LT3781
“Bootstrap” Start Dual
Transistor Synchronous Forward Controller
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FEATURES
DESCRIPTIO
The LT®3781 controller simplifies the design of high
power synchronous dual transistor forward DC/DC converters. The part employs fixed frequency current mode
control and supports both isolated and nonisolated topologies. The IC drives external N-channel power MOSFETs
and operates with input voltages up to 72V.
High Voltage Operation up to 72V
Synchronizable Operating Frequency and Output
Switch Phase for Multiple Controller Systems
Synchronous Switch Output
Undervoltage Lockout Protection with 6V Hysteresis
for Self-Biased Power
Fixed Frequency Operation to 350kHz
Local ±1% Voltage Reference
Input Overvoltage Protection
Low Start-Up Current
Programmable Start Inhibit for Power Supply
Sequencing and Protection
Optocoupler Support
Soft-Start Control
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■
■
■
■
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The LT3781 is ideal for output derived power schemes,
through the use of a large undervoltage lockout hysteresis
range. The part is also equipped with an 18V VCC shunt
regulator, which prevents exceeding absolute maximum
ratings while in trickle start applications.
The LT3781’s operating frequency is programmable and
can be synchronized up to 350kHz. Switch phase is also
controlled during synchronized operation to accommodate multiple-converter systems. Internal logic guarantees 50% maximum duty cycle operation to prevent transformer saturation.
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APPLICATIO S
Isolated Telecommunication Systems
Personal Computers and Peripherals
Distributed Power Step-Down Converters
Lead Acid Battery Backup Systems
Automotive and Heavy Equipment
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The LT3781 is available in a 20-lead SSOP package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
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TYPICAL APPLICATIO
36V-72V to 5V/7A DC/DC
Synchronous Forward Converter (Quarter-Brick Footprint)
L1
4.7µH
6
Q1
5
MURS120T3
ZVN3310F
73.2k
1%
1
0.1µF
1nF
10k
19
18
14
11
PGND 12
SG
LT3781
SHDN 5VREF
FSET
THERM SYNC SGND SS VC VFB
6
5
52.3k
1µF
15
TG BSTREF BG SENSE
OVLO
1.24k
1%
+
4.7Ω
3
7
4
8
100Ω
150pF
4.7nF
3.3k
0.047µF
0.01µF
3.01k
1%
51Ω
6.8k
1k
1%
LTC1693-2 6
VCC1
VCC2
5
IN2
OUT2
7
1
IN1
OUT1
2
4
GND2 GND1
2k
0.22µF
50V
CMPZ5242B
12V
8
3
9
10
100Ω
FZT690
4.7µF
16V
5V
OUT
BAS21
2
1OV
BIAS
330pF
20
C5
330µF
10V
VOUT–
0.1µF
100V
VCC VBST
+
Q6
9
10k
MMBD914LT1
13
10
11
12
MBR0540T1
1nF
100V
1nF
100V 10Ω
0.25W
BAT54
1OV
BIAS
20k
4
10Ω
0.25W
Q5
VIN–
270k
0.25W
8
R9
0.025Ω
1/2W
Q3
C3
1.5µF
100V
•
•
C4
1.5µF
100V
•
C2
22µF
100V
1
2
3
7
MURS120T3
+
68µF
20V
VOUT = 5V
IOUT = 7A
VOUT+
L2
4.7µH
T1
•
VIN+
C2:SANYO 100MV22AX
C3, C4: VITRAMON VJ1825Y155MXB
C5: 4X KEMET T510X337KO10AS
L1: COILCRAFT DO1608C-472
L2: PANASONIC ETQP6F4R1LF4
Q1,Q3:100V SILICONIX SUD40N10-25
Q5,Q6: SILICONIX Si4450
T1:COILTRONICS VP5-1200
3781f
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LT3781
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ABSOLUTE
AXI U RATI GS
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PACKAGE/ORDER I FOR ATIO
(Note 1)
Power Supply (VCC)
Low Impedance Source Voltage ............. –0.3V to 20V
Shutdown Mode:
(Supply Self-Regulates to 18V)
Maximum Input Current ............................... 20mA
Topside Supply (VBST) ....................................................
