Datasheet LTC3806 (Analog Devices) - 10

LTC3806
U U W U APPLICATIO S I FOR ATIO
Cross regulation and tracking between the master and slave Select a value for R7 less than or equal to 120k. Now outputs are impacted by transformer and secondary-side choose the fraction K of the total feedback taken from power MOSFET selection. Select a power MOSFET with VOUT1. The higher the fraction used, the tighter VOUT1 is low on resistance. In addition, a transformer with low controlled, but the poorer VOUT2 is controlled (since it winding resistances and highest coupling coefficient will contributes less to the total feedback). The values for R6A have better cross regulation and tracking. and R6B can now be calculated:
Composite Feedback
R7 ⎛ V ⎞ R6A OUT1 = – 1 K ⎝⎜ V ⎠⎟ In applications where accuracy is important on more than REF one output, composite feedback may be used. This sacri- R7 ⎛ V ⎞ OUT2 = fices some of the accuracy of one output for improved R B 6 – 1 1– K ⎝⎜ VREF ⎠⎟ accuracy on the other output(s). Figure 2 shows how composite feedback can be applied to two outputs. This technique can easily be extended to more outputs if needed. OUT1 OUT2 R6A R6B
Programming Turn-On and Turn-Off Thresholds with the RUN Pin
FB The LTC3806 leaves a comparator detection circuit and R7 the voltage reference active even when the device is shut 3806 F02 down (Figure 3). This allows users to accurately program
Figure 2. Composite Feedback
an input voltage at which the converter will turn on and off. VIN + R2 RUN RUN + COMPARATOR BIAS AND 6V START-UP CONTROL INPUT – SUPPLY OPTIONAL FILTER R1 CAPACITOR 1.230V REFERENCE GND – 3806 F03a
Figure 3a. Programming the Turn-On and Turn-Off Thresholds Using the RUN Pin
VIN + R2 RUN 1M RUN RUN + COMPARATOR COMPARATOR INPUT RUN SUPPLY 6V + 6V – EXTERNAL GND LOGIC CONTROL 1.230V – – 1.230V 3806 F03b 3806 F03c
Figure 3b. On/Off Control Using External Logic Figure 3c. External Pull-Up Resistor On RUN Pin for “Always On” Operation
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