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PDF LTC3726 Data sheet ( Hoja de datos )
| Número de pieza | LTC3726 | |
| Descripción | Secondary-Side Synchronous Forward Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LTC3726www.DataSheet4U.com
Secondary-Side Synchronous
Forward Controller
FEATURES
■ Secondary-Side Control for Fast Transient Response
■ Self-Starting Architecture Eliminates Need for
Separate Bias Regulator
■ Proprietary Gate Drive Encoding Scheme Reduces
System Complexity
■ Current Mode Control Ensures Current Sharing
■ PLL Fixed Frequency: 100kHz to 500kHz
■ ±1% Output Voltage Accuracy
■ OPTI-LOOP Compensation
■ Available in a Narrow 16-Lead SSOP Package
U
APPLICATIO S
■ Isolated 48V Telecommunication Systems
■ Internet Servers and Routers
■ Distributed Power Step-Down Converters
■ Automotive and Heavy Equipment
DESCRIPTIO
The LTC®3726 is a secondary-side controller for synchro-
nous forward converters. When used in conjunction with
the LTC3705/LTC3725 gate driver and primary-side con-
trollers, the part creates a complete isolated power supply
that combines the simplicity of OPTI-LOOP® compensa-
tion with the speed of secondary-side control.
The LTC3726 has been designed to simplify the design of
highly efficient, secondary-side forward converters. Work-
ing in concert with the LTC3705 or LTC 3725, the LTC3726
forms a robust, self-starting converter that eliminates the
need for the separate bias regulator that is commonly used
in secondary-side control applications. In addition, a pro-
prietary scheme is used to multiplex gate drive signals and
DC bias power across the isolation barrier through a
single, tiny pulse transformer.
The LTC3726 is available in a 16-lead SSOP package.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
OPTI-LOOP is a registered trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Protected by U.S. Patents, including 6144194. Other patents pending.
TYPICAL APPLICATIO
36V-72V to 3.3V/20A Isolated Forward Converter
VIN+
1µF
100V
x3
VIN–
Si7852DP
MURS120
T1
••
Si7852DP
Si7336ADP
30mΩ
1W
MURS120
2mΩ
2W
FQT7N10
100k
BAS21 0.22µF
L1: COILCRAFT SER2010-122
T1: PULSE PA0807
T2: PULSE PA0297
365k
1%
NDRV
UVLO
BOOST TG TS BG IS
FB/IN+
T2
••
1µF
2.2µF
25V
VCC
SS/FLT
LTC3705
FS/IN–
15k
1% 33nF
GND PGND VSLMT
162k
33nF
CMPSH1-4
Si7336ADP
×2
L1
1.2Ω 1.2µH
5k
FZT690B
10µF
25V
7.5V
2.2µF
VOUT+
330µF
6.3V
×3
VOUT–
IS– FG
IS+
PT+
SW SG
VCC
LTC3726
PT–
RUN/SS GND
PGND
SLP
FS/SYNC
FB/PHASE
ITH
MODE
680pF
20k
102k
1%
22.6k
1%
3726 TA01
3726fb
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LTC3726www.DataSheet4U.com
FB Voltage vs Temperature
601.0
FB Voltage Line Regulation
601.0
600.5
600.5
600.0
600.0
599.5
599.5
599.0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3726 G15
599.0
0
5 10 15 20 25 30
VIN SUPPLY VOLTAGE (V)
3726 G11
Undervoltage Lockout
vs Temperature
4.65
4.60
4.55 VCC RISING
4.50
4.45
4.40
4.35
4.30
4.25
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3726 G16
Gate Driver On-Resistance vs VCC
1.8
1.7
Gate Driver On-Resistance
vs Temperature
2.50
VCC = 7V
2.25
1.6 2.00
1.5
PULL-DOWN
1.4
PULL-UP
1.3
1.75
1.50
1.25
1.2
5
6 7 8 9 10
VCC VOLTAGE (V)
3726 G12
1.00
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
3726 G13
Efficiency (Figure 5)
95
VIN = 36V
90
VIN = 72V
85
80
0 5 10 15 20 25
LOAD CURRENT (A)
3726 G17
Load Step (Figure 5)
VOUT
100mV/DIV
IOUT
10A/DIV
20µs/DIV
VIN = 48V
VOUT = 3.3V
LOAD STEP = 0A TO 20A
3726 G18
3726fb
5
5 Page 
U
OPERATIO
Slave Mode Operation
When two or more LTC3726 devices are used in PolyPhase
systems, one device becomes the “Master” controller,
while the others are used as “Slaves.” Slave mode is
activated when the FB/PHASE pin is greater than approxi-
mately 2V. In this mode, the ITH pin becomes a high-
impedance input, allowing it to be driven by the Master
controller. In this way, equal inductor currents are estab-
lished in each of the individual phases. Also, in slave
mode, the soft-start charge/discharge currents are dis-
abled, allowing the Master device to control the charging
and discharging of the soft-start capacitor.
