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LTC4350 Datasheet(PDF) 9 Page - Linear Technology |
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LTC4350 Datasheet(HTML) 9 Page - Linear Technology |
9 / 18 page LTC4350 4350fb applicaTions inForMaTion SB pin (or share bus). If the FB pin is above the reference, the COMP1 pin is grounded and the SB pin is disconnected from the COMP1 pin using the series diode. The LTC4350 with the highest reference will drive the SB pin and the 20k loads connected to the SB pin. All of the other LTC4350’s COMP1 pins are pulled low because their FB pins are at a higher voltage than their references. The series diode between the COMP1 pin and the SB pin is actually a low impedance buffer amplifier with a diode in the output stage. Therefore, the master LTC4350’s E/A1 drives the share bus to the proper value that keeps the output voltage tightly regulated. The buffer amplifier is capable of driving at least fifty 20k loads (each 20k load represents an LTC4350). OUTPUT VOLTAGE ADjUSTMENT The LTC4350 is designed to work with supplies featuring remote sense. The output voltage of each power supply needs to be adjusted below the final output voltage at the commonload.Forexample,a5Vsystemwouldrequirethe power supply output be set to 4.90V or some value below 5V. This is normally done using the trim pin of the module. The power supply output is then increased by artificially reducing the positive sense voltage by a small amount. The LTC4350 would then adjust the output voltage to 5V, an increase of 2%. The maximum range of adjustment can be set from 2% to 5% to compensate for voltage drops in the wiring, but no more than 300mV. In most power supplies, the voltage sense is tied directly to the output voltage. If a small valued resistor, ROUT, is placed in series with the power supply sense line, a voltage drop across ROUT appears as a lower sensed voltage. This requires the power supply to increase its output voltage to compensate. Thus, the LTC4350 exercises complete control of the final output voltage. The IOUT block converts the E/A2 output (COMP2 pin) to a current that flows through ROUT (see Figure 3). As the voltage at COMP2 increases, the current in ROUT Figure 3. Simplified Block Diagram GATE DRIVE RSENSE REF SHARE BUS 4350 F03 IADJ SENSE+ IOUT PIN OUT+ CG LOAD ROUT+ – THIS RESISTOR CONVERTS IADJ TO A VOLTAGE TO MODIFY THE REMOTE SENSE INPUT OF THE POWER SUPPLY (SENSE+). IT CREATES AN ARTIFICIAL SENSE+ VOLTAGE THAT ADJUSTS THE POWER SUPPLY’S OUTPUT VOLTAGE UP OR DOWN THIS AMPLIFIER CONVERTS THE E/A2 VOLTAGE OUTPUT TO A CURRENT OUTPUT (IADJ) THIS AMPLIFIER FORCES THE POWER SUPPLY CURRENT TO EQUAL THE REFERENCE CURRENT VALUE (i.e., SHARE BUS) THIS VOLTAGE REPRESENTS THE REFERENCE CURRENT VALUE (i.e., SHARE BUS) NEEDED TO FORCE THE OUTPUT VOLTAGE TO EQUAL THE REF THIS VOLTAGE REPRESENTS THE POWER SUPPLY CURRENT MEASURED USING A SENSE RESISTOR PASS-FET USED TO DISCONNECT A BAD POWER SUPPLY AND TO HOT-SWAP A POWER SUPPLY – + – + – + ISENSE RGAIN E/A1 COMP1 PIN FB PIN COMP2 PIN E/A2 – + IOUT RSET PIN RSET 4 3 2 1 20k gm = 1m |
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