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參數(shù)資料
| 型號： | R1280D002C |
| 文件頁數(shù)： | 10/26頁 |
| 文件大?。?/td> | 344K |
| 代理商： | R1280D002C |
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Rev.1.10 - 10 -
ILxmax
ILxmin
Ton
Toff
T=1/fosc
Tf
IL
Discontinuous Mode
t
ILxmax
ILxmin
Ton
Toff
T=1/fosc
t
IL
Iconst
Continuous Mode
<Current through L>
Step 1. Lx Tr. turns on, current, IL=i1 flows, energy is charged in L. In proportion to the on time, Ton, of Lx Tr. IL=i1
increases from IL=ILxmin=0 and reaches ILxmax.
Step 2. When the Lx Tr. turns off, L turns on Shottky diode (SD) and flow IL=i2 to maintain IL = ILxmax.
Step 3. IL=i2 decreases gradually, after Tf passes, IL=ILxmin=0 is true, then SD turns off. Note that in the case of
continuous mode, before IL=ILxmin=0 is true, Toff passes and next cycle starts, then Lx Tr. turns on. In this case,
ILxmin>0, therefore IL increases from IL=ILxmin>0.
With the PWM controller, switching time (fosc) in the time unit is fixed, and by controlling Ton, output voltage is
maintained.
I
Output Current and Selection of External Components
There are also two modes, or discontinuous mode and continuous mode for the PWM inverting 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 V
IN
, the current is V
IN
×
t/L.
Therefore, the electric power, P, which is supplied with input side, can be described as in next formula.
T
ON
V
IN
2
×
t/L dt
0
Formula 9
Thus average electric power in one cycle, P
AV
is described as in the next formula.
T
ON
P
AV
=1/(Ton +Toff)
V
IN
2
×
t/L dt =V
IN
2
×
Ton
2
/(2
×
L
×
(Ton + Toff))
0
Formula 10
This electric power P
AV
equals to output electric power V
OUT
×
I
OUT,
thus,
I
OUT
= f
OSC
×
V
IN
2
×
T
ON
2
/(2
×
L
×
V
OUT
)
When I
OUT
becomes more than formula 11, the current flows through the inductor continuously, then the mode
becomes continuous. In the continuous mode, the deviation of the current equals between Ton and Toff, therefore,
V
IN
×
Ton/L=V
OUT
×
Toff/L
Formula 12
In this moment, the current flowing continuously through L, is assumed as Iconst, I
OUT
is described as in the next
formula:
I
OUT
= f
OSC
×
V
IN
2
×
T
ON
2
/(2
×
L
×
V
OUT
)+Ton/(Ton + Toff)
×
V
IN
×
Iconst /V
OUT
In this moment, the peak current, ILxmax flowing through the inductor and the driver Tr. is described as follows:
ILxmax = Iconst +V
IN
×
Ton/L
Formula 14
With the formula 12,13, ILxmax is,
ILxmax = (Ton+Toff)/Toff
×
I
OUT
+V
IN
×
Ton/(2
×
L)
Therefore, peak current is more than I
OUT
. Considering the value of ILxmax, the condition of input and output, and
external components should be selected.
In the formula 14, 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 V
IN
is low, the loss of V
IN
is generated with the on resistance of the switch. As for V
OUT,
Vf (as much as
0.3V) of the diode should be considered.
Formula 11
Formula 13
Formula 15
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| R1280D002C-TR | 制造商:RICOH 制造商全稱:RICOH electronics devices division 功能描述:2CH PWM DC/DC Controller |
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