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LTM4646 数据表(PDF) 17 Page - Linear Technology |
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LTM4646 数据表(HTML) 17 Page - Linear Technology |
17 / 34 page LTM4646 17 4646f For more information www.linear.com/LTM4646 APPLICATIONS INFORMATION Output Voltage Soft Starting and Tracking Output voltage tracking can be programmed externally using the TRACK/SS pins. The output can be tracked up and down with another regulator. The master regulator’s output is divided down with an external resistor divider that is the same as the slave regulator’s feedback divider for to implement coincident tracking. The LTM4646 uses an accurate 60.4k resistor internally for the top feedback resistor for each channel. Figure 5 shows an example of coincident tracking. SLAVE 1 60.4k RFB • VTRACK VTRACK is the track ramp applied to the slave’s track pin. VTRACK has a control range of 0V to 0.6V, or the internal reference voltage. When the master’s output is divided down with the same resistor values used to set the slave’s output, then the slave will coincident track with the master until it reaches its final value. The master will continue to its final value from the slave’s regulation point. Voltage tracking is disabled when VTRACK/SS is more than 0.6V. RTB in Figure 5 will be equal to the RFB for coincident track- ing. Figure 6 shows the coincident tracking waveforms. The TRACK/SS pins can be controlled by a capacitor placed on the regulator TRACK/SS pin to ground. A 1.0A current source will charge the TRACK/SS pin up to the voltage reference and then proceed up to INTVCC. After the 0.6V ramp, the TRACK/SS pin will no longer be in control, and the internal voltage reference will control output regu- lation from the feedback divider. Foldback current limit is disabled during this sequence of turn-on during tracking or soft-starting. The TRACK/SS pins are pulled low when the RUN pin is below 1.2V. The total soft-start time can be calculated as: tSOFT-START CSS 1.0µA • 0.6V Ratiometric tracking can be achieved by a few simple calculations and the slew rate value applied to the mas- ter’s TRACK/SS pin. As mentioned above, the TRACK/SS pin has a control range from 0 to 0.6V. The master’s TRACK/SS pin slew rate is directly equal to the master’s output slew rate in volts per second. The equation: MR SR • 60.4k RTA where MR is the master’s output slew rate and SR is the slave’s output slew rate in volts per second. When coincident tracking is desired, then MR and SR are equal, thus RTA is equal to 60.4k. RTB is derived from equation: RTB 0.6V VFB 60.4k VFB RFB VTRACK RTA where VFB is the feedback voltage reference of the regula- tor, and VTRACK/SS is 0.6V. Since RTA is equal to the 60.4k top feedback resistor of the slave regulator in equal slew rate or coincident tracking, then RTB is equal to RFB with VFB = VTRACK/SS. In ratiometric tracking, a different slew rate may be desired for the slave regulator. RTB can be solved for when SR is slower than MR. Make sure that the slave supply slew rate is chosen to be fast enough so that the slave output voltage will reach it final value before the master output. For example, MR = 20V/s, and SR = 15V/s. Then RTA = 80.6k. Solve for RTB to equal to 80.6k. Each of the TRACK/SS pins will have the 1.0A current source on when a resistive divider is used to implement tracking on that specific channel. This will impose an off- set on the TRACK/SS pin input. Smaller values resistors with the same ratios as the resistor values calculated from the above equation can be used. For example, where the 60.4k is used then a 6.04k can be used to reduce the TRACK/SS pin offset to a negligible value. Power Good Each PGOOD pin is connected to an internal open-drain N-channel MOSFET. An external resistor or current source can be used to pull this pin up to 6V (e.g., VOUT1,2 or DRVCC). Overvoltage or undervoltage comparators (OV, UV) turn on the MOSFET and pull the PGOOD pin low when the feedback voltage is outside the ±7.5% window of the reference voltage. The PGOOD pin is also pulled low when the channel’s RUN pin is below the 1.2V threshold |
类似零件编号 - LTM4646 |
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类似说明 - LTM4646 |
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