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LTC4221 Datasheet(PDF) 18 Page - Linear Technology |
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LTC4221 Datasheet(HTML) 18 Page - Linear Technology |
18 / 24 page LTC4223-1/LTC4223-2 18 422312f The first step is to select the appropriate value of RSENSE for the 12V supply. Calculating RSENSE value is based on the maximum load current and the lower limit for the circuit breaker threshold, ΔVSENSE(CB)(MIN). R V I mV SENSE SENSE CB MIN LOAD MAX = Δ = ()( ) () . . 47 5 74A A m = 6 Ω If a 1% tolerance is assumed for the 6mΩ sense resistor, the minimum and maximum circuit breaker trip current is calculated as follows: 422312 F11 C2 330nF VCC AUXIN R2 51 Ω Figure 11. RC Network for VCC Filtering APPLICATIONS INFORMATION PCB Layout Considerations For proper operation of the LTC4223’s circuit breaker, Kelvin-connection to the sense resistor is strongly rec- ommended. The PCB layout should be balanced and symmetrical to minimize wiring errors. In addition, the PCB layout for the sense resistor and the power MOSFET should include good thermal management techniques for optimal device power dissipation. A recommended PCB layout for the 12V sense resistor and the power MOSFET is illustrated in Figure 12. In applications where load current exceeds 10A, wide PCB traces are recommended to minimize resistance and temperature rise. The suggested trace width for 1 oz copper foil is 0.03” for each ampere of DC current to keep PCB trace resistance, voltage drop and temperature rise to a minimum. Note that the sheet resistance of 1 oz copper foil is approximately 0.5mΩ/square, and voltage drops due to trace resistance add up quickly in high cur- rent applications. In most applications, it will be necessary to use plated- through via to make circuit connections from component layers to power and ground layers internal to the PCB. For 1 oz copper foil plating, a general rule is 1A of DC current per via. Consult your PCB fabrication facility for design rules pertaining to other plating thicknesses. It is important to place the VCC bypass capacitor C2 as close as possible between VCC and GND. The transient voltage suppressors Z1 and Z2 are also placed between the supply inputs and ground using short wide traces. Design Example As a design example, consider the AMC Hot Swap ap- plication shown earlier in Figure 1 with the power supply requirements given in Table 2. CURRENT FLOW TO LOAD TRACK WIDTH W: 0.03" PER AMPERE ON 1OZ Cu FOIL SENSE RESISTOR POWER PAK SO-8 12VOUT 12VIN GND 422312 F12 GND CG 12VGATE VIA TO GND PLANE 16 1 2 15 LTC4223CGN* • • CURRENT FLOW TO LOAD W CURRENT FLOW TO SOURCE *ADDITIONAL DETAILS OMITTED FOR CLARITY, DRAWING NOT TO SCALE! VIA TO GND PLANE W W RG R3 R1 Z1 C1 8 Figure 12. Recommended Layout for Power MOSFET, Sense Resistor and GATE Components on 12V Rail Table 2. AMC Power Supply Requirements SUPPLY VOLTAGE MAXIMUM LOAD CURRENT MAXIMUM LOAD CAPACITANCE 12V 7.4A 800μF 3.3VAUX 150mA 150μF Table 3. MicroTCA Power Supply Requirements SUPPLY VOLTAGE MAXIMUM LOAD CURRENT MAXIMUM LOAD CAPACITANCE 12V 7.6A 1600μF 3.3VAUX 150mA 150μF |
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