Datasheet LTC3873 (Analog Devices) - 9

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