AN296 Silicon_Laboratories, AN296 Datasheet - Page 8

• Manufacturer Part Number: AN296
• Description: Using THE Si3400 AND Si3401 POE PD Controllers IN Isolated AND Non-isolated Designs
• Manufacturer: Silicon_Laboratories

AN296
3.6. Signature Resistors and Capacitors and Component Selection Criteria
The Si3400/01 is designed to meet 802.3af signature requirements with RDET (connected to pin RDET) = 25.5 kΩ,
±1%. Recommended resistor values for RCLASS (connected to pin RCL) are listed in Table 2.
The voltage across these resistors is limited so that 0603 or larger surface mount resistors may be used.
CSIG should be a 100 V X7R type surface mount with tolerance of ±10%. While this type of capacitor exhibits a
strong voltage and temperature dependence, the 50–120 nF requirement of 802.3af will be met.
3.7. Input Filter
802.3af requires that the input filter capacitor be greater than 5 µF. Additionally, the ripple at the RJ-45 input at the
switching frequency of approximately 350 kHz must be less than 150 mV. To reduce the chance of conducted or
radiated emissions due to induced common-mode voltage on the Ethernet cable pairs, it may be desirable to
further reduce the input ripple.
To maintain the >5 µF input capacitance, an aluminum electrolytic capacitor, such as a Sanyo 100ME12AX or
Panasonic EEUFC120, is used. This capacitor has an ESR of up to 0.4 Ω and results in excessive input ripple
because of the ripple current from the switcher, which can be as much as 3 A. To keep input ripple down, it is
recommended that additional X7R surface mount capacitors be used in parallel (for example, a 1 µF, 100 V, X7R
1210 capacitor with an ESR of less than 0.6 Ω is available from several vendors).
As will be explained in the section about surge protection, the input filter capacitor also helps absorb surge current.
If the input capacitor is too large, the reaction time of the hot swap switch current limit function increases. An input
filter capacitor of 15 µF total has been found to be a good compromise.
8
Class
Table 2. RCLASS Recommended Resistor Values
0
1
2
3
4
Reserved—treat as Class 0
Power Level that the PSE
Must Support
15.4 W
15.4 W
4.0 W
7 W
Rev. 0.8
RCLASS ±1%
Open circuit
69.8 Ω
45.3 Ω
30.9 Ω
127 Ω