LINEAR TECHNOLOGY LT3508 Technical data

LT3508

Dual Monolithic 1.4A

Step-Down Switching

Regulator

FEATURES

Wide Input Voltage Range: 3.7V to 36V

Two 1.4A Output Switching Regulators with Internal

Power Switches

Adjustable 250kHz to 2.5MHz Switching Frequency

Synchronizable over the Full Frequency Range

Anti-Phase Switching Reduces Ripple

Uses Small Inductors and Ceramic Capacitors

Accurate Programmable Undervoltage Lockout

Independent Tracking, Soft-Start and Power Good

Circuits Ease Supply Sequencing

Output Adjustable Down to 800mV

Small 4mm × 4mm 24-Pin QFN or 16-Pin Thermally

Enhanced TSSOP Surface Mount Packages

APPLICATIONS

Automotive

DSP Power Supplies

Wall Transformer Regulation

DSL and Cable Modems

PCI Express

DESCRIPTION

The LT®3508 is a dual current mode PWM step-down DC/DC converter with internal power switches capable of generating two 1.4A outputs. The wide input voltage range of 3.7V to 36V makes the LT3508 suitable for regulating power from a wide variety of sources, including automotive batteries, 24V industrial supplies and unregulated wall adapters. Both converters are synchronized to a single oscillator programmable up to 2.5MHz and run with opposite phases, reducing input ripple current. Its high operating frequency allows the use of small, low cost inductors and ceramic capacitors, resulting in low, predictable output ripple. Each regulator has independent tracking and softstart circuits and generates a power good signal when its output is in regulation, easing power supply sequencing and interfacing with microcontrollers and DSPs.

Cycle-by-cycle current limit, frequency foldback and thermal shutdown provide protection against shorted outputs, and soft-start eliminates input current surge during startup. The low current (<2A) shutdown mode enables easy power management in battery-powered systems.

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.

TYPICAL APPLICATION

3.3V and 5V Dual Output Step-Down Converter with Output Sequencing

5.6V TO 36V

OUT1

3.3V

1.4A

35.7k

11.5k

22µF

6.8µH

4.7µF

0.22µF 0.22µF

51k

150pF

1nF

VINSHDN

BOOST1

SW1

LT3508

TRACK/SS1

GND

= 700kHz

BOOST2

SW2

RT/SYNC

FB2FB1

PG1 PG2TRACK/SS2

52.3k

ON OFF

43k

100pF

10µH

56.2k

10.7k

100k

3508 TA01a

OUT2 5V

1.4A

10µF

POWER GOOD

VIN = 12V

EFFICIENCY (%)

Efﬁ ciency

= 5V

OUT2

= 3.3V

OUT1

0.5 1

LOAD CURRENT (A)

1.5

3508 TA01b

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LT3508

ABSOLUTE MAXIMUM RATINGS

VIN Pin Voltage ............................................(–0.3V), 40V

BOOST Pin Voltage ...................................................60V

BOOST Above SW Voltage ........................................30V

SHDN, PG Voltage .....................................................40V

TRACK/SS, FB, R

/SYNC, VC Voltage ..........................6V

Operating Junction Temperature Range (Note 2)

LT3508E ............................................. –40°C to 125°C

LT3508I .............................................. –40°C to 125°C

LT3508H ............................................ –40°C to 150°C

PIN CONFIGURATION

TOP VIEW

TRACK/SS1

BOOST1

BOOST2

TRACK/SS2

EXPOSED PAD (PIN 17) IS GND AND MUST BE SOLDERED TO PCB

SW1

IN1

IN2

SW2

16-LEAD PLASTIC TSSOP

θJA = 40°C/W, θJC = 10°C/W

FE PACKAGE

FB1

PG1

/SYNC

SHDN

PG2

FB2

(Note 1)

Storage Temperature Range

QFN .................................................... –65°C to 150°C

TSSOP ............................................... –65°C to 150°C

Lead Temperature (Soldering, 10 sec)

