ANALOG DEVICES LT 3508 EUF Datasheet

LT3508

Dual Monolithic 1.4A

Step-Down Switching

Regulator

FEATURES

n

Wide Input Voltage Range: 3.7V to 36V

n

Two 1.4A Output Switching Regulators with Internal

Power Switches

n

Adjustable 250kHz to 2.5MHz Switching Frequency

n

Synchronizable over the Full Frequency Range

n

Anti-Phase Switching Reduces Ripple

n

Uses Small Inductors and Ceramic Capacitors

n

Accurate Programmable Undervoltage Lockout

n

Independent Tracking, Soft-Start and Power Good

Circuits Ease Supply Sequencing

n

Output Adjustable Down to 800mV

n

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

Enhanced TSSOP Surface Mount Packages

APPLICATIONS

n

Automotive

n

DSP Power Supplies

n

Wall Transformer Regulation

n

DSL and Cable Modems

n

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.

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

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

No R

SENSE

property of their respective owners.

TYPICAL APPLICATION

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

V

5.6V TO 36V

OUT1

3.3V

1.4A

IN

35.7k

11.5k

22µF

6.8µH

4.7µF

V

IN1VIN2

BOOST1

0.22µF 0.22µF

SW1

V

51k

150pF

1nF

C1

TRACK/SS1

GND

f

SW

SHDN

BOOST2

SW2

LT3508

RT/SYNC

= 700kHz

FB2FB1

V

C2

PG1
PG2TRACK/SS2

52.3k

ON OFF

43k

100pF

10µH

56.2k

10.7k

100k

3508 TA01a

OUT2
5V

1.4A

10µF

POWER
GOOD

95

VIN = 12V

90

85

80

EFFICIENCY (%)

75

70

65

0

Effi ciency

V

= 5V

OUT2

V

= 3.3V

OUT1

0.5 1

LOAD CURRENT (A)

1.5

3508 TA01b

3508fd

1

LT3508

ABSOLUTE MAXIMUM RATINGS

(Notes 1, 7)

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

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

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

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

TRACK/SS, FB, RT/SYNC, V

Voltage ..........................6V

C

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

1

2

3

SW1

4

V

IN1

V

IN2

SW2

16-LEAD PLASTIC TSSOP

θJA = 40°C/W, θJC = 10°C/W

5

6

7

8

FE PACKAGE

17

GND

16

15

14

13

12

11

10

9

FB1

V

C1

PG1

RT/SYNC

SHDN

PG2

V

C2

FB2

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

VC1PG1

RT/SYNC

24 23 22 21 20 19

1

FB1

TRACK/SS1

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

2

3

GND

4

GND

5

GND

6

GND

24-LEAD (4mm × 4mm) PLASTIC QFN

θJA = 40°C/W, θJC = 10°C/W

7 8 9

SW1

BOOST1

UF PACKAGE

25

GND

10 11 12

IN1VIN2

V

SHDN

PG2

SW2

C2

V

18

17

16

15

14

13

BOOST2

FB2

TRACK/SS2

GND

GND

GND

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 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:
For more information on tape and reel specifi cations, go to:

http://www.linear.com/leadfree/

http://www.linear.com/tapeandreel/

3508fd

2

LT3508

ELECTRICAL CHARACTERISTICS

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

PARAMETER CONDITIONS MIN TYP MAX UNITS

Minimum Operating Voltage, V

Minimum Operating Voltage, V

Quiescent Current Not Switching 4.3 5.2 mA

V

IN1

Quiescent Current Not Switching 320 500 µA

V

IN2

Shutdown Current (V

IN1

FB Voltage

FB Pin Bias Current (Note 3) V

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

V

C

Switching Frequency R

Switching Phase R

Maximum Duty Cycle (Note 4) R

Foldback Frequency R

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

Switch V

CESAT

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

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

IN1

IN2

+ V

)V

IN2

= 25°C. VIN = 12V unless otherwise noted. (Note 2)

A

l

V

= 12V

IN1

= 0.3V 0.1 2 µA

SHDN

l

0.790

l

0.784

= 0.800V, VC = 0.5V

FB

= 33.2k

T

= 33.2k 150 180 210 Deg

T

= 33.2k

T

R

= 7.50k

T

R

= 169k

T

= 33.2k, VFB = 0V 120 kHz

T

l

l

0.92 1 1.06 MHz

l

84 90

l

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

ISW = 1.5A 300 mV

= 1.5A, V

SW

= 0V 0.8 1.2 2.2 µA

TRACK/SS

Rising 56 75 110 mV

FB

= 0.6V, IPG = 250µA 0.13 0.4 V

FB

= 2V 0.01 1 µA

PG

= 60mV Above Threshold Voltage 6 8 10 µA

SHDN

= 17V 35 50 mA

BOOST

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 the

BOOST

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.

