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PDF LTC1698 Data sheet ( Hoja de datos )
| Número de pieza | LTC1698 | |
| Descripción | Isolated Secondary Synchronous Rectifier Controller | |
| Fabricantes | Linear | |
| Logotipo |  |
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LTC1698
Isolated Secondary
Synchronous Rectifier Controller
FEATURES
s High Efficiency Over Wide Load Current Range
s ±0.8% Output Voltage Accuracy
s Dual N-Channel MOSFET Synchronous Drivers
s Pulse Transformer Synchronization
s Optocoupler Feedback Driver
s Programmable Current Limit Protection
s ±5% Margin Output Voltage Adjustment
s Adjustable Overvoltage Fault Protection
s Power Good Flag
s Auxiliary 3.3V Logic Supply
s Available in 16-Lead SSOP and SO Packages
U
APPLICATIO S
s 48V Input Isolated DC/DC Converters
s Isolated Telecommunication Power Systems
s Distributed Power Step-Down Converters
s Industrial Control Systems
s Automotive and Heavy Equipment
DESCRIPTIO
The LTC®1698 is a precision secondary-side forward
converter controller that synchronously drives external
N-channel MOSFETs. It is designed for use with the
LT®3781 primary-side synchronous forward converter
controller to create a completely isolated power supply.
The LT3781 synchronizes the LTC1698 through a small
pulse transformer and the LTC1698 drives a feedback
optocoupler to close the feedback loop. Output accuracy
of ±0.8% and high efficiency over a wide range of load
currents are obtained.
The LTC1698 provides accurate secondary-side current
limit using an external current sense resistor. The input
voltage at the MARGIN pin provides ±5% output voltage
adjustment. A power good flag and overvoltage input are
provided to ensure proper power supply conditions. An
auxiliary 3.3V logic supply is included that supplies up to
10mA of output current.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
VIN
36V to 72V
Q1
L1
VDD BIAS +
+
COUT
R2
R1
VOUT
D1
T1
Q2 D2
RPRISEN
TG BG
CSG
SG
T2
Q4
Q3
RSECSEN
CSYNC
RSYNC
1 10
2 VDD PWRGD 8
CG VFB
16
FG LTC1698
12
ISNS
6
VCOMP
RC
CC
CFB
11
ISNSGND
13 RCILM
ICOMP
CCILM
15
SYNC
9
OVPIN
7
MARGIN
VMARGIN
R5
R4
LT3781
+ REF
– VFB
VC
RF
CF
RE
ISOLATION
BOUNDARY
RK 5
14 VAUX
OPTODRV
VAUX
3.3V
CK PGND GND O.1µF 10mA
34
1681 F01
PLEASE REFER TO FIGURE 12 IN THE TYPICAL APPLICATIONS
SECTION FOR THE COMPLETE 3.3V/15A APPLICATION SCHEMATIC
Figure 1. Simplified 2-Transistor Isolated Forward Converter
1698f
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LTC1698
VAUX vs Temperature
3.465
VDD = 8V
3.424 ILOAD = 0mA
3.383
3.341
3.300
3.259
3.218
3.176
3.135
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
1698 G10
VAUX vs Line Voltage
3.465
VDD = 8V
3.424 ILOAD = 0mA
3.383
3.341
3.300
3.259
3.218
3.176
3.135
5 6 7 8 9 10 11 12 13 14
VDD (V)
1698 G11
VAUX vs Load Current
3.465
VDD = 8V
3.424 TA = 25°C
3.383
3.341
3.300
3.259
3.218
3.176
3.135
0 1 2 3 4 5 6 7 8 9 10
LOAD CURRENT (mA)
1698 G12
VAUX Short-Circuit Current
vs Temperature
0
VDD = 8V
–10
–20
–30
–40
–50
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
1698 G13
Maximum OPTO Driver Output
Voltage vs Load Current
8
VDD = 10V
6
VDD = 8V
VDD = 7V
4
VDD = 6V
VDD = 5V
2
TA = 25°C
VCOMP = 0V
0
0 1234567
LOAD CURRENT (mA)
8
9 10
1698 G22
VAUX Short-Circuit Current
vs VDD
0
TA = 25°C
–10
–20
–30
–40
–50
5 6 7 8 9 10 11 12 13 14
VDD (V)
1698 G14
Opto Driver Load Regulation
3.030
3.024
3.018
VDD = 8V
TA = 25°C
1.0
0.8
0.6
3.012
0.4
3.006
0.2
3.000
0
2.994
–0.2
2.988
–0.4
2.982
–0.6
2.976
–0.8
2.970
–1.0
0 1 2 3 4 5 6 7 8 9 10
LOAD CURRENT (mA)
1698 G15
Maximum OPTO Driver Output
Voltage vs Temperature
8
VDD = 10V
6
VDD = 8V
VDD = 7V
4
VDD = 6V
VDD = 5V
2
Opto Driver Short-Circuit Current
vs Temperature
–10
VDD = 8V
–15 VOPTODRV = 1.233V
–20
–25
–30
–35
–40
VCOMP = 0V
IOPTODRV = –10mA
0
–50 –25 0 25
50
75 100 125 150
TEMPERATURE (°C)
1698 G23
–45
–50
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
1698 G16
1698f
5
5 Page 
LTC1698
APPLICATIO S I FOR ATIO
drivers TG and BG go high. The pulse transformer T2
generates a negative slew at the SYNC pin and forces the
secondary MOSFET driver FG to go high and CG to go low.
