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ISL62773 Datasheet(PDF) 30 Page - Renesas Technology Corp |
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ISL62773 Datasheet(HTML) 30 Page - Renesas Technology Corp |
30 / 37 page ISL62773 FN8263 Rev 1.00 Page 30 of 37 Dec 16, 2015 Resistor Current-Sensing Network Figure 30 shows the resistor current-sensing network for a 3-phase solution. Each inductor has a series current sensing resistor, Rsen. Rsum and Ro are connected to the Rsen pads to accurately capture the inductor current information. The Rsum and Ro resistors are connected to capacitor Cn. Rsum and Cn form a filter for noise attenuation. Equations 23 through 25 give the VCn(s) expression. Transfer function ARsen(s) always has unity gain at DC. Current-sensing resistor Rsen value does not have significant variation over-temperature, so there is no need for the NTC network. The recommended values are Rsum = 1kΩ and Cn = 5600pF. Overcurrent Protection Refer to Equation 1 on page 17 and Figures 24, 28 and 30; resistor Ri sets the droop current, Idroop. Tables 1 and 2 on page 19 show the internal OCP threshold. It is recommended to design Idroop without using the Rcomp resistor. For example, the OCP threshold is 1.5V on the IMON pin. This translates to 45µA of Isum current or 56.25µA of droop current. Idroop is designed to be 45µA at full load, so the OCP trip level is 1.25x of the full load current. For inductor DCR sensing, Equation 26 gives the DC relationship of Vcn(s) and Io(s): Substitution of Equation 26 into Equation 1 gives Equation 27: Therefore: Substitution of Equation 18 and application of the OCP condition in Equation 28 gives Equation 29: Where Iomax is the full load current and Idroopmax is the corresponding droop current. For example, given N = 3, Rsum = 3.65kΩ, Rp = 11kΩ, Rntcs = 2.61kΩ, Rntc = 10kΩ, DCR = 0.9mΩ, Iomax = 65A and Idroopmax = 45µA. Equation 29 gives Ri = 359Ω. For resistor sensing, Equation 30 gives the DC relationship of Vcn(s) and Io(s). Substitution of Equation 30 into Equation 1 gives Equation 31: Therefore: Substitution of Equation 32 and application of the OCP condition in Equation 28 gives Equation 33: Where Iomax is the full load current and Idroopmax is the corresponding droop current. For example, given N = 3, Rsen =1mΩ, Iomax = 65A and Idroopmax = 45µA, Equation 33 gives Ri = 481Ω Load Line Slope See Figure 16 for load line implementation. For inductor DCR sensing, substitution of Equation 27 into Equation 2 gives the load line slope expression in Equation 34: For resistor sensing, substitution of Equation 31 into Equation 2 gives the load line slope expression in Equation 35: FIGURE 30. RESISTOR CURRENT-SENSING NETWORK CN RSUM RO DCR L DCR L RSUM RO PHASE2 PHASE3 IO DCR L PHASE1 RO RSUM RI ISUM+ ISUM- VCN RSEN RSEN RSEN + - VCn s Rsen N ------------- Io s ARsen s = (EQ. 23) ARsen s 1 1 s sns ------------- + ----------------------- = (EQ. 24) Rsen 1 Rsum N --------------- Cn ----------------------------- = (EQ. 25) VCn Rntcnet Rntcnet Rsum N --------------- + ------------------------------------------ DCR N ------------- Io = (EQ. 26) Idroop 1 Ri ----- Rntcnet Rntcnet Rsum N --------------- + ------------------------------------------ DCR N ------------- Io = (EQ. 27) Ri Rntcnet DCR Io NRntcnet Rsum N --------------- + Idroop ---------------------------------------------------------------------------------- = (EQ. 28) Ri Rntcs Rntc + R p Rntcs Rntc Rp ++ ---------------------------------------------------- DCR Iomax N Rntcs Rntc + R p Rntcs Rntc Rp ++ ---------------------------------------------------- Rsum N --------------- + Idroopmax ----------------------------------------------------------------------------------------------------------------------------- = (EQ. 29) VCn Rsen N ------------- Io = (EQ. 30) Idroop 1 Ri ----- Rsen N ------------- Io = (EQ. 31) Ri Rsen Io NIdroop --------------------------- = (EQ. 32) Ri Rsen Iomax NIdroopmax -------------------------------------- = (EQ. 33) LL Vdroop Io ------------------- Rdroop Ri ------------------- Rntcnet Rntcnet Rsum N --------------- + ------------------------------------------ DCR N ------------- == (EQ. 34) LL Vdroop Io ------------------- Rsen Rdroop NRi --------------------------------------- == (EQ. 35) |
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