VBSTREF – 0.3V to VBSTREF +20V (VBST(MAX) = 90V)
Topside Reference Pin (VBSTREF) ............... –0.6V to 75V
SHDN Pin Voltage ........................... –0.3V to VCC + 0.3V
All Other Input Voltages .............. –0.3V to 5VREF + 0.3V
5VREF Pin Sink Current ......................................... 10mA
FSET Pin Current ...................................... –2mA to 5mA
All Other Input Pin Currents ...................... –2mA to 2mA
Operating Ambient
Temperature Range (Note 4) ...............–40°C to 85°C
Operating Junction
Temperature Range ...............................–40°C to 125°C
Storage Temperature Range ..................–65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
TOP VIEW
SHDN
1
20 VBST
OVLO
2
19 TG
THERM
3
18 BSTREF
SGND
4
17 NC
5VREF
5
16 NC
FSET
6
15 BG
SYNC
7
14 PWRGND
SS
8
13 VCC
VFB
9
12 SG
VC 10
LT3781EG
LT3781IG
11 SENSE
G PACKAGE
20-LEAD PLASTIC SSOP
TJMAX = 125°C, θJA = 90°C/W
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 TA = 25°C.
VCC = VBST = 12V, VBSTREF = 0V, VVC = 2V, VFB = VREF = 1.25V, CTG = CBG = CSG = 1000pF.
SYMBOL PARAMETER
Supply and Protection
VCCUVLO Undervoltage Lockout Threshold
VCCSHDN
ICC
Shutdown Mode Shunt Regulator
DC Active Supply Current
CONDITIONS
Falling Edge
Rising Edge
100µA < IVCC ≤ 10mA
(Note 2)
●
●
●
MIN
TYP
MAX
UNITS
8.0
13
16.5
8.4
14.5
18
17
8.6
16
19.9
22
25
1200
30
8.5
V
V
V
mA
mA
µA
µA
mA
µA
V
mV
µA
mV
V
V
V
●
DC Active UVLO Supply Current
DC Standby Supply Current
VBST
DC Active Supply Current
DC Standby Supply Current
VSHDN
Shutdown Rising Threshold
Shutdown Threshold Hysteresis
ISS
Soft-Start Charge Current
VSS
Soft-Start Reset Threshold
VBSTUVLO Boost Undervoltage Lockout
(VBST-BSTREF)
Boost UVLO Hysteresis
5V External Reference
V5VREF
5V Reference Voltage
VSHDN = 1.35V, VCC = 8V
VSHDN < 0.3V
TG Logic High (Note 2)
VSHDN < 0.3V
●
●
●
●
VSS = 2V
●
Falling Edge
Rising Edge
●
●
●
0 ≤ (I5VREF – IVC) < 20mA
●
I5VREFSC
R5VREF
Short-Circuit Current
Output Impedance
Source, IVC = 0
0 ≤ (I5VREF – IVC) < 20mA
●
1.15
100
–14
5.7
6.5
0.3
4.85
4.80
20
800
16
5.0
0.1
1.25
150
– 10
225
6.4
7.0
0.6
5.0
45
1
1.35
200
–6
7.1
7.5
5.10
5.15
V
V
mA
Ω
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LT3781
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C.
VCC = VBST = 12V, VBSTREF = 0V, VVC = 2V, VTS = 0V, VFB = VREF = 1.25V, CTG = CBG = CSG = 1000pF.