TSSOP .............................................................. 300°C

TOP VIEW

/SYNC

VC1PG1

24 23 22 21 20 19

FB1

TRACK/SS1

EXPOSED PAD (PIN 25) IS GND AND MUST BE SOLDERED TO PCB

GND

24-LEAD (4mm s 4mm) PLASTIC QFN

7 8 9

BOOST1

θJA = 40°C/W, θJC = 10°C/W

10 11 12

IN1VIN2

SW1

UF PACKAGE

SHDN

PG2

SW2

BOOST2

FB2

TRACK/SS2

GND

ORDER INFORMATION

LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE

LT3508EFE#PBF LT3508EFE#TRPBF 3508FE 16-Lead Plastic TSSOP –40°C to 125°C

LT3508IFE#PBF LT3508IFE#TRPBF 3508FE 16-Lead Plastic TSSOP –40°C to 125°C

LT3508HFE#PBF LT3508HFE#TRPBF 3508HFE 16-Lead Plastic TSSOP –40°C to 150°C

LT3508EUF#PBF LT3508EUF#TRPBF 3508

LT3508IUF#PBF LT3508IUF#TRPBF 3508

LT3508HUF#PBF LT3508HUF#TRPBF 3508H

24-Lead (4mm × 4mm) Plastic QFN 24-Lead (4mm × 4mm) Plastic QFN 24-Lead (4mm × 4mm) Plastic QFN

–40°C to 125°C

–40°C to 150°C

Consult LTC Marketing for parts speciﬁ ed with wider operating temperature ranges.*Temperature grades are identiﬁ ed by a label on the shipping container. Consult LTC Marketing for information on non-standard lead based ﬁ nish parts.

For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel speciﬁ cations, go to: http://www.linear.com/tapeandreel/

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LT3508

ELECTRICAL CHARACTERISTICS

The l denotes the speciﬁ cations which apply over the full operating temperature range, otherwise speciﬁ cations are at T

= 25°C. VIN = 12V, V

PARAMETER CONDITIONS MIN TYP MAX UNITS

Minimum Operating Voltage, V

Quiescent Current Not Switching 4.3 5.2 mA

IN1

Quiescent Current Not Switching 320 500 µA

IN2

Shutdown Current (V

IN1

IN2

+ V

IN2

= 12V

IN1

= 0.3V 0.1 2 µA

SHDN

FB Voltage

FB Pin Bias Current (Note 3) V

= 0.800V, VC = 0.4V

FB Voltage Line Regulation 5V < VIN < 40V 0.01 %/V

Error Amp Transconductance 300 µS

Error Amp Voltage Gain 600 V/V

to Switch Current Gain 2.5 A/V

Switching Frequency R

Switching Phase R

Maximum Duty Cycle (Note 4) R

Foldback Frequency R

= 33.2k

= 33.2k 150 180 210 Deg

= 33.2k

= 7.50k

= 169k

= 33.2k, VFB = 0V 120 kHz

Switch Current Limit (Note 5) Duty Cycle = 15%

Switch V

CESAT

ISW = 1.5A 300 mV

Switch Leakage Current 0.01 1 µA

Minimum Boost Voltage 1.7 2.5 V

Boost Pin Current I

TRACK/SS Pin Current V

PG Threshold Offset V

PG Voltage Output Low V

PG Pin Leakage V

= 1.5A 35 50 mA

= 0V 0.8 1.2 2.2 µA

TRACK/SS

Rising 56 75 110 mV

= 0.6V, IPG = 250µA 0.13 0.4 V

= 2V 0.01 1 µA

SHDN Threshold Voltage 2.53 2.63 2.73 V SHDN Input Current (Note 6) V

= 60mV Above Threshold Voltage 6 8 10 µA

SHDN

SHDN Threshold Current Hysteresis 5.5 7.5 9.5 µA

SYNC Threshold Voltage 1 1.25 1.5 V

SYNC Input Frequency 0.25 2.5 MHz

= 17V unless otherwise noted. (Note 2)

