Classifcation Load Current of LTC4278

Question

Hello; 
 we got some question regarding the Classifcation Load Current of LTC4278; could you kindly help on it; please? 
 As shown in figure 1; if the PSE input port voltage is between 12.5V to On; the Classifcation Load Current will constantly be there? 
 In our test shown in Figure 3; we found when port voltage <=33V; 40+ mA current into LTC4278; which cause the chip temp rising to 85C @ 25C ambient temp. 
 If so; any method to relieve this issue? Many thanks! 
 
 Figure 3: Red - Input voltage; Purple - current 
 
 Regards; 
 Bell

EHorsma · Accepted Answer

Hello Bell, 
 The LTC4278 and all PD Controllers consume power during classification, up to approximately 0.7W for a Class 4 Signature. How does the IEEE standard minimize this power loss? An IEEE 802.3at-compliant PSE will only provide a Class event for between 6ms to 30ms, and there&rsquo;s only up to two Class events with a 6ms to 12ms Mark event between them. This restriction limits the heat generated within the IC. The IEEE standard defines PD behavior with a state diagram. While the Class event voltage range is defined as 14.5V to 20.5V, the PD State Machine exits its Class Event state with two conditions: (1) the port voltage decreases to the Mark threshold, or (2) when the port voltage increases to Vport_PD, defined as 37V to 57V. 
 So, what&rsquo;s happening in the waveform shared here? The PSE is successfully detecting, classifying, and powering on the port. However, the PSE&rsquo;s supply voltage is only 33V. This is below the allowed PSE voltage, which is 44V minimum for 802.3af (Type 1, 13W) PSEs and 50V minimum for 802.3at (25.5W, Type 2) PSEs. 30V is below VPort_PD minimum, so the PD never exits Class and turns on. This is not a valid state for an IEEE 802.3at system. The PSE voltage should be increased to be compliant with the desired power level. 
 
 Best Regards, 
 Eric)

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Classifcation Load Current of LTC4278