R1211D002D 查看數據表（PDF） - RICOH Co.,Ltd.
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R1211D002D Datasheet PDF : 40 Pages
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R1211x
Output Current and Selection of External Components
<Basic Circuit>
i2
Inductor Diode
IOUT
VIN
i1
VOUT
LX Tr CL
GND
<Circuit through L>
Discontinuous Mode
IL
IL
ILxmax
Continuous Mode
ILxmax
ILxmin
Ton
T=1/fosc
Tf
t
Toff
ILxmin
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 =
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.
∫ POFF =
Tf
0
VIN
×
(VOUT
−
VIN) × t/L
dt
........................................................................................................
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.
∫ ∫ PAV = 1/(TON + TOFF) × {
V Ton 2
0
IN
× t/L
dt
+
Tf
0
VIN
×
(VOUT
−
VIN) × t/L
dt}
...................................................
Formula
3
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 × VIN2 { × TON2 / 2 ×L × (VOUT − VIN)} = VIN2 × TON/(2×L × VOUT) .................................................... Formula 5
10
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