LT3999 - Low Noise, 1A, 1MHz Push-Pull DC/DC Driver with Duty Cycle Control

LT3999
Low Noise, 1A, 1MHz
Push-Pull DC/DC Driver
with Duty Cycle Control
FEATURES
DESCRIPTION
Wide Input Operating Range: 2.7V to 36V
n Dual 1A Switches with Programmable Current Limit
n Programmable Switching Frequency: 50kHz to 1MHz
n Frequency Synchronization Up to 1MHz
n ∆V Compensation Using Duty Cycle Control
IN
n Low Noise Topology
n Programmable Input Over and Undervoltage Lockout
n Cross Conduction Prevention Circuitry
n Programmable Soft-Start
n Low Shutdown Current: <1µA
n10-Lead MSOP and DFN Packages
The LT®3999 is a monolithic, high voltage, high frequency
DC/DC transformer driver providing isolated power in a
small solution footprint.
n
The LT3999 has two 1A current limited power switches
that switch out of phase. The duty cycle is programmable
to adjust the output voltage. The switching frequency is
programmed up to 1MHz and can be synchronized to an
external clock for more accurate placement of switcher
harmonics. The input operating range is programmed
with the precision undervoltage and overvoltage lockouts.
The supply current is reduced to less than 1µA during
shutdown. A user-defined RC time constant provides an
adjustable soft-start capability by limiting the inrush current at start-up.
APPLICATIONS
n
n
n
n
n
n
Low Noise Isolated Supplies
Medical Instrument and Safety
Distributed Power
Multiple Output Supplies
Positive-to-Negative Supplies
Noise Immunity in Data Acquisition, RS232
and RS485
The LT3999 is available in a 10-lead MSOP and 3mm ×
3mm DFN package with exposed pad.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
TYPICAL APPLICATION
LT3999 Line Regulation with
Duty Cycle Control
12V to 12V, 10W Low Noise Isolated DC/DC Converter
16
10µF
16V
VIN
15.3µH
SYNC
255k
SWA
UVLO
•
•
OVLO/DC
RDC
10k
•
LT3999
15.8k
SWB
RT
10µF
16V
VOUT
12V
0.8A
•
3999 TA01a
ILIM/SS
28k
500kHz
RBIAS
0.1µF
49.9k
15
OUTPUT VOLTAGE (V)
VIN
12V
14
13
IOUT = 200mA
IOUT = 400mA
12
IOUT = 800mA
11
10
9
GND
8
10
11
12
13 14 15 16
INPUT VOLTAGE (V)
17
18
3999 TA01b
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1
LT3999
ABSOLUTE MAXIMUM RATINGS
(Note 1)
SWA, SWB.................................................. –0.3V to 80V
VIN, UVLO................................................... –0.3V to 60V
OVLO/DC, SYNC .......................................... –0.3V to 8V
Operating Junction Temperature Range (Note 2)
LT3999E............................................. –40°C to 125°C
LT3999I.............................................. –40°C to 125°C
LT3999H............................................. –40°C to 150°C
LT3999MP.......................................... –55°C to 150°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec)
MSOP................................................................ 300°C
PIN CONFIGURATION
TOP VIEW
TOP VIEW
SWA
RBIAS
VIN
UVLO
OVLO/DC
1
2
3
4
5
11
GND
10
9
8
7
6
SWB
ILIM/SS
SYNC
RT
RDC
MSE PACKAGE
10-LEAD PLASTIC MSOP
