LINER LT1681ESW Dual transistor synchronous forward controller Datasheet

LT1681
Dual Transistor
Synchronous Forward Controller
DESCRIPTIO
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FEATURES
High Voltage: Operation Up to 72V
Synchronizable Operating Frequency and Output
Switch Phase for Multiple Controller Systems
Fixed Frequency Operation to 350kHz
Adaptive and Adjustable Blanking
Synchronous Rectifier Driver
Local 1% Voltage Reference
Undervoltage Lockout Protection with Hysteresis
Input Overvoltage Protection
Programmable Start Inhibit
Transformer Primary Saturation Protection
Optocoupler Feedback Support
Soft-Start Control
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The LT ®1681 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.
The LT1681’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.
The LT1681 incorporates a soft-start feature that provides a
controlled increase in supplied current during start-up and
after an undervoltage lockout or overvoltage/overcurrent
event.
The part is available in a 20-lead wide SO package to support
high voltage pin-to-pin clearance.
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APPLICATIO S
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Isolated Telecommunication Systems
Personal Computers and Peripherals
Lead Acid Battery Backup Systems
Automotive and Heavy Equipment
, LTC and LT are registered trademarks of Linear Technology Corporation.
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TYPICAL APPLICATIO
36V-72V DC to 5V/7A Synchronous Forward Converter (Half-Brick Footprint)
L2
4.1µH
L1
4.7µH
VOUT = 5V
IOUT = 7A
VOUT+
•
VIN+
6
Q1
C4
1.5µF
100V
5
MURS120T3
C3
1.5µF
100V
•
•
MURS120T3
C2
22µF
100V
8
4
Q3
0.025Ω
1/2W
•
+
7
1
2
3
10Ω
0.25W
MBR0540T1
1nF
100V
10 1nF
11 100V 10Ω
0.25W
12
+
Q6
9
4.7Ω
T1 Q5
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
Q10: ON SEMI MMBT3906LTI
73.2k
1%
270k
0.25W
20k
VOUT–
ZVN3310F
1OV
BIAS
CMPZ5248B
18V
0.1µF
68µF
20V
10k
+
1nF 24k
BAT54
10k
MMBD914LT1
0.1µF
100V
20
17
19
18
16
11
12
15
VCC VBST BLKSENS TG BSTREF BG SENSE TMAX PGND 13
SG
LT1681
OVLO
9
1
THERM SYNC SGND SS VC VFB
SHDN 5VREF FSET
1.24k
1%
6
5
52.3k
Q10
1µF
3
7
4
8
10
100Ω
150pF
4.7nF
0.01µF
100Ω
FZT690
4.7µF
16V
5V
OUT
2
56k
1OV
BIAS
330pF
BAS21
14
CMPZ5248B
15V
C5
330µF
10V
3.3Ω
0.047µF
3.01k
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
51Ω
1681 TA01
1k
1%
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LT1681
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ABSOLUTE MAXIMUM RATINGS
PACKAGE/ORDER INFORMATION
(Note 1)
Supply Voltages
Power Supply (VCC) ............................. – 0.3V to 20V
Topside Supply (VBST) ................... VBSTREF – 0.3V to
VBSTREF + 20V (VBST(MAX) = 90V)
Topside Reference Pin (VBSTREF) .......... – 0.6V to 75V
Input Voltages
SHDN Pin .................................. – 0.3V to VCC + 0.3V
All Other Inputs ..................... – 0.3V to 5VREF + 0.3V
Maximum Currents
5VREF Pin ........................................ – 85mA to 10mA
FSET Pin ............................................. – 2mA to 5mA
All Other Inputs .................................. – 2mA to 2mA
Operating Ambient Temperature Range
LT1681E (Note 4) .............................. – 40°C to 85°C
LT1681I ............................................. – 40°C to 85°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
LT1681ESW
LT1681ISW
18 BSTREF
SGND 4
17 BLKSENS
5VREF 5
16 BG
FSET 6
15 PWRGND
SYNC 7
14 VCC
SS 8
13 SG
VFB 9
12 IMAX
VC 10
11 SENSE
SW PACKAGE
20-LEAD PLASTIC SO
TJMAX = 125°C, θJA = 85°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
CONDITIONS
MIN
TYP
MAX
UNITS
9
12
18
V
17
22
25
mA
mA
800
1200
µA
Supply and Protection
VCC
Operating Supply Voltage Range
ICC
DC Active Supply Current
●
(Note 2)
●
IBST
VSHDN
DC Active UVL Supply Current
VSHDN > 1.35V, VCC = 8V
DC Standby Supply Current
VSHDN < 0.3V
DC Active Supply Current
TG Logic High (Note 2)
DC Standby Supply Current
VSHDN < 0.3V
Shutdown Rising Threshold
Shutdown Threshold Hysteresis
ISS
Soft-Start Charge Current
VSS
Soft-Start Reset Threshold
VCCUVLO
Undervoltage Lockout Threshold
VSS = 2V
Boost UVLO Hysteresis
µA
0.5
5
●
8.5
●
1.15
●
●
Falling Edge
Rising Edge
Falling Edge
Rising Edge
mA
µA
0.1
1.25
1.35
V
100
150
200
mV
–14
– 10
–6
µA
225
Undervoltage Lockout Hysteresis
VBSTUVLO Boost Undervoltage Lockout
(VBST-BSTREF)
●
●
●
8.0
8.3
8.40
8.75
●
0.25
0.35
●
●
5.7
6.5
6.4
7.0
●
0.3
0.6
mV
8.60
8.95
V
V
V
7.1
7.5
V
V
V
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LT1681
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
CONDITIONS
MIN
TYP
MAX
UNITS
4.85
4.80
5
●
5.10
5.15
V
V
●
20
45
mA
1
Ω
5V External Reference
V5VREF
5V Reference Voltage
0 ≤ (I5VREF – IVC) < 20mA
I5VREFSC
Short-Circuit Current
Source, IVC = 0
R5VREF
Output Impedance
0 ≤ (I5VREF – IVC) < 20mA
Error Amplifier Reference Voltage
Measured at Feedback Pin
Error Amp
VFB
●
IFB
Feedback Input Current
AV
Error Amplifier Voltage Gain
IVC
Error Amplifier Current Limit
VVC
GBW
1.242
1.225
VFB = VREF
1.250
1.258
1.265
–50
V
V
nA
72
dB
25
1
mA
mA
Zero Current Output Voltage
1.4
V
Maximum Output Voltage
3.2
V
Gain Bandwidth Product
Source
Sink
●
●
10
0.5
(Note 3)
1
MHz
12
V/V
Current Sense and Blanking
AV
Amplifier DC Gain
ISENSE
Input Bias Current
VSENSE
Current Limit Threshold
tD
●
135
130
●
4.5
Current Sense to Switch Delay
Blanking Input Bias Current
tMIN
