Datasheet

LTC3805
12
3805fg
applications inForMation
C
VCC
is charged to V
TURNON
. At this point, assuming V
RUN
> V
RUNON
, the converter begins switching the external
MOSFET and ramps up the converter output voltage at
a rate set by the capacitor C
SS
on the SSFLT pin. Since
R
START
cannot supply enough current to operate the ex-
ternal MOSFET, C
VCC
begins discharging and V
CC
drops.
The soft-start must be fast enough and the discharge of
C
VCC
slow enough so that the output voltage reaches its
target value of 5V before V
CC
drops to V
TURNOFF
or the
converter would fail to start.
The typical application circuit in Figure 9 shows a different
flyback converter bias power strategy for a case in which
neither the input or output voltage is suitable for provid-
ing bias power to the LTC3805. A small NPN preregulator
transistor and a Zener diode are used to accelerate the
rise of V
CC
and reduce the value of the V
CC
bias capacitor.
The flyback transformer has an additional bias winding to
provide bias power. Note that this topology is very power-
ful because, by appropriate choice of transformer turns
ratio, the output voltage can be chosen without regard to
the
value of the input voltage or the V
CC
bias power for
the LTC3805. The number of turns in the bias winding is
chosen according to
N
BIAS
= N
SEC
V
CC
+
V
D4
V
OUT
− V
D1
where N
BIAS
is the number of turns in the bias winding,
N
SEC
is the number of turns in the secondary winding,
V
CC
is the desired voltage to power the LTC3805, V
OUT
is
the converter output voltage, V
D1
is the forward voltage
drop of D1 and V
D4
is the forward voltage drop of D4.
Note that since V
OUT
is regulated by the converter control
loop, V
CC
is also regulated although not as precisely. The
value of V
CC
is often constrained since N
BIAS
and N
SEC
are
often a limited range of small integer numbers. For proper
operation, the value of V
CC
must be between V
TURNON
and
V
TURNOFF
. Since the ratio of V
TURNON
to V
TURNOFF
is over
two to one, this requirement is relatively easy to satisfy.
Figure 9 shows a similar low power nonisolated telecom
converter using a trickle charger.
Transformer Design Considerations
Transformer specification and design is perhaps the
most critical part of applying the LTC3805 successfully.
In addition to the usual list of caveats dealing with high
frequency power transformer design, the following should
prove useful.
Turns Ratios
Due to the use of the external feedback resistor divider
ratio to set output
voltage, the user has relative freedom
in selecting transformer turns ratio to suit a given ap-
plication. Simple ratios of small integers, e.g., 1:1, 2:1,
3:2, etc. can be employed which yield more freedom in
setting total turns and transformer inductance. Simple
integer turns ratios also facilitate the use of “off-the-shelf”
configurable transformers. Turns ratio can be chosen on
the basis of desired duty cycle. However, remember that
the input supply voltage plus the secondary-to-primary
referred version of the flyback pulse (including leakage
spike) must not exceed the allowed external MOSFET
breakdown rating.