Circuit Note

CN-0575

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Rev. 0 | 5 of 7

ISOLATED FLYBACK CONVERTER CIRCUIT

DESIGN

An LT8304 flyback converter is used to supply the 5 V power

required by the host Raspberry Pi. This device can accept an input

voltage between 3 V and 100 V and deliver up to 24 W of isolated

output power, allowing it to easily operate in SPoE PD Classes 12

and 13. Figure 6 shows the isolated flyback circuit configuration for

the CN0575.

Figure 6. LT8304 Flyback Converter Circuit

In CN0575, the LT8304 uses a flyback transformer with a turns

ratio of 6:1 and a primary inductance of 40 μH to produce the

isolated power. As recommended in the LT8304 data sheet, an RC

and Schottky/Zener snubber is additionally connected across the

primary windings of the transformer to protect the device from high

voltage spikes generated by leakage inductance. A diode is used

to provide rectification on the secondary side, and is rated for a

forward current of 10 A and a reverse voltage of 45 V. Three 100 μF

capacitors are used to reduce the ripple of the output voltage. This

combination of components allows the design to meet the power

requirements for Class 12 and Class 13 devices, as well as support

the operating requirements of the LT8304. For the general design

procedure in selecting these components, refer to the LT8304 data

sheet.

The output voltage of the LT8304 is set by an external feedback

resistor connected between the SW and RFB pins; the value of

which is calculated using Equation 2:

RFB=RREF×NPS×VOUT+VF

VREF

(2)

where:

RFB is the feedback resistance in kΩ.

RREF is the reference resistance in kΩ.

VREF is the internal reference voltage in V.

NPS is the transformer effective primary-to-secondary turns ratio.

VOUT is the output voltage.

VF is the output diode forward voltage.

For the required output voltage of 5 V, a 6:1 transformer turns ratio,

1 V VREF, 10 k RREF, and an output diode forward voltage of 0.3 V,

the equation results in an RFB value of 318 kΩ; the closest standard

1% value of 316 kΩ is used.

As shown in Figure 7, these specifications allow the LT8304 to

supply more than 2 A of load current — more than adequate

for Classes 12 and 13, which only require 1.68 A and 1.54 A,

respectively for 5 V.

Figure 7. LT8304 Load Regulation for Class 12 and Class 13 Operation

MINIMAL FIELD DEVICE

The CN0575 can be used as a basic temperature sensing field

device using an on-board ADT75 digital temperature sensor. This

device is ideal for a minimal complete temperature monitoring

application. The only external components required for its operation

are a decoupling capacitor for the supply pin, and pull-up resistors

for the I2C lines.

In addition to the ADT75, the CN0575 also includes a push button

and an LED as basic GPIO devices that can be used to provide

remote user input/output. Additional Raspberry Pi peripheral hard-

ware devices can be used to prototype other field device applica-

tions once basic connectivity has been established.

COMMON VARIATIONS

While the CN0575 is intended to operate in either SPoE PD classes

other than 12 or 13, the LTC9111 PD controller supports all PD

classes from 10 and 15. The isolated power supply can then be

optimized for the power level required by the end application.

For applications not requiring galvanic isolation, or where the isolat-

ed circuitry is low power, the ADIN1110 PHY can be capacitively

coupled to the MDI. In this topology, the isolation barrier can be

placed between the microprocessor and sensor, ADC, or DAC

using a digital isolator such as the ADuM5411.

For applications such as field switches and building/room control-

lers where the processor has an integrated MAC, the ADIN1100

can be used instead. This is a 10BASE-T1L PHY device with

MII, RMII, and RGMII MAC drivers for development with existing

interfaces.

CIRCUIT EVALUATION AND TEST

This section covers the setup and procedure for evaluating the

CN0575 circuit evaluation board using a Raspberry Pi. For com-

plete details, refer to the EVAL-CN0575-RPIZ User Guide.