LINEAR TECHNOLOGY LT3439 Technical data

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FEATURES

■

Reduced Conducted and Radiated EMI

■

Single Resistor Control of Output Switch Voltage
and Current Slew Rates

■

Cross Conduction Prevention Circuitry

■

Two 1A Current Limited Power Switches

■

Low Minimum Supply Voltage: 2.8V

■

Low Shutdown Current: <20µA

■

50% Duty Cycle

■

20kHz to 250kHz Oscillator Frequency

■

Synchronizable to 300kHz

■

Overcurrent and Overtemperature Protected

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APPLICATIO S

■

Low Noise Isolated Supplies

■

Radio and Telecom Supplies

■

Distributed Supplies

■

Medical Instruments

■

Precision Instruments

■

Low Noise Filament Supplies

LT3439

Slew Rate Controlled

Ultralow Noise1A Isolated

DC/DC Transformer Driver

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DESCRIPTIO

The LT®3439 is a push-pull DC/DC transformer driver that
reduces conducted and radiated electromagnetic interfer­ence (EMI). Ultralow noise and EMI are achieved by
controlling the output switch voltage and current slew
rates. Slew rates are user adjustable to optimize output
noise versus efficiency. The LT3439 can reduce high
frequency harmonic content by as much as 40dB with only
a minor decrease in efficiency.

The LT3439 includes two 1A current limited power switches
to ensure start-up under heavy loads. It also includes an
oscillator that can be synchronized to an external clock for
more accurate placement of switcher harmonics. Protec­tion features include current limiting, undervoltage lock­out, thermal shutdown and cross conduction prevention
circuitry.

The LT3439 is available in a thermally enhanced 16-pin
TSSOP with an exposed backside.

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

TYPICAL APPLICATIO

Low Noise 5V to 5V Push-Pull DC/DC Transformer

V

IN

5V

C

IN

22µF
10V

680pF

16.9k

11

5
6

V

SHDN

LT3439
SYNC
C

T

R

T

GND PGND

10

13

IN

COL A

COL B

R

1, 16

3

14

47

SL

34k

3439 TA01

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OPTIONAL

D1

T1

•

•

•

•

A

D2

C
C1, C2: TDK C4532X5R0J476M
D1, D2: MBR0520
L1: TDK SLF6028T-330MR69
T1: COILTRONICS 15835

L1 33µH

C1
47µF

6.3V

: MURATA GRM235Y5V226Z10

IN

47µF

6.3V

B

C2

V

OUT

5V
500mA

200µV/DIV

20mV/DIV

B

A

5V Output Noise

5µs/DIV 3439 TA01b

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LT3439

FE PACKAGE

16-LEAD PLASTIC TSSOP

1
2
3
4
5
6
7
8

TOP VIEW

16
15
14
13
12
11
10

9

PGND

NC

COL A

R

SL

SYNC

C

T

R

T

NC

PGND
NC
COL B
V

IN

NC
SHDN
GND
NC

WWWU

ABSOLUTE AXI U RATI GS

PACKAGE/ORDER I FOR ATIO

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(Note 1)

VIN Voltage ............................................................. 20V

COL A, COL B Voltage............................................. 35V

ORDER PART

NUMBER

SHDN, SYNC Voltage.............................................. 20V

Maximum Junction Temperature ......................... 150°C

LT3439EFE

Operating Junction Temperature Range

(Note 2) ............................................ –40°C to 125°C

Storage Temperature Range ................ –65°C to 150°C

Lead Temperature (Soldering, 10 sec)................. 300°C

FE PART

MARKING

3439EFE

T

= 125°C, θJA = 40°C/W

JMAX

NOTE: BACKSIDE OF PACKAGE CONNECTED TO GND

Consult LTC Marketing for parts specified with wider operating temperature ranges.

ELECTRICAL CHARACTERISTICS

The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C.
VIN = 5V; RT = 16.9k; CT = 680pF; RSL = 16.9k; COL A, COL B, SHDN pins open, unless otherwise noted.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Supply and Shutdown

V

IN

V

IN(MIN)

I

VIN

I

VIN(SHDN)

V

SHDN(ON)

V

SHDN(OFF)

I

SHDN

Oscillator and Sync

f

MAX

f

SYNC

V

SYNC

R

SYNC

Output Switches (COL A, COL B)

DC Switch Duty Cycle ● 50 %
BV Output Switch Breakdown Voltage 2.8V ≤ VIN ≤ 17.5V ● 35 50 V
R

ON

I

LIM(MAX)

Slew Control

V

SLEWR

V

SLEWF

I

SLEWR
SLEWF

I

Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.

Operating Range ● 2.8 17.5 V
Minimum Input Voltage 2.55 2.7 V
Supply Current 2.8V ≤ VIN ≤ 17.5V ● 12 mA
Supply Current in Shutdown Mode 2.8V ≤ VIN ≤ 17.5V, V

= 0V 5 20 µA

SHDN

Shutdown Turn-On Threshold 2.8V ≤ VIN ≤ 17.5V ● 1.3 1.4 V
Shutdown Turn-Off Threshold 2.8V ≤ VIN ≤ 17.5V ● 1.20 1.26 V
Shutdown Pin Current Hysteresis 2.8V ≤ VIN ≤ 17.5V, V

= 1.4V ● 10 20 40 µA

SHDN

Oscillator Frequency ● 250 kHz
Synchronization Frequency Range ● 300 kHz
SYNC Pin Threshold ● 1.4 2.25 V
SYNC Pin Input Resistance 40 kΩ

Output Switch On Resistance I

COLA

or I

= 0.75A ● 0.5 0.95 Ω

COLB

Switch Current Limit 1.2 1.4 1.65 A

Output Voltage Slew Rising Edge Collector A or B 17 V/µs
Output Voltage Slew Falling Edge Collector A or B 17 V/µs
Output Current Slew Rising Edge Collector A or B 5 A/µs
Output Current Slew Falling Edge Collector A or B 5 A/µs

Note 2: The LT3439E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 125°C operating
junction temperature range are assured by design, characterization and
correlation with statistical process controls.