θJA = 40°CW, θJC = 10°CW
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
SWA
1
RBIAS
2
VIN
3
UVLO
4
OVLO/DC
5
10 SWB
11
GND
9 ILIM/SS
8 SYNC
7 RT
6 RDC
DD PACKAGE
10-LEAD (3mm × 3mm) PLASTIC DFN
θJA = 43°C/W, θJC = 5.5°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3999EMSE#PBF
LT3999EMSE#TRPBF
LTGKR
10-Lead Plastic MSOP
–40°C to 125°C
LT3999IMSE#PBF
LT3999IMSE#TRPBF
LTGKR
10-Lead Plastic MSOP
–40°C to 125°C
LT3999HMSE#PBF
LT3999HMSE#TRPBF
LTGKR
10-Lead Plastic MSOP
–40°C to 150°C
LT3999MPMSE#PBF
LT3999MPMSE#TRPBF
LTGKR
10-Lead Plastic MSOP
–55°C to 150°C
LT3999EDD#PBF
LT3999EDD#TRPBF
LGKQ
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LT3999IDD#PBF
LT3999IDD#TRPBF
LGKQ
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Consult LTC Marketing for information on nonstandard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
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LT3999
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 15V
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Input Supply and Shutdown
VIN Minimum Operating Voltage
2.7
l
VIN Overvoltage Lockout
Internal, Rising
VIN Supply Current
(Note 3)
VIN Shutdown Current
VUVLO = 0.3V
l
36
UVLO Threshold (Rising)
l
1.15
UVLO Hysteresis
UVLO Pin Current
42
VUVLO = 1.25V
l
1.15
OVLO/DC Hysteresis
0.1
1
μA
1.25
1.35
V
mV
10
100
nA
1.25
1.35
V
125
VOVLO/DC = 1.25V
V
mA
125
OVLO/DC Threshold (Rising)
OVLO/DC Pin Current
40
4.3
V
10
mV
100
nA
Power Switches (SWA, SWB)
Switch Saturation Voltage
ISW = 1A
Switch Current Limit
Internal Default
350
l
1.0
Non Overlap Time
Switch Base Drive Current
ISW = 1A
1.4
mV
1.7
A
70
ns
35
mA
Oscillator/Sync
Switching Frequency
RT = 316k
RT = 49.9k
RT = 12.1k
l
Synchronization Frequency Range
280
50
300
1000
100
320
kHz
kHz
kHz
1000
kHz
SYNC Voltage Threshold
1.5
V
SYNC Pin Input Resistance
200
kΩ
ILIM/SS
SWA and SWB Current Limit
RILIM/SS = 43.2k
l
0.4
ILIM/SS Pin Current
0.5
0.6
10
A
μA
Duty Cycle
Switch Duty Cycle
OVLO/DC = 0.8V, RDC = 24.3k, RT = 49.9k
OVLO/DC = 0.612V, RDC = 24.3k, RT = 49.9k
OVLO/DC = 0.3V, RDC = 24.3k, RT = 49.9k
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 the device
reliability and lifetime.
Note 2: The LT3999E is guaranteed to meet performance specifications
from 0°C to 125°C junction temperature. Specifications over the –40°C
to 125°C operating junction temperature range are assured by design,
characterization, and correlation with statistical process controls. The
LT3999I Is guaranteed over the –40°C to 125°C operating junction
temperature range. The LT3999H is guaranteed over the full –40°C to
150°C operating junction temperature range. The LT3999MP is 100%
tested and guaranteed over the –55°C to 150°C junction temperature
range. High junction temperatures degrade operating lifetimes; operating
lifetime is derated for junction temperatures greater than 125°C.
l
22
20
25
48
30
%
%
%
Note 3: Supply current specification does not include switch drive
currents. Actual supply currents will be higher.
