Evaluation Board User Guide UG-403
Rev. 0 | Page 7 of 12
DOUBLE SUPPLY
The ADuM4070 switching regulator on this evaluation board can
be configured for double-supply output (for more information
about the transformer, see the Transformer Selection section).
When the ADuM4070 is set to double-supply mode, the feedback
divider resistors should be configured as described in Table 1
and the output mode resistors should be configured as described
in Table 2.
By default, the double-supply configuration provides a regulated
15 V output and an unregulated 7.5 V output, which are isolated
from the 5 V primary input supply. The double supply is capable
of delivering up to 140 mA to external loads. The double supply
can be reconfigured as 12 V (regulated) and 6 V (unregulated)
secondary isolated supplies or as positive and negative supplies.
See the Other Secondary Isolated Supply Configurations section
for more information.
TERMINALS
In the double-supply configuration, the EVAL -ADuM4070EBZ
board has terminal blocks on Side 1 (the primary/power supply
input side) and Side 2 (the secondary/power supply output side).
An 8.0 mm isolation barrier separates Side 1 from Side 2. Table 5
summarizes the functions of the terminal connections. These
connections are described in detail in the Input Power Connections
and Output Power Connections sections.
Input Power Connections
Connect 5 V to P1 (labeled 5V), and connect the negative end
of the supply to P2 (labeled GND). These are the only off-board
connections required for the board to function in double-supply
configuration.
V
DD1
is the ADuM4070 transformer driver supply, and V
DDA
is
the primary supply voltage (see the ADuM4070 data sheet for
additional information). V
DD1
and V
DDA
are bypassed by a 47 µF
ceramic capacitor (C3) and a 0.1 µF local bypass capacitor (C8)
located close to the ADuM4070. R4, R3, C4, and C5 are provided
for an optional and unpopulated snubber, which can be used to
reduce radiated emissions.
Output Power Connections
Output loads can be connected to P9 (labeled OUT2) and P7
(labeled OUT1), which are the isolated, unregulated 7.5 V and
regulated 15 V output supplies, respectively. Connect the return
of the load to P10 and P8, which are labeled ISO_GND.
Side 2 is powered by the secondary isolated 15 V supply. The
ADuM4070 internal low dropout regulator converts this voltage
to 5 V. The regulated 5 V supply powers the ADuM4070 secondary
side. Therefore, the ADuM4070 V
REG
pin is 15 V, and the V
DD2
pin
is 5 V. The 15 V supply connects to P7 (labeled OUT1). The
7.5 V supply connects to P9 (labeled OUT2). The Side 2 ground
reference is tied to P10. See the ADuM4070 data sheet for an
explanation of the double-supply theory of operation. Figure 8
through Figure 11 show efficiency curves for the double supply
with the 15 V or 12 V isolated output supply connected to V
REG
.
Powering V
REG
from the Unregulated 7.5 V Supply
V
REG
can be powered from the unregulated 7.5 V supply, which
results in higher efficiency. However, when the 15 V supply is
unloaded, the unregulated 7.5 V supply is approximately 3 V,
which is insufficient for powering the ADuM4070 secondary
side. This causes the double supply to run open-loop, leaving
the 15 V supply unregulated. Using 15 V for V
REG
ensures that
the secondary side of the ADuM4070 powers up under light
load conditions. Move the 0 Ω, 0603 resistor from R8 to R9 to
power Side 2 from the 7.5 V supply.
Care must be taken to avoid driving an output pin because this
can result in permanent damage to the ADuM4070.
Table 5. Double-Supply Terminal Function Descriptions
Terminal Label Description
P1 5V Side 1—5 V primary input supply
P2 GND Side 1—ground reference
P9 OUT2
Side 2—7.5 V secondary isolated supply
(unregulated)
P10 ISO_GND Side 2—ground reference
P7 OUT1
Side 2—15 V secondary isolated supply
(regulated)
P8 ISO_GND Side 2—ground reference
TRANSFORMER SELECTION
The E VA L-ADuM4070EBZ supports multiple transformer
options. In the double-supply configuration, the board must be
equipped with a Halo Electronics TGRAD-590V8LF (T2) or a
Coilcraft CR7984-CL (T3) 1CT:3CT turns ratio transformer
(see the ADuM4070 data sheet for details on transformer
selection with the ADuM4070). Figure 8 and Figure 10 show
the efficiency of the supply using a 1CT:3CT Coilcraft transformer
(CR7984-CL) and a 1CT:3CT Halo Electronics transformer
(TGRAD-590V8LF), respectively, at various switching frequencies.
Figure 9 shows how temperature affects efficiency.
SWITCHING FREQUENCY OPTIONS
The resistance connected from the ADuM4070 oscillator
control pin (OC) to ground sets the double-supply switching
frequency. Figure 3 shows the relationship between this
resistance and the converter switching frequency. The EVA L-
ADuM4070EBZ can be configured with 0 Ω, 0603 resistors to
set one of four preset switching frequencies. Table 6 lists the
switching frequencies that can be selected by short- or open-
circuiting R12, R13, and R14.
The user can select a different switching frequency by removing
R12 and R13 and then choosing R18 based on Figure 3. The board
is configured for the 500 kHz setting by default. Figure 8 and
Figure 10 show how the switching frequency affects the efficiency
of the supply using a Coilcraft transformer (CR7984-CL) and a
Halo Electronics transformer (TGRAD-590V8LF), respectively.
