Datasheet

LTC3766
40
3766fa
For more information www.linear.com/LTC3766
applicaTions inForMaTion
A useful variant of the inductor over-winding bias supply
is shown in Figure 27, where a discrete transformer T
OW
has been used instead of an additional winding on the
main inductor L
F
. This is often more convenient because
standard parts can readily be used.
In the circuit of Figure 27, a second diode D
OW2
has been
added to prevent DC bias current from being carried in
the transformer T
OW
. This transformer can be either a
gate-drive or flyback-style transformer, which are widely
available in a range of turns ratios. Note that transformer
T
OW
requires only functional isolation and can be physically
very small. This circuit produces a bias voltage given by:
V
OW
= V
OUT
– 0.5
( )
N
L2
N
L1
– 0.5
During an output overload condition, the voltage generated
by a either a buck supply or inductor overwinding sup-
ply will drop as the converter output voltage decreases.
If this happens and there is no peak charge bias supply
,
then the L
TC3766 will have a UVLO fault that will cause
both the LTC3765 and LTC3766 to shut down and attempt
a restart. If a peak charge supply is used together with a
buck or inductor overwinding supply, then the LTC3766 will
automatically re-energize the high voltage linear regulator
when the V
AUX
pin gets too low. If continuous operation of
the peak charge and high voltage regulator is not needed,
then the REGSD pin can be used to limit the total time that
this regulator is allowed to operate (shown as an option
in Figures 23 and 24). This enables a low power pass
transistor to be used. See Linear Regulator Operation for
more information on using the REGSD feature.
Soft-Start Ramp Time and Control Hand-Off
The soft-start ramp time on the LTC3766 is set by placing a
capacitor between the SS pin and GND. This secondary-side
soft-start capacitor only controls the output voltage ramp
after control hand-off has taken place. Consequently, its
effect on the overall output voltage start-up will depend on
the primary to secondary hand-off voltage in the particular
application. Choose a soft-start capacitor using:
C
SS
=
5µA
( )
t
SS
1.83 0.6 – V
FB(HO)
( )
where t
SS
is the soft-start time after control hand-off to
the secondary and V
FB(HO)
is the voltage on the FB pin at
control hand-off. The total soft-start time will be the sum
of t
SS
and the open-loop soft-start time prior to control
hand-off set by the LTC3765. Note that during the open-
loop soft-start time, the output voltage ramp will vary
significantly with load, since the synchronous MOSFET is
not enabled and the converter may operate in discontinu
-
ous current mode. If precise control over the soft-start
time is desired, use a secondar
y-side bias scheme that
provides control hand-off at the lowest possible output
voltage. See above sections on generating secondar
y-side
bias for details.
Just prior to control hand-off, the LTC3766 rapidly pre
-
sets the soft-start capacitor so that the internal soft-start
voltage is equal to V
FB(HO)
, ensuring a smooth transition
from primary to secondary control. Due to the dielectric
absorption of the soft-start capacitor, however, the voltage
on the soft-start capacitor may droop somewhat following
the initial preset. This can result in a small step down in the
output voltage ramp after control hand-off, and an associ
-
ated negative current transient in the output inductor. To
minimize this effect, use a soft-start capacitor with a low
dielectric absorption, such as an NPO ceramic capacitor.
Pulse T
ransformer Selection
The pulse transformer that connects the LTC3766
PT
+
/PT
−
outputs to the LTC3765 IN
+
/IN
−
inputs functions
V
IN
V
AUX
NDRV
LTC3766
V
CC
C
VCC
3766 F27
C
OW
D
OW1
D
OW2
N
L2
SW
MAIN
XFMR
N
L1
L
F
N
S
N
P
V
OW
C
OUT
V
OUT
•
•
Figure 27. Inductor Overwinding Using Standard Parts