SYMBOL
PARAMETER
CONDITIONS
Error Amplifier Reference Voltage
Measured at Feedback Pin
MIN
TYP
MAX
UNITS
1.242
1.225
1.250
1.258
1.265
V
V
Error Amp
VFB
●
IFB
Feedback Input Current
AV
Error Amplifier Voltage Gain
IVC
Error Amplifier Current Limit
VVC
GBW
–50
nA
72
dB
25
1
mA
mA
Zero Current Output Voltage
1.4
V
Maximum Output Voltage
3.2
V
Gain Bandwidth Product
VFB = VREF
Source
Sink
●
●
10
0.5
(Note 3)
1
MHz
12
V/V
Current Sense
AV
Amplifier DC Gain
ISENSE
Input Bias Current
VSENSE
Current Limit Threshold
tD
Current Sense to Switch Delay
tMIN
Switch Minimum On Time
µA
– 275
Measured at SENSE Pin
●
135
130
Measured at BG Output
150
165
170
mV
mV
175
ns
250
ns
THERM and OVLO Fault Detectors
VTHERM/
Threshold (Rising Edge)
VOVLO
Threshold Hysteresis
tD
Fault Delay to Output Disable
●
1.2
●
20
>50mV Overdrive
1.25
1.3
V
40
60
mV
650
ns
Oscillator and Synchronization Decoder
fOSC
Oscillator Frequency, Free Run
Measured at FSET Pin
Frequency Programming Error
fOSC ≤ 500kHz (Note 3)
IFSET
FSET Input Bias Current
FSET Charging, VFSET = 2V
VSYNC
SYNC Logic High Input Threshold
SYNC Logic Low Input Threshold
Positive-Going Edge
Negative-Going Edge
fSYNC
SYNC Frequency
tH, L
Maximum SYNC Pulse Width
(Logic High or Logic Low)
●
–10
700
kHz
5
%
50
●
●
0.8
●
fOSC/2
1.4
1.4
nA
2
350
fOSC = Oscillator Free-Run Frequency
V
V
kHz
1/fOSC
s
0.5
V
V
Output Drivers
VTG
TG On Voltage
TG Off Voltage
●
●
tTGr/f
TG Rise/Fall Time
VBG
BG On Voltage
BG Off Voltage
tBGr/f
BG Rise/Fall Time
VSG
SG On Voltage
SG Off Voltage
tSGr/f
SG Rise/Fall Time
10% to 90%/90% to 10%
tSG-BG
SG to BG Enable Lag Time
4V On/Off Thresholds
tTG-BG
TG to BG Enable Lag Time
4V On/Off Thresholds
11
10% to 90%/90% to 10%
11.5
0.1
35
●
●
11
10% to 90%/90% to 10%
11.5
0.1
ns
12
0.5
35
●
●
11
11.5
0.1
ns
12
0.5
35
●
80
150
100
V
V
V
V
ns
300
ns
ns
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LT3781
ELECTRICAL CHARACTERISTICS
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: Supply current specification does not include external FET gate
charge currents. Actual supply currents will be higher and vary with
operating frequency, operating voltages, and the type of external switch
elements used. See Applications Information.
Note 3: Guaranteed but not tested.
Note 4: The LT3781E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C operating
ambient temperature range are assured by design, characterization and
correlation with statistical process controls. For guaranteed performance
to specifications over the –40°C to 85°C operating ambient temperature
range, the LT3781I is available.
