LT3439
Slew Rate Controlled
Ultralow Noise1A Isolated
DC/DC Transformer Driver
FEATURES
s
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DESCRIPTIO
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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
The LT
®
3439 is a push-pull DC/DC transformer driver that
reduces conducted and radiated electromagnetic interfer-
ence (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. Protec-
tion features include current limiting, undervoltage lock-
out, 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.
APPLICATIO S
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Low Noise Isolated Supplies
Radio and Telecom Supplies
Distributed Supplies
Medical Instruments
Precision Instruments
Low Noise Filament Supplies
TYPICAL APPLICATIO
Low Noise 5V to 5V Push-Pull DC/DC Transformer
5V Output Noise
OPTIONAL
V
IN
5V
T1
C
IN
22µF
10V
13
11
5
6
680pF 7
16.9k
V
IN
SHDN
SYNC
C
T
R
T
GND
10
R
SL
PGND
1, 16
3439 TA01
D1
A
C1
47µF
6.3V
L1 33µH
C2
47µF
6.3V
B
•
3
•
•
V
OUT
5V
500mA
COL A
LT3439
COL B
•
14
4
34k
B
200µV/DIV
D2
C
IN
: MURATA GRM235Y5V226Z10
C1, C2: TDK C4532X5R0J476M
D1, D2: MBR0520
L1: TDK SLF6028T-330MR69
T1: COILTRONICS 15835
A
20mV/DIV
U
5µs/DIV
3439 TA01b
U
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sn3439 3439fs
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LT3439
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
PGND
NC
COL A
R
SL
SYNC
C
T
R
T
NC
1
2
3
4
5
6
7
8
16 PGND
15 NC
14 COL B
13 V
IN
12 NC
11 SHDN
10 GND
9
NC
V
IN
Voltage ............................................................. 20V
COL A, COL B Voltage ............................................. 35V
SHDN, SYNC Voltage .............................................. 20V
Maximum Junction Temperature ......................... 150°C
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
ORDER PART
NUMBER
LT3439EFE
FE PART
MARKING
3439EFE
FE PACKAGE
16-LEAD PLASTIC TSSOP
T
JMAX
= 125°C,
θ
JA
= 40°C/ W
NOTE: BACKSIDE OF PACKAGE CONNECTED TO GND
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications which apply over the full operating temperature range, otherwise specifications are at T
A
= 25°C.
V
IN
= 5V; R
T
= 16.9k; C
T
= 680pF; R
SL
= 16.9k; COL A, COL B, SHDN pins open, unless otherwise noted.
SYMBOL
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
DC
BV
R
ON
I
LIM(MAX)
Slew Control
V
SLEWR
V
SLEWF
I
SLEWR
I
SLEWF
Output Voltage Slew Rising Edge
Output Voltage Slew Falling Edge
Output Current Slew Rising Edge
Output Current Slew Falling Edge
Collector A or B
Collector A or B
Collector A or B
Collector A or B
17
17
5
5
V/µs
V/µs
A/µs
A/µs
Oscillator Frequency
Synchronization Frequency Range
SYNC Pin Threshold
SYNC Pin Input Resistance
Switch Duty Cycle
Output Switch Breakdown Voltage
Output Switch On Resistance
Switch Current Limit
2.8V
≤
V
IN
≤
17.5V
I
COLA
or I
COLB
= 0.75A
q
q
q
q
q
q
PARAMETER
Operating Range
Minimum Input Voltage
Supply Current
Supply Current in Shutdown Mode
Shutdown Turn-On Threshold
Shutdown Turn-Off Threshold
Shutdown Pin Current Hysteresis
CONDITIONS
q
MIN
2.8
TYP
MAX
17.5
UNITS
V
V
mA
µA
V
V
µA
kHz
kHz
V
kΩ
%
V
Supply and Shutdown
2.55
2.8V
≤
V
IN
≤
17.5V
2.8V
≤
V
IN
≤
17.5V, V
SHDN
= 0V
2.8V
≤
V
IN
≤
17.5V
2.8V
≤
V
IN
≤
17.5V
2.8V
≤
V
IN
≤
17.5V, V
SHDN
= 1.4V
q
q
q
q
2.7
20
1.4
40
250
300
12
5
1.3
1.20
10
1.26
20
1.4
40
50
35
1.2
50
0.5
1.4
2.25
Output Switches (COL A, COL B)
0.95
1.65
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
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.
