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LTC4221 Datasheet(PDF) 14 Page - Linear Technology |
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LTC4221 Datasheet(HTML) 14 Page - Linear Technology |
14 / 24 page LTC4223-1/LTC4223-2 14 422312f APPLICATIONS INFORMATION There are two different modes of fault time-out for the 3.3V auxiliary supply: adjustable delay through TIMER capacitor during power-up when ⎯A⎯U⎯X⎯P⎯G⎯O⎯O⎯D not asserted; fixed 25μs delay after power-up when ⎯A⎯U⎯X⎯P⎯G⎯O⎯O⎯D asserted low. Under the situation whereby AUXON toggles low then high for short duration after power-up while ⎯A⎯U⎯X⎯P⎯G⎯O⎯O⎯D still pulling low due to output load capacitor, 25μs fault time-out applies. When the auxiliary supply is powered up into an output short, the ACL amplifier will regulate the gate of the internal pass transistor to produce 240mA output cur- rent. At this time a 10μA pull-up current starts charging up the TIMER pin capacitor until it exceeds its threshold (1.235V). The internal pass transistor then turns off and ⎯F⎯A⎯U⎯L⎯T pulls low. Thereafter, the TIMER is discharged by a 2μA pull-down current. The fault filter delay is given by CT • 123[ms/μF]. After a successful power-up cycle, the ACL amplifier pro- tects the auxiliary supply from overcurrent by pulling down the gate of the internal pass transistor rapidly as shown in Figure 7. Thereafter, the gate recovers and servos the output current to about 240mA for 25μs before pulling down to ground gently, turning the transistor off. At this time, ⎯F⎯A⎯U⎯L⎯T pulls low and the 12V external MOSFET is also turned off by the 1mA GATE pull-down current. Whenever the 3.3V auxiliary supply trips off due to an overcurrent fault, the 12V supply also shuts off. The auxiliary supply is, however, unaffected by faults on the 12V supply. In either case ⎯F⎯A⎯U⎯L⎯T latches low when the affected channels turn off, and ⎯F⎯A⎯U⎯L⎯T is cleared by tog- gling the ON pins. Faults are cleared automatically in the LTC4223-2 auto-retry version. If there is significant supply lead inductance, a severe output short may collapse the input to ground before the LTC4223 can bring the current under control. In this case the undervoltage lockout will activate after a 12μs filter delay, and pull the gate down. Then the ACL amplifier will take control and regulate the output in active current limit. Under this situation, the fault time-out is set by TIMER delay instead of 25μs filter delay. Undervoltage Fault An undervoltage fault occurs if either AUXIN or 12VIN falls below its undervoltage threshold for longer than 12μs. This turns off the affected supply’s switch instantly, but does not clear the fault latches. Further, an undervoltage fault on one supply does not affect the operation of the other supply. If the bias supply input, VCC falls below its UVLO threshold for more than 80μs, all supply switches are turned off and the fault latches are cleared. Operation resumes from a fresh start-up cycle when VCC is restored. Figure 7. Short-Circuit Fault on 3.3VAUX Output Figure 6. Short-Circuit Fault on 12V Output 5 μs/DIV 12VGATE 5V/DIV 12VOUT 5V/DIV 422312 F06 ΔVSENSE 200mV/DIV 5 μs/DIV AUXOUT 5V/DIV FAULT 5V/DIV 422312 F07 ILOAD 1A/DIV |
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