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PTB48540CAS Datasheet(PDF) 6 Page - Texas Instruments |
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PTB48540CAS Datasheet(HTML) 6 Page - Texas Instruments |
6 / 13 page Application Notes continued For technical support and further information visit http://power.ti.com D P s s a l Ce g a s U ) s t t a w ( d n a m e D D P m a r g o r P s s a l C e c n a t s i s e R l a n r e t x E r o t s i s e R n i Mx a M 0t l u a f e D4 4 . 05 9 . 2 1k 2 4 . 4 Ω e n o N 1l a n o i t p O4 4 . 04 8 . 30 5 9 Ω k 1 2 . 1 Ω 2l a n o i t p O4 8 . 39 4 . 63 4 5 Ω 9 1 6 Ω 3l a n o i t p O9 4 . 65 9 . 2 10 6 3 Ω 2 9 3 Ω 4d e w o l l A t o N— e s U e r u t u F —2 5 2 Ω 7 6 2 Ω Table 1-1; PD Class Programming Resistance for the different types of PD class defined in the IEEE 802.3af Standard. The power modules support the PD classification protocol with a default ‘Class 0’ signature. ‘Class 0’ corresponds to a 4.42-k Ω programming resis- tance, which is set by an internal resistor located between the ‘Class A’ (pin 12) and ‘Class B’ (pin 11) terminals. By placing an external resistor across the Class A/B pins (in parallel with the internal 4.42-k Ω resistor) the power module can be made to communicate one of the alternate classifications to the PSE. Consult Table 1-1 for the external resistance values. Under-Voltage Lockout The UVLO prevents the internal DC/DC converter from seeing an input voltage until the voltage applied to either the ‘Data Line A/B’ or ‘Spare Line A/B’ pair of Ethernet connections approaches 42 V. The UVLO threshold correlates to a voltage between the ‘Detect B’ (pin 6) and ‘Class B’ (pin 11) terminals of approximately 39.3 V. Only after the voltage applied from the Ethernet is above the UVLO threshold is the module’s internal bus voltage allowed to rise. The internal bus powers the DC/DC converter and can be measured between the ‘Detect B’ (pin 6) and ‘Input Ref’ (pin 4) terminals. Input Capacitance In accordance with the IEEE 802.3af Standard, the PTB48540 power modules provide an input capacitance of 0.1 µF to the PSE when communicating the required detection and classification signatures. Once fully pow- ered (Vin ≥42 V), the PSE will see the combined input capacitance of the EMI filter and DC/DC converter; approximately 7 µF. This capacitance is sufficient to operate the module’s internal DC/DC converter, and satisfies the 5 µF minimum capacitance required by the IEEE 802.3af Standard. For improved hold-up capability, this input capacitance can be increased with an external capacitor. Connect the anode of the external capacitor to ‘Detect B’ (pin 6), and the cathode to the ‘Input Ref’ (pin 4). During power up the power interface IC limits the inrush current for charging the input capacitance. Additional capacitance increases the power dissipation in the IC. For this reason the maximum recommended value of external capacitance is 220 µF (100-V electrolytic). Startup Startup of the module in a PoE application consists of a complex process of handshaking states between the module and PSE. During the PD detection state the PSE uses a low voltage (<10 V) to detect the module’s “valid device” resistance signature. This is followed by the classification detection state where the PSE applies a voltage of 15 V to 20 V to detect the module’s PD class. The PSE con- tinues to raise the input voltage, but the input voltage to the internal DC/DC converter is held at zero until the voltage from the PSE approaches 42 V. At an input voltage of 42 V or higher, the module’s power interface IC allows the internal bus voltage to rise using a limited amount of inrush current. Approximately 50 ms after the DC/DC converter input filter capacitors are fully charged, the module is able to produce a regulated output voltage. Converter Over-Current Protection The internal DC/DC converter has inherent protection against an output load fault. Whenever its load current exceeds the over-current protection threshold (see speci- fication table) the converter momentarily turns its output off. After a short period (<100 ms), the regulator will attempt to power up again by executing a soft-start power up. The converter will continue in a successive cycle of shutdown, followed by soft-start power up until the load fault is removed. When the DC/DC converter is powered from just the Ethernet source, a load current above its rated output (but below its over-current threshold) will likely activate the over-current protection offered by the power interface IC. This is especially at input voltages lower than 48 V. Ethernet Over-Current Protection Protection is also provided for the Ethernet power source equipment (PSE). In event of a fault across the module’s internal bus, the onboard power interface IC limits the maximum current that may be drawn from the PSE to no less than 405 mA. This prevents the DC/DC con- verter from drawing excessive input current and also safeguards against external faults that may occur across the ‘Detect B’ (pin 6) and ‘Input Ref’ (pin 4) terminals. Note: These terminals can be used to add capacitance to the module’s internal bus. Load faults applied to the DC/DC converter’s output will most often trigger the power interface IC’s protec- tion mechanism prior to activating the converter’s own current limit threshold. In these instances the power interface IC will completely shut down the module’s inter- nal bus. This is a latched condition. It is reset by the PoE source when it attempts another power-up cycle after it senses loss of the PD’s Maintain Power Signature (10 mA). PTB48540 Series Not Recommended for New Designs |
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