Switch Minimum On Time
150
165
170
175
VBLKSENS Blanking Input Threshold
IBLKSENS
µA
– 275
Measured at SENSE Pin
VBLKSENS = VBG, Measured at BG Output
5
mV
mV
ns
5.5
V
–2
µA
250
ns
IMAX Sense
IIMAX
Input Bias Current
VIMAX
IMAX Threshold (Rising Edge)
IMAX Threshold Hysteresis
Measured at IMAX Input
Measured at IMAX Input
tP
IMAX Output Switch Disable Delay
Measured at BG Output
µA
– 250
●
320
360
140
400
130
mV
mV
ns
THERM and OVLO Fault Detectors
VTHERM/
VOVLO
Threshold (Rising Edge)
Threshold Hysteresis
tP
Fault Delay to Output Disable
●
●
1.2
20
50mV Overdrive
1.25
40
1.3
60
650
V
mV
ns
Oscillator and Synchronization Decoder
fOSC
Oscillator Frequency, Free Run
Measured at FSET Pin
Frequency Programming Error, Free Run
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)
fOSC = Oscillator Free-Run Frequency
700
●
–10
5
50
●
●
0.8
●
fOSC/2
1.4
1.4
kHz
%
nA
2
350
1/fOSC
V
V
kHz
s
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LT1681
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
MIN
TYP
MAX
UNITS
11
11.5
0.1
0.5
V
V
Output Drivers
VTG
TG On Voltage
TG Off Voltage
tTGr/f
TG Rise/Fall Times
VBG
BG On Voltage
BG Off Voltage
tBGr/f
BG Rise/Fall Times
VSG
SG On Voltage
SG Off Voltage
●
●
10% to 90%/90% to 10%
35
11
●
●
10% to 90%/90% to 10%
SG Rise/Fall Times
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%
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.
0.5
35
●
●
tSGr/f
11.5
0.1
ns
11.5
0.1
ns
0.5
35
80
●
150
V
V
V
V
ns
300
100
ns
ns
Note 3: Guaranteed but not tested.
Note 4: The LT1681E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the – 40°C to 85°C operating
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 range, the LT1681I is available.
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TYPICAL PERFOR A CE CHARACTERISTICS
ICC Supply Current
vs Temperature
ICC Supply Current
vs SHDN Pin Voltage
1100
20
17
16
15
–55 –40
ICC SUPPLY CURRENT (mA)
ICC SUPPLY CURRENT (nA)
ICC SUPPLY CURRENT (mA)
18
18
TA = 25°C
VCC = 12V
19
ICC Supply Current
vs VCC Supply Voltage
900
700
500
0
40
80
TEMPERATURE (°C)
125
1681 G01
0
100
200
300
400
SHDN PIN VOLTAGE (mV)
500
1681 G02
TA = 25°C
17
16
15
9
10
12
14
16
SUPPLY VOLTAGE (V)
18
1681 G03
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LT1681
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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 CURRENT (V)
1681 G04
5.00
4.95
125
1.260
50
40
30
–55 –40
0
40
80
TEMPERATURE (°C)
0
40
80
TEMPERATURE (°C)
125
1681 G10
1.245
1.240
–55 –40
0
40
80
TEMPERATURE (°C)
125
1681 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
1681 G08
VC Pin Short-Circuit Current Limit
vs Temperature
125
Error Amp Reference
vs Temperature
60
1681 G07
20
0
40
80
TEMPERATURE (°C)
1681 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
1681 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
1681 G11
0
100
200
300
400
SOFT-START PIN VOLTAGE (mV)
500
1681 G12
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LT1681
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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
1681 G13
7
6
5
4
3
2
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
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SHDN (Pin 1): Shutdown Pin. Pin voltages exceeding
positive-going threshold of 1.25V enables the LT1681.
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. All 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, the LT1681 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, the LT1681
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
the 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 LT1681 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,
LT1681 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.
The LT1681 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 0.8mA. Once the
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LT1681
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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  

FSET 
–
6
–
4

≅ 0.5 • 10 + C FSET
+  8 • 10 +
 

 
R
3

FSET



–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 the synchronization function is not desired, this pin may be shorted to
ground.
The LT1681 internal oscillator drives a toggle flip-flop that
assures ≤ 50% duty cycle operation during oscillator freerun. The oscillator, therefore, runs at twice the operating
frequency of the converter. 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 converter 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 LT1681 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 LT1681 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 soft-start function incorporates a 1VBE “dead zone”
such that a zero current condition is maintained on the V C
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 the 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 LT1681 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.
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.
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LT1681
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The error amplifier can also be configured as a
transimpedance amplifier for use in secondary-side controller applications. (See 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 limit circuitry provides for a maximum peak value of 150mV across
the sense resistor during normal operation.