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LT3999
TYPICAL PERFORMANCE CHARACTERISTICS
VIN Shutdown Current
Switching Frequency
1.5
1.0
VCESAT vs Switch Current
400
400
375
350
350
300
SWITCH VCESAT (mV)
2.0
FREQUENCY (kHz)
SHUTDOWN CURRENT (µA)
2.5
325
300
275
250
0.5
225
0
–50 –25
0
0
Switch Leakage Current
1800
400
300
200
1600
CURRENT LIMIT (mA)
SWITCH CURRENT (µA)
SWITCH VCESAT (mV)
Switch Current Limit
2000
2.5
500
2.0
1.5
1.0
0.5
0
1.35
1.35
OVLO PIN VOLTAGE (V)
UVLO PIN VOLTAGE (V)
UVLO FALLING
1.10
1.05
25 50 75 100 125 150
TEMPERATURE (°C)
1.20
OVLO FALLING
1.15
1.10
1.05
1.00
0.95
0.95
25 50 75 100 125 150
TEMPERATURE (°C)
OVLO RISING
1.25
1.00
0
0
3999 G06
1.30
UVLO RISING
1.20
0.90
–50 –25
RILIM/SS = 43.2k
600
OVLO Threshold Voltage
1.40
1.15
800
3999 G05
UVLO Threshold Voltage
1.25
RILIM/SS = 80.6k
1000
0
–50 –25
25 50 75 100 125 150
TEMPERATURE (°C)
1.40
1.30
1200
200
0
–50 –25
25 50
75 100 125 150
TEMPERATURE (°C)
RILIM/SS = OPEN
1400
400
3999 G04
0.90
–50 –25
3999 G07
4
0 100 200 300 400 500 600 700 800 900 1000
SWITCH CURRENT (mA)
3999 G03
3.0
SWITCH CURRENT = 1A
0
100
3999 G02
Switch VCESAT
100
–50 –25
150
0
25 50 75 100 125 150
TEMPERAURE (°C)
3999 G01
600
200
50
200
–50 –25
25 50
75 100 125 150
TEMPERATURE (°C)
250
0
25 50 75 100 125 150
TEMPERATURE (°C)
3999 G08
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LT3999
TYPICAL PERFORMANCE CHARACTERISTICS
Soft-Start (ILIM/SS) Current
Switch Duty Cycle
14
40
35
12
11
DUTY CYCLE (%)
SOFT-START CURRENT (µA)
13
10
9
8
7
6
25
20
15
5
4
–50 –25
30
0
25 50 75 100 125 150
TEMPERATURE (°C)
10
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3999 G10
3999 G09
PIN FUNCTIONS
SWA, SWB (Pin 1, Pin 10): SWA and SWB pins are the
open-collector nodes of the power switches. These pins
drive the transformer and are connected to the outer terminals of the center tapped transformer. Large currents
flow through these pins so keep PCB traces short and wide.
RBIAS (Pin 2): The RBIAS pin sets the bias current of
the power switches (SWA and SWB). Connect the pin to
a 49.9k resistor to GND.
VIN (Pin 3): The VIN pin is the main supply pin for the
switch driver and internal regulator. Short duration, high
current pulses are produced during the turn on and turn
off of the power switches. Connect a low ESR capacitor
of 4.7µF or greater.
UVLO (Pin 4): The UVLO pin has a precision threshold
with hysteresis to implement an accurate VIN undervoltage lockout. The UVLO function disables switching and
sets the part into a low current shutdown mode. Connect
the UVLO pin directly to VIN or to a resistor divider string.
OVLO/DC (Pin 5): The OVLO/DC pin has a precision threshold with hysteresis to implement an accurate VIN overvoltage lockout. The OVLO function disables the switching.
Connect OVLO/DC pin to ground to disable the function
or to a resistor divider string to program the duty cycle.
RDC (Pin 6): The RDC pin is the duty cycle control pin. A
resistor to ground sets the duty cycle. If unused leave the
pin floating or connect to the OVLO/DC pin.
RT (Pin 7): The RT pin sets the switching frequency of
the power switches.
SYNC (Pin 8): The SYNC pin synchronizes the part to an
external clock. Set the internal oscillator frequency below
the external clock frequency. Synchronizing the clock to
an external reference is useful for creating more stable
positioning of the switcher voltage or current harmonics.
Connect the SYNC pin to ground if not used.
ILIM/SS (Pin 9): The ILIM/SS pin sets a threshold level
for the cycle by cycle maximum switch current. Implement soft-start with a capacitor, CSS, placed on this pin to
ground. An internal current source charges the capacitor.
The RILIM, CSS time constant sets the soft-start time and
ramps the maximum switch current threshold at start-up.
If the ILIM/SS function is not used, float this pin and the
current limit will default to the internal limit.
GND (Pin 11): The ground pin is the exposed pad of the
package. Solder the exposed pad directly to the ground
plane.