BOOST

0.790

0.784

0.92 1 1.06 MHz

84 90

2.0 2.6 3.2 A

3.4 3.7 V

2.5 3.0 V

0.800 0.814

0.816

50 300 nA

80 98

% % %

V V

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 LT3508E is guaranteed to meet performance speciﬁ cations from 0°C to 125°C junction temperature. Speciﬁ cations over the –40°C to 125°C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LT3508I is guaranteed over the full –40°C to 125°C operating junction temperature range. The LT3508H is guaranteed over the full –40°C to 150°C operating junction temperature range. High junction temperatures degrade operating lifetimes. Operating lifetime is derated at junction temperatures greater than 125°C.

Note 3: Current ﬂ ows out of pin. Note 4: V

LT3508 when V

=12V. Circuitry increases the maximum duty cycle of the

BOOST

> VIN + 2.5V. See “Minimum Operating Voltage” in

BOOST

the Applications Information section for details. Note 5: Current limit is guaranteed by design and/or correlation to static

test. Slope compensation reduces current limit at higher duty cycles.

Note 6: Current ﬂ ows into pin. Note 7: This IC includes overtemperature protection that is intended

to protect the device during momentary overload conditions. Junction temperature will exceed the maximum operating junction temperature range when overtemperature protection is active. Continuous operation above the speciﬁ ed maximum operating junction temperature may impair device reliability.

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LT3508

TYPICAL PERFORMANCE CHARACTERISTICS

Efﬁ ciency, V

TA = 25°C f = 700kHz

EFFICIENCY (%)

Feedback Voltage

0.810

0.805

0.800

= 5V

OUT

VIN = 12V

VIN = 24V

VIN = 32V

0.5 1

LOAD CURRENT (A)

3508 G01

1.5

Efﬁ ciency, V

TA = 25°C f = 700kHz

EFFICIENCY (%)

0.5 1

LOAD CURRENT (A)

Switch Current Limit vs Temperature

3.0

2.5

2.0

1.5

= 3.3V Efﬁ ciency, V

OUT

VIN = 12V

VIN = 24V

VIN = 32V

1.5

3508 G02

TA = 25°C f = 1MHz

EFFICIENCY (%)

Switch Current Limit vs Duty Cycle

3.0 TA = 25°C

2.5

2.0

1.5

= 1.8V

OUT

VIN = 3.3V

VIN = 5V

VIN = 12V

0.5 1

LOAD CURRENT (A)

TYPICAL

MINIMUM

1.5

3508 G03

FEEDBACK VOLTAGE (V)

0.795

0.790 –50

–25 0 25 50

TEMPERATURE (°C)

Switching Frequency vs R

1000

TA = 25°C

100

(k)

0.1 FREQUENCY (MHz)

1.0

CURRENT LIMIT (A)

0.5

75 100 125 150

3508 G04

–50 –25 0 25 50 75 100 125 150

Switching Frequency

110

3508 G07

vs Temperature

1.2 RT = 33.2k

1.0

0.8

0.6

0.4

SWITCHING FREQUENCY (MHz)

0.2

–50 –25 0 25 50 75 100 125 150

TEMPERATURE (°C)

3508 G05

3508 G08

1.0

CURRENT LIMIT (A)

0.5

20 40 60 80

DUTY CYCLE (%)

Switching Frequency Foldback

3.0 TA = 25°C

2.5

2.0

1.5

1.0

SWITCHING FREQUENCY (MHz)

0.5

200 400 600 800

100 300 500 700

FEEDBACK VOLTAGE (mV)

RT = 7.50k

RT = 33.2k

100

3508 G06

RT = 169k

3508 G09

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TYPICAL PERFORMANCE CHARACTERISTICS

LT3508

Quiescent Current VC Voltages

5.0 TA = 25°C

4.5

4.0

3.5

3.0

2.5

2.0

1.5

INPUT CURRENT (mA)

1.0

0.5

515

IN1

IN2

INPUT VOLTAGE (V)