IMAX (Pin 12): Primary Current Runaway Protection. The
IMAX pin is used to detect primary-side switch currents
and shuts down the primary switches if a current runaway
condition is detected. The IMAX function is not disabled
during the current sense blanking interval. The pin is
typically connected to the primary bottom-side switch
source and monitors switch current via a ground-referenced current sense resistor. If the pin voltage exceeds
360mV, LT1681 switching function is disabled in 130ns.
Exceeding the IMAX threshold also triggers a soft-start
reset, resulting in a graceful recovery from a current
runaway event. For single-sense resistor systems, this pin
can be shorted to SENSE for protection during the blanking interval or shorted to SGND if not used.
SG (Pin 13): Synchronous Switch Output Driver. This pin
can be connected directly to the gate of the 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 causes
its transition to “lead” the primary switch path during turnon by 150ns.
VCC (Pin 14): IC Local Power Supply Input. Bypass with a
capacitor at least 10 times greater than C5VREF to PGND.
The LT1681 incorporates undervoltage lockout that disables switching functions if VCC is below 8.4V. The LT1681
supports operational VCC power supply voltages from 9V
to 18V (20V absolute maximum).
PWRGND (Pin 15): Output Driver Ground Reference.
Connect through low impedance trace to VIN decoupling
capacitor.
BG (Pin 16): Bottom-Side Primary Switch/Forward Switch
Output Driver. Pin can be connected directly to gate 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.
BLKSENS (Pin 17): Blanking Sense Input. The current
sense function (via SENSE pin) is disabled while the
BLKSENS pin is below 5V. BLKSENS is typically connected to the gate of the bottom-side primary switch
MOSFET.
BSTREF (Pin 18): VBST Supply Reference. Typically connects to source of topside external power FET switch.
TG (Pin 19): Topside (Boosted) Primary Output Driver. 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 1µF is generally adequate for most
applications. The bootstrap diode must have a reversebreakdown voltage greater than the converter VIN. The
LT1681 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.
1681f
8
×4
ILIM
1.25V
Q
T
–
+
SGND 4
OVLO 2
–
+
REFERENCE
GENERATOR
1.25V
UVL
(<8V)
1.25V
350mV
S
5VREF 5
1.25V
–
+
ERROR AMP
×12
PHASE
DETECT
f = ×2
THERM 3
SHDN 1
VCC 14
VFB 9
IMAX 12
SENSE 11
+
VC 10
–
FSET 6
Q
5VREF
R
S
NOL
LOGIC
R
S
Q
+
–
+
–
+
–
+
–
SYNC 7
1681 BD
225mV
10µA
8 SS
17 BLKSENS
15 PWRGND
13 SG
16 BG
18 BSTREF
19 TG
20 VBST
LT1681
BLOCK DIAGRA
1681f
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APPLICATIO S I FOR ATIO
Overview
The LT1681 is a high voltage, high current synchronous
regulator controller, optimized for use with dual transistor
forward topologies. The IC uses a constant frequency,
current mode architecture with internal logic that prevents
operation over 50% duty cycle. A unique synchronization
scheme allows the system clock to be synchronized up to
an operational frequency of 350kHz, along with phase
control for easy integration of multicontroller systems. A
local precision 5V supply is available for external support
circuitry and can be loaded up to 20mA.
Internal fault detection circuitry disables switching when
a variety of system faults are detected such as: input
supply overvoltage or undervoltage faults, excessive system temperature, transformer primary-side saturation and
local supply overcurrent conditions. The LT1681 has a
current limit soft-start feature that gradually increases the
current drive capability of a converter system to yield a
smooth start-up with minimal overshoot. The soft-start
circuitry is also used for smooth recoveries from system
fault conditions.
External FET switches are employed for the switch elements, and hearty switch drivers allow implementation of
high current designs. An adaptive blanking scheme built
into the LT1681 allows for correct current-sense blanking
regardless of switch size and even while using external
switch drive buffers. The LT1681 employs a voltage output
error amplifier, providing superior integrator linearity and
allowing easy high bandwidth integration of optocoupler
feedback for fully isolated solutions.
Theory of Operation (See Block Diagram)
The LT1681 senses the output voltage of its associated
converter via the VFB pin. The difference between the
voltage on this pin and an internal 1.25V reference is
amplified to generate an error voltage on the VC pin, which
is used as a threshold for the current sense comparator.
The current sense comparator gets its information from
the SENSE pin, which monitors the voltage drop across an
external current sense resistor. When the detected switch
current increases to the level corresponding to the error
voltage on the VC pin, the switches are disabled until the
next switch cycle.
During normal operation, the LT1681 internal oscillator
runs at twice the switching frequency. The oscillator
output toggles a T flip-flop, generating a 50% duty cycle
pulse that is used internally as the system clock for the IC.
When the output of this flip-flop transitions high, the
primary switches are enabled. The primary-side switches
stay enabled until the transformer primary current, sensed
via the SENSE pin, connected to a ground-referenced
resistor in series with the bottom-side switch FET, is
sufficient to trip the current sense comparator and, in turn,
reset the RS latch. When the RS latch resets, the primary
switches are disabled and the synchronous switch is
enabled. The adaptive blanking circuit senses the bottomside gate voltage via the BLKSENS pin and prevents
current sensing until the FET is fully enabled, preventing
false triggering due to a turn-on transition glitch. If the
current comparator threshold is not obtained when the
flip-flop output transitions low, the RS latch is bypassed
and the primary switches are disabled until the next flipflop output transition, forcing a maximum switch duty
cycle less than 50%.
