LTC3706_15 Datasheet, PDF (15/22 Pages) Linear Technology – Secondary-Side Synchronous Forward Controller with PolyPhase Capability

LTC3706
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 q_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
q_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
3706fd
15