LTC3805 Linear Technology, LTC3805 Datasheet - Page 12

• Manufacturer Part Number: LTC3805
• Description: Adjustable Frequency Current Mode Flyback DC/DC Controller
• Manufacturer: Linear Technology

www.datasheet4u.com
APPLICATIONS INFORMATION
C
> V
MOSFET and ramps up the converter output voltage at
a rate set by the capacitor C
R
ternal MOSFET, C
The soft-start must be fast enough and the discharge of
C
target value of 5V before V
converter would fail to start.
The typical application circuit in Figure 9 shows a different
fl yback 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
The fl yback 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
the LTC3805. The number of turns in the bias winding is
chosen according to
where N
N
V
the converter output voltage, V
drop of D1 and V
LTC3805
12
VCC
VCC
CC
START
SEC
N
RUNON
is the desired voltage to power the LTC3805, V
BIAS
is charged to V
slow enough so that the output voltage reaches its
is the number of turns in the secondary winding,
cannot supply enough current to operate the ex-
CC
BIAS
= N
, the converter begins switching the external
and reduce the value of the V
is the number of turns in the bias winding,
SEC
VCC
V
D4
V
OUT
TURNON
CC
is the forward voltage drop of D4.
begins discharging and V
+ V
− V
D4
. At this point, assuming V
D1
CC
SS
drops to V
D1
on the SSFLT pin. Since
is the forward voltage
CC
CC
TURNOFF
bias power for
bias capacitor.
CC
drops.
or the
OUT
RUN
is
Note that since V
loop, V
value of V
often a limited range of small integer numbers. For proper
operation, the value of V
V
two to one, this requirement is relatively easy to satisfy.
Figure 10 shows a similar low power nonisolated telecom
converter using a trickle charger.
Transformer Design Considerations
Transformer specifi cation 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”
confi gurable 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 fl yback pulse (including leakage
spike) must not exceed the allowed external MOSFET
breakdown rating.
TURNOFF
CC
. Since the ratio of V
CC
is also regulated although not as precisely. The
is often constrained since N
OUT
is regulated by the converter control
CC
must be between V
TURNON
to V
BIAS
TURNOFF
and N
TURNON
SEC
is over
3805fb
and
are