LTC3803 Datasheet, PDF (8/12 Pages) Linear Technology – Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT

LTC3803
APPLICATIO S I FOR ATIO
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
flux level and winding insulation breakdown voltages, C_IN
and C_OUT’s maximum working voltage, ESR, and maxi-
mum ripple current ratings, and D1 and R_SENSE’s power
ratings.
T1
D2
V_IN
L_BIAS
•
D1
C_VCC
C_C
R3 R_START
•
C_IN L_PRI
L_SEC
•
5
1
V_CC
6
I_TH/RUN NGATE
Q1
2
LTC3803
4 ^RSL
GND SENSE
V_FB
R1 ³ R2
R_SENSE
C_OUT
V_OUT
3803 F02
Figure 2. Typical LTC3803 Application Circuit
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 = ^{VOUT – 0.8V} • R1
0.8V
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_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.
TRANSFORMER DESIGN CONSIDERATIONS
Transformer specification 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 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^TM 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
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.
VERSA-PAC is a trademark of Coiltronics, Inc.
3803i
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