Datasheet LTC3803-5 (Analog Devices) - 9

LTC3803-5
APPLICATIONS INFORMATION
Many LTC3803-5 application circuits can be derived from Choose resistance values for R1 and R2 to be as large as the topology shown in Figure 2. possible in order to minimize any effi ciency loss due to the static current drawn from V The LTC3803-5 itself imposes no limits on allowed power OUT, but just small enough so that when V output, input voltage V OUT is in regulation, the error caused by IN or desired regulated output voltage the nonzero input current to the V V FB pin is less than 1%. OUT; these are all determined by the ratings on the external A good rule of thumb is to choose R1 to be 80k or less. power components. The key factors are: Q1’s maximum drain-source voltage (BVDSS), on-resistance (RDS(ON)) and maximum drain current, T1’s saturation fl ux level and
TRANSFORMER DESIGN CONSIDERATIONS
winding insulation breakdown voltages, CIN and COUT’s Transformer specifi cation and design is perhaps the most maximum working voltage, ESR, and maximum ripple critical part of applying the LTC3803-5 successfully. In current ratings, and D1 and RSENSE’s power ratings. addition to the usual list of caveats dealing with high fre- quency power transformer design, the following should VIN D1 prove useful. T1 VOUT • RVCC C L C IN L PRI SEC OUT
Turns Ratios
• 5 Due to the use of the external feedback resistor divider CVCC VCC ratio to set output voltage, the user has relative freedom 1 6 ITH/RUN NGATE Q1 in selecting transformer turns ratio to suit a given appli- CC LTC3803-5 R 2 4 SL cation. Simple ratios of small integers, e.g., 1:1, 2:1, 3:2, GND SENSE etc. can be employed which yield more freedom in setting VFB RSENSE 3 total turns and mutual inductance. Simple integer turns R1 R2 38035 F02 ratios also facilitate the use of “off-the-shelf” confi gu- rable transformers such as the Coiltronics VERSA-PAC™
Figure 2. Typical LTC3803-5 Application Circuit
series in applications with high input to output voltage ratios. For example, if a 6-winding VERSA-PAC is used
SELECTING FEEDBACK RESISTOR DIVIDER VALUES
with three windings in series on the primary and three windings in parallel on the secondary, a 3:1 turns ratio The regulated output voltage is determined by the resistor will be achieved. divider across VOUT (R1 and R2 in Figure 2). The ratio of R2 to R1 needed to produce a desired VOUT can be Turns ratio can be chosen on the basis of desired duty calculated: cycle. However, remember that the input supply voltage plus the secondary-to-primary referred version of the V – 0.8V R2 = OUT • R1 fl yback pulse (including leakage spike) must not exceed 0.8V the allowed external MOSFET breakdown rating. 38035fd 9 Document Outline FEATURES DESCRIPTION APPLICATIONS TYPICAL APPLICATION ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION ORDER INFORMATION ELECTRICAL CHARACTERISTICS TYPICAL PERFORMANCE CHARACTERISTICS PIN FUNCTIONS BLOCK DIAGRAM OPERATION APPLICATIONS INFORMATION TYPICAL APPLICATIONS PACKAGE DESCRIPTION REVISION HISTORY TYPICAL APPLICATION RELATED PARTS