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LTC3803-5 Datasheet(PDF) 9 Page - Linear Technology |
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LTC3803-5 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  LTC3803-5 9 38035fd 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 VINordesiredregulatedoutputvoltage VOUT; these are all determined by the ratings on the external power components. The key factors are: Q1’s maximum drain-source voltage (BVDSS), on-resistance (RDS(ON)) and maximum drain current, T1’s saturation flux level and winding insulation breakdown voltages, CIN and COUT’s maximum working voltage, ESR, and maximum ripple current ratings, and D1 and RSENSE’s power ratings. 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 VOUT, but just small enough so that when VOUT is in regulation, the error caused by the nonzero input current to the VFB 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. Figure 2. Typical LTC3803-5 Application Circuit SELECTING FEEDBACK RESISTOR DIVIDER VALUES The regulated output voltage is determined by the resistor divider across VOUT (R1 and R2 in Figure 2). The ratio of R2 to R1 needed to produce a desired VOUT can be calculated: R2 = V OUT – 0.8V 0.8V •R1 VCC ITH/RUN LTC3803-5 GND NGATE SENSE VFB 5 6 4 1 2 3 D1 • • COUT CIN LSEC LPRI CVCC CC VOUT 38035 F02 RSENSE RSL R1 RVCC R2 Q1 T1 VIN |
Similar Part No. - LTC3803-5_15 |
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Similar Description - LTC3803-5_15 |
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