LM3524 National Semiconductor, LM3524 Datasheet - Page 14

• Manufacturer Part Number: LM3524
• Description: Regulating Pulse Width Modulator
• Manufacturer: National Semiconductor

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Typical Applications
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
and energy is drawn from V
biased and I
Q1 opens, t
where D1 turns ON. The output current is now supplied
through L1, D1 to the load and any charge lost from C
ing t
lator, the current through L1 has a DC component plus some
ure 19 shows the inductor’s current in relation to Q1’s ON
and OFF times.
Since I
and neglecting V
The above equation shows the relationship between V
and duty cycle.
In calculating input current I
tor’s DC current, assume first 100% efficiency:
for
This equation shows that the input, or inductor, current is
larger than the output current by the factor (1 + t
Since this factor is the same as the relation between V
V
I
IN
L
. I
, I
ON
IN(DC)
= 100%, P
L
IN
is replenished. Here also, as in the step-down regu-
is again selected to be approximately 40% of I
L
+ = I
across inductor L1. During the time, t
can also be expressed as:
o
OFF
is supplied from the charge stored in C
L
, voltage V1 will rise positively to the point
OUT
−, V
SAT
FIGURE 19. Relation of Switch Timing to Inductor Current in Step-Up Regulator
IN
= P
and V
t
ON
IN
= V
D1
IN
IN(DC)
and stored in L1; D1 is reverse
o
t
OFF
, which equals the induc-
(Continued)
− V
IN
t
OFF
,
ON
, Q1 is ON
ON
o
. When
L
/t
o
IN
o
. Fig-
OFF
dur-
, V
and
).
o
14
So far it is assumed
Q1 and forward on voltage of D1. The internal power loss
due to these voltages is the average I
through either V
loss becomes I
This equation assumes only DC losses, however
ther decreased because of the switching time of Q1 and D1.
MAX
FIGURE 18. Basic Step-Up Switching Regulator
will be somewhat less due to the saturation voltage of
IN(DC)
SAT
or V
(1V).
= 100%, where the actual efficiency or
D1
. For V
MAX
DS008650-24
is then:
SAT
L
= V
current flowing, or I
D1
= 1V this power
DS008650-23
MAX
is fur-
IN
,