R1211X Datasheet, PDF (11/33 Pages) RICOH electronics devices division – Step-up DC/DC Controller

s Output Current and Selection of External Components
Inductor
<Basic Circuit>
i2
Diode
IOUT
VIN
VOUT
i1
Lx Tr
CL
GND
Discontinuous Mode
<Current through L>
IL
IL
ILxmax
Continuous Mode
ILxmax
ILxmin
ILxmin
Tf
Ton
T=1/fosc
t
Toff
Iconst
Ton
T=1/fosc
t
Toff
There are two modes, or discontinuous mode and continuous mode for the PWM step-up switching regulator
depending on the continuous characteristic of inductor current.
During on time of the transistor, when the voltage added on to the inductor is described as VIN, the current is VIN ×t/L.
Therefore, the electric power, PON, which is supplied with input side, can be described as in next formula.
TON
PON=∫VIN2×t/L dt
0
Formula 1
With the step-up circuit, electric power is supplied from power source also during off time. In this case, input current is
described as (VOUT-VIN)×t/L, therefore electric power, POFF is described as in next formula.
Tf
POFF=∫VIN×(VOUT-VIN)×t/L dt
0
Formula 2
In this formula, Tf means the time of which the energy saved in the inductance is being emitted. Thus average
electric power, PAV is described as in the next formula.
TON
Tf
PAV=1/(Ton+Toff)×{∫VIN²Ã—t/L dt + ∫VIN×(VOUT-VIN)×t/L dt} Formula 3
0
0
In PWM control, when Tf=Toff is true, the inductor current becomes continuos, then the operation of switching
regulator becomes continuous mode.
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 × VIN²Ã—TON²/{2×L ×(VOUT-VIN)}=VIN²Ã—TON/(2×L×VOUT)
Formula 5
When IOUT becomes more than formula 5, the current flows through the inductor, then the mode becomes
Rev. 1.10
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