3508 G10

2.5

2.0

1.5

VOLTAGE (V)

1.0

0.5

–50 –25 0 25 50 75 100 125 150

Switch Voltage Drop

350

TA = 25°C

300

250

200

150

CLAMP VOLTAGE

TO SWITCH

TEMPERATURE (°C)

3508 G11

Boost Pin Current

TA = 25°C

Error Amp Output Current

OUTPUT CURRENT (µA)

–50 –25 0 25 50 75 100 125 150

SINKING

SOURCING

TEMPERATURE (°C)

3508 G12

100

SWITCH VOLTAGE (mV)

SHDN Pin Current

120

100

SHDN PIN CURRENT (µA)

515

0.5

SWITCH CURRENT (A)

TA = –45°C

TA = 25°C

10 20

SHDN PIN VOLTAGE (V)

1 1.5

3508 G13

TA = 125°C

3508 G15

BOOST PIN CURRENT (mA)

0.5

SWITCH CURRENT (A)

1 1.5

3508 G14

Undervoltage Lockout

4.0

3.5

3.0

2.5

2.0

1.5

INPUT VOLTAGE (V)

1.0

0.5

–50 –25 0 25 50 75 100 125 150

IN1

IN2

TEMPERATURE (°C)

3508 G16

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LT3508

PIN FUNCTIONS

BOOST1, BOOST2: The BOOST pins are used to provide drive voltages, higher than the input voltage, to the internal NPN power switches. Tie through a diode to a 2.8V or higher supply, such as V

OUT

or VIN.

Exposed Pad: The Exposed Pad metal of the package provides both electrical contact to ground and good thermal contact to the printed circuit board. The Exposed Pad must be soldered to the circuit board for proper operation.

FB1, FB2: The LT3508 regulates each feedback pin to

0.800V. Connect the feedback resistor divider taps to these pins.

GND: Tie the GND pins directly to the Exposed Pad and ground plane.

PG1, PG2: The power good pins are the open-collector outputs of an internal comparator. PG remains low until the FB pin is within 10% of the ﬁ nal regulation voltage. As well as indicating output regulation, the PG pins can be used to sequence the two switching regulators. These pins can be left unconnected. The PG outputs are valid when V

is greater than 2.4V and SHDN is high. The PG

IN1

comparators are disabled in shutdown.

/SYNC: The RT/SYNC pin is used to set the internal

oscillator frequency. Tie a 33.2k resistor from R

/SYNC

to GND for a 1MHz switching frequency. To synchronize the part to an external frequency, drive the R

/SYNC pin

with a logic-level signal with positive and negative pulse widths of at least 80ns.

SHDN: The shutdown pin is used to put the LT3508 in shutdown mode. Pull the pin below 0.3V to shut down the LT3508. The 2.63V threshold can function as an accurate undervoltage lockout (UVLO), preventing the regulator

from operating until the input voltage has reached the programmed level. Do not drive SHDN more than 6V above V

IN1

SW1, SW2: The SW pins are the outputs of the internal power switches. Connect these pins to the inductors, catch diodes and boost capacitors.

TRACK/SS1, TRACK/SS2: The TRACK/SS pins are used to soft-start the two channels, to allow one channel to track the other output, or to allow both channels to track another output. For tracking, tie a resistor divider to this pin from the tracked output. For soft-start, tie a capacitor to this pin. An internal 1.2µA soft-start current charges the capacitor to create a voltage ramp at the pin. Leave these pins disconnected if unused.

, VC2: The VC pins are the outputs of the internal error

amps. The voltages on these pins control the peak switch currents. These pins are normally used to compensate the control loops, but can also be used to override the loops. Pull these pins to ground with an open drain to shut down each switching regulator.

IN1

: The V

pin supplies current to the LT3508 internal

IN1

circuitry and to the internal power switch connected to SW1 and must be locally bypassed. V

must be greater

IN1

than 3.7V for channel 1 or channel 2 to operate.