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TYPICAL PERFOR A CE CHARACTERISTICS
Shutdown Mode: VCC vs
Temperature (ICC = 1mA)
Shutdown Mode: VCC vs ICC
18.4
18.20
TA = 25°C
18.15
VCC (V)
VCC (V)
18.2
18.0
18.10
17.8
18.05
17.6
100µ
1m
300µ
3m
10m
18.00
–55 –40
ICC (A)
40
80
0
TEMPERATURE (°C)
3781 • G01a
3781 • G01b
ICC Supply Current
vs Temperature
ICC Supply Current
vs VCC Supply Voltage
18
20
TA = 25°C
19
ICC SUPPLY CURRENT (mA)
ICC SUPPLY CURRENT (mA)
VCC = 12V
18
17
16
15
–55 –40
0
125
40
80
TEMPERATURE (°C)
125
3781 G01
17
16
15
9
10
12
14
16
SUPPLY VOLTAGE (V)
18
3781 G03
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LT3781
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TYPICAL PERFOR A CE CHARACTERISTICS
IBST Boost Supply Current
vs Temperature
ICC Supply Current
vs SHDN Pin Voltage
UVLO ICC Supply Current
vs Temperature
60
5.2
1
5.1
5.0
4.9
4.8
–55 –40
0
40
80
TEMPERATURE (°C)
UVLO ICC SUPPLY CURRENT (mA)
ICC SUPPLY CURRENT (µA)
IBST BOOST SUPPLY CURRENT (mA)
TA = 25°C
40
20
0
125
0
0.2
0.4
0.6
0.8
1.0
SHDN PIN VOLTAGE (V)
3781 G04
5.00
4.95
125
1.260
50
40
30
–55 –40
0
40
80
TEMPERATURE (°C)
0
40
80
TEMPERATURE (°C)
125
3781 G10
1.245
1.240
–55 –40
0
40
80
TEMPERATURE (°C)
125
3781 G09
Soft-Start Output Current
vs Soft-Start Pin Voltage
60
TA = 25°C
VSS = 2V
SOFT-START OUTPUT CURRENT (µA)
12
SOFT-START OUTPUT CURRENT (µA)
VC PIN SHORT-CIRCUIT CURRENT LIMIT (mA)
10
–55 –40
1.250
Soft-Start Output Current
vs Temperature
25
15
125
1.255
3781 G08
VC Pin Short-Circuit Current Limit
vs Temperature
125
Error Amp Reference
vs Temperature
60
3781 G07
20
0
40
80
TEMPERATURE (°C)
3781 G06
ERROR AMP REFERENCE (V)
5VREF SHORT-CIRCUIT CURRENT LIMIT (mA)
5VREF VOLTAGE (V)
5.05
40
80
TEMPERATURE (°C)
0.5
–55 –40
1.2
5VREF Short-Circuit Current Limit
vs Temperature
5.10
0
0.6
3781 G05
5VREF Voltage vs Temperature
4.90
–55 –40
0.8
11
10
9
8
–55 –40
40
20
0
0
40
80
TEMPERATURE (°C)
125
3781 G11
0
100
200
300
400
SOFT-START PIN VOLTAGE (mV)
500
3781 G12
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LT3781
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TYPICAL PERFOR A CE CHARACTERISTICS
Soft-Start Output Current
vs Soft-Start Pin Voltage
Current Sense Amplifier
Bandwidth vs Temperature
60
8
CURRENT SENSE AMP BANDWIDTH (MHz)
SOFT-START OUTPUT CURRENT (µA)
TA = 25°C
40
20
0
0
1
2
3
4
SOFT-START PIN VOLTAGE (V)
5
3781 G13
7
6
5
4
3
2
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
3781 G14
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SHDN (Pin 1): Shutdown Pin. Pin voltages exceeding
positive going threshold of 1.25V enables the LT3781.
150mV of input hysteresis resists mode switching instability.
The SHDN pin can be controlled by either a logic level input
or with an analog signal. This shutdown feature is typically
used for input supply undervoltage protection. A resistor
divider from the converter input supply to the SHDN pin
monitors that supply for control of system power-up
sequencing, etc.
An 18V clamp on the VCC pin is enabled during shutdown
mode, preventing a trickle start circuit from pulling that pin
above maximum operational levels. All other internal
functions are disabled during shutdown.
OVLO (Pin 2): Overvoltage Shutdown Sense. Typically
connected to input supply through a resistor divider. If pin
voltage exceeds 1.25V, LT3781 switching function is
disabled to protect boosted circuitry from exceeding absolute maximum voltage. 40mV of input hysteresis resists
mode switching instability. Exceeding the OVLO threshold
also triggers soft-start reset, resulting in a graceful recovery from an input transient event.
THERM (Pin 3): System Thermal Shutdown. Auxiliary
shutdown pin that is typically used for system thermal
protection. If pin voltage exceeds 1.25V, LT3781
switching function is disabled. 40mV of input hysteresis
resists mode switching instability. Exceeding the THERM
threshold also triggers soft-start reset, resulting in a
graceful recovery.
SGND (Pin 4): Signal Ground Reference. Careful board
layout techniques must be used to prevent corruption of
signal ground reference. High-current switching paths
must be oriented on the converter ground plane such that
currents to/from the switches do not affect the integrity of
the LT3781 signal ground reference.