System Fault Detection—The General Fault Condition
(GFC)
The LT1681 contains circuitry for detecting internal and
system faults. Detection of a fault triggers a “general fault
condition” or GFC. When a GFC is detected, the LT1681
disables switching and discharges the soft-start capacitor. When the GFC subsides, the LT1681 initiates a startup cycle via the soft-start circuitry to assure a graceful
recovery. Recovery from a GFC is gated by the soft-start
capacitor discharge. The capacitor must be discharged to
a threshold of 225mV before the GFC can be concluded. As
the zero output current threshold of the SS pin is typically
a transistor VBE, or 0.7V, latching the GFC until a 225mV
threshold is achieved assures a zero output current state
is obtained in the event of a short-duration fault. A GFC is
also triggered during a system state change event, such as
entering shutdown mode, to prevent any mode transition
abnormalities.
1681f
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Events that trigger a GFC are:
a) Exceeding the current limit of the 5VREF pin
causing excessive power dissipation due to inadequate
gate drive during start-up.
b) Detecting an undervoltage condition on VCC
Error Amplifier Configurations
c) Detecting an undervoltage condition on 5VREF
The converter output voltage information is fed back to the
LT1681 onto the VFB pin where it is transformed into an
output current control voltage by the error amplifier. The
error amplifier is generally configured as an integrator and
is used to create the dominant pole for the main converter
feedback loop. The LT1681 error amplifier is a true high
gain voltage amplifier. The amplifier noninverting input is
internally referenced to 1.25V; the inverting input is the
VFB pin and the output is the VC pin. Because both low
frequency gain and integrator frequency characteristics
can be controlled with external components, this amplifier
allows far greater flexibility and precision compared with
use of a transconductance error amplifier.
d) Pulling the SHDN pin below the shutdown threshold
e) Exceeding the IMAX pin threshold
f) Exceeding the 1.25V fault detector threshold on either
the OVLO or THERM pins
The OVLO and THERM pins are used to directly trigger a
GFC. If either of these pins are not used, they can be
disabled by connecting the pin to SGND. The intention of
the OLVO pin is to allow monitoring of the input supply to
protect from an overvoltage condition. Monitoring of
system temperature (THERM) is possible through use of
a resistor divider using a thermistor as a resistor divider
component. The 5VREF pin can provide the precision
supply required for these applications. When these fault
detection circuits are disabled during shutdown or VCC pin
UVLO conditions, a reduction in OVLO and THERM pin
input impedance to ground will occur. To prevent excessive pin input currents, low impedance pull-up devices
must not be used on these pins.
In a nonisolated converter configuration where a resistor
divider is used to program the desired output voltage, the
error amplifier can be configured as a simple active
integrator, forming the system dominant pole (see Figure␣ 1). Placing a capacitor CERR from the VFB pin to the VC
pin will set the single-pole crossover frequency at
(2πRFBCERR)–1. Additional poles and zeros can be added
by increasing the complexity of the RC network.
Undervoltage Lockout
RFB
9
VFB
CERR
10
VC
+
The function of the high side switch output (TG) is also
gated by UVLO circuitry monitoring the bootstrap supply
(VBST-BSTREF). Switching of the TG pin is disabled until
the voltage across the bootstrap supply is greater than
7.4V. This helps prevent the possibility of forcing the high
side switch into a linear operational region, potentially
VOUT
–
The LT1681 maintains a low current operational mode
when an undervoltage condition is detected on the VCC
supply pin, or when VCC is below the undervoltage lockout
(UVLO) threshold. During a UVLO condition on the VCC
pin, the LT1681 disables all internal functions with the
exception of the shutdown and UVLO circuitry. The external 5VREF supply is also disabled during this condition.
Disabling of all switching control circuity reduces the
LT1681 supply current to < 1mA, simplifying integration
of trickle charging in systems that employ output feedback
supply generation.
1.25V
LT1681
1681 F01
Figure 1. Nonisolated Error Amp Configuration
Another common error amplifier configuration is for
optocoupler use in fully isolated converters with secondary-side control (see Figure 2). In such a system, the
dominant pole for the feedback loop is created at the secondary-side controller, so the error amplifier needs only to
1681f
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Figure 3 is a plot of oscillator frequency vs CFSET and
RFSET. Typical values for 300kHz operation (150kHz system frequency) are CFSET = 150pF and RFSET = 51k.
600
550
OSCILLATOR FREQUENCY (kHz)
translate the optocoupler information. The bandwidths of
the optocoupler and amplifier should be as high as possible to simplify system compensation. This high bandwidth operation is accomplished by using the error amplifier as a transimpedance amplifier, with the optocoupler
transistor emitter providing feedback information directly
into the VFB pin. A resistor from VFB to ground provides the
DC bias condition for the optocoupler. Connecting the
optocoupler transistor collector to the local 5VREF supply
reduces Miller capacitance effects and maximizes the bandwidth of the optocoupler. Higher optocoupler current also
means higher bandwidth, and the 5VREF supply can provide collector currents up to 10mA.
500
450
100pF
400
150pF
350
300
200pF
250
330pF
200
150
100
20
VOUT
SENSE
5
5VREF
30
40 50 60 70 80
TIMING RESISTOR (kΩ)
90
100
1681 F03
5V
10
Figure 3. Oscillator Frequency vs Timing Components
VFB
VC
–
9
+
1.25V
LT1681
1681 F01
Figure 2. Optocoupler High BW Configuration
Oscillator Frequency Programming
and Synchronization
The LT1681 internal oscillator runs at twice the system
switching frequency. The oscillator output toggles a T flipflop, generating a 50% duty cycle pulse that is used
internally as the system clock for the IC. Free-run frequency for the internal oscillator is programmed via an RC
timing network connected to the FSET pin. A pull-up
resistor RFSET, connected from the 5VREF pin to FSET,
provides current to charge a timing capacitor CFSET connected from the FSET pin to ground. The oscillator operates by allowing RFSET to charge CFSET up to 2.5V at which
point RFSET is pulled back toward ground by a 2.5k resistor
internal to the LT1681. When the voltage across CFSET is
pulled down to 1.5V, the FSET pin becomes high impedance, once again allowing RFSET to charge CFSET.
