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LTC3884 Datasheet(PDF) 11 Page - Linear Technology |
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LTC3884 Datasheet(HTML) 11 Page - Linear Technology |
11 / 22 page LTC3874-1 11 38741f For more information www.linear/LTC3874-1 applicaTions inForMaTion Current Limit Programming To match the master controller current limit, each channel of the LTC3874-1 can be programmed separately with the ILIM and LOWDCR pins. The 4-level logic input pin ILIM setup summary is shown in Table 2. When ILIM is grounded, both channels are set to be low current range. When ILIM is tied to INTVCC, both channels are set to be high current range. Which setting should be used? For balanced load current sharing, use the same current range setting as in the master controller. Note, the LTC3874-1 does not have active clamping circuit on ITH pin for peak current limit and over current protection. Over current protection relies on the master controller to drive the ITH pin not to exceed the clamped voltage. The relationship between the current sense threshold and ITH voltage can be found in Table 3. Table 2. ILIM vs Range ILIM CHANNEL 0 CURRENT LIMIT CHANNEL 1 CURRENT LIMIT GND Range Low Range Low 1/3 INTVCC Range High Range Low 2/3 INTVCC or Float Range Low Range High INTVcc Range High Range High Table 3. Current Sense Threshold vs ITH Voltage ITH (V) CURRENT SENSE THRESHOLD (mV) LOWDCR = H LOWDCR = L RANGE = H RANGE = L RANGE = H RANGE = L 2.40 32.5 18.1 81.3 45.1 2.33 31.4 17.4 78.4 43.6 2.26 30.2 16.8 75.6 42.0 2.20 29.1 16.2 72.7 40.4 2.18 28.8 16.0 72.0 40.0 2.13 28.0 15.5 69.9 38.8 2.06 26.8 14.9 67.1 37.3 1.99 25.7 14.3 64.2 35.7 1.92 24.6 13.6 61.4 34.1 1.85 23.4 13.0 58.5 32.5 1.79 22.3 12.4 55.7 30.9 1.72 21.1 11.7 52.8 29.4 1.68 20.4 11.3 51.0 28.4 Table 3. Current Sense Threshold vs ITH Voltage (continued) ITH (V) CURRENT SENSE THRESHOLD (mV) LOWDCR = H LOWDCR = L RANGE = H RANGE = L RANGE = H RANGE = L 1.58 18.9 10.5 47.2 26.2 1.51 17.7 9.9 44.3 24.6 1.45 16.6 9.2 41.5 23.0 1.38 15.5 8.6 38.6 21.4 ISENSE+ and ISENSE− Pins ISENSE+ and ISENSE– are the inputs to the current com- parators. When the LOWDCR pin is high, their common mode input voltage range is 0V to 3.5V. ISENSE– should be connected directly to VOUT of the master controller. ISENSE+ is connected to an R • C filter with time constant one-fifth of L/DCR of the output inductor. Care must be taken not to float these pins during normal operation. Filter components, especially capacitors, must be placed close to the LTC3874-1, and the sense lines should run close together to a Kelvin connection underneath the current senseelement.TheLTC3874-1isdesignedtobeusedwith a sub-milliohm DCR value; without proper care, parasitic resistance, capacitance and inductance will degrade the current sense signal integrity, making the programmed current limit unpredictable. In Figure 2, resistor R must be placedclosetotheoutputinductorandcapacitorCcloseto the IC pins to prevent noise coupling to the sense signal. The LTC3874-1 can also be used like any conventional current mode controller by disabling the LOWDCR pin, connecting it to ground. An RC filter can be used to sense the output inductor signal and connects to the ISENSE+ pin. Its time constant, R • C, should equal to L/DCR of the output inductor. By pulling down the LOWDCR pin, the current limit increases by 2.5 times. See Table 3 for details. In these applications, the common mode operat- ing voltage range of ISENSE+, ISENSE– is from 0V to 5.5V. Table 4. Output Voltage Range vs LOWDCR Pin LOWDCR OUTPUT VOLTAGE Low 0V to 5.5V High 0V to 3.5V |
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Similar Description - LTC3884 |
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