參數(shù)資料
型號(hào)： LM3524DMWC
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類(lèi)： 穩(wěn)壓器
英文描述： 0.2 A SWITCHING CONTROLLER, 350 kHz SWITCHING FREQ-MAX, UUC
封裝： WAFER
文件頁(yè)數(shù)： 6/22頁(yè)
文件大?。?/td> 728K
代理商： LM3524DMWC
Typical Applications (Continued)
THE STEP-UP SWITCHING REGULATOR
Figure 18 shows the basic circuit for a step-up switching
regulator. In this circuit Q1 is used as a switch to alternately
apply V
IN across inductor L1. During the time, tON,Q1isON
and energy is drawn from V
IN and stored in L1; D1 is reverse
biased and I
o is supplied from the charge stored in Co. When
Q1 opens, t
OFF, voltage V1 will rise positively to the point
where D1 turns ON. The output current is now supplied
through L1, D1 to the load and any charge lost from C
o dur-
ing t
ON is replenished. Here also, as in the step-down regu-
lator, the current through L1 has a DC component plus some
I
L. IL is again selected to be approximately 40% of IL. Fig-
ure 19 shows the inductor’s current in relation to Q1’s ON
and OFF times.
Since
I
L+ = IL, VINtON = VotOFF VINtOFF,
and neglecting V
SAT and VD1
The above equation shows the relationship between V
IN,Vo
and duty cycle.
In calculating input current I
IN(DC), which equals the induc-
tor’s DC current, assume first 100% efficiency:
for
η = 100%,P
OUT = PIN
This equation shows that the input, or inductor, current is
larger than the output current by the factor (1 + t
ON/tOFF).
Since this factor is the same as the relation between V
o and
V
IN,IIN(DC) can also be expressed as:
So far it is assumed
η = 100%, where the actual efficiency or
η
MAX will be somewhat less due to the saturation voltage of
Q1 and forward on voltage of D1. The internal power loss
due to these voltages is the average I
L current flowing, or IIN,
through either V
SAT or VD1. For VSAT = VD1 = 1V this power
loss becomes I
IN(DC) (1V). ηMAX is then:
This equation assumes only DC losses, however
η
MAX is fur-
ther decreased because of the switching time of Q1 and D1.
DS008650-23
FIGURE 18. Basic Step-Up Switching Regulator
DS008650-24
FIGURE 19. Relation of Switch Timing to Inductor Current in Step-Up Regulator
www.national.com
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