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LT3999
BLOCK DIAGRAM
•
VIN
CIN
T1
•
D1
VOUT
•
•
D2
3
1
VIN
RA2
4
UVLO
BANDGAP
+
+
SWB
LINEAR
REGULATOR
INTERNAL
BIAS
–
RA1
10
SWA
+
+
5
OVLO/DC
SWITCH
CONTROL
–
SWITCH A
RB
+
2
RBIAS
SWITCH B
+
OSCILLATOR
–
+
–
DUTY CYCLE
CONTROL
+
RSENSE
RDC
6
RT
SYNC
8
RDC
6
RBIAS
7
RT
+
–
+
–
–
GND ILIM/SS
11
9
RILIM
3999 BD
CSS
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LT3999
OPERATION
Overview
Current Limit and Soft-Start
The LT3999 is a monolithic isolated push-pull DC transformer driver. It includes functions such as duty cycle
control, soft-start and protection features.
The LT3999 ILIM/SS pin programs the cycle-by-cycle
switch current limit and the soft-start time. A resistor on
the ILIM/SS pin sets the current limit. A capacitor on the
pin in conjunction with the resistor sets the soft-start time.
Push-Pull Topology
In a push-pull topology, a pair of switches operating out
of phase generate a square wave voltage pulse on the
primary side of a center tapped transformer. The diodes
on the secondary side rectify the voltage and generate
the output voltage. This voltage is simply VIN times the
transformer turns ratio.
Duty Cycle Control
The LT3999 duty cycle control provides, to a degree, line
regulation. The duty cycle is programmed by a resistor
on the RDC pin and the OVLO/DC voltage. By making the
OVLO/DC voltage a function of VIN the duty cycle will
adjust with varying VIN thereby keeping VOUT constant.
This feature is useful in cases where an LDO is used to post
regulate the output of the LT3999. By pseudo regulating
the output with the duty cycle control the power dissipation in the LDO is minimized.
Leaving the RDC pin floating or connecting it to the OVLO/
DC pin disables the duty cycle function and the LT3999
operates at close to 50% duty cycle.
When the programmed current limit is reached the switch
is immediately turned off and remains off for the remainder
of the cycle. Leaving the ILIM/SS pin unconnected will
disable the programmable current limit and the LT3999
will default to its internal current limit.
The soft-start function ramps the maximum switch current
over the programmed soft-start time. The purpose of the
soft-start is to reduce inrush current from the input supply.
Other Features
The LT3999 protection features include overvoltage lockout (OVLO), undervoltage lockout (UVLO) and thermal
shutdown.
The OVLO function is programmed with the OVLO/DC pin.
Switching is disabled during an OVLO event. An internal
overvoltage lockout on the VIN pin is also provided to
protect the LT3999.
The UVLO function is programmed with the UVLO pin.
Switching is disabled during a UVLO event. The UVLO
pin is also used to put the LT3999 into a low quiescent
shutdown state.
At a junction temperature above the operating temperature range the thermal shutdown function turns off both
switches.
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LT3999
APPLICATIONS INFORMATION
Switching Frequency
Oscillator Sync
The LT3999 drives two output power switches out of
phase, thus the oscillator frequency is two times the actual
switching frequency of each power switch. The choice
of switching frequency is a trade-off between power efficiency and the size of capacitive and inductive storage
components.
In applications where a more precise frequency is desired
to accurately place high frequency harmonics, the LT3999
oscillator can be synchronized to an external clock. Set
the internal oscillator frequency 10% to 50% lower than
the external sync frequency. The switching frequency is
one-half the sync frequency.
Operating at low switching frequency reduces the switching losses (transient losses) and consequently improves the
power converter efficiency. However, the lower switching
frequency requires greater inductance for a given amount
of ripple current, resulting in a larger design footprint and
higher cost.
Drive the SYNC pin with a 2V or greater square wave.
The rising edge of the sync square wave will initiate clock
discharge. If unused, connect the SYNC pin to ground.
