R1212D Datasheet, PDF (12/41 Pages) RICOH electronics devices division – STEP-UP DC/DC CONTOLLER

R1212D
Output Current and Selection of External Components
<Basic Circuit>
Inductor
Diode
i2
IOUT
VIN
i1
VOUT
Lx Tr
CL
<Circuit through L>
Discontinuous Mode
Continuous Mode
IL
IL
ILxmax
ILxmax
ILxmin
Tf
ILxmin
t
Iconst
Ton
Toff
T=1/fosc
1/ton
Ton
Toff
T=1/fosc
1/ton
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 =}
V
2
IN
× t /L
dt
..................................................................................................... Formula
1
0
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 ................................................................................Formula 2
0
In this formula, Tf means the time of which the energy saved in the inductance is being emitted. Thus average
electric power, or PAV is described as in the next formula.
Ton
Tf
∫ ∫ ^{PAV = 1/(TON + TOFF) × {}
V
2
IN
×
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.
12