSW 20V/DIV VSW 20V/DIV IL 500mA/DIV IL 500mA/DIV VOUT 100mV/DIV VOUT 100mV/DIV 1µs/DIV VIN = 40V, VOUT = 3.3V L = 6.8µH, COUT = 10µF, IOUT = 250mA 1µs/DIV 3502 F02 VIN = 40V, VOUT = 3.3V L = 6.8µH, COUT = 10µF, IOUT = 500mA Figure 2. Pulse-Skipping Occurs when Required On-Time is Below 60ns Figure 3. Pulse-Skipping with Large Load Current Will be Limited by the DA Valley Current Limit.
PAGE 12
LT3502/LT3502A Applications Information tion, which is okay, but further reduces maximum load current. For details of the maximum output current and discontinuous mode operation, see Linear Technology Application Note 44. Catch Diode A low capacitance 500mA Schottky diode is recommended for the catch diode, D1. The diode must have a reverse voltage rating equal to or greater than the maximum input voltage. The Diodes Inc. SBR1U40LP, ON Semi MBRM140, and Diodes Inc.
PAGE 13
LT3502/LT3502A Applications Information Table 2 VENDOR Panasonic PHONE (714) 373-7366 URL www.panasonic.com Kemet (864) 963-6300 www.kemet.com Sanyo (408)794-9714 www.sanyovideo.com Murata AVX (404) 436-1300 www.murata.com www.avxcorp.com Taiyo Yuden (864) 963-6300 www.taiyo-yuden.com Figure 4 shows the transient response of the LT3502A with several output capacitor choices. The output is 3.3V.
PAGE 14
LT3502/LT3502A Applications Information VOUT 32.4k FB IL 0.2A/DIV 10µF VOUT 0.1V/DIV AC COUPLED 10k 10µs/DIV 3502 F04a 10µs/DIV 3502 F04b 10µs/DIV 3502 F04c VOUT 32.4k 50pF IL 0.2A/DIV 10µF ×2 FB VOUT 0.1V/DIV AC COUPLED 10k VOUT 32.4k FB 10k + IL 0.2A/DIV 100µF VOUT 0.1V/DIV AC COUPLED SANYO 4TPB100M Figure 4. Transient Load Response of the LT3502A with Different Output Capacitors as the Load Current is Stepped from 150mA to 400mA. VIN = 12V, VOUT = 3.3V, L = 6.
PAGE 15
LT3502/LT3502A Applications Information 7 8 6 7 6 START 4 3 RUN 4 3 2 2 1 0 0.001 START RUN 5 VIN (V) VIN (V) 5 1 0.1 0.01 LOAD CURRENT (A) 0 0.001 1 0.01 0.1 LOAD CURRENT (A) 3502 G20 3502 G19 (6a) LT3502A Typical Minimum Input Voltage, VOUT = 3.3V (6b) LT3502A Typical Minimum Input Voltage, VOUT = 5V 7 8 6 7 6 RUN START 3 START 4 3 2 2 1 0 0.001 RUN 5 VIN (V) VIN (V) 5 4 1 1 0.1 0.01 LOAD CURRENT (A) 0 0.001 1 0.01 0.
PAGE 16
LT3502/LT3502A Applications Information VSW 10V/DIV RUN SHDN IL 500mA/DIV GND 3502 F07a VOUT 2V/DIV VIN = 12V VOUT = 3.3V L = 6.8µH COUT = 10µF 5µs/DIV VIN = 12V VOUT = 3.3V L = 6.8µH COUT = 10µF 50µs/DIV RUN VSW 10V/DIV 50k SHDN 0.1µF IL 500mA/DIV GND 3502 F07b VOUT 2V/DIV 3502 F07 Figure 7. To Soft-Start the LT3502A, Add a Resistor and Capacitor to the SHDN Pin D4 VIN VSW 10V/DIV BD BOOST VIN VOUT SW LT3502A DA IL 500mA/DIV SHDN VIN = 40V VOUT = 0V L = 6.
PAGE 17
LT3502/LT3502A Applications Information or because it is tied to VIN), then the LT3502/LT3502A’s internal circuitry will pull its quiescent current through its SW pin. This is fine if your system can tolerate a few mA in this state. If you ground the SHDN pin, the SW pin current will drop to essentially zero. However, if the VIN pin is grounded while the output is held high, then parasitic diodes inside the LT3502/LT3502A can pull large currents from the output through the SW pin and the VIN pin.