sn3439 3439fs
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Ω
A
W
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W W
W
LT3439
TYPICAL PERFOR A CE CHARACTERISTICS
Minimum Input Voltage
vs Temperature
2.750
2.700
SUPPLY CURRENT (µA)
2.650
INPUT VOLTAGE (V)
12
17.5V
10
8
5V
6
4
2
2.7V
OSCILLATOR FREQUENCY (kHz)
2.600
2.550
2.500
2.450
2.400
2.350
2.300
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3439 G01
Switch Voltage Drop
vs Switch Current
0.9
0.8
125°C
SHDN PIN CURRENT (µA)
SUPPLY CURRENT (mA)
0.7
SWITCH VOLTAGE (V)
0.6
0.5
0.4
0.3
0.2
0.1
0
0.1 0.2 0.3
0.4 0.5 0.6 0.7 0.8 0.9 1.0
SWITCH CURRENT (A)
3439 G04
Current Limit vs Temperature
2.0
1.8
THRESHOLD VOLTAGE (V)
CURRENT LIMIT (A)
1.35
1.34
1.33
1.32
1.31
1.30
1.29
1.28
VOLTAGE THRESHOLD (V)
1.6
1.4
1.2
1.0
0.8
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3439 G07
U W
85°C
25°C
Supply Current in Shutdown Mode
vs Temperature
16
14
120
Oscillator Frequency
vs Temperature
R
T
= 16.9k
C
T
= 680pF
115
110
105
100
95
90
85
80
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3439 G03
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3439 G02
Supply Current vs Slew Resistor
25
T
A
= 25°C
SHDN Pin Hysteresis Current
vs Temperature
24
22
20
18
16
14
12
20
15
10
5
0
0
10000
30000
20000
SLEW RESISTOR (Ω)
40000
3439 G05
10
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3439 G06
SHDN Pin Voltage Threshold
vs Temperature
1.6
1.5
1.4
1.3
1.2
1.1
1.27
–50 –25
SYNC Pin Voltage Threshold
vs Temperature
0
25 50 75 100 125 150
TEMPERATURE (°C)
3439 G08
1.0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3439 G09
sn3439 3439fs
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LT3439
PI FU CTIO S
PGND (Pins 1, 16):
Power ground is connected to the
emitter of the power switches via an internal sense resis-
tor. 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 cur-
rents flow into these pins so external traces should be kept
as short as possible.
R
SL
(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 Infor-
mation section of this data sheet.
SYNC (Pin 5):
The SYNC pin can be used to synchronize
the oscillator to an external clock. R
T
and C
T
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.
C
T
(Pin 6):
The oscillator capacitor pin is used in conjunc-
tion with the R
T
pin to set the oscillator frequency. For
R
T
= 16.9k, C
T
can be calculated as follows:
C
T
(nF) = 70/f
OSC
(kHz)
The transformer operating frequency and the frequency
of each output is one half of the frequency of the oscillator.
R
T
(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, approxi-
mately 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.
V
IN
(Pin 13):
This is the supply pin for the part and should
be bypassed with a 4.7µF or greater, low ESR capacitor.
When V
IN
≤
2.5V, an internal undervoltage lockout circuit
will trip and turn both outputs off.
4
U
U
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sn3439 3439fs
LT3439
BLOCK DIAGRA
11
LDO
REGULATOR
T
INTERNAL V
CC
R
T
7
C
T
6
5
R
T
C
T
SYNC
OSCILLATOR
FF
OPERATIO
Push-Pull Topology
The push-pull DC transformer topology is a very straight-
forward switching power supply. The two switches are
turned on out of phase at 50% duty cycles. During the
switch on time, V
IN
is applied across the primary side of
the transformer. The voltage on the secondary side of the
transformer is simply V
IN
times the turns ratio. The diodes
rectify the secondary voltage and generate the output
voltage. The output capacitor is for hold-up and filtering.
Some of the topology’s advantages are: 1) Stepping up or
down the input voltage can easily be done by setting the
turns ratio. 2) The transformer provides isolation between
the input and output. 3) Each switch cycle applies V
IN
across the transformer in opposite polarities. Therefore,
the transformer core never saturates and a separate reset
circuit is not necessary.
The push-pull topology is not without its concerns. An
imbalance in the two sides of the transformer can
W
T1
V
IN
D1
V
OUT
C
OUT
D2
•
•
•
•
13
LT3439
COLA
3
14
COLB
SHDN V
IN
Q
SLEW
CONTROL
QB
OUTPUT
DRIVERS
+
R
SENSE
16
PGND
1
–
10
GND
4
R
SL
3439 BD
R
SL
U
eventually cause the transformer to saturate. Also, dur-
ing the turn-off of the switches, the leakage inductance
causes a large undesirable voltage spike. The LT3439
slew control feature addresses both of these concerns
and is discussed in the Applications Information section.
Slew Control
Control of voltage and current slew rate is maintained via
two feedback loops. One loop controls the output switch
collector voltage dV/dt and the other loop controls the
emitter current dI/dt. Output slew control is achieved by
comparing the two currents generated by these slewing
events to a current set by the external resistor R
SL
. The two
control loops work together to provide a smooth transition
from voltage slew control to current slew control.
Internal Regulator
Most of the control circuitry operates from an internal 2.4V
low dropout regulator that is powered from V
IN
. V
IN
can
sn3439 3439fs
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