The LT3999 switching frequency is set in the range of 50kHz
to 1MHz. The value of RT for a given operating frequency
is chosen from Table 1 or from the following equation:
Table 1. Recommended 1% Standard Values
RT
316kΩ
158kΩ
76.8kΩ
49.9kΩ
36.5kΩ
28kΩ
22.6kΩ
19.1kΩ
16.2kΩ
14kΩ
12.1kΩ
fSW
50kHz
100kHz
200kHz
300kHz
400kHz
500kHz
600kHz
700kHz
800kHz
900kHz
1000kHz
 1

R T (kΩ) = 
– 70ns • 3.25 •1010
 2 • fSW

Duty Cycle
To run the LT3999 at full duty cycle leave the RDC pin
unconnected.
Variations in VIN are, to a first order, compensated with
the LT3999 duty cycle control function. The duty cycle
function is implemented with a resistor divider on VIN
connected to the OVLO/DC pin and a resistor to ground
on the RDC pin. Use the following formula to calculate
the RDC resistor or duty cycle:
Duty Cycle (DC) =
RDC =
VIN •
1.25 •RDC
RB
VIN •
•R T • 4
R A +RB
RB
•R T •DC • 4
R A +RB
1.25
where RA and RB are the resistors from the VIN to OVLO/
DC resistor divider and RT is the frequency setting resistor. See Figure 1. Setting the OVLO/DC pin to be 0.612V
at the nominal VIN voltage yields good line regulation over
a wide input range.
The duty cycle refers to the duty cycle of the individual
switch. Normally each switch operates at close to 50%
duty cycle.
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LT3999
APPLICATIONS INFORMATION
Soft-Start and Current Limit
VIN
The LT3999 soft-start ramps the peak switch current over
a time programmed by either a capacitor or a resistor and
capacitor on the ILIM/SS pin.
VIN
RA
RB
RA2
UVLO
OR
OVLO/DC
UVLO
RA1
OVLO/DC
When programming the soft-start time with a capacitor only
the soft-start time is calculated with the following formula:
RB
3999 F01
Figure 1. Precision UVLO and OVLO Resistor Divider
tSS (ms) = CSS • 80
where CSS is in µF.
The current limit defaults to the internally set value because
there is no resistor on the pin.
When programming the soft-start time with a resistor
and capacitor on the ILIM/SS pin the soft-start time is
calculated with the following formula:
τ = RC
Resistors are chosen by first selecting RB. Then calculate
RA with the following formula:
 V

R A =RB  TH – 1
 1.25V 
where VTH is the VIN referred voltage at which the supply
is enabled (UVLO) or disabled (OVLO/DC).
where 3τ will be 95% of the maximum current.
Transformer Design
The cycle-by-cycle current limit of the LT3999 is set with
a resistor on the ILIM/SS pin. Use the following formula
to calculate the value of the resistor:
Table 3 lists recommended center tapped transformers
for a variety of input voltage, output voltage and power
combinations. These transformers will yield slightly high
output voltages so that they can accommodate an LDO
regulator on the output.
RILIM (kΩ) = ILIM • 86.4
OVLO/DC and UVLO
The UVLO pin has a precision voltage threshold with
hysteresis to enable the LT3999. The pin is typically connected to VIN through a resistor divider; however, it can
be directly connected to VIN.
The OVLO/DC pin has a precision voltage threshold with
hysteresis to disable the LT3999 switching operation. The
pin is typically connected to VIN through a resistor divider.
The OVLO/DC pin can be directly connected to GND to
disable the function. It is possible to use two separate
resistor divider strings for OVLO/DC and UVLO pins or
combine them together and use one resistor divider string
to drive both pins. See Figure 1.
If your application is not listed, the LTC Applications group
is available to assist in the choice and/or the design of the
transformer. In the design/selection of the transformer
the following characteristics are critical and should be
considered:
Table 3. Recommended Center Tapped Transformers
NOMINAL
INPUT
VOLTAGE (V)
NOMINAL
OUTPUT
VOLTAGE (V)
OUTPUT
POWER (W)
PART NUMBER
5
5
5
Coilcraft PA6383
5
12
1
Coilcraft PA6381
5
12
3
Cooper Bussmann
CTX02-19064
12
12
10
Coilcraft PA6384
24
24
20
Cooper Bussmann
CTX02-19061
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LT3999
APPLICATIONS INFORMATION
Turns Ratio
Winding Resistance
The turns ratio of the transformer determines the output
voltage. The following equation is used as a first pass to
calculate the turns ratio:
Resistance in either the primary or secondary winding
reduces overall efficiency and degrades load regulation.
