R1212D Datasheet, PDF (13/41 Pages) RICOH electronics devices division – STEP-UP DC/DC CONTOLLER

R1212D
In the continuous mode, the deviation of the current is equal between on time and off time.
VIN = TON / L = (VOUT − VIN) × Toff / L ............................................................................. Formula 4
Further, the electric power, PAV is equal to output electric power, VOUT × IOUT, thus,
{ } IOUT = fOSC × VIN2 × TON2 / 2 × L × (VOUT − VIN) = VIN2 × TON /(2 × L × VOUT) ..................... Formula 5
When IOUT becomes more than formula 5, the current flows through the inductor, then the mode becomes
continuous. The continuous current through the inductor is described as Iconst, then,
{ } IOUT = fOSC × VIN2 × TON2 / 2 × L × (VOUT − VIN) + VIN × Iconst / VOUT ................................ Formula 6
In this moment, the peak current, ILxmax flowing through the inductor and the driver Tr. is described as
follows:
ILx max = Iconst + VIN × TON / L ..................................................................................... Formula 7
With the formula 4,6, and ILxmax is,
ILx max = VOUT / VIN × IOUT + VIN × TON /(2 × L) ................................................................ Formula 8
Therefore, peak current is more than IOUT. Considering the value of ILxmax, the condition of input and output,
and external components should be selected.
In the formula 7, peak current ILxmax at discontinuous mode can be calculated. Put Iconst=0 in the formula.
The explanation above is based on the ideal calculation, and the loss caused by LX switch and external
components is not included. The actual maximum output current is between 50% and 80% of the calculation.
Especially, when the ILX is large, or VIN is low, the loss of VIN is generated with the on resistance of the switch. As
for VOUT, Vf (as much as 0.3V) of the diode should be considered.
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