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LT3509

3509fd

For more information www.linear.com/LT3509

Typical applicaTion

FeaTures

applicaTions

DescripTion

Dual 36V, 700mA

Step-Down Regulator

The LT

3509 is a dual, current mode, step-down switching

regulator, with internal power switches each capable of

providing 700mA output current. This regulator provides a

compact and robust solution for multi-rail systems in harsh

environments. It incorporates several protection features

including overvoltage lockout and cycle-by-cycle current

limit. Thermal shutdown provides additional protection.

The loop compensation components and the boost diodes

are integrated on-chip. Switching frequency is set by a

single external resistor. External synchronization is also

possible. The high maximum switching frequency allows

the use of small inductors and ceramic capacitors for low

ripple. Constant frequency operation above the AM band

avoids inter

ference with radio reception, making the LT3509

well suited for automotive applications. Each regulator

has an independent shutdown and soft-start control pin.

When both converters are powered down, the common

circuitry enters a low current shutdown state.

3.3V and 5V Dual Output Step-Down Converter

Two 700mA Switching Regulators with Internal

Power Switches

Wide 3.6V to 36V Operating Range

Overvoltage Lockout Protects Circuit Through 60V

Supply Transients

Short-Circuit Robust

Low Dropout Voltage: 95% Maximum Duty Cycle

Adjustable 300kHz to 2.2MHz Switching Frequency

Synchronizable Over the Full Range

Uses Small Inductors and Ceramic Capacitors

Integrated Boost Diodes

Internal Compensation

Thermally Enhanced 14-Lead (4mm × 3mm)

DFN and 16 Lead MSOP Packages

Automotive Electronics

Industrial Controls

Wall Transformer Regulation

Networking Devices

CPU, DSP, or FPGA Power

3509 TA01a

LT3509

GND

DA1

FB1

RUN/SS1

SYNC

DA2

FB2

RUN/SS2

BDV

BOOST2BOOST1

SW2SW1

10µH

= 700kHz

0.1µF

6.5V TO 36V

(TRANSIENT TO 60V)

2.2µF

6.8µH

3.3V

700mA

10µF

10.2k

31.6k

53.6k

MBRM140MBRM140

1nF

22µF

10.2k

60.4k

1nF

LOAD CURRENT (A)

0.0

EFFICIENCY (%)

0.7

3509 TA01b

0.1

0.2

0.6

0.50.40.3

OUT

= 3.3V

OUT

= 5V

= 12V

= 700kHz

Efﬁciency

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.

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Summary of content (24 pages)

