ltc3803es6-5 Linear Technology Corporation, ltc3803es6-5 Datasheet - Page 10

• Manufacturer Part Number: ltc3803es6-5
• Description: Constant Frequency Current Mode Flyback Dc/dc Controller In Thinsot
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC3803-5
Many LTC3803-5 application circuits can be derived from
the topology shown in Figure 2.
The LTC3803-5 itself imposes no limits on allowed power
output, input voltage V
voltage V
external power components. The key factors are: Q1’s
maximum drain-source voltage (BV
(R
flux level and winding insulation breakdown voltages, C
and C
mum ripple current ratings, and D1 and R
ratings.
SELECTING FEEDBACK RESISTOR DIVIDER VALUES
The regulated output voltage is determined by the resistor
divider across V
R2 to R1 needed to produce a desired V
calculated:
10
C
VCC
DS(ON)
C
R
C
2
OUT
=
Figure 2. Typical LTC3803-5 Application Circuit
) and maximum drain current, T1’s saturation
V
OUT
’s maximum working voltage, ESR, and maxi-
OUT
1
2
; these are all determined by the ratings on the
0 8
I
GND
TH
.
R
R1
– .
/RUN
VCC
LTC3803-5
V
OUT
0 8
V
V
V
CC
FB
IN
U
5
3
NGATE
SENSE
V
(R1 and R2 in Figure 2). The ratio of
R2
•
R
IN
1
U
6
4
C
IN
or desired regulated output
R
SL
L
PRI
•
T1
Q1
R
W
SENSE
•
L
DSS
SEC
D1
), on-resistance
SENSE
C
OUT
OUT
U
’s power
38035 F02
can be
V
OUT
IN
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.
TRANSFORMER DESIGN CONSIDERATIONS
Transformer specification and design is perhaps the most
critical part of applying the LTC3803-5 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 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.
OUT
is in regulation, the error caused by the
OUT
, but just small enough so
FB
pin is less than 1%. A
TM
series in
38035fb