If efficiency or load regulation is unsatisfactory, verify
that the voltage drops in the transformer windings are
not excessive.
NS
VOUT + VF
=
NP 2 ( VIN – VSW ) DC
Capacitors
where VF is the forward voltage of the output diode, VSW
is the voltage drop across the internal switches (see the
Typical Performance curves) and DC is the duty cycle.
Sufficient margin should be added to the turns ratio to
account for voltage drops due to transformer winding
resistance.
Magnetizing Current
The magnetizing inductance of the transformer causes
a ripple current that is independent of load current. This
ripple current is calculated by:
∆I=
VIN •DC
fSW •LM
where ∆I and LM are primary ripple current and magnetizing
inductance referred to the primary side of the transformer,
respectively. Increasing the transformer magnetizing inductance, LM, reduces the ripple current. The ripple current
formula shows the effect of the switching frequency on
the magnetizing inductance. Setting the LT3999 at high
switching frequency reduces the ripple current for the
same magnetizing inductance. Therefore, it is possible to
reduce the transformer turns and still achieve low ripple
current. This helps to reduce the power converter footprint
as well. The transformer magnetizing inductance should
be designed for the worst-case duty cycle and input line
voltage combination.
A good rule of thumb is to set the primary current ripple
amplitude 10% to 30% of the average primary current, IP:
IP =
POUT
VIN • eff
In applications with full duty cycle operation, the input
supply current is approximately constant. Therefore, large
input “hold-up type” capacitors are not necessary. A low
value (>4.7µF), low ESR ceramic will be adequate to filter
high frequency noise at the input. The output capacitors
supply energy to the output load only during switch
transitions. Therefore, large capacitance values are not
necessary on the output.
Transformer winding capacitance between the isolated
primary and secondary has parasitic currents that can
cause noise on the grounds. Providing a high frequency,
low impedance path between the primary and secondary
gives the parasitic currents a local return path. A 2.2nF,
1kV ceramic capacitor is recommended.
Optional LC Filter
An optional LC filter, as shown on the Typical Application
on the first page of this data sheet, should be included if
ultralow noise and ripple are required. It is recommended
that the corner frequency of the filter should be set a
decade below the switching frequency so that the switch
noise is attenuated by a factor of 100. For example, if the
fOSC = 100kHz, then fCORNER = 10kHz where:
fCORNER =
1
2 • π LC
Switching Diode Selection
A fast recovery, surface mount diode such as a Schottky
is recommended. The proximity of the diodes to the
transformer outputs is important and should be as close
as possible with short, wide traces connecting them.
where POUT is the output power of the converter and eff
is the converter efficiency, typically around 85%.
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LT3999
APPLICATIONS INFORMATION
Output Voltage Regulation
The junction temperature is computed as:
The output voltage of the DC transformer topology is
unregulated. Variations in the input voltage will cause
the output voltage to vary because the output voltage is
a function of the input voltage and the transformer turn
ratio. Also, variations in the output load will cause the
output voltage to change because of circuit parasitics,
such as the transformer DC resistance and power switch
on resistance. If regulation is necessary, a post regulator
such as a linear regulator can be added to the output of
the supply. See the Typical Applications for examples of
adding a linear regulator.
TJ = TAMB + PD • θJA
Power Consideration
The current derived from the VIN pin and the SWA and
SWB switching currents are the sources of the LT3999
power dissipation. The power dissipation is the sum of:
where:
PD = PVIN + PVCESAT + PSW and θJA is the package
thermal resistance.
Layout Consideration Check List
The following is a list of recommended layout considerations:
• Locate the bypass capacitor on the VIN pin of the transformer close to the transformer.
• Create a solid GND plane, preferably on layer two of
the PCB.
• Use short wide traces to connect to the transformer.
1)The quiescent current and switch drive power
dissipation:
• The transformer and PCB routing should be carefully designed to maximize the symmetry between two
switching half cycles.
