Datasheet

LTC3803
9
3803fc
APPLICATIONS INFORMATION
Many LTC3803 application circuits can be derived from
the topology shown in Figure 2.
The LTC3803 itself imposes no limits on allowed power
output, input voltage V
IN
or desired regulated output voltage
V
OUT
; these are all determined by the ratings on the external
power components. The key factors are: Q1’s maximum
drain-source voltage (BV
DSS
), on-resistance (R
DS(ON)
)
and maximum drain current, T1’s saturation fl ux level and
winding insulation breakdown voltages, C
IN
and C
OUT
’s
maximum working voltage, ESR, and maximum ripple
current ratings, and D1 and R
SENSE
’s power ratings.
TRANSFORMER DESIGN CONSIDERATIONS
Transformer specifi cation and design is perhaps the
most critical part of applying the LTC3803 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 appli-
cation. 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” confi gu-
rable transformers such as the Coiltronics VERSA-PAC™
series in 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
fl yback 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 prominent
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 bifi lar 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 bifi lar winding is not always
practical.
SELECTING FEEDBACK RESISTOR DIVIDER VALUES
The regulated output voltage is determined by the resistor
divider across V
OUT
(R1 and R2 in Figure 2). The ratio
of R2 to R1 needed to produce a desired V
OUT
can be
calculated:
R2 =
V
OUT
– 0.8V
0.8V
•R1
Choose resistance values for R1 and R2 to be as large as
possible in order to minimize any effi ciency loss due to
the static current drawn from V
OUT
, but just small enough
so that when V
OUT
is in regulation, the error caused by
the nonzero input current to the V
FB
pin is less than 1%.
A good rule of thumb is to choose R1 to be 80k or less.
Figure 2. Typical LTC3803 Application Circuit
V
CC
I
TH
/RUN
LTC3803
GND
NGATE
SENSE
V
FB
5
R3
6
4
1
2
3
D1
D2
•
•
C
OUT
C
IN
L
SEC
L
PRI
L
BIAS
C
VCC
C
C
V
OUT
3803 F02
R
SENSE
R
SL
R1
R
START
R2
Q1
T1
V
IN
•