LTC3803-5_15 Datasheet, PDF (9/16 Pages) Linear Technology – Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT

LTC3803-5
APPLICATIONS INFORMATION
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_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 maximum ripple
current ratings, and D1 and R_SENSE’s power ratings.
C_VCC
C_C
V_IN
R_VCC
D1
T1
•
C_IN L_PRI
L_SEC
•
5
1
V_CC
6
I_TH/RUN NGATE
Q1
2
LTC3803-5
4 ^RSL
GND SENSE
V_FB
R1 ³ R2
R_SENSE
C_OUT
V_OUT
38035 F02
Figure 2. Typical LTC3803-5 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 LTC3803-5 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 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” configu-
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
flyback pulse (including leakage spike) must not exceed
the allowed external MOSFET breakdown rating.
38035fd
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