PAGE 1
LT3509 Dual 36V, 700mA Step-Down Regulator Features n n n n n n n n n n Description Two 700mA Switching Regulators with Internal Power Switches Wide 3.6V to 36V Operating Range Overvoltage Lockout Protects Circuit Through 60V Supply Transients Short-Circuit Robust Low Dropout Voltage: 95% Maximum Duty Cycle Adjustable 300kHz to 2.
PAGE 2
LT3509 Absolute Maximum Ratings (Note 1) VIN Pin (Note 2).........................................................60V BD Pin........................................................................20V BOOST Pins...............................................................60V BOOST Pins above SW..............................................30V RUN/SS, FB, RT, SYNC pins.........................................6V Operating Junction Temperature Range (Notes 3, 6) LT3509E.......................................
PAGE 3
LT3509 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C, VIN = 12V. (Note 3) PARAMETER CONDITIONS MIN TYP MAX 3.3 3.6 V 37 38.5 40 V 1.9 2.2 mA 9 15 µA 0.8 0.816 V VIN Undervoltage Lockout VIN Overvoltage Lockout Input Quiescent Current Not Switching VFB > 0.8V Input Shutdown Current V(RUN/SS[1,2]) < 0.3V Feedback Pin Voltage Reference Voltage Line Regulation l 0.784 3.
PAGE 4
LT3509 Typical Performance Characteristics Efficiency vs Load Current VOUT = 5V, fSW = 2.0MHz 95 95 TA = 25ºC 85 EFFICIENCY (%) 80 VIN = 24V 75 70 65 0 0.4 0.2 0.6 75 70 75 65 60 60 55 0 0.2 0.4 0.6 VIN = 12V 70 65 50 0.8 0 0.4 0.2 ILOAD(A) ILOAD(A) 0.6 0.8 ILOAD(A) 3509 G02 3509 G01 Switch VCE(SAT) vs ISW 0.35 80 VIN = 12V 80 55 0.8 TA = 25ºC 85 EFFICIENCY (%) VIN = 12V 85 EFFICIENCY (%) 90 TA = 25ºC 90 90 60 Efficiency vs Load Current VOUT = 1.8V, fSW = 0.
PAGE 5
LT3509 Typical Performance Characteristics fSW vs Temperature (Measured at 1MHz) FB Pin Voltage vs Temperature 0.810 1.04 30 0.800 0.795 1.02 1.01 1.00 0.99 0 0.97 –50 –25 25 50 75 100 125 150 TEMPERATURE (°C) 0 45 160 TA = 25ºC 140 TA = 85ºC 120 MINIMUM ON-TIME (ns) MAXIMUM VIN (V) 30 25 20 15 10 10 0 0.4 ILOAD (A) 0.6 0.8 3509 G10 Minimum On-Time vs Temperature, ILOAD = 0.3A 100 80 60 40 0 0.2 0.4 0.6 0 –50 –25 0.
PAGE 6
LT3509 Pin Functions (DFN/MSOP) DA1, DA2 (Pins 1, 7/Pins 1, 8): The DA pins are the anode connections for the catch diodes. These are connected internally to the exposed ground pad by current sensing resistors. BOOST1, BOOST2 (Pins 2, 6/Pins 2, 7): The BOOST pins are used to dynamically boost the power transistor base above VIN to minimize the voltage drop and power loss in the switch. These should be tied to the associated switch pins through the boost capacitors.
PAGE 7
LT3509 Block Diagram COMMON CIRCUITRY 1 OF 2 REGULATOR CHANNELS SHOWN VIN BD C1 OVERVOLTAGE DETECT BOOST RUN/SS1 MAIN CURRENT COMPARATOR SHUTDOWN AND SOFT-START CONTROL C3 VREF AND CORE VOLTAGE REGULATOR SYNC RT RT SWITCH LOGIC RUN/SS2 C2 NOTE: THE BD PIN IS COMMON TO BOTH CHANNELS BOOST DIODE SWITCH DRIVER DA CURRENT COMPARATOR – POWER SWITCH ERROR AMPLIFIER OSCILLATOR L1 SW VREF 0.8V VOUT D1 –17mV DA VC SLOPE C4 C5 R1 18mΩ FB R2 CLAMP GND 3509 BD Figure 1.
PAGE 8
LT3509 Operation Overview The LT3509 is a dual, constant frequency, current mode switching regulator with internal power switches. The two independent channels share a common voltage reference and oscillator and operate in phase. The switching frequency is set by a single resistor and can also be synchronized to an external clock. Operation can be best understood by referring to the Block Diagram (Figure 1). Startup and Shutdown When the RUN/SS[1,2] pins are pulled low (<0.
PAGE 9
LT3509 APPLICATIONS INFORMATION Shutdown and Soft Start Setting The Output Voltage When the RUN/SS pins are pulled to ground, the part will shut down to its lowest current state of approximately 9µA. If driving a large capacitive load it may be desirable to use the current limiting soft-start feature. Connecting capacitors to ground from the RUN/SS pins will control the delay until full current is available.
PAGE 10
LT3509 APPLICATIONS INFORMATION External Synchronization The external synchronization provides a trigger to the internal oscillator. As such, it can only raise the frequency above the free-run value. To allow for device and component tolerances, the free run frequency should be set to at least 12% lower than the lowest supplied external synchronization reference. The oscillator and hence the switching frequency can then pushed up from 12% above the free-run frequency, set by the selected RT.
PAGE 11
LT3509 APPLICATIONS INFORMATION In any step-down switcher the duty cycle when operating in continuous, or fixed frequency, mode is dependent on the step-down ratio. This is because for a constant average load current the decay of the inductor current when the switch is off must match the increase in inductor current when the switch is on.
PAGE 12
LT3509 APPLICATIONS INFORMATION Minimum Input Voltage and Boost Architecture The minimum operating voltage is determined either by the LT3509’s internal undervoltage lockout of ~3.6V or by its maximum duty cycle. The maximum duty cycle for fixed frequency operation is given by: DCMAX = 1− tOFF(MIN) • fSW It follows that: VOUT + VF − VF + VSW DCMAX VIN(MIN) = If a reduction in switching frequency can be tolerated the minimum input voltage can drop to just above output voltage.
