ADUM3470CRSZ Analog Devices Inc, ADUM3470CRSZ Datasheet - Page 26

• Manufacturer Part Number: ADUM3470CRSZ
• Description: IC, DIGITAL ISOLATOR, 60NS, SSOP-20
• Manufacturer: Analog Devices Inc
• Series: iCoupler®r

Specifications of ADUM3470CRSZ

• No. Of Channels: 4
• Propagation Delay: 60ns
• Supply Current: 33mA
• Supply Voltage Range: 3V To 3.6V, 4.5V To 5.5V
• Digital Ic Case Style: SSOP
• No. Of Pins: 20
• Operating Temperature: -40°C ~ 105°C
• Inputs - Side 1/side 2: 4/0
• Number Of Channels: 4
• Isolation Rating: 2500Vrms
• Voltage - Supply: 3.3V, 5V
• Data Rate: 25Mbps
• Output Type: Logic
• Package / Case: 20-SSOP (0.200", 5.30mm Width)
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

ADuM3470/ADuM3471/ADuM3472/ADuM3473/ADuM3474
TRANSFORMER DESIGN
Transformers have been designed for use in the circuits shown
in Figure 38, Figure 39, and Figure 40 and are listed in Table 18.
The design of a transformer for the ADuM347x can differ from
some isolated dc-to-dc converter designs that do not regulate the
output voltage. The output voltage is regulated by a PWM controller
in the ADuM347x that varies the duty cycle of the primary side
switches in response to a secondary side feedback voltage, V
received through an isolated digital channel. The internal
controller has a limit of 40% maximum duty cycle.
TRANSFORMER TURNS RATIO
To determine the transformer turns ratio, and taking into
account the losses for the primary switches and the losses for
the secondary diodes and inductors, the external transformer
turns ratio for the ADuM347x can be calculated by
where:
N
V
V
V
D is the duty cycle = 0.30 for a 30% typical duty cycle, 40% is
maximum, and a multiplier factor of 2 is used for the push-pull
switching cycle.
For Figure 38, the 5 V to 5 V reference design in Table 18, with
V
For a similar 3.3 V to 3.3 V single power supply and with V
3.0 V, the turns ratio is also N
transformer turns ratio N
power applications (5 V to 5 V, 5 V to 3.3 V, and 3.3 V to 3.3 V).
For Figure 39, the circuit uses double windings and diode pairs
to create a doubler circuit; therefore, half the output voltage, V
is used in the equation.
N
V
of diodes creating a doubler circuit.
V
V
D is duty cycle which equals 0.30 for a 30% typical duty cycle,
40% is maximum, and a multiplier factor of 2 is used for the
push-pull switching cycle.
For Figure 39, the 5 V to 15 V reference design in Table 18, with
V
S
ISO
D
CC (MIN)
CC (MIN)
S
ISO
D
CC (MIN)
CC (MIN)
/N
/N
is the Schottky diode voltage drop (0.5 V maximum).
is the Schottky diode voltage drop (0.5 V maximum).
/2 is used in the equation because the circuit uses two pairs
is the isolated output supply voltage.
P
P
N
N
N
N
is the primary to secondary turns ratio.
is the primary to secondary turns ratio.
S
P
P
S
is the minimum input supply voltage.
is the minimum input supply voltage.
= 4.5 V, the turns ratio is N
= 4.5 V, the turns ratio is N
=
=
V
V
CC
CC
V
V
(
(
ISO
MIN
ISO
2
MIN
+
)
)
+
×
×
V
V
D
D
D
D
S
×
/N
×
2
2
P
= 2 can be used for the three single
S
/N
P
= 2. Therefore, the same
S
S
/N
/N
P
P
= 2.
= 3.
CC (MIN)
ISO
FB
Rev. 0 | Page 26 of 32
,
/2,
=
For Figure 40, the circuit also uses double windings and diode pairs
to create a doubler circuit; however, because a positive and negative
output voltage is created, V
where:
N is the primary to secondary turns ratio.
V
because the circuit uses two pairs of diodes creating a doubler
circuit with a positive and negative output.
V
V
factor of 2 is used for the push-pull switching cycle. A higher
duty cycle of D = 0.35 for a 35% typical duty cycle (40% is
maximum) was used in the Figure 40 circuit to reduce the
maximum voltages seen by the diodes for a ±15 V supply.
For Figure 40, the +5 V to ±15 V reference design in Table 18,
with V
TRANSFORMER ET CONSTANT
The next transformer design factor to consider is the ET constant.
This constant determines the minimum V × μs constant of
the transformer over the operating temperature. ET values of
14 V × μs and 18 V × μs were selected for the ADuM347x designs
listed in Table 18 using the following equation:
where:
V
f
in startup, and a multiplier factor of 2 is used for the push-pull
switching cycle.
TRANSFORMER PRIMARY INDUCTANCE AND
RESISTANCE
Another important characteristic of the transformer for designs
with the ADuM347x is the primary inductance. Transformers
for the ADuM347x are recommended to have between 60 μH to
100 μH of inductance per primary winding. Values of primary
inductance in this range are needed for smooth operation of the
ADuM347x pulse-by-pulse current-limit circuit, which can help
protect against build up of saturation currents in the transformer. If
the inductance is specified for the total of both primary windings,
for example, as 400 μH, the inductance of one winding is ¼ of two
equal windings, or 100 μH.
Another important characteristic of the transformer for designs
with the ADuM347x is primary resistance. Primary resistance as
low as is practical (less than 1 Ω) helps reduce losses and improves
efficiency. The dc primary resistance can be measured and specified,
and is shown for the transformers in Table 18.
SW (MIN)
ISO
D
CC (MIN)
CC (MAX)
is the Schottky diode voltage drop (0.5 V maximum).
is the isolated output supply voltage and is used in the equation
ET
N
N
CC (MIN)
is the minimum primary switching frequency = 300 kHz
P
S
is the minimum input supply voltage, and a multiplier
(
is the maximum input supply voltage.
Min
=
V
CC
= 4.5 V, the turns ratio is N
)
V
=
(
MIN
ISO
f
V
SW
+
)
CC
×
(
V
MIN
(
D
MAX
D
)
×
×
2
)
ISO
2
is used in the equation.
S
/N
P
= 5.