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TEMPERATURE (°C)

–50

1.0

VOLTAGE THRESHOLD (V)

1.1

1.2

1.3

1.4

050

100

150

3439 G09

1.5

1.6

–25 25

75

125

TEMPERATURE (°C)

–50

SHDN PIN CURRENT (µA)

18

20

22

25

3439 G06

16

14

–25 0 15050 75 100 125

12

10

24

TYPICAL PERFOR A CE CHARACTERISTICS

LT3439

Minimum Input Voltage
vs Temperature

2.750

2.700

2.650

2.600

2.550

2.500

2.450

INPUT VOLTAGE (V)

2.400

2.350

2.300
–50

–25

0

TEMPERATURE (°C)

Switch Voltage Drop
vs Switch Current

0.9

0.8

0.7

0.6

0.5

0.4

0.3

SWITCH VOLTAGE (V)

0.2

0.1

0

0.1 0.2 0.3

0.4 0.5 0.6 1.0

SWITCH CURRENT (A)

25 50 75 100

125°C

85°C
25°C

0.7 0.8 0.9

125 150

3439 G01

3439 G04

Supply Current in Shutdown Mode
vs Temperature

16

14

12

10

8

6

SUPPLY CURRENT (µA)

4

2

0

–50

–25

0

17.5V

5V

2.7V

25 50 75 100

TEMPERATURE (°C)

Supply Current vs Slew Resistor

25

20

15

10

SUPPLY CURRENT (mA)

5

0

10000

0

20000

SLEW RESISTOR (Ω)

30000

125 150

3439 G02

TA = 25°C

3439 G05

40000

Oscillator Frequency
vs Temperature

120

RT = 16.9k

= 680pF

C

T

115

110

105

100

95

90

OSCILLATOR FREQUENCY (kHz)

85

80

–50

–25

0

25 50 75 100

TEMPERATURE (°C)

SHDN Pin Hysteresis Current
vs Temperature

125 150

3439 G03

2.0

1.8

1.6

1.4

1.2

CURRENT LIMIT (A)

1.0

0.8
–50

Current Limit vs Temperature

50

–25 0

25 75 150

TEMPERATURE (°C)

100 125

3439 G07

SHDN Pin Voltage Threshold
vs Temperature

1.35

1.34

1.33

1.32

1.31

1.30

1.29

THRESHOLD VOLTAGE (V)

1.28

1.27
–50

–25

0

25 50 75 100

TEMPERATURE (°C)

SYNC Pin Voltage Threshold
vs Temperature

125 150

3439 G08

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LT3439

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PI FU CTIO S

PGND (Pins 1, 16): Power ground is connected to the
emitter of the power switches via an internal sense resis­tor. It has large currents flowing through it and should be
connected to a good quality ground plane.

COL A, COL B (Pins 3, 14): These are the open collectors
of the output power switches. They are connected to the
outer terminals of the center tap transformer. Large cur­rents flow into these pins so external traces should be kept
as short as possible.

RSL (Pin 4): The slew control resistor sets the maximum
current and voltage slew rate for the collectors A and B.
The minimum resistor value is 3.4k for fast slewing and the
maximum resistor is 34k for slow slewing. For more
details, see “Slew Rate Setting” in the Applications Infor­mation section of this data sheet.

SYNC (Pin 5): The SYNC pin can be used to synchronize
the oscillator to an external clock. RT and CT should be set
such that the oscillator clock frequency is approximately
10% below the external clock frequency. If unused, this
pin should be tied to GND. For more details, see “Oscillator
Sync” in the Applications Information section of this data
sheet.

CT (Pin 6): The oscillator capacitor pin is used in conjunc­tion with the RT pin to set the oscillator frequency. For
RT = 16.9k, CT can be calculated as follows:

CT(nF) = 70/f

OSC

(kHz)

RT (Pin 7): The oscillator resistor pin is used to set the
charge and discharge currents of the oscillator capacitor.
The nominal value is 16.9k. The resistance can be adjusted
between ±25% of nominal for better frequency accuracy.

SHDN (Pin 11): The SHDN pin is used to shut down the
part. Grounding this pin will disable all internal circuitry.
Increasing the SHDN voltage above the turn-on threshold
will enable the part. At the turn-on threshold, approxi­mately 20µA of current is sourced out of the pin. This
current, in conjunction with the Thevenin resistance on the
pin, sets up the hysteresis. This allows the user to set the
undervoltage lockout (UVLO) of the supply and the amount
of start-up hysteresis with a resistor divider off of the input
voltage. Above 2.1V on the SHDN pin, the hysteresis
current is reduced to zero. If unused the pin can be left
floating or tied directly to the input voltage.

GND (Pin 10): Signal Ground. The oscillator, slew control
circuitry and the internal regulator are referred to signal
ground. Internally, signal ground is tied to substrate and
the exposed backside of the device. Connect the GND pin
to the ground plane and keep the connection free of large
currents.

VIN (Pin 13): This is the supply pin for the part and should
be bypassed with a 4.7µF or greater, low ESR capacitor.
When VIN ≤ 2.5V, an internal undervoltage lockout circuit
will trip and turn both outputs off.

The transformer operating frequency and the frequency
of each output is one half of the frequency of the oscillator.

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