ltc3803es6-3 Linear Technology Corporation, ltc3803es6-3 Datasheet - Page 10

• Manufacturer Part Number: ltc3803es6-3
• Description: Constant Frequency Current Mode Flyback Dc/dc Controller In Thinsot
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Many LTC3803-3 application circuits can be derived from
the topology shown in Figure 2.
The LTC3803-3 itself imposes no limits on allowed power
output, input voltage V
age V
external power components. The key factors are: Q1’s
maximum drain-source voltage (BV
(R
level and winding insulation breakdown voltages, C
C
ripple current ratings, and D1 and R
LTC3803-3
SELECTING FEEDBACK RESISTOR DIVIDER VALUES
The regulated output voltage is determined by the resistor
divider across V
to R1 needed to produce a desired V
Choose resistance values for R1 and R2 to be as large as
possible in order to minimize any efficiency loss due to the
static current drawn from V
that when V
nonzero input current to the V
good rule of thumb is to choose R1 to be 80k or less.
10
OUT
DS(ON)
C
R
VCC
C
2
’s maximum working voltage, ESR, and maximum
C
OUT
=
Figure 2. Typical LTC3803-3 Application Circuit
) and maximum drain current, T1’s saturation flux
V
; these are all determined by the ratings on the
R3
D2
OUT
1
2
OUT
0 8
R
I
GND
.
TH
START
– .
R1
OUT
/RUN
LTC3803-3
V
0 8
is in regulation, the error caused by the
V
V
V
CC
FB
IN
U
(R1 and R2 in Figure 2). The ratio of R2
5
3
V
NGATE
SENSE
R2
IN
•
R
or desired regulated output volt-
1
U
6
4
C
IN
OUT
R
L
SL
L
BIAS
PRI
, but just small enough so
FB
•
•
T1
Q1
R
pin is less than 1%. A
SENSE
SENSE
W
•
OUT
L
DSS
SEC
D1
can be calculated:
’s power ratings.
), on-resistance
C
OUT
U
38033 F02
IN
V
OUT
and
TRANSFORMER DESIGN CONSIDERATIONS
Transformer specification and design is perhaps the most
critical part of applying the LTC3803-3 successfully. In
addition to the usual list of caveats dealing with high fre-
quency 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 applica-
tion. 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 mutual inductance. Simple integer turns ratios
also facilitate the use of “off-the-shelf” configurable trans-
formers such as the Coiltronics VERSA-PAC
applications with high input to output voltage ratios. For
example, if a 6-winding VERSA-PAC is used with three
windings in series on the primary and three windings in
parallel on the secondary, a 3:1 turns ratio will be achieved.
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.
Leakage Inductance
Transformer leakage inductance (on either the primary or
secondary) causes a voltage spike to occur after the
output switch (Q1) turn-off. This is increasingly promi-
nent at higher load currents, where more stored energy
must be dissipated. In some cases a “snubber” circuit will
be required to avoid overvoltage breakdown at the
MOSFET’s drain node. Application Note 19 is a good
reference on snubber design.
A bifilar or similar winding technique is a good way to
minimize troublesome leakage inductances. However,
remember that this will limit the primary-to-secondary
breakdown voltage, so bifilar winding is not always
practical.
TM
series in
38033fa