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LTC4211 Datasheet(PDF) 21 Page - Linear Technology |
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LTC4211 Datasheet(HTML) 21 Page - Linear Technology |
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21 / 36 page  21 LTC4211 4211f Table 2. tSLOWCOMP vs CFILTER CFILTER tSLOWCOMP 100pF 82 µs 220pF 156 µs 330pF 224 µs 470pF 310 µs 680pF 440 µs 820pF 527 µs 1000pF 638 µs For proper circuit breaker operation, Kelvin-sense PCB connections between the sense resistor and the LTC4211’s VCC and SENSE pins are strongly recommended. The drawing in Figure 11 illustrates the correct way of making connections between the LTC4211 and the sense resistor. PCB layout should be balanced and symmetrical to mini- mize wiring errors. In addition, the PCB layout for the sense resistor should include good thermal management techniques for optimal sense resistor power dissipation. The power rating of the sense resistor should accommo- date steady-state fault current levels so that the compo- nent is not damaged before the circuit breaker trips. Table 4 in the Appendix lists sense resistors that can be used with the LTC4211’s circuit breaker. IRC-TT SENSE RESISTOR LR251201R010F OR EQUIVALENT 0.01 Ω, 1%, 1W CURRENT FLOW TO LOAD CURRENT FLOW TO LOAD TO VCC TO SENSE TRACK WIDTH W: 0.03" PER AMP ON 1 OZ COPPER W 4211 F11 Figure 11. Making PCB Connections to the Sense Resistor OPERATIO CALCULATING CIRCUIT BREAKER TRIP CURRENT For a selected RSENSE value, the nominal load current that trips the circuit breaker is given by Equation 10: I V R mV R TRIP NOM CB NOM SENSE NOM SENSE NOM () () () () == 50 (10) The minimum load current that trips the circuit breaker is given by Equation 11. I V R mV R TRIP MIN CB MIN SENSE MAX SENSE MAX () () () () == 40 (11) where RR R SENSE MAX SENSE NOM TOL () ( ) • =+ 1 100 SENSE RESISTOR CONSIDERATIONS The fault current level at which the LTC4211’s internal electronic circuit breaker trips is determined by a sense resistor connected between the LTC4211’s VCC and SENSE pins and two separate trip points. The first trip point is set by the SLOW COMP’s threshold, VCB(SLOW) = 50mV, and occurs should a load current fault condition exist for more than 20 µs. The current level at which the electronic circuit breaker trips is given by Equation 8: I V R mV R TRIP SLOW CB SLOW SENSE SENSE () () == 50 (8) The second trip point is set by the FAST COMP’s threshold, VCB(FAST) = 150mV, and occurs during fast load current transients that exist for 300ns or longer. The current level at which the circuit breaker trips in this case is given by Equation 9: I V R mV R TRIP FAST CB FAST SENSE SENSE () () == 150 (9) As a design aid, the currents at which electronic circuit breaker trips for common values for RSENSE are shown in Table 3. Table 3. ITRIP(SLOW) and ITRIP(FAST) vs RSENSE RSENSE ITRIP(SLOW) ITRIP(FAST) 0.005 Ω 10A 30A 0.006 Ω 8.3A 25A 0.007 Ω 7.1A 21A 0.008 Ω 6.3A 19A 0.009 Ω 5.6A 17A 0.01 Ω 5A 15A |
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