PAGE 13
LT3509 APPLICATIONS INFORMATION Boost Pin Considerations Figure 7 through Figure 9 show several ways to arrange the boost circuit. The BOOST pin must be more than 2V above the SW pin for full efficiency. For outputs of 3.3V and higher, the standard circuit Figure 7 is best. For lower output voltages, the boost diode can be tied to the input Figure 8. The circuit in Figure 7 is more efficient because the boost pin current comes from a lower voltage source.
PAGE 14
LT3509 APPLICATIONS INFORMATION When the switch is off, the potential across the inductor is the output voltage plus the catch diode forward voltage.
PAGE 15
LT3509 APPLICATIONS INFORMATION Table 2. Recommended Inductors MANUFACTURER/ PART NUMBER VALUE (µH) ISAT (A) DCR (W) HEIGHT (mm) LPS4018-222ML 2.2 2.8 0.07 1.7 LPS5030-332ML 3.3 2.5 0.066 2.9 LPS5030-472ML 4.7 2.5 0.083 2.9 LPS6225-682ML 6.8 2.7 0.095 2.4 LPS6225-103ML 10 2.1 0.105 2.4 CDRH4D22/HP-2R2N 2.2 3.2 0.0035 2.4 CDRH4D22/HP-3R5N 3.5 2.5 0.052 2.4 CDRH4D22/HP-4R7N 4.7 2.2 0.066 2.4 CDRH5D28/HP-6R8N 6.8 3.1 0.049 3.0 CDRH5D28/HP-8R2N 8.2 2.7 0.
PAGE 16
LT3509 APPLICATIONS INFORMATION or from RC. This model works well as long as the inductor current ripple is not too low (∆IRIPPLE > 5% IOUT) and the loop crossover frequency is less than fSW/5. An optional phase lead capacitor (CPL) across the feedback divider may improve the transient response. LT3509 1.1S VOUT VIN CPL RC 260µS R1 75k COUT VREF = 0.8V 95pF 1.73M R2 3509 F10 Figure 10.
PAGE 17
LT3509 APPLICATIONS INFORMATION and Z5U types lose a large fraction of their capacitance with applied voltage and at temperature extremes. Because loop stability and transient response depend on the value of COUT, this loss may be unacceptable. Use X7R and X5R types. the top feedback resistor. The small-signal model shown in Figure 10 can be used to model this in a simulator or to give insight to an empirical design.
PAGE 18
LT3509 APPLICATIONS INFORMATION Diode Selection The catch diode (D1 from Figure 1) conducts current only during switch off time. Average forward current in normal operation can be calculated from: ID( AVG) = IOUT ( VIN – VOUT ) / VIN The only reason to consider a diode with a larger current rating than necessary for nominal operation is for the worst-case condition of shorted output. The diode current will then increase to the typical peak switch current limit.
PAGE 19
LT3509 APPLICATIONS INFORMATION Hot Plugging Considerations The small size, reliability and low impedance of ceramic capacitors make them attractive for the input capacitor. Unfortunately they can be hazardous to semiconductor devices if combined with an inductive supply loop and a fast power transition such as through a mechanical switch or connector.
PAGE 20
LT3509 Typical Applications 1.8V and 3.3V Outputs, Synchronized to 300kHz to 600kHz VIN 4.5V TO 36V (TRANSIENT TO 60V) 2.2µF BD BOOST1 BOOST2 0.22µF 15µH 0.22µF 10µH VOUT 1.8V 0.7A VIN SW1 LT3509 UPS140 12.4k CLOCK 22µF 1.6V UPS140 DA1 DA2 FB1 FB2 31.6k RUN/SS1 RUN/SS2 10k SYNC 22nF VOUT 3.3V 0.7A SW2 RT 10k GND 22nF 22µF 178k 0.
PAGE 21
LT3509 package description Please refer to http://www.linear.com/product/LT3509#packaging for the most recent package drawings. DE Package 14-Lead Plastic DFN (4mm × 3mm) (Reference LTC DWG # 05-08-1708 Rev B) 0.70 ±0.05 3.30 ±0.05 3.60 ±0.05 2.20 ±0.05 1.70 ±0.05 PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC 3.00 REF RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 4.00 ±0.10 (2 SIDES) R = 0.05 TYP 3.00 ±0.10 (2 SIDES) R = 0.115 TYP 8 0.40 ±0.10 14 3.30 ±0.
PAGE 22
LT3509 package description Please refer to http://www.linear.com/product/LT3509#packaging for the most recent package drawings. MSE Package 16-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1667 Rev F) BOTTOM VIEW OF EXPOSED PAD OPTION 2.845 ±0.102 (.112 ±.004) 5.10 (.201) 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 4.039 ±0.102 (.
PAGE 23
LT3509 Revision History REV DATE DESCRIPTION C 4/10 Changed Pin Name to RT D 01/16 (Revision history begins at Rev C) PAGE NUMBER 1, 2, 6, 7, 20, 21, 24 Revised Absolute Maximum Ratings 2 Updated Notes and Change/Add Values in Electrical Characteristics 3 Revised Values in Typical Performance Characteristics 5 Revised Values in Pin Functions 6 Revised Values in Startup and Shutdown Section 8 Revised Values in Shutdown and Soft-Start, Frequency Setting Sections, and Table 1 9 Clarified S
PAGE 24
LT3509 Typical Applications 2MHz, 5V and 3.3V Outputs VIN 6.5V TO 16V (TRANSIENT TO 60V) 2.2µF BD BOOST2 0.1µF 0.1µF 6.8µH VOUT 5V 0.7A VIN BOOST1 SW1 MBRM140 52.3k 4.7µH SW2 MBRM140 LT3509 DA1 DA2 FB1 FB2 VOUT 3.3V 0.7A 31.6k RUN/SS1 RUN/SS2 GND SYNC RT 10k 10µF 16.9k 22nF 22nF 10µF 10k fSW = 2MHz 3509 TA02 Related Parts PART NUMBER DESCRIPTION COMMENTS LT1766 60V, 1.2A (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter VIN: 5.5V to 60V, VOUT = 1.20V, IQ = 2.