Is o la t e d Tra n s fo rm e r Drive r
fo r P CMCIA Ap p lic a t io n s
MAX845
1
6
5V
IN
D1
V
CC
0.33µF
MBR0520L*
78L05
MAX845
3
4
N.C.
IN
OUT
FS
GND
0.33µF
8
D2
SD
HALO
TGM-030P3
-5V
100mA
GND1 GND2 N.C.
2
7
5
*1N914 POSSIBLE FOR LOWER CURRENTS
Figure 5. Low-Noise Supply
remains in the shutdown mode until the voltage is high
enough to allow proper operation.
Is o la t e d Da t a Co n ve rs io n
Almost any serial-interface device is a candidate for
operation across an isolation barrier; Figure 6 illustrates
one example. The MAX176 analog-to-digital converter
(ADC) operates from +5V and -12V supplies, provided
by the multiple-tapped secondary and linear regulators.
This circuit easily supplies several hundred milliwatts of
additional isolated power for signal conditioning, multi-
plexing, or sensors. A +12V supply can be generated
by adding two more diodes from the ends of the sec-
ondary, and a -5V supply can be generated by con-
Is o la t e d 4 m A t o 2 0 m A An a lo g In t e rfa c e
The 4mA to 20mA current loop is widely used in the
process-control industry for transducer and actuator
control signals. These signals are commonly referred to
a distant ground that may be at a considerably higher
voltage with respect to the local ground. The circuit in
Figure 8 generates an isolated 4mA to 20mA current
from a 5V supply.
1
3
necting additional diodes to the ⁄ and ⁄ tap points on
4
4
Is o la t e d RS -4 8 5 Da t a In t e rfa c e
The MAX845 power-supply transformer driver also pro-
vides isolated power for RS-485 data-interface applica-
tions. The application circuit of Figure 9 combines the
MAX845 with a low-dropout linear regulator, a trans-
former, several high-speed optocouplers, and a Maxim
RS-485 interface device.
the secondary. The MAX845 supplies sufficient power
for almost any Maxim ADC.
Te le p h o n e -S u b s c rib e r-Lin e P o w e r S u p p ly
The standard telephone system is placed in the “off
hook” state by placing a load on the line to signal the
central office that service is requested. Normally, most of
this power is wasted in a load resistor, but some systems
can benefit from utilizing this free power. Figure 7 shows
one way to transform the wasted telephone power to an
isolated, regulated 5V at currents up to 50mA.
Is o la t e d RS -2 3 2 Da t a In t e rfa c e
The MAX845 is ideal for isolated RS-232 data-interface
applications requiring more than four transceivers. Its
750mW output power capability enables it to drive 10
transceivers simultaneously. Figure 10 shows the typi-
cal application circuit for a complete 120kbps isolated
RS-232 data interface. This figure also shows how the
Sharp PC417 optocouplers can be replaced by the
lowe r-c os t Qua lity Te c hnolog ie s 4N25 d e vic e s to
achieve data transfer rates up to 19.2kbps.
Be c a us e the te le p hone line is a hig h-imp e d a nc e
source, there can be a start-up problem with any DC-
to-DC converter; when the line voltage is low during
start-up, the frequency can be too low for the trans-
former, causing it to saturate. This excess saturation
current can keep the voltage from climbing to normal
operating levels. Thus the purpose of Q1, Q2, and the
a s s oc ia te d re s is tors is to e ns ure tha t the MAX845
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7
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