5VREF (Pin 5): 5V Local Reference. Allows connection of
external loads up to 20mA DC. Typically bypassed with
1µF ceramic capacitor to SGND. Reference output is
current limit protected to a typical value of 45mA. If the
load on the 5V reference exceeds the current limit value,
LT3781 switching function is disabled and the soft-start
function is reset.
FSET (Pin 6): Oscillator Timing Pin. Connect a resistor
(RFSET) from the 5VREF pin to this pin and a capacitor
(CFSET) from this pin to ground.
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LT3781
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The LT3781 oscillator operates by monitoring the voltage
on CFSET as it is charged via RFSET. When the voltage on the
FSET pin reaches 2.5V, the oscillator rapidly discharges
the capacitor with an average current of about 0.8mA.
Once the voltage on the pin is reduced to 1.5V, the pin
becomes high-impedance and the charging cycle repeats.
The oscillator operates at twice the switching frequency of
the controller.
Oscillator frequency fOSC can be approximated by the
relation:
fOSC
–1
R
2
≅ 0.5 • 10–6 + C FSET FSET + 8 • 10– 4 +
RFSET
3
–1
SYNC (Pin 7): Oscillator Synchronization Input Pin with
TTL-Level Compatible Input. The SYNC input signal (at the
desired synchronized operating frequency) controls both
the internal oscillator (running at twice the SYNC frequency) and the output switch phase. If synchronization
function is not desired, this pin may be floated or shorted
to ground.
The LT3781 internal oscillator drives a toggle flip-flop that
assures a ≤50% duty-cycle condition during oscillator
free-run. The oscillator, therefore, runs at twice the operating frequency of the controller. The SYNC input decoder
incorporates a frequency doubling circuit for oscillator
synchronization, resetting the internal oscillator on both
the rising and falling edges of the input signal.
The SYNC input decoder also differentiates transition
phase and forces the toggle flip-flop to phase-lock with the
SYNC input. A transition to logic high on the SYNC input
signal corresponds to the initiation of a new switching
cycle (primary switches turning on pending current control) and a transition to logic low forces a primary switch
off state. As such, the maximum operating duty cycle is
equal to the duty cycle of the SYNC signal. The SYNC input
can therefore be used to reduce the maximum duty cycle
of the controller by reducing the duty cycle of the SYNC
input.
SS (Pin 8): Soft-Start. Connect a capacitor (CSS) from this
pin to ground.
The output voltage of the LT3781 error amplifier corresponds to the peak current sense amplifier output detected before resetting the switch outputs. The soft-start
circuit forces the error amplifier output to a zero sense
current for start-up. A 10µA current is forced from this pin
onto an external capacitor. As the SS pin voltage ramps up,
so does the LT3781 internally sensed current limit. This
effectively forces the internal current limit to ramp from
zero, allowing overall converter current to slowly increase
until normal output regulation is achieved. This function
reduces output overshoot on converter start-up. The softstart functions incorporate a 1VBE “dead zone” such that
a zero-current condition is maintained on the VC pin until
the SS pin rises to 1VBE above ground.
The SS pin voltage is reset to start-up condition during
shutdown, undervoltage lockout, and overvoltage or
overcurrent events, yielding a graceful converter output
recovery from these events.
VFB (Pin 9): Error Amplifier Inverting Input. Typically
connected to a resistor divider from the output and compensation components to the VC pin.
The VFB pin is the converter output voltage feedback node.
Input bias current of ~50nA forces pin high in the event of
an open feedback path condition. The error amplifier is
internally referenced to 1.25V.
Values for the VOUT to VFB feedback resistor (RFB1) and
the VFB to ground resistor (RFB2) can be calculated to
program converter output voltage (VOUT) via the following
relation:
VOUT = 1.25 • (RFB1 + RFB2)/RFB2
VC (Pin 10): Error Amplifier Output. The LT3781 error
amplifier is a low impedance output inverting gain stage.