 I •DC

PVIN = VIN  SW
+ 4mA
 30

• Solder the LT3999 exposed pad to the PCB. Add multiple
vias to connect the exposed pad to the GND plane.
where ISW is the average switch current.
More Help
2)The conducting power dissipation of the switches during
on state:
AN70: “A Monolithic Switching Regulator with 100mV
Output Noise” contains much information concerning
applications and noise measurement techniques.
PVCESAT = VCESAT • ISW • 2DC
where DC is the duty cycle and VCESAT is the collector
to emitter voltage drop during the switch saturation.
3)The dynamic power dissipation due to the switching
transitions:
PSW = VIN • ISW • fOSC • (tr + tf)
where tr and tf are the rise and fall times.
3999fa
For more information www.linear.com/LT3999
11
LT3999
TYPICAL APPLICATIONS
30V to 12V, 10W Push-Pull DC Transformer
VIN
30V
CIN
10µF
50V
VIN
R1
499k
SYNC
SWA
UVLO
R2
19.1k
LT3999
•
•
•
SWB
ILIM/SS
RBIAS
RBIAS
49.9k
VOUT
12V
COUT 0.8A
10µF
16V
D2
RT
C1
0.1µF
•
OVLO/DC
RDC
RT
28k
500kHz
L1
OPTIONAL
D1
T1
3999 TA02
D1, D2: DIODES INC. B260
L1: COILCRAFT M56132-153
T1: COOPER BUSSMANN CTX02-19062
GND
5V to 5V, 4W Low Part Count Push-Pull DC Transformer
VIN
5V
CIN
47µF
10V
VIN
UVLO
D1
T1
SWA
SYNC
•
•
•
•
VOUT
5V
COUT 0.8A
10µF
10V
OVLO/DC
LT3999
RDC
D2
RT
ILIM/SS
RT
12.1k
1MHz
RBIAS
49.9k
RBIAS
SWB
3999 TA03
D1, D2: CENTRAL SEMI. CMSH1-20M
T1: COILCRAFT PA6383
GND
10V-15V to ±12V, 200mA Isolated Switching Regulator
VIN
10V TO 15V
CIN
10µF
100V
R1
715k
VIN
SYNC
R2
36.5k
SWA
UVLO
ILIM/SS
12
CSS
0.01µF
RBIAS
49.9k
RBIAS
•
•
D2
•
•
D3
SWB
RT
RDC
13.3k
R4
39k
LT3999
RDC
RT
12.1k
1MHz
C3
180pF
OVLO/DC
R3
66.5k
D1
T1
GND
D4
D1-D4: CENTRAL SEMI. CMSH1-200HE
L1, L2: COILCRAFT XFL3012-393MEG
T1: WÜRTH 750314781
L1
39µH
C1
10µF
R7 50V
10k
0.01µF
L2
39µH
C2
10µF
50V
SHDN OUT
IN
LT3065
ADJ
REF/BYP
SHDN OUT
IN
ILIM
LT3090
SET
GND
1M
COUT1
10µF
25V
VOUT
12V
200mA
R8
52.3k
COUT2
10µF
25V
–VOUT
–12V
R6 200mA
10k
R10
243k
3999 TA04
3999fa
For more information www.linear.com/LT3999
LT3999
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
MSE Package
10-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1664 Rev I)
BOTTOM VIEW OF
EXPOSED PAD OPTION
1.88 ±0.102
(.074 ±.004)
5.10
(.201)
MIN
1
0.889 ±0.127
(.035 ±.005)
1.68 ±0.102
(.066 ±.004)
0.05 REF
10
0.305 ± 0.038
(.0120 ±.0015)
TYP
RECOMMENDED SOLDER PAD LAYOUT
3.00 ±0.102
(.118 ±.004)
(NOTE 3)
DETAIL “B”
CORNER TAIL IS PART OF
DETAIL “B” THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
NO MEASUREMENT PURPOSE
10 9 8 7 6
DETAIL “A”
0° – 6° TYP
1 2 3 4 5
GAUGE PLANE
0.53 ±0.152
(.021 ±.006)
DETAIL “A”
0.18
(.007)
0.497 ±0.076
(.0196 ±.003)
REF
3.00 ±0.102
(.118 ±.004)
(NOTE 4)
4.90 ±0.152
(.193 ±.006)
0.254
(.010)
0.29
REF
1.68
(.066)
3.20 – 3.45
(.126 – .136)
0.50
(.0197)
BSC
1.88
(.074)
SEATING
PLANE
0.86
(.034)
REF
1.10
(.043)
MAX
0.17 – 0.27
(.007 – .011)
TYP
0.50
(.0197)
BSC
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
6. EXPOSED PAD DIMENSION DOES INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD
SHALL NOT EXCEED 0.254mm (.010") PER SIDE.
