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LTC3873EDDB Datasheet(PDF) 9 Page - Linear Technology |
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LTC3873EDDB Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  LTC3873 9 3873fa VCC Bias Power The VCC pin must be bypassed to the GND pin with a minimum 10μF ceramic or tantalum capacitor located immediately adjacent to the two pins. Proper supply by- passing is necessary to supply the high transient currents required by the MOSFET gate driver. For maximum flexibility, the LTC3873 is designed so that it can be operated from voltages well beyond the LTC3873’s absolute maximum ratings. In the simplest case, the LTC3873 can be powered with a resistor connected between the input voltage and VCC. The built-in shunt regulator limits the voltage on the VCC pin to around 9.3V as long as the shunt regulator is not forced to sink more than 25mA. This powering scheme has the drawback that the power loss in the resistor reduces converter efficiency and the 25mA shunt regulator maximum may limit the maximum-minimum range of input voltage. In some cases, the input or the output voltage is within the operational range of VCC for the LTC3873. In this case, the LTC3873 is operated directly from either the input or output voltage. The typical application circuit on the first page of this data sheet shows a 5V output converter in which RSTARTandCVCCformastart-uptricklechargerwhile D1 powers VCC from the output once the converter is in normal operation. Note that RSTART need only supply the very small 55μA micropower start-up current while CVCC is charged to VTURNON. At this point, VRUN/SS > VSHDN, the converter begins switching the external MOSFET and ramps up the converter output voltage at a rate set by the capacitor CRUN/SS on the RUN/SS pin. Since RSTART cannot supply enough current to operate the external MOSFET, CVCC begins discharging and VCC drops. The soft-start must be fast enough so that the output voltage reaches its target value of 5V before VCC drops to VTURNOFF or the converter will fail to start. Otherwise more CVCC capacitor is needed to hold the input voltage when soft-start is too long. Figure 5 shows a different flyback converter bias power strategy for a case in which neither the input or the output is suitable for providing the bias power to the LTC3873. The trickle charger is identical to that described in the prior paragraph. However, the flyback transformer has an additional bias winding to provide bias power. Note that this topology is very powerful because, by appropriate choice of the transformer turn ratio, the output voltage can be chosen without regard to the value of the input voltage or the VCC bias power for the LTC3873. The number of the turns in the bias winding is chosen according to: NN VV VV BIAS SEC CC D OUT D = + + 2 1 where NBIAS is the number of turns in the bias winding, NSEC is the number of turns in the secondary winding, VCC is the desired voltage to power the LTC3873, VOUT is the converter output voltage, VD1 is the forward drop voltage of D1 and VD2 is the forward drop voltage of D2. Note that since VOUT is regulated by the converter control loop, VCC is also regulated although not precisely. The value of VCC is often constrained since NBIAS and NSEC are often a limited range of small integer numbers. For proper operation, the value of VCC must be between VTURNON and VTURNOFF. Since the ratio of VTURNON to VTURNOFF is over two to one, the requirement is relative easy to satisfy. Finally, as with all trickle charger start-up schemes, the soft-start must be fast enough so that the power supplied by the bias winding is available before the discharge of CVCC down to VTURNOFF. APPLICATIONS INFORMATION VCC RUN/SS NGATE LTC3873 ITH GND CC COUT NSEC NPRI NBIAS VOUT CVIN CIN CVCC R3 RSTART D2 T1 D1 VIN RSENSE 3873 F05 Q1 • • SW VFB R1 R2 RSL • Figure 5. Typical LTC3873 Application Circuit |
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