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AV34063
Note 5 : If the output switch is driven into hard saturation (non-Darlington configuration) at low switch
currents (≤300mA) and high driver currents (≥30mA), it may take up to 2.0µs to come out of sturation.
This condition will shorten the off time at frequencies ≥30KHz, and is magnified at high temperatures.
This condition does not occur with a darlington configuration, since the output switch cannot saturate.
If a non-darlington is used, the following output drive condition is recommended.
Figure13. Design Formula Table
Vsat=Saturation voltage of the output switch.
VF =Forward voltage drop of the output rectifier.
The following power supply characteristics must be chosen:
Vin -Nominal input voltage.
Vout -Desired output voltage, Vout = 1.25(1+R2/R1).
Iout - Desired output current.
Fmin -Minimum desired output switching frequency at the selected valued of Vin an Io.
Vripple(p-p) - Desired peak-to-peak output ripple voltage.In practice, the calculated capacitor
value will need to be increased due to its equivalent series resistance and board layout.The
ripple voltage should be kept to a low value since it will directiy affect the line and load
regulation.
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