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USBLC6-2P6 Datasheet(PDF) 4 Page - STMicroelectronics |
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USBLC6-2P6 Datasheet(HTML) 4 Page - STMicroelectronics |
4 / 11 page USBLC6-2 4/11 TECHNICAL INFORMATION 1. SURGE PROTECTION The USBLC6-2 is particularly optimized to perform surge protection based on the rail to rail topology. The clamping voltage VCL can be calculated as follow : VCL+ = VBUS + VF for positive surge VCL- = - VF for negative surge with: VF = VT + Rd.Ip (VF forward drop voltage) / (VT threshold voltage) We assume that the value of the dynamic resistance of the clamping diode is typically: Rd = 0.5Ω and VT = 1.2V. For an IEC61000-4-2 surge Level 4 (Contact Discharge: Vg=8kV, Rg=330Ω), VBUS = +5V, and if in first approximation, we assume that : Ip = Vg / Rg = 24A. So, we find: VCL+ = +17V VCL- = -12V Note: the calculations do not take into account phenomena due to parasitic inductances. 2. SURGE PROTECTION APPLICATION EXAMPLE If we consider that the connections from the pin VBUS to VCC and from GND to PCB GND are done by two tracks of 10mm long and 0.5mm large; we assume that the parasitic inductances Lw of these tracks are about 6nH. So when an IEC61000-4-2 surge occurs, due to the rise time of this spike (tr=1ns), the voltage VCL has an extra value equal to Lw.dI/dt. The dI/dt is calculated as: dI/dt = Ip/tr = 24 A/ns The overvoltage due to the parasitic inductances is: Lw.dI/dt = 6 x 24 = 144V By taking into account the effect of these parasitic inductances due to unsuitable layout, the clamping voltage will be : VCL+ = +17 + 144 = 161V VCL- = -12 - 144 = -156V We can reduce as much as possible these phenomena with simple layout optimization. It’s the reason why some recommendations have to be followed (see paragraph “How to ensure a good ESD protection”). Figure 6: ESD behavior; parasitic phenomena due to unsuitable layout Lw VI/O ESD SURGE GND I/O +VCC VBUS VF Lw di dt Lw di dt V+ = CL V +V +Lw BUS F di dt surge >0 di dt surge <0 V- = CL -V -Lw F t tr=1ns VV CC F + Lw di dt VCL+ POSITIVE SURGE 183V -Lw di dt t tr=1ns - VF VCL- NEGATIVE SURGE -178V |
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