■ FIGURE 3.
SMPS WITH 555
QI remember seeing a SMPS (switched-mode power supply) boost converter based on a 555 (two
actually) awhile back. I was
wondering if you had the schematic
still. I am challenged to get the classic
timer chip to do a modern day
function. I want to see how much
efficiency I can get out of a simple
design. There is one thing that makes
the 555 desirable in some sense; the
CMOS version can go down to 1.8V
(don’t quote me on this) or so.
However, the TTL version can go
down to 3V which is good for battery
powered boost converters.
AActually, the TTL 555 is only specified to work from 5V to 15V. If you use it at hree volts, you do it at
your own risk because it is not
guaranteed to work. Most CMOS
555s are specified to work from 2V
to 15V, but the LMC555 is specified
to work from 1.5V to 15V.
Figure 3. IC1 sets the frequency,
triggering IC2 which is PWM’d
by feedback to the control pin
(pin 5). Q1 limits the peak
current by resetting IC2.
Since this is a similar circuit, I will
use the design equations for the
Ton/Toff = (Vo+Vd-Vi)/(Vi-Vsat)
where: Vo = output V = 12
If the frequency (F) is 50 kHz,
then Ton + Toff = 1/50K = .02 mS.
For simplicity, let Ton = 9*Toff = 18
µs. This ratio is set by R1/R2.
The capacitor, C1, of IC1 is =
1.44/F/R1 = 2,880 pF when R1
Let C = 2,700 pF (an available
value), so then F = 53 kHz.
Ip = 2*Io*(Ton+Toff)/Toff =
2*0.025(.305+1) = 0.065A when
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