LM3524 datasheet(17/22 Pages) NSC

17 / 22 page

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

during t

ON is replenished. Here also, as in the step-down

regulator, the current through L1 has a DC component plus

some

∆I

L is again selected to be approximately 40% of

L. Figure 19 shows the inductor’s current in relation to Q1’s

ON and OFF times.

Since

∆I

L+=

∆I

L−, 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

00865023

FIGURE 18. Basic Step-Up Switching Regulator

00865024

FIGURE 19. Relation of Switch Timing to Inductor Current in Step-Up Regulator

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