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LTC2926IUFD Datasheet(PDF) 11 Page - Linear Technology |
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LTC2926IUFD Datasheet(HTML) 11 Page - Linear Technology |
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11 / 28 page  LTC2926 11 2926fa RFA and RFB. The slave output voltage varies as a function of the master signal with terms set by RTA and RTB. By selecting appropriate values of RTA and RTB, it is possible to generate any of the profiles in Figures 1 to 4. Controlling the Ramp-Up and Ramp-Down Behavior The operation of the LTC2926 is most easily understood by referring to the simplified functional diagram in Figure 6. When the ON pin is low, the remote sense switch is opened and the MGATE pin is pulled to ground causing the master signal to remain low. Since the current through RTB1 is at its maximum when the master signal is low, the current sourced by FB1 is also at its maximum. The current forces the FB1 pin voltage above 0.8V, which pulls the SGATE1 pin low and disconnects the slave’s supply generator. The minimum voltage across the slave load is a function of the maximum FB1 current, the feedback divider resistors, and the load resistance (see Load Requirements). When the ON pin rises above 1.23V, the master signal ramps up, and the slave supply tracks the master signal. The master ramp rate is set by an external capacitor driven by a 10µA current source from an internal charge pump. If no external MOSFET is used for the master signal, the ramp rate is set by tying the MGATE and RAMP pins together at one terminal of the external capacitor (see Ratiometric Tracking Example or Supply Sequencing Example). The MGATE pin voltage will be limited to VCC + 1V (max) by the weak internal clamp on the RAMP pin. The rising master signal decreases the tracking current mirrored out of the FB1 pin. The gate controller circuitry maintains 0.8V at FB1 by driving the SGATE1 voltage and, via the external MOSFET source-follower, the slave supply output. When the slave supply output reaches the slave supply module voltage, the FB1 pin will fall below 0.8V and the gate controller will drive the SGATE1 pin above VCC to fully enhance the MOSFET. After the MGATE, SGATE1 and SGATE2 pins reach their maximum voltages, the RSGATE pin is pulled up by a 10µA current source from an internal charge pump, which closes the integrated remote sense switches. The integrated remote sense switch allows the slave supply generator to compensate for voltage drop across the slave’s MOSFET (Q1). When the ON pin falls below VON(TH) – ΔVON(TH), typically 1.16V, the remote sense switch opens and the MGATE pin pulls down with 10µA. The master signal and the slave supplies will fall at the same rate as they rose previously, following the tracking or sequencing profile in reverse. The ON pin can be controlled by a digital I/O pin or it can be used to monitor an input supply. By connecting a resistive voltage divider from an input supply to the ON pin, the supplies will ramp up only after the monitored supply reaches a preset voltage. Figure 5. Simplified Tracking Cell and Gate Controller Cell Combination APPLICATIO S I FOR ATIO RTB RTA 2926 F05 LTC2926 GATE CONTROLLER CELL SGATE 0.8V 10 µA FB SUPPLY TRACK IFB ITRACK MASTER RAMP SLAVE RFB RFA 10 µA 0.8V VCC QEXT VCC + 5V – + TRACKING CELL – + |
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