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LTC4229 Datasheet(PDF) 14 Page - Linear Technology |
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LTC4229 Datasheet(HTML) 14 Page - Linear Technology |
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14 / 28 page  LTC4229 14 4229f For more information www.linear.com/LTC4229 applicaTions inForMaTion In the event of a severe short-circuit fault on the 12V output as shown in Figure 4, the output current can surge to tens of amperes. The LTC4229 responds within 1µs to bring the current under control by pulling the HGATE to OUT voltage down to zero volts. Almost immediately, the gate of the Hot Swap MOSFET recovers rapidly due to the charge stored in the RHG and CHG network, and current is actively limited until the fault filter times out. Due to parasitic supply lead inductance, an input supply without any bypass capacitor may collapse during the high cur- rent surge and then spike upwards when the current is interrupted. Figure 13 shows the input supply transient suppressors consisting of Z1, RSNUB and CSNUB for the supply if there is no input capacitance. FTMR Pin Functions An external capacitor CFT connected from the FTMR pin to GND serves as fault filtering when the supply output is in active current limit. When the voltage across the sense Figure 3. Overcurrent Fault on 12V Output Figure 4. Severe Short-Circuit on 12V Output resistorexceedsthefoldbackcurrentlimitthreshold(from 25mV to 8.3mV), FTMR pulls up with 100µA. Otherwise, it pulls down with 2µA. The fault filter times out when the 1.235V FTMR threshold is exceeded, causing the FAULT pin to pull low. For a given fault filter delay, the equation for setting the external capacitor CFT value is: CFT = tFF • 0.083 [µF/ms] After the fault filter timeout, the FTMR pin capacitor pulls down with 2µA from the 1.235V FTMR threshold until it reaches0.2V.Then,itcompletes14coolingcyclesconsist- ing of the FTMR pin capacitor charging to 1.235V with a 100µA current and discharging to 0.2V with a 2µA current. At that point, the HGATE pin voltage is allowed to start up if the fault has been cleared as described in the Resetting Fault section. When the latched fault is cleared during the cool-off period, the FAULT pin pulls high. The total cool-off time for the MOSFET after an overcurrent fault is: tCOOL = CFT • 11 [s/µF] After the cool-off period, the HGATE pin is only allowed to pull up if the fault has been cleared for the latchoff configuration.Fortheauto-retryconfiguration,thelatched fault is cleared automatically following the extended cool- off period and the HGATE pin voltage is allowed to restart. Resetting Fault For the latchoff configuration with the RTMR pin tied to INTVCC, an overcurrent fault is latched after fault filter times out, and the FAULT pin is asserted low. To reset a latched fault and restart the output, pull the UV pin below 0.6V for more than 100µs and then high above 1.235V. The fault latch resets and the FAULT pin deasserts on the falling edge of the UV pin. When UV goes high again, a debounce timing cycle is initiated before the HGATE pin voltagerestarts.TogglingtheENpinhighandthenlowagain also resets a fault, but the FAULT pin pulls high at the end of the debounce cycle before the HGATE pin voltage starts up. Bringing the supply below the INTVCC undervoltage lockout threshold (2.2V) shuts off all the MOSFETs and resets the fault latch. A debounce cycle is initiated before a normal start-up when the supply is restored above the INTVCC UVLO threshold. 200µs/DIV OUT 10V/DIV HGATE 10V/DIV ILOAD 20A/DIV 4229 F03 5µs/DIV OUT 10V/DIV HGATE 10V/DIV ILOAD 20A/DIV 4229 F04 |
Similar Part No. - LTC4229_15 |
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Similar Description - LTC4229_15 |
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