0.1016 ±0.0508
(.004 ±.002)
MSOP (MSE) 0213 REV I
3999fa
For more information www.linear.com/LT3999
13
LT3999
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
DD Package
10-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1699 Rev C)
0.70 ±0.05
3.55 ±0.05
1.65 ±0.05
2.15 ±0.05 (2 SIDES)
PACKAGE
OUTLINE
0.25 ± 0.05
0.50
BSC
2.38 ±0.05
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
3.00 ±0.10
(4 SIDES)
R = 0.125
TYP
6
0.40 ± 0.10
10
1.65 ± 0.10
(2 SIDES)
PIN 1 NOTCH
R = 0.20 OR
0.35 × 45°
CHAMFER
PIN 1
TOP MARK
(SEE NOTE 6)
0.200 REF
0.75 ±0.05
0.00 – 0.05
5
1
(DD) DFN REV C 0310
0.25 ± 0.05
0.50 BSC
2.38 ±0.10
(2 SIDES)
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2).
CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
TOP AND BOTTOM OF PACKAGE
14
3999fa
For more information www.linear.com/LT3999
LT3999
REVISION HISTORY
REV
DATE
DESCRIPTION
A
04/15
Corrected pin assignments
PAGE NUMBER
Revised schematics
5
13, 16
3999fa
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection
of its circuits
as described
herein will not infringe on existing patent rights.
For more
information
www.linear.com/LT3999
15
LT3999
TYPICAL APPLICATION
5V to 12V, 1W Low Power Push-Pull DC Transformer
VIN
5V
CIN
10µF
10V
VIN
R1
261k
SYNC
SWA
UVLO
R2
100k
LT3999
ILIM/SS
RILIM
40.3k
•
•
COUT
2.2µF
16V
RBIAS
49.9k
RBIAS
VOUT
12V
0.08A
D1B
RT
CSS
0.1µF
•
OVLO/DC
RDC
RT
12.1k
1MHz
D1A
T1
•
GND
SWB
3999 TA05
D1, D2: VISHAY BAT54C
T1: COOPER BUSSMANN CTX02-19065R
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT3439
Slew Rate Controlled Ultralow Noise 1A Isolated DC/DC
Transformer Driver
VIN: 2.7V to 17.5V, IQ (Supply) = 12mA, ISD < 12mA, SO-16,
Low Noise: <100mVP-P, Independent Control of Switch Voltage
and Current Slew Rates
LT1533
Slew Rate Controlled Ultralow Noise 1A Switching Regulator
VIN: 2.7V to 23V, IQ (Supply) = 12mA, ISD < 12mA, SO-16,
Low Noise: <100mVP-P, Independent Control of Switch Voltage
and Current Slew Rates
LT1683
Slew Rate Controlled Ultralow Noise Push-Pull Controller
VIN: 2.7V to 20V, IQ (Supply) = 25mA, ISD < 24mA, SSOP-20,
Low Noise: <200mVP-P, Independent Control of Switch Voltage
and Current Slew Rates
LT1738
Slew Rate Controlled Ultralow Noise DC/DC Controller
VIN: 2.7V to 20V, IQ (Supply) = 12mA, ISD < 24mA, SSOP-20,
Greatly Reduced Conducted and Radiated EMI, Independent
Control of Switch Voltage and Current Slew Rates
16 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
For more information www.linear.com/LT3999
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com/LT3999
3999fa
LT 0415 REV A • PRINTED IN USA
 LINEAR TECHNOLOGY CORPORATION 2014