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LTM4619IVPBF Datasheet(PDF) 11 Page - Linear Technology |
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LTM4619IVPBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  LTM4619 11 4619f APPLICATIONS INFORMATION Figure 3. Example of Coincident Tracking Output voltage tracking can be programmed externally using the TK/SS pin. The master channel is divided down with an external resistor divider that is the same as the slave channel’s feedback divider to implement coincident tracking. The LTM4619 uses an accurate 60.4k resistor internally for the top feedback resistor. Figure 3 shows an example of coincident tracking. Figure 4 shows the output voltages with coincident tracking. VSLAVE = 1+ R1 R2 ⎛ ⎝⎜ ⎞ ⎠⎟ •VTRACK VTRACK is the track ramp applied to the slave’s TK/SS2 pin. VTRACK has a control range of 0V to 0.8V. 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. Ratiometric modes of tracking can be achieved by select- ing different divider resistors values to change the output tracking ratio. The master output must be greater than the slave output for the tracking to work. Master and slave data inputs can be used to implement the correct resistors values for coincident or ratio tracking. Soft-Start and Tracking The LTM4619 has the ability to either soft-start by itself with a capacitor or track the output of another channel or external supply. When one particular channel is configured to soft-start by itself, a capacitor should be connected to its TK/SS pin. This channel is in the shutdown state if its RUN pin voltage is below 1.2V. Its TK/SS pin is actively pulled to ground in this shutdown state. Once the RUN pin voltage is above 1.2V, the channel pow- ers up. A soft-start current of 1.3μA then starts to charge its soft-start capacitor. Note that soft-start or tracking is achieved not by limiting the maximum output current of the controller but by controlling the output ramp voltage according to the ramp rate on the TK/SS pin. Current foldback is disabled during this phase to ensure smooth soft-start or tracking. The soft-start or tracking range is defined to be the voltage range from 0V to 0.8V on the TK/SS pin. The total soft-start time can be calculated as: tSOFT-START = 0.8V •CSS 1.3µA Figure 4. Coincident Tracking 4619 F03 VIN VFB1 COMP1 VOUT1 TK/SS1 RUN1 PGOOD FREQ/PLLFLTR VFB2 COMP2 VOUT2 TK/SS2 RUN2 EXTVCC INTVCC MODE/PLLIN LTM4619 SGND PGND R3 19.1k R4 28k COUT2 COUT1 VOUT1 3.3V VOUT2 2.5V C1 0.1μF CIN VIN 5.5V TO 28V C2 22pF C3 22pF R1 60.4k R2 28k VOUT1 TIME OUTPUT VOLTAGE 4619 F04 MASTER OUTPUT SLAVE OUTPUT |
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