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LTC3850IGN-TRPBF 数据表(PDF) 18 Page - Linear Technology |
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LTC3850IGN-TRPBF 数据表(HTML) 18 Page - Linear Technology |
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18 / 36 page  LTC3850/LTC3850-1 18 38501fb selected mode once TK/SS > 0.74V. The output ripple is minimized during the 100mV forced continuous mode window ensuring a clean PGOOD signal. When the channel is configured to track another supply, the feedback voltage of the other supply is duplicated by a resistor divider and applied to the TK/SS pin. Therefore, the voltage ramp rate on this pin is determined by the ramp rate of the other supply’s voltage. Note that the small soft-start capacitor charging current is always flowing, producing a small offset error. To minimize this error, select the tracking resistive divider value to be small enough to make this error negligible. In order to track down another channel or supply after the soft-start phase expires, the LTC3850 is forced into continuous mode of operation as soon as VFB is below the undervoltage threshold of 0.74V regardless of the setting on the MODE/PLLIN pin. However, the LTC3850 should always be set in force continuous mode tracking down when there is no load. After TK/SS drops below 0.1V, its channel will operate in discontinuous mode. Output Voltage Tracking The LTC3850 allows the user to program how its out- put ramps up and down by means of the TK/SS pins. Through these pins, the output can be set up to ei- ther coincidentally or ratiometrically track another supply’s output, as shown in Figure 5. In the following discussions, VOUT1 refers to the LTC3850’s output 1 as a master channel and VOUT2 refers to the LTC3850’s output 2 as a slave channel. In practice, though, either phase can be used as the master. To implement the coincident track- ing in Figure 5a, connect an additional resistive divider to VOUT1 and connect its midpoint to the TK/SS pin of the slave channel. The ratio of this divider should be the same as that of the slave channel’s feedback divider shown in Figure 6a. In this tracking mode, VOUT1 must be set higher than VOUT2.Toimplementtheratiometrictracking,theratio of the VOUT2 divider should be exactly the same as the master channel’s feedback divider. By selecting different resistors, the LTC3850 can achieve different modes of tracking including the two in Figure 5. So which mode should be programmed? While either mode in Figure 5 satisfies most practical applications, some tradeoffs exist. The ratiometric mode saves a pair of resistors, but the coincident mode offers better output regulation. This can be better understood with the help of Figure 7. At the input stage of the slave channel’s error amplifier, two common anode diodes are used to clamp the equivalent reference voltage and an additional diode is used to match the shifted common mode voltage. The top two current sources are of the same amplitude. In the coincident mode, the TK/SS voltage is substantially higher than 0.8V at steady state and effectively turns off D1. D2 and D3 will therefore conduct the same current and offer tight matching between VFB2 and the internal precision 0.8V reference. In the ratiometric mode, however, TK/SS equals 0.8V at steady state. D1 will divert part of the bias current to make VFB2 slightly lower than 0.8V. Although this error is minimized by the exponential I-V characteristic of the diode, it does impose a finite amount of output voltage deviation. Furthermore, when the master channel’s output experiences dynamic excursion (under load transient, for example), the slave channel output will be affected as well. For better output regulation, use the coincident tracking mode instead of ratiometric. INTVCC Regulators and EXTVCC The LTC3850 features an NPN linear regulator that supplies power to INTVCC from the VIN supply. INTVCC powers the gate drivers and much of the LTC3850’s internal circuitry. The linear regulator regulates the voltage at the INTVCC pin to 5V when VIN is greater than 6.5V. EXTVCC connects to INTVCC through a P-channel MOSFET and can supply the needed power when its voltage is higher than 4.7V. Each of these can supply a peak current of 100mA and must be bypassed to ground with a minimum of 1μF ceramic capacitor or low ESR electrolytic capacitor. No matter what type of bulk capacitor is used, an additional 0.1μF ceramic capacitor placed directly adjacent to the INTVCC and PGND pins is highly recommended. Good bypassing is needed to supply the high transient currents required by the MOSFET gate drivers and to prevent interaction between the channels. High input voltage applications in which large MOSFETs are being driven at high frequencies may cause the maxi- mum junction temperature rating for the LTC3850 to be APPLICATIONS INFORMATION |
类似零件编号 - LTC3850IGN-TRPBF |
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类似说明 - LTC3850IGN-TRPBF |
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