______________Component Selection
Transformer
The MAX845 drives any transformer that has a center-
tapped primary and a saturation rating of at least 5V-µs
(ET product) per side. The oscillator frequency varies
linearly with V
CC
. The transformer is most vulnerable to
saturation at the minimum frequency, because the
switches are on for the longest period. At V
CC
= 4.5V,
the transformer must withstand at least:
1 1
4.5V x ———–——— x
—
= 5V-µs
450kHz min 2
And at V
CC
= 5.5V, the transformer must withstand
at least:
1 1
5.5V x ———–——— x — = 5V-µs
550kHz min 2
Thus, the required ET product is constant over the
entire 5V ±10% range.
Select either a toroid or a gapped core. Although some
applications will require custom transformers, many
can use standard transformer designs, such as those
listed in Table 1. Some of these manufacturers have
standard products designed for the MAX845, while
some have standard products that can be adapted for
specific customer requirements. Table 1 also lists some
suppliers of suitable magnetic cores.
An ungapped toroid core must never be allowed to sat-
urate. An empirical way to measure a toroid’s ET prod-
uct is to wind 20 turns on the bare core and observe
the current waveform on an oscilloscope while driving
the winding with a function generator. Generate a 50%
duty-cycle square wave at a test frequency of 500kHz,
with no DC offset. Gradually increase the driving volt-
age until the waveform suddenly begins to draw more
current. At this point, the core is saturating, so reduce
the driving voltage until the core just barely stops satu-
rating. The ET product indicated is simply the maxi-
mum voltage that can be applied without saturation,
multiplied by 1µs (the time of half of the period of the
input signal). Because the ET product varies linearly
with the number of turns, this test winding can be
scaled up or down to act as a suitable primary for that
particular core.
A gapped core, such as a bobbin or drum core, is not
limited by ET product, but rather by inductance and
winding resistance. The primary inductance must be
high enough to prevent excessive current flow under
light-load conditions, yet low enough that it can be
wound on the core. Good results can be achieved by
using a primary inductance between 50µH and 200µH.
Calculate the number of turns required by using the
manufacturer’s A
L
(inductance per turn squared) value,
or measure a test winding with an inductance meter.
Inductance varies with the square of the number of turns.
While most MAX845 applications will use a toroid trans-
former for highest efficiency and lowest EMI, there may
be applications that can utilize less expensive trans-
formers, such as E, I, or U-shaped cores, magnetic
bobbins, or etched windings on a printed circuit board.
Table 1 lists some transformer and core suppliers who
can assist with your magnetics design.
The secondary or secondaries can be scaled to produce
whatever output is required for the application at hand,
taking into account the rectifier topology to be used and
the forward voltage loss of the diodes selected.
Step-by-Step Transformer
Design Procedure
Before starting the design, determine the minimum and
maximum output voltage requirement, the minimum
and maximum load current, the physical size con-
straints, and the cost budget.
1) Select an appropriate core shape and material from
core vendors’ data sheets; trade-off EMI vs. space
and cost. Since the MAX845’s output waveform is a
square wave, it is rich in harmonics, so choose a
material with low losses at up to several MHz.
MAX845
Isolated Transformer Driver
for PCMCIA Applications
12 ______________________________________________________________________________________
Table 1. Transformer and Transformer-Core
Suppliers
TRANSFORMERS TRANSFORMER CORES
Halo Electronics
Phone: (415) 969-7313
FAX: (415) 367-7158
Ask for MAX845 Transformer
Magnetics Inc.
Phone: (412) 282-8282
FAX: (412) 282-6955
Coilcraft
Phone: (708) 639-6400
FAX: (708) 639-1469
Ask for MAX845 Transformer
Fair-Rite Products
Phone: (914) 895-2055
FAX: (914) 895-2629
BH Electronics
Phone: (612) 894-9590
FAX: (612) 894-9380
Ask for MAX845 Transformer
Philips Components
Phone: (401) 762-3800
FAX: (401) 762-3805, ext. 324
MMG (Magnetic Materials Group)
Phone: (201) 345-8900
FAX: (201) 345-1172
Amidon Associates
Phone: (714) 850-4660
FAX: (714) 850-1163
Sumida USA
Phone: (708) 956-0666
FAX: (708) 956-0702