When TG and BG go low, SG goes high and the secondary
controller forces CG high and FG low.
For a given pulse transformer, a bigger capacitor CSG
generates a higher and wider SYNC pulse. The peak of this
pulse should be much higher than the SYNC threshold.
Amplitudes greater than ±5V help to speed up the SYNC
comparator and reduce the SYNC to FG and CG drivers
propagation delay. The minimum pulse width is 75ns.
Overshoot during the pulse transformer reset interval
must be minimized and kept below the minimum com-
parator thresholds of ±1V. The amount of overshoot can
be reduced by having a smaller reset resistor RSYNC. For
nonisolated applications, the SYNC input can be driven
directly by a square pulse. To reduce the propagation
delay, make the positive and negative magnitude of the
square wave much greater than the ±2.2V maximum
threshold.
In addition to the simple driver synchronization, the sec-
ondary controller requires a driver disable signal. Loss of
synchronization while CG is high will cause Q4 to dis-
charge the output capacitor. This produces a negative
output voltage transient and possible damage to the load
circuitry connected to VOUT. To overcome this problem,
the LTC1698 comes with a unique adaptive time-out
circuit. It works well within the 50kHz to 400kHz frequency
range. At every positive SYNC pulse, the internal timer
resets. If the SYNC signal is missing, the internal timer
loses its reset command, and eventually exceeds the
internal time-out limit. This forces both the FG and CG
drivers to go low immediately.
The time-out duration varies linearly with the LT3781
primary controller clocking frequency. Upon power up,
the time-out circuitry takes a few clock cycles to adapt to
the input clock frequency. During this time interval, the
drivers pulse width might be prematurely terminated, and
the inductor current flows through the MOSFETs body
diode. Once the LTC1698 timer locks to the clocking
frequency, the LTC1698 drivers follow the SYNC signal
without fail. Figure 5 shows the SYNC time-out wave-
SG
SYNC
FG
CG
RESET
(INTERNAL)
DISDRI
(INTERNAL)
1698 F05
Figure 5. SYNC Time-Out Waveforms
forms. The time-out circuit guarantees that if the SYNC
pulse is missing for more than one period, both the
drivers will be shut down preventing the output voltage
from going below ground. The wide synchronization
frequency range adds flexibility to the forward converter
and allows this converter chip set to meet different
application requirements.
Under normal operating conditions, the time-out circuitry
adapts to the switching frequency within a few cycles.
Once synchronized, internal circuitry ensures the maxi-
mum time that the Catch FET (Q4) could be left turned on
is typically just over one switching period. This is particu-
larly important with high output voltages that can generate
significant negative output inductor currents if the Catch
FET Q4 is left on. Poor feedback loop performance includ-
ing output voltage overshoot can cause the primary con-
troller to interrupt the synchronization pulse train. While
this generally is not a problem, it is possible that low
frequency interruptions could lead to a time-out period
longer than a switching period, limited only by the internal
timer clamp (50µs typical).
Output Voltage Programming
The switching regulator output voltage is programmed
through a resistor feedback network (R1 and R2 in
Figure 1) connected to VFB. If the output is at its nominal
value, the divider output is regulated to the error amplifier
threshold of 1.233V.
The output voltage is thus set according to the relation:
VOUT = 1.233 • (1 + R2/R1)
1698f
11
11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LTC1698.PDF ] | |
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