Datasheet LTC3803-3 (Analog Devices) - 9

LTC3803-3
APPLICATIONS INFORMATION
Many LTC3803-3 application circuits can be derived from
TRANSFORMER DESIGN CONSIDERATIONS
the topology shown in Figure 2. Transformer specifi cation and design is perhaps the most The LTC3803-3 itself imposes no limits on allowed power critical part of applying the LTC3803-3 successfully. In output, input voltage VIN or desired regulated output voltage addition to the usual list of caveats dealing with high fre- VOUT ; these are all determined by the ratings on the external quency 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 LBIAS total turns and mutual inductance. Simple integer turns D2 V • IN D1 ratios also facilitate the use of “off-the-shelf” confi gu- VOUT rable transformers such as the Coiltronics VERSA-PAC™ • C R3 IN R series in applications with high input to output voltage START L L C PRI SEC OUT • ratios. For example, if a 6-winding VERSA-PAC is used 5 CVCC with three windings in series on the primary and three VCC 1 6 I windings in parallel on the secondary, a 3:1 turns ratio TH/RUN NGATE Q1 CC LTC3803-3 R will be achieved. 2 4 SL GND SENSE V Turns ratio can be chosen on the basis of desired duty FB RSENSE 3 cycle. However, remember that the input supply voltage R1 R2 38033 F02 plus the secondary-to-primary referred version of the fl yback pulse (including leakage spike) must not exceed
Figure 2. Typical LTC3803-3 Application Circuit
the allowed external MOSFET breakdown rating.
SELECTING FEEDBACK RESISTOR DIVIDER VALUES Leakage Inductance
The regulated output voltage is determined by the resistor Transformer leakage inductance (on either the primary divider across V or secondary) causes a voltage spike to occur after the OUT (R1 and R2 in Figure 2). The ratio of R2 to R1 needed to produce a desired V output switch (Q1) turn-off. This is increasingly prominent OUT can be calculated: at higher load currents, where more stored energy must be dissipated. In some cases a snubber circuit will be V – 0 8 . V R OUT 2 = • R1 required to avoid overvoltage breakdown at the MOSFET’s 0 8 . V drain node. Application Note 19 is a good reference on Choose resistance values for R1 and R2 to be as large as snubber design. possible in order to minimize any effi ciency loss due to A bifi lar or similar winding technique is a good way to the static current drawn from VOUT, but just small enough minimize troublesome leakage inductances. However, so that when VOUT is in regulation, the error caused by remember that this will limit the primary-to-second- the nonzero input current to the VFB pin is less than 1%. ary breakdown voltage, so bifi lar winding is not always A good rule of thumb is to choose R1 to be 80k or less. practical. 38033fd 9 Document Outline FEATURES DESCRIPTION APPLICATIONS TYPICAL APPLICATION ABSOLUTE MAXIMUM RATINGS PACKAGE/ORDER INFORMATION ORDER INFORMATION ELECTRICAL CHARACTERISTICS TYPICAL PERFORMANCE CHARACTERISTICS PIN FUNCTIONS BLOCK DIAGRAM OPERATION APPLICATIONS INFORMATION TYPICAL APPLICATIONS PACKAGE DESCRIPTION REVISION HISTORY TYPICAL APPLICATION RELATED PARTS