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LTC4227 Datasheet(PDF) 11 Page - Linear Technology |
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LTC4227 Datasheet(HTML) 11 Page - Linear Technology |
11 / 22 page LTC4227 11 422712fa For more information www.linear.com/LTC4227 applicaTions inForMaTion During a normal power-up, the ideal diode MOSFETs turn onfirst.Assoonastheinternallygeneratedsupply,INTVCC, rises above its 2.2V undervoltage lockout threshold, the internal charge pump is allowed to charge up the CPO pins. Because the ideal diode MOSFETs are connected in parallelasadiode-OR,theSENSE+pinvoltageapproaches the highest of the supplies at the IN1 and IN2 pins. The MOSFET associated with the lower input supply voltage willbeturnedoffbythecorrespondinggatedriveamplifier. Before the Hot Swap MOSFET can be turned on, EN must remain low and ON must remain high for a tD(HGATE) de- bounce timing cycle to ensure that any contact bounces duringtheinsertionhaveceased.Attheendofthedebounce cycle, the internal fault latches are cleared. The Hot Swap MOSFET is then allowed to turn on by charging up HGATE with a 10µA current source from the charge pump. The voltage at the HGATE pin rises with a slope equal to 10µA/ CHG and the supply inrush current flowing into the load capacitor, CL, is limited to: IINRUSH = CL CHG • 10µA The OUT voltage follows the HGATE voltage when the Hot Swap MOSFET turns on. If the voltage across the current sense resistor, RS, becomes too high, the inrush current willbelimitedbytheinternalcurrentlimitingcircuitry.Once the MOSFET gate overdrive exceeds 4.2V, the PWRGD pin pulls low to indicate that the power is good. Once OUT reaches the input supply voltage, HGATE continues to ramp up. An internal 12V clamp limits the HGATE voltage above OUT. When the ideal diode MOSFET is turned on, the gate drive amplifier controls the gate of the MOSFET to servo the forward voltage drop across the MOSFET to 25mV. If the load current causes more than 25mV of drop, the MOSFET gate is driven fully on and the voltage drop is equal to ILOAD • RDS(ON). Turn-Off Sequence The external MOSFETs can be turned off by a variety of conditions. A normal turn-off for the Hot Swap MOSFET is initiated by pulling the ON pin below its 1.155V threshold (80mV ON pin hysteresis), or pulling the EN pin above its 1.235V threshold. Additionally, an overcurrent fault of sufficient duration to trip the circuit breaker also turns off the Hot Swap MOSFET. Normally, the LTC4227 turns off the MOSFET by pulling the HGATE pin to ground with a 300µA current sink. All of the MOSFETs turn off when INTVCC falls below its undervoltage lockout threshold (2.2V). The DGATE pin is pulled down with a 100µA current to one diode voltage below the IN pin, while the HGATE pin is pulled down to the OUT pin by a 200mA current. When D2ON is pulled high above 1.235V, the ideal diode MOSFET in the IN2 supply path is turned off with DGATE2 pulled low by a 100µA current. The gate drive amplifier controls the ideal diode MOSFET to prevent reverse current when the input supply falls below SENSE+. If the input supply collapses quickly, the gate drive amplifier turns off the MOSFET with a fast pull- down circuit as soon as it detects that IN is 25mV below SENSE+. If the input supply falls at a more modest rate, the gate drive amplifier controls the MOSFET to maintain SENSE+ at 25mV below IN. Figure 2. Ideal Diode Controller Start-Up Waveforms Figure 3. LTC4227-1/LTC4227-2 Hot Swap Controller Power-Up Sequence VOLTAGE 10V/DIV 5ms/DIV 4227 F02 IN1 SENSE+ DGATE1 DGATE2 CP01 CP02 ON 5V/DIV HGATE 10V/DIV OUT 10V/DIV PWRGD 10V/DIV 50ms/DIV 4227 F03 |
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