Due the relatively fast fall time of the oscillator waveform,
the FSET pin is held at its 1.5V threshold by an internal lowimpedance clamp to reduce undershoot error. If this pin is
externally forced low for any reason, external current
limiting is required to prevent damage to the LT1681.
Continuous source current from the FSET pin should not
exceed 1mA. Putting a 2k resistor in series with any low
impedance pull-down device will assure proper function
and protect the IC from damage.
Oscillator Synchronization
Synchronization of the LT1681 system clock is accomplished by driving a TTL level logic pulse train at the
desired system switching frequency into the SYNC pin. In
order to assure proper synchronization, each phase of the
synchronization signal must be less then an oscillator
free-run cycle.
The SYNC input pulse controls the phasing as well as the
frequency of controller switching. The SYNC circuit functions by forcing the phase of the oscillator output flip-flop
to match the phase of the SYNC pulse and prematurely
ending the oscillator charge cycle on each transition
edge. At the SYNC low-to-high transition, the LT1681
starts a switch-on cycle and the minimum switch-off
period is forced during the SYNC logic low period.
Because the SYNC logic low period corresponds directly
1681f
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APPLICATIO S I FOR ATIO
to the minimum off time, the converter maximum duty
cycle can be forced using the SYNC input. For example, a
30% duty cycle SYNC pulse forces 30% maximum duty
cycle operation for the converter. Because the logic low
pulse width exceeds the logic high pulse width in < 50%
duty cycle operation, the oscillator free-run cycle time
must be programmed to exceed the logic low duration.
The LT1681 enters an ultralow current shutdown mode
when the SHDN pin is below 350mV. During this mode,
total supply current drops to a typical value of less than
1µA. When SHDN rises above 350mV, the IC will draw
increasing amounts of supply current until just before the
1.25V turn-on threshold is achieved, when the typical
supply current reaches 60µA.
The shutdown function can be disabled by connecting the
SHDN pin to VCC. This pin is internally clamped to 2.5V
through a 20k series input resistance and can therefore
draw almost 1mA when tied directly to the VCC supply. This
additional current can be minimized by making the connection through an external series resistor (100k is typically used).
2.5V
FSET
1.5V
SYNC
SYSTEM
CLOCK
(INTERNAL)
1681 F04
Figure 4. Oscillator/SYNC Waveforms
It is also possible to run the LT1681 in a SYNC-only mode
by disabling the oscillator completely. Connecting a resistor divider from the 5VREF pin to the FSET pin, forcing a
voltage within the charge range of 1.5V to 2.5V, will allow
the oscillator to follow the SYNC input exclusively with no
provision for free-run. Setting values to force a voltage as
close to 2V as possible is recommended.
5 5VREF
75k
LT1681
6 FSET
50k
100pF
1681 F05
Figure 5. Oscillator Connection for Sync-Only Mode Operation
Shutdown
The LT1681 SHDN pin will support TTL and CMOS logic
signals and also analog inputs. The SHDN pin turn-on
(rising) threshold is 1.25V with 150mV of hysteresis. A
common use of the SHDN pin is for undervoltage detection on the input supply. Driving the SHDN pin with a
resistor divider connected from the input supply to ground
will prevent switching until the desired input supply voltage is achieved.
Soft-Start
The LT1681 current control pin (VC) limits sensed current
to zero at voltages less than 1.4V through full current limit
at VC = 3.2V, yielding 1.8V over the full regulation range.
The voltage on the VC pin is internally forced to be less than
or equal to SS + 0.7V. As such, the SS pin has a “dead
zone” between 0V and 0.7V, where a zero sensed current
condition is maintained. At SS voltages above 0.7V, the
sensed current limit threshold on pin VC may rise as
needed up to the SS maintained current limit value. Once
the SS pin rises to the VC pin maximum value less 0.7V, or
2.5V, the SS circuit has no effect.
The SS pin sources a typical current of 10µA. Placing a
capacitor (CSS) from the SS pin to ground will cause the
voltage on the SS pin to ramp up at a controlled rate,
allowing a graceful increase of maximum converter output
current during a start-up condition. The start-up delay
time to full available current limit is:
tSS = 2.5 • 105 • CSS (sec)
The LT1681 internally pulls the SS pin below the zero
current threshold during any fault condition to assure
graceful recovery. The SS circuit also acts as a fault control
latch to assure a full-range recovery from a short duration
fault. Once a fault condition is detected, the LT1681 will
suspend switching until the SS pin has discharged to
approximately 225mV.
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Layout Considerations—Grounding
The LT1681 is typically used in high current converter
designs that involve substantial switching transients. The
switch drivers on the IC are designed to drive large
capacitances and, as such, generate significant transient
currents. Careful consideration must be made regarding
input and local power-supply bypassing to avoid corrupting the ground references used by the error amplifier and
current sense circuitry.
Effective grounding of the two-transistor synchronous
forward topology where the LT1681 is used is inherently
difficult. The situation is complicated further by the number of bypass elements that must be considered.
Typically, high current paths and transients from the input
supply and any local drive supplies must be kept isolated
from SGND, to which sensitive circuits such as the error
amp reference and the current sense circuits, as well as the
local 5VREF supply, are referred. By virtue of the topologies
used in LT1681 applications, the large currents from the
primary switches, as well as the switch drive transients,
pass through the sense resistor to ground. This defines
the ground connection of the sense resistor as the reference point for both SGND and PGND. In nonisolated
applications where SGND is the output reference, we
now have a condition where every bypass capacitor in
the converter is referenced to the same point.
Effective grounding can be achieved by considering the
return current paths from the sense resistor to each
respective bypass capacitor. Don’t be tempted to run
small traces to separate the grounds. A power ground
plane is important as always in high-power converters, but
bypass elements must be oriented such that transient
currents in the return paths of VIN and VCC do not mix. Care
must be taken to keep these transients away from the
SGND reference. An effective approach is to use a 2-layer
ground plane, reserving an entire layer for SGND. The
5VREF and non-isolated converter output bypasses can
then be directly connected to the SGND plane.