In slave mode, the phase angle of each LTC3726 can be set
by using the FB/PHASE pin. This pin can be tied to VCC, or
LTC3726www.DataSheet4U.com
have a single resistor to VCC to activate Slave mode and set
the phase angle (delay) of the internal oscillator relative to
the incoming sync signal on the FS/SYNC pin. Any one of
six preset values can be selected as summarized in Table 4.
Table 4
FB/PHASE PIN
VFB/PHASE < 2V
VFB/PHASE = VCC
200kΩ to VCC
100kΩ to VCC
50kΩ to VCC
PHASE DELAY
0°
180°
60°
90°
120°
OPERATING MODE
Master
Slave
Slave
Slave
Slave
APPLICATIO S I FOR ATIO
Start-Up Considerations
In self-starting applications, the LTC3705/LTC3725 will
initially begin the soft-start of the converter in an open-
loop fashion. After bias is obtained on the secondary side,
the LTC3726 assumes control and completes the soft-
start interval. In order to ensure that control is properly
transferred from the LTC3705/LTC3725 (primary-side) to
the LTC3726 (secondary-side), it is necessary to limit the
rate of rise on the primary-side soft-start ramp so that the
LTC3726 has adequate time to wake up and assume
control before the output voltage gets too high. This
condition is satisfied for many applications if the following
relationship is maintained:
CSS,SEC ≤ CSS PRI
However, care should be taken to ensure that soft-start
transfer from primary-side to secondary-side is com-
pleted well before the output voltage reaches its target
value. A good design goal is to have the transfer completed
when the output voltage is less than one-half of its target
value. Note that the fastest output voltage rise time during
primary-side soft-start mode occurs with maximum input
voltage and minimum load current.
The open-loop start-up frequency on the LTC3705/LTC3725
is set by placing a resistor from the FB/IN+ pin to GND.
Although the exact start-up frequency on the primary side
is not critical, it is generally good practice to set this
approximately equal to the operating frequency on the
secondary side. The FS/IN– start-up resistor for the
LTC3705/LTC3725 may be selected using the following:
fPRI(Hz)
=
3.2 • 1010
RFS/IN– + 10k
In the event that the secondary-side circuitry fails to
properly start up and assume control of switching, there
are several fail-safe mechanisms to help avoid overvoltage
conditions. First, the LTC3705/LTC3725 contains a volt-
second clamp that will keep the primary-side duty cycle at
a level that cannot produce an overvoltage condition.
Second, the LTC3705/LTC3725 contains a time-out fea-
ture that will detect a FAULT if the LTC3726 fails to start up
and deliver PWM signals to the primary side. Finally, the
LTC3726 has an independent overvoltage detection circuit
that will crowbar the output of the DC/DC converter using
the synchronous MOSFET switch.
In the event that a short circuit is applied to the output of
the DC/DC converter prior to start-up, the LTC3726 will
generally not receive enough bias voltage to operate. In
this case, the LTC3705/LTC3725 will detect a FAULT for
3726fb
11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LTC3726.PDF ] | |
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