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LTC3873IDDB-5-TRPBF Datasheet(PDF) 10 Page - Linear Technology |
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LTC3873IDDB-5-TRPBF Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  LTC3873-5 10 38735fb Turns Ratios Due to the use of the external feedback resistor divider ratio to set output voltage, the user has relative freedom in selecting a transformer turns ratio to suit a given ap- plication. 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” configurable 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. Leakage Inductance Transformer leakage inductance (on either the primary or secondary) causes a voltage spike to occur after the output switch (Q1) turn-off. This is increasingly prominent at higher load currents where more stored energy must be dissipated. In some cases a “snubber” circuit will be required to avoid overvoltage breakdown at the MOSFET’s drain node. Application Note 19 is a good reference on snubber design. A bifilar or similar winding technique is a good way to minimize troublesome leakage inductances. However, remember that this will limit the primary-to- secondary breakdown voltage, so bifilar winding is not always practical. Power MOSFET Selection The power MOSFET serves two purposes in the LTC3873-5: it represents the main switching element in the power path and its RDS(ON) represents the current sensing element for the control loop. Important parameters for the power MOSFET include the drain-to-source breakdown voltage (BVDSS), the threshold voltage (VGS(TH)), the on-resistance (RDS(ON))versusgate-to-sourcevoltage,thegate-to-source APPLICATIONS INFORMATION and gate-to-drain charges (QGS and QGD, respectively), the maximum drain current (ID(MAX)) and the MOSFET’s thermal resistances (RTH(JC) and RTH(JA)). Current Limit Programming During the switch on-time, the control circuit limits the maximum voltage drop across the current sense com- ponent to about 270mV, 100mV and 170mV at low duty cycle with IPRG tied to VIN, GND or left floating respec- tively. For boost applications with RDS(ON) sensing, refer to the LTC3872 data sheet for the selection of MOSFET RDS(ON). MOSFETs have conduction losses (I2R) and switching losses. For VDS < 20V, high current efficiency generally improves with large MOSFETs with low RDS(ON), while for VDS > 20V the transition losses rapidly increase to the point that the use of a higher RDS(ON) device with lower reverse transfer capacitance, CRSS, actually provides higher efficiency. Output Capacitors The output capacitor is normally chosen by its effective series resistance (ESR), which determines output ripple voltage and affects efficiency. Low ESR ceramic capaci- tors are often used to minimize the output ripple. Boost regulators have large RMS ripple current in the output capacitor that must be rated to handle the current. The output ripple current (RMS) is: II VV V RMS COUT OUT MAX OUT IN MIN IN MIN () ( ) () () • – ≈ Output ripple is then simply: VOUT = RESR(ΔIL(RMS)) The output capacitor for flyback converter should have a ripple current rating greater than: II D D RMS OUT MAX MAX = • – 1 |
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Similar Description - LTC3873IDDB-5-TRPBF |
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