VBST
LT1681
VIN
VBST
BSTREF
VCC
VCC
5VREF
SGND
PGND
1681 F06
Figure 6. High Current Transient Return Paths
1681f
14
C3
1.5µF
100V
68µF
20V
0.1µF
+
10k
MMBZ5248B-7
18V
MMBZ5245LT1
15V
267k
0.25W
1nF
24k
1.24k
56k
20k
73.2k
BAS21
20
17
19
18
16
11
12
15
BAS21
BAS21
BAT54
330pF
10k
BAT54
MMBT3906LT1
ZVN3310F
10Ω
5VREF
1µF
MMBT3906LT1
2
6
82pF
3
4
4700pF
7
8
3k
3300pF
10Ω
10
3
1•
4
•6
1k
5
8
S
2
3 T1
5
4
6
2
14
5
10k
15
1
4700pF
1k
220pF
S
Q1
12
11
16 2
100Ω
0.25W
6
0.1µF
VAUX
OPTODRV
SYNC
MARGIN
OVPIN
VFB
3
4
10
PGND GND PWRGD ICOMP
LTC1698
S
1.24k
1%
13
7
9
8
3.3k
L2
4.8µH
1k
976Ω
4.22k
1%
1681 F07
0.1µF
3.01k
Q14, Q15
FDS6680A
×2
MMBZ5240BLT1
10V
0.22µF
1000pF
2k
4.7Ω
MBR0530
0.022µF
1000pF
100V 10Ω
0.25W
1nF
100V
10Ω
0.25W
VDD ISNS ISNSGND FG CG VCOMP
1
4.7µF
FZT690B
2.2nF
250V
Q5, Q6
FDS6680A
×2
NC
12
11
10
9
8
7
Figure 7. 36V-72V DC in to 5V/10A Isolated Synchronous Forward Converter
52.3k
5
0.1µF
T2
ISO1
5VREF MOC207
4 3 1
7
6
3.3Ω
BAT54
0.030Ω
1/2W
Q3
MURS120T3
MMBT3906LT1
MURS120T3
10Ω
VCC VBST BLKSENS TG BSTREF BG SENSE IMAX PGND 13
SG
LT1681SW
OVLO
9
1
THERM SYNC SGND SS VC VFB
SHDN 5VREF FSET
14
L3 1mH
0.1µF
C4
1.5µF
100V
C1: MURATA ERIE GHM3045X7R222K-GC
C2, C3, C4: VITRAMON VJ1825Y155MXB
C5 TO C8: 330µF 10V KEMET T510X337K010AS
OR 330µF 6.3V KEMET T520D337M006AS
ISO1: FAIRCHILD MOC207
L1: COILCRAFT DO1608C-472
L2: PANASONIC ETQPAF4R8HFA
L3: COILCRAFT DO1608C-105
Q1, Q3: SILICONIX Si4486EY
Q5, Q6, Q14,Q15: FAIRCHILD FDS6680A
T1: MIDCOM 31267R OR COILTRONICS CTX02-14675
(FUNCTIONAL INSULATION) OR
MIDCOM 31322R (BASIC INSULATION)
T2: MIDCOM 31264R
(FUNCTIONAL INSULATION) OR
MIDCOM 31323R (BASIC INSULATION)
VIN–
C2
1.5µF
100V
•
L1
4.7µH
•
•
•
VIN+
+
S
VOUT–
C5 TO C8
330µF
10V
×4
VOUT+
LT1681
TYPICAL APPLICATIO S
1681f
15
U
C3
1.5µF
100V
C4
1.5µF
100V
C26
68µF
20V
0.1µF
+
10k
MMBZ5248B-7
18V
MMBZ5245LT1
15V
267k
0.25W
1nF
24k
1.24k
1%
56k
20k
73.2k
1%
BAS21
20
17
19
18
16
11
12
15
BAS21
BAS21
BAT54
330pF
10k
BAT54
MMBT3906LT1
Q12
ZVN3310F
10Ω
5VREF
1µF
MMBT3906LT1
2
6
82pF
3
4
4700pF
7
8
3k
3300pF
10Ω
10
4
•6
1k
5
8
S
2
1 T1
4
2
14
5
10k
15
3
4700pF
1k
220pF
S
Q1
12
11
16 2
100Ω
2k
0.25W
6
0.1µF
VAUX
OPTODRV
SYNC
MARGIN
OVPIN
VFB
3
4
10
1.24k
1%
13
7
9
8
1k
1000pF
S
L2
2.35µH
2.43k
1%
1698 F11
1k
0.33µF
1.78k
1%
3.01k
Q6, Q15, Q17
FDS6680A
×3
MMBZ5240BLT1
10V
0.22µF
50V
MBR0530
PGND GND PWRGD ICOMP
LTC1698
4.7Ω
1000pF
100V
0.022µF
1000pF
100V 10Ω
0.25W
10Ω
0.25W
VDD ISNS ISNSGND FG CG VCOMP
1
4.7µF
16V
Q13
FZT690B
Q5, Q14
FDS6680A
×2
C1
2200pF
250V
5
7
VSEC
Figure 8. 36V-72V DC in to 3.3V/20A Isolated Synchronous Forward Converter
52.3k
1%
5
3
1•
T2
ISO1
5VREF MOC207
4 3 1
7
6
3.3Ω
0.1µF
BAT54
0.025Ω
1/2W
Q3
MURS120T3
MMBT3906LT1
MURS120T3
10Ω
VCC VBST BLKSENS TG BSTREF BG SENSE IMAX PGND 13
SG
LT1681SW
OVLO
9
1
THERM SYNC SGND SS VC VFB
SHDN 5VREF FSET
14
0.1µF 100V
L3 1mH
VIN–
C1: MURATA ERIE GHM3045X7R222K-GC
C2, C3, C4: VITRAMON VJ1825Y155MXB
C5 TO C8: 330µF 10V KEMET T510X337K010AS
OR 330µF 6.3V KEMET T520D337M006AS
C26: AVX TPSE686M020R0150
ISO1: FAIRCHILD MOC207
L1: COILCRAFT DO1608C-332
L2: PULSE P1977 PLANAR INDUCTOR
L3: COILCRAFT DO1608C-105
Q1, Q3: SILICONIX Si4486EY
Q5, Q6, Q14,Q15,Q17: FAIRCHILD FDS6680A
Q7: FAIRCHILD NDT410EL
Q12: ZETEX ZVN3310F
Q13: ZETEX FZT690
T1: PULSE P1976 PLANAR TRANSFORMER
(FUNCTIONAL INSULATION) OR
PULSE PA-0191 (BASIC INSULATION)
