Datasheet LTC3803 (Analog Devices) - 9

LTC3803
APPLICATIONS INFORMATION
Many LTC3803 application circuits can be derived from
TRANSFORMER DESIGN CONSIDERATIONS
the topology shown in Figure 2. Transformer specifi cation and design is perhaps the The LTC3803 itself imposes no limits on allowed power most critical part of applying the LTC3803 successfully. output, input voltage VIN or desired regulated output voltage In addition to the usual list of caveats dealing with high VOUT; these are all determined by the ratings on the external frequency power transformer design, the following should power components. The key factors are: Q1’s maximum prove useful. drain-source voltage (BVDSS), on-resistance (RDS(ON)) and maximum drain current, T1’s saturation fl ux level and
Turns Ratios
winding insulation breakdown voltages, CIN and COUT’s Due to the use of the external feedback resistor divider maximum working voltage, ESR, and maximum ripple ratio to set output voltage, the user has relative freedom current ratings, and D1 and RSENSE’s power ratings. in selecting transformer turns ratio to suit a given appli- cation. Simple ratios of small integers, e.g., 1:1, 2:1, 3:2, T1 etc. can be employed which yield more freedom in setting total turns and mutual inductance. Simple integer turns LBIAS • ratios also facilitate the use of “off-the-shelf” confi gu- D2 VIN D1 rable transformers such as the Coiltronics VERSA-PAC™ VOUT • series in applications with high input to output voltage R3 RSTART C L C IN L PRI SEC OUT ratios. For example, if a 6-winding VERSA-PAC is used • with three windings in series on the primary and three 5 CVCC V windings in parallel on the secondary, a 3:1 turns ratio CC 1 6 ITH/RUN NGATE Q1 will be achieved. CC LTC3803 R 2 4 SL GND SENSE Turns ratio can be chosen on the basis of desired duty VFB R cycle. However, remember that the input supply voltage SENSE 3 R1 R2 plus the secondary-to-primary referred version of the 3803 F02 fl yback pulse (including leakage spike) must not exceed
Figure 2. Typical LTC3803 Application Circuit
the allowed external MOSFET breakdown rating.
Leakage Inductance SELECTING FEEDBACK RESISTOR DIVIDER VALUES
Transformer leakage inductance (on either the primary The regulated output voltage is determined by the resistor or secondary) causes a voltage spike to occur after the divider across VOUT (R1 and R2 in Figure 2). The ratio output switch (Q1) turn-off. This is increasingly prominent of R2 to R1 needed to produce a desired VOUT can be at higher load currents, where more stored energy must calculated: be dissipated. In some cases a “snubber” circuit will be V – 0.8V OUT required to avoid overvoltage breakdown at the MOSFET’s R2 = • R1 0.8V drain node. Application Note 19 is a good reference on snubber design. Choose resistance values for R1 and R2 to be as large as A bifi lar or similar winding technique is a good way to possible in order to minimize any effi ciency loss due to minimize troublesome leakage inductances. However, the static current drawn from VOUT, but just small enough remember that this will limit the primary-to-secondary so that when VOUT is in regulation, the error caused by breakdown voltage, so bifi lar winding is not always the nonzero input current to the VFB pin is less than 1%. practical. A good rule of thumb is to choose R1 to be 80k or less. 3803fc 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 APPLICATIONS RELATED PARTS