IN2

: The V

pin supplies current to the internal power

IN2

switch connected to SW2 and must be locally bypassed. Connect this pin directly to V nel 2 is coming from a different source. V greater than 3V and V

must be greater than 3.7V for

IN1

unless power for chan-

IN1

must be

IN2

channel 2 to operate.

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BLOCK DIAGRAM

SHDN

LT3508

IN1

RT/SYNC

TRACK/SS

1.2µA

0.75V

INT REG

AND REF

MASTER

OSC

CLK1

CLK2

SLOPE

SLAVE

CLK

OSC

0.625V

–

LIMIT

CLAMP

ERROR

AMP

– +

TRACK/SS

75mV

0.80V

BOOST

OUT

GND

–

3508 F01

Figure 1. Block Diagram of the LT3508 with Associated External Components (One of Two Switching Regulators Shown)

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LT3508

OPERATION

The LT3508 is a dual constant frequency, current mode regulator with internal power switches. Operation can be best understood by referring to the Block Diagram. If the SHDN pin is tied to ground, the LT3508 is shut down and draws minimal current from the input source tied to the

pins. If the SHDN pin exceeds 1V, the internal bias

circuits turn on, including the internal regulator, reference and oscillator. The switching regulators will only begin to operate when the SHDN pin exceeds 2.63V.

The switcher is a current mode regulator. Instead of directly modulating the duty cycle of the power switch, the feedback loop controls the peak current in the switch during each cycle. Compared to voltage mode control, current mode control improves loop dynamics and provides cycle-bycycle current limit. A pulse from the oscillator sets the RS ﬂ ip-ﬂ op and turns on the internal NPN power switch. Current in the switch and the external inductor begins to increase. When this current exceeds a level determined by the voltage at V

, current comparator C1 resets the

ﬂ ip-ﬂ op, turning off the switch. The current in the inductor ﬂ ows through the external Schottky diode and begins to decrease. The cycle begins again at the next pulse from the oscillator. In this way, the voltage on the V

pin controls

the current through the inductor to the output. The internal error ampliﬁ er regulates the output current by continually adjusting the V on the V

pin voltage. The threshold for switching

pin is 0.8V, and an active clamp of 1.75V limits

the output current.

The switching frequency is set either by the resistance to GND at the R signal driving the R

/SYNC pin or the frequency of the logic-level

/SYNC pin. A detection circuit monitors

for the presence of a SYNC signal on the pin and switches

between the two modes. Unique circuitry generates the appropriate slope compensation ramps and generates the 180° out-of-phase clocks for the two channels.

The switching regulator performs frequency foldback during overload conditions. An ampliﬁ er senses when

is less than 0.625V and begins decreasing the oscil-

lator frequency down from full frequency to 12% of the nominal frequency when V

= 0V. The FB pin is less than

0.8V during start-up, short-circuit and overload conditions. Frequency foldback helps limit switch current under these conditions.

The switch driver operates either from V

or from the

BOOST pin. An external capacitor and Schottky diode are used to generate a voltage at the BOOST pin that is higher than the input supply. This allows the driver to saturate the internal bipolar NPN power switch for efﬁ cient operation.

The TRACK/SS pin serves as an alternative input to the error ampliﬁ er. The ampliﬁ er will use the lowest voltage of either the reference of 0.8V or the voltage on the TRACK/SS pin as the positive input of error ampliﬁ er. Since the TRACK/SS pin is driven by a constant current source, a single capacitor on the pin will generate a linear ramp on the output voltage. Tying the TRACK/SS pin to a resistor divider from the output of one of the switching regulators allows one output to track another.

The PG output is an open-collector transistor that is off when the output is in regulation, allowing an external resistor to pull the PG pin high. Power good is valid when the LT3508 is enabled (SHDN is high) and V

is greater

IN1

than ~2.4V.

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LINEAR TECHNOLOGY LT3508 Technical data

Specifications and Main Features

Frequently Asked Questions

User Manual