T2: MIDCOM 31264R (FUNCTIONAL INSULATION) OR
MIDCOM 31323R (BASIC INSULATION)
C2
1.5µF
100V
•
L1
3.3µH
•
16
•
VIN+
+
TRIM
S
VOUT–
C5 TO C8
330µF
10V
×4
VOUT+
LT1681
TYPICAL APPLICATIO S
1681f
U
Q7
NDT410EL
0.1µF
C26
68µF
20V
+
10k
MMBZ5245LT1
15V
267k
0.25W
4.7µF
47k
1.5k
0.25W
MMBD914LT1
5VREF
VIN–
20
17
19
18
16
11
12
BAT54
15
BAS21
BAS21
330pF
10k
BAT54
MMBT3906LT1
Q12
ZVN3310F
10Ω
1µF
MMBT3906LT1
5VREF
52.3k
1%
5
6
82pF
3
4
4700pF
7
8
3k
3300pF
10Ω
10
0.1µF
3
1•
T2
4
•6
1k
5
8
S
2
S
Q1
1 T1
4
2
14
5
10k
15
3
4700pF
1k
220pF
ISO1
5VREF MOC207
4 3 1
7
6
3.3Ω
BAT54
0.025Ω
1/2W
Q3
MURS120T3
MMBT3906LT1
MURS120T3
VCC VBST BLKSENS TG BSTREF BG SENSE IMAX PGND 13
SG
2
LT1681SW
OVLO
9
1
V
SS
F
THERM
SYNC
SGND
5V
V
FB
REF
SET
C
SHDN
14
BAS21
0.1µF 100V
L3 1mH
MMBZ5248LT1
18V
62k
0.25W
C3
1.5µF
100V
C4
1.5µF
100V
12
4.7µF
16V
Q13
FZT690
11
16 2
100Ω
2k
0.25W
6
0.1µF
VAUX
OPTODRV
SYNC
MARGIN
OVPIN
VFB
3
4
10
1.24k
1%
13
7
9
8
1k
1000pF
S
L2
2.35µH
1k
1.78k
1%
3.01k
1%
2.43k
1%
+
S
VOUT–
C5 TO C8
330µF
10V
×4
VOUT+
7
+
5
4
–
LT1006S8
0.33µF
3.01k
1%
6
1
9V
2
3
1698 F12
3.01k
1%
3.01k
1%
3.01k
1%
100Ω
0.25W
100Ω
0.25W
TRIM
SENSE–
SENSE+
VOUT+
C1: MURATA ERIE GHM3045X7R222K-GC
C2, C3, C4: VITRAMON VJ1825Y155MXB
C5 TO C8: 330µF 10V KEMET T510X337K010AS
OR 330µF 6.3V KEMET T520D337M006AS
C26: AVX TPSE686M020R0150
ISO1: FAIRCHILD MOC207
L1: COILCRAFT DO1608C-332
L2: PULSE P1977 PLANAR INDUCTOR
L3: COILCRAFT DO1608C-105
Q1, Q3: SILICONIX Si4486EY
Q5, Q6, Q14,Q15,Q17: FAIRCHILD FDS6680A
Q7: FAIRCHILD NDT410EL
Q12: ZETEX ZVN3310F
Q13: ZETEX FZT690
T1: PULSE P1976 PLANAR TRANSFORMER
(FUNCTIONAL INSULATION) OR
PULSE PA-0191 (BASIC INSULATION)
T2: MIDCOM 31264R (FUNCTIONAL INSULATION) OR
MIDCOM 31323R (BASIC INSULATION)
Q6, Q15, Q17
FDS6680A
×3
MMBZ5240BLT1
10V
0.22µF
50V
MBR0530
PGND GND PWRGD ICOMP
LTC1698
4.7Ω
1000pF
100V
0.022µF
1000pF
100V 10Ω
0.25W
10Ω
0.25W
VDD ISNS ISNSGND FG CG VCOMP
1
9V
Q5, Q14
FDS6680A
×2
C1
2200pF
250V
5
7
VSEC
Figure 9. 36V-72V DC in to 3.3V/20A Isolated Synchronous Forward Converter with Fast Start and Differential Sense
1nF
24k
1.24k
1%
56k
20k
73.2k
1%
MMBT3904LT1
1.5k
0.25W
C2
1.5µF
100V
10Ω
•
L1
3.3µH
•
•
VIN+
LT1681
TYPICAL APPLICATIO S
1681f
17
U
LT1681
U
TYPICAL APPLICATIO S
LT1681/LTC1698 36V-72V VIN to 5V/10A Module
(See Figure 7 for Application Schematic)
LT1681/LTC1698 Isolated 5V/10A Converter
Efficiency vs Load Current
100
95
36V
EFFICIENCY (%)
90
48V
85
72V
80
75
70
65
60
1
2
3
4
5
6
7
CURRENT (A)
8
9
10
1681 TA04
LT1681/LTC1698 36V-72V VIN to 3.3V/20A Module
(See Figure 9 for Application Schematic)
LT1681/LTC1698 Isolated 3.3V/20A Converter
Efficiency vs Load Current
100
95
EFFICIENCY (%)
36V
90
48V
72V
85
80
75
70
2
4
6
8
10 12 14
CURRENT (A)
16
18
20
1681 TA05
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18
LT1681
U
PACKAGE DESCRIPTION
SW Package
20-Lead Plastic Small Outline (Wide .300 Inch)
(Reference LTC DWG # 05-08-1620)
0.496 – 0.512*
(12.598 – 13.005)
20
19
18
17
16
15
14
13
12
11
0.394 – 0.419
(10.007 – 10.643)
NOTE 1
0.291 – 0.299**
(7.391 – 7.595)
0.010 – 0.029 × 45°
(0.254 – 0.737)
1
2
3
4
5
6
7
8
9
0.093 – 0.104
(2.362 – 2.642)
10
0.037 – 0.045
(0.940 – 1.143)
0° – 8° TYP
0.009 – 0.013
(0.229 – 0.330)
NOTE 1
0.016 – 0.050
(0.406 – 1.270)
0.050
(1.270)
BSC
0.014 – 0.019
(0.356 – 0.482)
TYP
0.004 – 0.012
(0.102 – 0.305)
S20 (WIDE) 1098
NOTE:
