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LTC4211 Datasheet(PDF) 11 Page - Linear Technology |
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LTC4211 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  LTC4223-1/LTC4223-2 11 422312f APPLICATIONS INFORMATION Several conditions must be met before the external and internal switches are allowed to turn on. First VCC and the input supplies (12VIN, AUXIN) must exceed their undervoltage lockout thresholds. Next TIMER must be <0.2V and ⎯E⎯N must be pulled low. Once these conditions are met, a debounce timing cycle is initiated when AUXON or 12ON pin is toggled from low to high. These two control pins turn on/off the 3.3V auxiliary and 12V supplies. At the end of the debounce cycle, the ON pins and fault status are checked. If both ON pins are high and fault is cleared, the 3.3V auxiliary supply starts up first followed by the 12V supply. Note that the turn-on delay for the AUXON and 12ON pins is 15μs and 30μs. Figure 2 shows the two supplies turning on in sequence after ⎯E⎯N goes low. By default, the internal pass transistor turns on first if both ON pins are high and start-up conditions met. The output is current limited at 240mA by its internal ACL amplifier as the load current charging the output capaci- tor increases. This causes the TIMER to ramp up with a 10μA pull-up. Normally the AUXOUT voltage exceeds its power-good threshold before TIMER time-out and then ⎯A⎯U⎯X⎯P⎯G⎯O⎯O⎯D pulls low. Once AUXOUT signals power is good and the TIMER pin returns to <0.2V, the external MOSFET is then allowed to turn on by charging up the GATE with a 10μA current source (Figure 2). The voltage at the GATE pin rises with a slope equal to 10μA/CG and the supply inrush current flowing into the load capacitor CL1 (see Figure 14) is limited to: I C C µA INRUSH L G = 1 10 • The 12V output follows the GATE voltage when the MOSFET turns on. If the voltage across the current sense resistor RS becomes too high, the inrush current is limited by the internal current limit circuitry. Once the output, 12VOUT exceeds its power-good threshold, ⎯1⎯2⎯P⎯G⎯O⎯O⎯D also pulls low. If only the 12ON pin is high at the end of debounce cycle, the external MOSFET turns on first. After that, if AUXON pulls high, the internal switch turns on only after the 12V output signals power is good and TIMER <0.2V. Figure 2. Normal Power-Up Sequence Table 1. 12V and Auxiliary Supply Turn-Off Conditions CONDITION RESULT CLEARED BY AUX 12V AUXON Goes Low Turns Off No Effect AUXON High 12ON Goes Low No Effect Turns Off 12ON High ⎯E⎯N Goes High Turns Off Turns Off ⎯E⎯N Low UVLO on VCC Turns Off Turns Off VCC > UVLO UVLO on AUXIN Turns Off No Effect AUXIN > UVLO UVLO on 12VIN No Effect Turns Off 12VIN > UVLO AUX Overcurrent Fault Turns Off Turns Off AUXON and 12ON Low, ⎯E⎯N High-to-Low, UVLO on VCC 12V Overcurrent Fault No Effect Turns Off 12ON Low, ⎯E⎯N High-to- Low, UVLO on VCC Thermal Shutdown Turns Off Turns Off AUXON and 12ON Low, ⎯E⎯N High-to-Low, UVLO on VCC, Temperature < 120°C 20ms/DIV 12PGOOD 5V/DIV EN 5V/DIV AUXOUT 5V/DIV TIMER 2V/DIV 12VOUT 5V/DIV AUXPGOOD 5V/DIV 422312 F02 |
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