3508fd

3

LT3508

TYPICAL PERFORMANCE CHARACTERISTICS

Effi ciency, V

95

TA = 25°C
f = 700kHz

90

85

80

EFFICIENCY (%)

75

70

65

0

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

90

TA = 25°C
f = 700kHz

85

80

75

EFFICIENCY (%)

70

65

60

0

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

85

TA = 25°C
f = 1MHz

80

75

70

EFFICIENCY (%)

65

60

55

0

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

175

150

125

100

(k)

T

R

75

50

25

0

0

FREQUENCY (MHz)

75 100 125 150

3508 G04

T

TA = 25°C

2.52.01.51.00.5

3508 G07

1.0

CURRENT LIMIT (A)

0.5

0

–50 –25 0 25 50 75 100 125 150

TEMPERATURE (°C)

Switching Frequency
vs Temperature

1.2
RT = 33.2k

1.0

0.8

0.6

0.4

SWITCHING FREQUENCY (MHz)

0.2

0

–50 –25 0 25 50

TEMPERATURE (°C)

75 100 125 150

3508 G05

3508 G08

1.0

CURRENT LIMIT (A)

0.5

0

0

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

0

200 400 600 800

100 300 500 700

0

FEEDBACK VOLTAGE (mV)

RT = 7.50k

RT = 33.2k

100

3508 G06

RT = 169k

3508 G09

4

3508fd

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

0

0

515

10

V

IN1

V

IN2

20

INPUT VOLTAGE (V)

25

35

30

40

3508 G10

2.5

2.0

1.5

VOLTAGE (V)

1.0

C

V

0.5

0

–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

35

TA = 25°C

30

25

20

15

Error Amp Output Current

35

30

25

20

15

10

OUTPUT CURRENT (µA)

5

0

–50 –25 0 25 50 75 100 125 150

SINKING

SOURCING

TEMPERATURE (°C)

3508 G12

100

SWITCH VOLTAGE (mV)

50

0

0

SHDN Pin Current

120

100

80

60

40

SHDN PIN CURRENT (µA)

20

0

515

0

0.5

SWITCH CURRENT (A)

TA = –45°C

10 20

SHDN PIN VOLTAGE (V)

25

1 1.5

3508 G13

TA = 125°C

TA = 25°C

30

35

3508 G15

10

BOOST PIN CURRENT (mA)

5

0

0

0.5

SWITCH CURRENT (A)

1 1.5

3508 G14

Undervoltage Lockout

4.0

V

3.5

3.0

2.5

2.0

1.5

INPUT VOLTAGE (V)

1.0

0.5

40

0

–50 –25 0 25 50 75 100 125 150

IN1

V

IN2

TEMPERATURE (°C)

3508 G16

3508fd

5

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 3.7V and SHDN is high. The PG

IN1

comparators are disabled in shutdown.

RT/SYNC: The RT/SYNC pin is used to set the internal
oscillator frequency. Tie a 33.2k resistor from RT/SYNC
to GND for a 1MHz switching frequency. To synchronize
the part to an external frequency, drive the RT/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. In the
TSSOP package, if these pins are unused, tie a 47pF or
greater capacitor from each pin to GND.

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

V

C1

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 separately.

V

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
than 3.7V for channel 1 or channel 2 to operate. If V
is greater than 3.7V, channel 2 can operate with V

must be greater

IN1

IN2

IN1

as

low as 3V.

V

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.

6

3508fd

BLOCK DIAGRAM

SHDN

LT3508

V

IN1

RT/SYNC

V

IN

V

IN

C

IN

TRACK/SS

1.2µA

0.75V

INT REG

AND REF

MASTER

OSC

CLK1

CLK2

+

Σ

SLOPE

C1

SLAVE

CLK

OSC

+

0.625V

–

V

C

R

C

F

C

C

C

–

+

I

LIMIT

CLAMP

R

SQ

ERROR

AMP

–
+

+

TRACK/SS

+

75mV

0.80V

BOOST

SW

FB

D2

C3

L1

OUT

D1

C1

R1

R2

PG

+

GND

–

3508 F01

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

3508fd

7

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

V

IN

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
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
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
the output current.

The switching frequency is set either by the resistance to
GND at the RT/SYNC pin or the frequency of the logic-level
signal driving the RT/SYNC pin. A detection circuit monitors
for the presence of a SYNC signal on the pin and switches

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

C

, current comparator C1 resets the

C

pin controls

C

pin voltage. The threshold for switching

C

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-

V

FB

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

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
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
than 3.7V.

= 0V. The FB pin is less than

FB

or from the

IN

is greater

IN1

8

3508fd