1. PIN 1 IDENT, NOTCH ON TOP AND CAVITIES ON THE BOTTOM OF PACKAGES ARE THE MANUFACTURING OPTIONS.
THE PART MAY BE SUPPLIED WITH OR WITHOUT ANY OF THE OPTIONS
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
1681f
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.
19
LT1681
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
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Current Limit Protection, 100% of Duty Cycle
LT1160
Half-Bridge N-Channel MOSFET Driver
Up to 60V Input Supply, No Shoot-Through
LT1162
Dual Half-Bridge N-Channel MOSFET Driver
VIN to 60V, Good for Full-Bridge Applications
LT1336
Half-Bridge N-Channel MOSFET Driver
Smooth Operation at High Duty Cycle (95% to 100%)
LT1339
High Power Synchronous DC/DC Controller
60V Dual N-Channel MOSFET Controller
LTC®1530
High Power Step-Down Switching Regulator Controller
Excellent for 5V to 3.x Up to 50A
LTC1622
550kHz Step-Down Controller
8-Pin MSOP; Synchronizable; Soft-Start; Current Mode
TM
LTC1625/LTC1775 No RSENSE Current Mode Synchronous Step-Down Controller
97% Efficiency; No Sense Resistor; 16-Pin SSOP
LTC1628-PG
Dual, 2-Phase Synchronous Step-Down Controller
Power Good Output; Minimum Input/Output Capacitors;
3.5V ≤ VIN ≤ 36V
LTC1628-SYNC
Dual, 2-Phase Synchronous Step-Down Controller
Synchronizable 150kHz to 300kHz, VIN to 36V
LT1680
High Power DC/DC Current Mode Step-Up Controller
High Side Current Sense, Up to 60V Input
LTC1698
Secondary Synchronous Rectifier Controller
Use with the LT1681, Isolated Power Supplies, Contains
Voltage Margining, Optocoupler Driver, Synchronization Circuit
with the Primary Side
LTC1709-7
High Efficiency, 2-Phase Synchronous Step-Down Controller
with 5-Bit VID
Up to 42A Output; 0.925V ≤ VOUT ≤ 2V
LTC1709-8
High Efficiency, 2-Phase Synchronous Step-Down Controller
Up to 42A Output; VRM 8.4; 1.3V ≤ VOUT ≤ 3.5V
LTC1735
High Efficiency, Synchronous Step-Down Controller
Burst Mode® Operation; 16-Pin Narrow SSOP;
3.5V ≤ VIN ≤ 36V
LTC1736
High Efficiency, Synchronous Step-Down Controller with 5-Bit VID
Mobile VID; 0.925V ≤ VOUT ≤ 2V; 3.5V ≤ VIN ≤ 36V
LTC1772
ThinSOTTM Step-Down Controller
Current Mode; 550kHz; Very Small Solution Size
LTC1773
Synchronous Step-Down Controller
Up to 95% Efficiency, 550kHz, 2.65V ≤ VIN ≤ 8.5V,
0.8V ≤ VOUT ≤ VIN, Synchronizable to 750kHz
LTC1778
Wide Operating Range, No RSENSE Step-Down Controller
GN16-Pin, 0.8V FB Reference
LTC1874
Dual, Step-Down Controller
Current Mode; 550kHz; Small 16-Pin SSOP, VIN < 9.8V
LTC1876
2-Phase, Dual Synchronous Step-Down Controller with
Step-Up Regulator
3.5V ≤ VIN ≤ 36V, Power Good Output, 300kHz Operation
LTC1922-1
Synchronous Phase Modulated Full-Bridge Controller
50W to 2kW Power Supply Design, Adaptive Direct Sense ZVS
LTC1929
2-Phase 42A Synchronous Controller
Minimizes CIN and COUT, 4V ≤ VIN ≤ 36V, 300kHz
LTC3714
Intel Compatible, Wide Operating Range, No RSENSE Step-Down
Controller with Internal Op Amp
G28 Package, VOUT = 0.6V to 1.75V 5-Bit Mobile VID,
Active Voltage Positioning IMVP2, VIN to 36V
LTC3716
High Efficiency, 2-Phase Synchronous Step-Down Controller
with 5-Bit Mobile VID
VOUT = 0.6V to 1.75V, Active Voltage Positioning IMVP2,
VIN to 36V
No RSENSE and ThinSOT are trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technolgy Corporation.
1681f
20
Linear Technology Corporation
LT/TP 0302 2K • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com
 LINEAR TECHNOLOGY CORPORATION 2001