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LTM4630 Datasheet(PDF) 11 Page - Linear Technology |
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LTM4630 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 34 page  LTM4630 11 4630fa For more information www.linear.com/LTM4630 The typical LTM4630 application circuit is shown in Fig- ure 23. External component selection is primarily deter- mined by the maximum load current and output voltage. RefertoTable4forspecificexternalcapacitorrequirements for particular applications. VIN to VOUT Step-Down Ratios There are restrictions in the maximum VIN and VOUT step- down ratio that can be achieved for a given input voltage. Each output of the LTM4630 is capable of 98% duty cycle, but the VIN to VOUT minimum dropout is still shown as a function of its load current and will limit output current capability related to high duty cycle on the top side switch. Minimum on-time tON(MIN) is another consideration in operating at a specified duty cycle while operating at a certain frequency due to the fact that tON(MIN) < D/fSW, where D is duty cycle and fSW is the switching frequency. tON(MIN) is specified in the electrical parameters as 90ns. Output Voltage Programming ThePWMcontrollerhasaninternal0.6Vreferencevoltage. AsshownintheBlockDiagram,a60.4kΩinternalfeedback resistor connects between the VOUTS1 to VFB1 and VOUTS2 to VFB2. It is very important that these pins be connected to their respective outputs for proper feedback regulation. Overvoltage can occur if these VOUTS1andVOUTS2pinsare left floating when used as individual regulators, or at least one of them is used in paralleled regulators. The output voltage will default to 0.6V with no feedback resistor on either VFB1 or VFB2. Adding a resistor RFB from VFB pin to GND programs the output voltage: VOUT = 0.6V • 60.4k +RFB RFB Table 1. VFB Resistor Table vs Various Output Voltages VOUT 0.6V 1.0V 1.2V 1.5V 1.8V RFB Open 90.9k 60.4k 40.2k 30.2k For parallel operation of multiple channels the same feed- back setting resistor can be used for the parallel design. This is done by connecting the VOUTS1 to the output as shown in Figure 2, thus tying one of the internal 60.4k resistors to the output. All of the VFB pins tie together with one programming resistor as shown in Figure 2. APPLICATIONS INFORMATION Inparalleloperation,theVFBpinshaveanIFBcurrentof20nA maximumeachchannel.Toreduceoutputvoltageerrordue to this current, an additional VOUTS pin can be tied to VOUT, andanadditionalRFBresistorcanbeusedtolowerthetotal Thevenin equivalent resistance seen by this current. For exampleinFigure2,thetotalTheveninequivalentresistance of the VFB pin is (60.4k//RFB), which is 30.2k where RFB is equal to 60.4k for a 1.2V output. Four phases connected in parallel equates to a worse case feedback current of 4 • IFB = 80nA maximum. The voltage error is 80nA • 30.2k = 2.4mV. If VOUTS2 is connected, as shown in Figure 2, to VOUT,andanother60.4kresistorisconnectedfromVFB2to ground, then the voltage error is reduced to 1.2mV. If the voltage error is acceptable then no additional connections arenecessary.Theonboard60.4kresistoris0.5%accurate and the VFB resistor can be chosen by the user to be as accurate as needed. All COMP pins are tied together for current sharing between the phases. The TRACK/SS pins can be tied together and a single soft-start capacitor can be used to soft-start the regulator. The soft-start equation willneedtohavethesoft-startcurrentparameterincreased by the number of paralleled channels. See Output Voltage Tracking section. Figure 2. 4-Phase Parallel Configurations 4630 F02 60.4k TRACK1 TRACK2 VOUT1 VOUTS1 VFB1 VFB2 COMP1 4 PARALLELED OUTPUTS FOR 1.2V AT 70A OPTIONAL CONNECTION COMP2 VOUTS2 VOUT2 60.4k 60.4k TRACK1 TRACK2 0.1µF VOUT1 VOUTS1 VFB1 VFB2 COMP1 COMP2 VOUTS2 VOUT2 60.4k LTM4630 LTM4630 RFB 60.4k OPTIONAL RFB 60.4k USE TO LOWER TOTAL EQUIVALENT RESISTANCE TO LOWER IFB VOLTAGE ERROR |
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