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LTC3706IGN

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P22
LTC3706
13
3706fd
APPLICATIONS INFORMATION
Start-Up Considerations
In self-starting applications, the LTC3705 will initially begin
the soft-start of the converter in an open-loop fashion.
After bias is obtained on the secondary side, the LTC3706
assumes control and completes the soft-start interval. In
order to ensure that control is properly transferred from the
LTC3705 (primary-side) to the LTC3706 (secondary-side),
it is necessary to limit the rate of rise on the primary-side
soft-start ramp so that the LTC3706 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:
C
SS,SEC
≤ C
SS PRI
However, care should be taken to ensure that soft-start
transfer from primary-side to secondary-side is completed
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 pri-
mary-side soft-start mode occurs with maximum input
voltage and minimum load current.
The open-loop start-up frequency on the LTC3705 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 approxi-
mately equal to the operating frequency on the secondary
side. The FS/IN
–
start-up resistor for the LTC3705 may be
selected using the following:
f
PRI
(Hz)=
3.2 • 10
10
R
FS/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 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 contains a time-out feature that will detect
a FAULT if the LTC3706 fails to start up and deliver PWM
signals to the primary side. Finally, the LTC3706 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 LTC3706 will
generally not receive enough bias voltage to operate. In
this case, the LTC3705 will detect a FAULT for one of two
reasons: 1) the start-up time-out feature will be activated
since the LTC3706 never sends signals to the primary side
or 2) the primary-side overcurrent circuit will be tripped
because of current buildup in the output inductor. In either
case, the LTC3705 will initiate a shutdown followed by a
soft-start retry. See the LTC3705 data sheet for further
details.
OPERATION
Slave Mode Operation
When two or more LTC3706 devices are used in PolyPhase
systems, one device becomes the “master” controller, while
the others are used as “slaves.” Slave mode is activated
by connecting the FB pin to V
CC
. 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 established in each of the individual phases. Also, in
slave mode the soft-start charge/discharge currents are
disabled, allowing the master device to control the charging
and discharging of the soft-start capacitor.
LTC3706
14
3706fd
APPLICATIONS INFORMATION
Bias Supply Generation
Figure 2 shows a commonly used method of developing
a V
CC
bias supply for the LTC3706. During start-up, bias
winding 1 uses a peak detection method to rapidly develop
a V
IN
voltage for the LTC3706, which in turn drives the
linear regulator that generates the V
CC
voltage (7V). When
the main output of the converter is in regulation, winding
2 (configured as a forward-style output) is designed to
produce a regulated auxiliary voltage of approximately
7.5V to 8.5V. Since the auxiliary voltage is greater than that
of the linear regulator, the linear regulator will effectively
be shut down. Note that the output inductor L1 must be
adequately large so that its ripple current is continuous
given the amount of V
CC
load current, thereby providing
a stable output voltage.
Figure 2. Typical Bias Supply Configuration
V
IN
LTC3706
4.7Ω
MBRO530
BAS21
FMMT491A
1mH
NDRVREGSD
R
REGSD
C
REGSD
4.7µF
16V
WINDING 1
NB1
3706 F02
MAIN
TRANSFORMER
WINDING 2
NB2
•
•
BAS21
1
1µF
50V
V
CC
•
The turns ratio (NB1) of the bias winding 1 should be cho-
sen to ensure that there is adequate voltage to operate the
LTC3706 over the entire range for the DC/DC converter’s
input bus voltage (V
BUS
). This may be calculated using:
NB1=
V
CC(MIN)
+ 1.5V
V
BUS(MIN)
V
CC(MIN)
can be as low as 5V (if this provides adequate
gate drive voltage to maintain acceptable efficiency) or as
high as 7V. For V
CC(MIN)
= 6V and V
BUS
= 36V to 72V, this
would mean a turns ratio of NB1 ≈ 0.21 and a V
IN
voltage
range at the LTC3706 of 7.5V to 15V.
Using the bias circuit of Figure 2, the linear regulator
would normally operate only for a brief interval during the
initial soft-start ramp of the main output voltage. Under
some fault conditions (e.g., output overload), the auxiliary
voltage produced by bias winding 2 may decrease below
7V, causing the linear regulator to again supply the V
CC
bias current. Since the amount of power dissipation in the
linear regulator pass device may be quite high, it can take
considerable board area when the linear regulator pass
device is sized to handle this power continuously. As an
alternative, the REGSD pin may be used to effectively detect
an overtemperature condition on the linear regulator pass
device and generate a shut down (soft-start retry) before
overheating occurs. This allows for the use of a small (e.g.,
SOT-23) package for the linear regulator pass device.
LTC3706
15
3706fd
APPLICATIONS INFORMATION
The REGSD resistor should be selected based upon the
steady-state (DC) thermal impedance of the linear regula-
tor pass device.
R
REGSD
= 960k
θ
JA
•I
CC(MAX)
T
RISE(MAX)
where θ
JA
is the DC thermal impedance of the linear
regulator pass device and T
RISE(MAX)
is the maximum
junction temperature rise desired for the pass device.
The value for I
CC(MAX)
depends heavily on the particular
switching MOSFETs used, as well as on the details of
overall system design. Note that it may include the bias
current associated with the primary-side gate driver and
controller, if the LTC3705 is being used. The value for I
CC
is best determined experimentally and then guard banded
appropriately to establish I
CC(MAX)
. Using the Typical Ap-
plication circuit on the first page of this data sheet as an
example, if a SOT-23 MOSFET is chosen, we might have
θ
JA
= 150°C/W, t
RISE(MAX)
= 50°C and I
CC(MAX)
= 35mA
so that R
REGSD
≈ 100kΩ. In this case, the linear regulator
can run continuously for any V
IN
voltage that is less than:
4V = (V
IN
– V
CC
)(5µs)(R
REGSD
)
V
IN(MAX)
=
640k
R
REGSD
+ 7V
or 13.4V. In addition, a capacitor may be added in parallel
with the REGSD resistor to delay the thermal shutdown
and thereby account for the thermal time constant of the
pass device. When using a delay capacitor, care must be
taken to ensure that the safe operating area (SOA) of the
pass device is not exceeded. The capacitor should be
chosen to provide a time constant that is somewhat faster
than the thermal time constant of the pass device in the
system. This technique will allow for much higher transient
power dissipation, which is particularly useful in larger
(PolyPhase) systems that have a higher V
CC
bias current.
For the above SOT-23 example, a capacitor C
REGSD
= 1µF
provides a linear regulator shutdown delay given by:
t
SHDN
= C
REGSD
( )
R
REGSD
( )
ln
1
1–
640k
V
IN
– 7
( )
R
REGSD
or 33ms at V
IN
= 30V. This delay provides ample time for
linear regulator operation during soft-start, while providing
protection for the pass device during fault conditions such
as input overvoltage or output overcurrent.
Current Sensing
The LTC3706 provides considerable flexibility in current
sensing techniques. It supports two main methods: 1)
resistive current sensing and 2) current transformer cur-
rent sensing. Resistive current sensing is generally simpler,
smaller and less expensive, while current transformer sens-
ing is more efficient and generally appropriate for higher
(>20A) output currents. For resistive current sensing, the
sense resistor may be placed in any one of three different
locations: high side inductor, low side inductor or low
side switch, as shown in Figure 3. Sensing the inductor
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LTC3706IGN

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Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators LTC3706 - Secondary-Side Synchronous Forward Controller with Polyphase Capability
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