UG-197
Evaluation Board User Guide
Rev. 0 | Page 10 of 16
OTHER SECONDARY ISOLATED SUPPLY
CONFIGURATIONS
The double supply can be configured for 12 V regulated and 6 V
unregulated secondary isolated supplies by short-circuiting R12
with a 0 Ω resistor for R25. The regulated supply voltage is set
by the fraction of it that is fed back to the ADuM3471 via the
voltage divider comprising R12, R13, R14, and R25. The voltage
at the feedback pin is 1.25 V. With R25 open-circuited, the
ADuM3471 feedback voltage is approximately 1.25 V if VISO2
is 15 V. When R25 is short-circuited, the feedback voltage is
approximately 1.25 V if VISO2 is 12 V (see the ADuM347x data
sheet for more details on setting the secondary isolated output
supply voltage). Figure 15 shows the efficiency curves for both
output settings at 500 kHz with the Coilcraft transformer.
Positive and Negative Outputs
The double supply can be set up as a positive and negative ±15 V
supply by changing the transformer to a turns ratio CT1:CT5
transformer (see the ADuM347x data sheet for more information
on these transformers). Other changes begin with removing the
0 Ω resistors from R24 and R22 to R23 and R21. Short-circuiting
R23 instead of R24 makes the +7.5 V/6 V pin of J6 become the
−15 V supply. Short-circuiting R21 instead of R22 connects the
transformer center tap to the ground plane instead of the node
where L3, C20, and C27 are connected. Figure 16 shows which
resistors should be short-circuited and open-circuited for the
double supply or positive and negative supply configurations.
Note that the negative supply is unregulated. The positive and
negative supply can be set for ±12 V instead of ±15 V by short-
circuiting R25.
Whereas the +15 V output can be regulated, the same problems
with regulation can happen as described in the Powering VREG
from the Unregulated 7.5 V section. In addition, the −15 V
supply can vary over a wide range because it is unregulated and
influenced by the changes that happen on the +15 V output.
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016
DOUBLE SUPPLY
POSITIVE AND NEGATIVE SUPPLY
Figure 16. Double Supply Configuration with 0 Ω Resistors (Red)
SCHEMATIC
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017
Figure 17. Double Supply Schematic