PAGE 18
LT3502/LT3502A Applications Information the circuit. In Figure 9b an aluminum electrolytic capacitor has been added. This capacitor’s high equivalent series resistance damps the circuit and eliminates the voltage overshoot. The extra capacitor improves low frequency ripple filtering and can slightly improve the efficiency of the circuit, though it is likely to be the largest component in the circuit. An alternative solution is shown in Figure 9c.
PAGE 19
LT3502/LT3502A Applications Information The loop formed by these components should be as small as possible and tied to system ground in only one place. 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, and tie this ground plane to system ground at one location, ideally at the ground terminal of the output capacitor C1.
PAGE 20
LT3502/LT3502A Typical Applications 0.8V Step-Down Converter VBD 3V TO 7V VBD 3V TO 7V 0.1µF BD VIN 3V TO 40V C2 1µF VIN BOOST L1 C3 0.1µF 3.3µH SW LT3502A D1 0.1µF BD VIN 3V TO 40V C2 1µF VOUT 0.8V 500mA VIN BOOST SW LT3502 DA OFF ON SHDN GND C3 0.1µF L1 10µH D1 VOUT 0.8V 500mA DA FB OFF ON C1 47µF C1: JMK212BJ476MG C3: HMK212BJ104MG L1: LQH43CN3R3M03 SHDN GND FB C1 100µF C1: JMK316BJ107ML L1: LQH43CN100K03 3502 TA02a 3502 TA02b 1.
PAGE 21
LT3502/LT3502A Typical Applications 2.5V Step-Down Converter VBD 3V TO 7V VBD 3V TO 7V 0.1µF BD VIN 3.5V TO 40V C2 1µF VIN BOOST SW LT3502A D1 DA OFF ON SHDN GND L1 C3 0.1µF 6.8µH FB C1: JMK212BJ226MG L1: LQH43DN6R8M03 BD VIN 3.5V TO 40V C2 1µF VOUT 2.5V 500mA VIN BOOST SW LT3502 SHDN OFF ON C1 22µF FB GND C3 0.1µF L1 15µH D1 DA R1 21.3k R2 10k 0.1µF R1 21.3k R2 10k C1 22µF C1: JMK212BJ226MG L1: LQH55DN150M03 3502 TA04a VOUT 2.5V 500mA 3502 TA04b 3.
PAGE 22
LT3502/LT3502A Package Description DC8 Package 8-Lead Plastic DFN (2mm × 2mm) (Reference LTC DWG # 05-08-1719 Rev A) 0.70 ±0.05 2.55 ±0.05 1.15 ±0.05 0.64 ±0.05 (2 SIDES) PACKAGE OUTLINE 0.25 ± 0.05 0.45 BSC 1.37 ±0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED R = 0.05 TYP 2.00 ±0.10 (4 SIDES) PIN 1 BAR TOP MARK (SEE NOTE 6) R = 0.115 TYP 5 8 0.40 ± 0.10 0.64 ± 0.10 (2 SIDES) PIN 1 NOTCH R = 0.20 OR 0.
PAGE 23
LT3502/LT3502A Package Description 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 ± 0.076 (.0196 ± .003) REF 0° – 6° TYP GAUGE PLANE 1 2 3 4 5 0.53 ± 0.152 (.021 ± .
PAGE 24
LT3502/LT3502A Typical Application 5V Step-Down Converter BD VIN 6.7V TO 40V VIN BOOST C2 1µF SW LT3502A C3 0.1µF D1 DA OFF ON SHDN GND L1 10µH FB BD VIN 6.4V TO 40V VIN BOOST C2 1µF VOUT 5V 500mA SW LT3502 R2 10k C1 10µF C1: LMK316BJ106ML-BR L1: LQH43CN100K03 OFF ON SHDN GND L1 22µH D1 DA R1 52.3k C3 0.1µF FB R1 52.3k R2 10k C1: LMK316BJ106ML-BR L1: LQH43CN100K03 3502 TA06a VOUT 5V 500mA C1 22µF 3502 TA06b Related Parts PART NUMBER DESCRIPTION COMMENTS LT1766 60V, 1.