The amplifier has ample current source capability to allow
easy integration of isolation optocouplers that require bias
currents up to 10mA. External DC loading of the VC pin
reduces the external current sourcing capacity of the
5VREF pin by the same amount as the load on the VC pin.
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LT3781
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The error amplifier is typically configured using a feedback
RC network to realize an integrator circuit. This circuit
creates the dominant pole for the converter regulation
feedback loop. Integrator characteristics are dominated
by the value of the capacitor connected from the VC pin to
the VFB pin and the feedback resistor connected to the VFB
pin. Specific integrator characteristics can be configured
to optimize transient response.
The error amplifier can also be configured as a
transimpedance amplifier for use in secondary-side controller applications. (See the Applications Information
section for configuration and compensation details)
SENSE (Pin 11): Current Sense Amplifier (CSA)
Noninverting Input. Current is monitored via a ground
referenced current sense resistor, typically in series with
the source of the bottom side switch FET. Internal current
limit circuitry provides for a maximum peak value of
150mV across the sense resistor during normal operation.
SG (Pin 12): Synchronous Switch Output Driver. This pin
can be connected directly to gate of synchronous switch
if small FETs are used (CGATE < 5000pF), however, the use
of a gate drive buffer is recommended for peak efficiencies.
The SG pin output is synchronized and out-of-phase with
the BG output. The control timing of the SG output cause
it to “lead” the primary switch path during turn-on by
150nS.
VCC (Pin 13): IC Local Power Supply Input. Bypass with at
a capacitor at least 10 times greater than C5VREF. LT3781
incorporates undervoltage lockout that disables switching
functions if VCC is below 8.4V. The LT3781 supports
operational VCC power supply voltages from 9V to 18V
(20V absolute maximum). An 18V clamp on the VCC pin is
enabled during shutdown mode, preventing a trickle start
circuit from pulling that pin above maximum operational
levels during IC shutdown.
PWRGND (Pin 14): Output Driver Ground Reference.
Connect through low impedance trace to VIN decoupling
capacitor.
BG (Pin 15): Bottom Side Primary Switch/Forward Switch
Output Driver. This pin can be connected directly to
gate(s) of primary bottom side and forward switches if
small FETs are used (CGATE total < 5000pF), however, the
use of a gate drive buffer is recommended for peak
efficiencies.
The BG output is enabled at the start of each oscillator
cycle in phase with the TG pin but is timed to “lag” the TG
output during turn-on and “lead” the TG output during
turn-off. These delays force the concentration of transitional losses onto the bottom side primary switch.
An adaptive blanking circuit disables the current sense
function (via the SENSE pin) while the BG pin is below 5V.
BSTREF (Pin 18): VBST Supply Reference. Typically connects to source of topside external power FET switch.
TG (Pin 19): Topside (Boosted) Primary Output Driver.
This pin can be connected directly to gate of primary
topside switch if small FETs are used (CGATE < 5000pF),
however, the use of a gate drive buffer is recommended for
peak efficiencies.
VBST (Pin 20): Topside Primary Driver Bootstrapped Supply. This “boosted” supply rail is referenced to the BSTREF
pin.
Supply voltage is maintained by a bootstrap capacitor tied
from the VBST pin to the boosted supply reference (BSTREF)
pin. The charge on the capacitor is refreshed each switch
cycle through a Schottky diode connected from the VCC
supply (cathode) to the VBST pin (anode). The bootstrap
capacitor (CBOOST) must be at least 100 times greater than
the total load capacitance on the TG pin. A capacitor in the
range of 0.1µF to 1.0µF is generally adequate for most
applications. The bootstrap diode must have a reverse
breakdown voltage greater than the converter VIN. The
LT3781 supports operational VBST supply voltages up to
90V (absolute maximum) referenced to ground.
Undervoltage Lockout disables the topside switch until
VBST – BSTREF > 7.0V for start-up protection of the
topside switch.
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×4
ILIM
+
1.25V
–
SGND 4
OVLO 2
–
+
REFERENCE
GENERATOR
1.25V
UVLO
(
工商网监
湘ICP备2023018690号
