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七,功能概述 VCC and VIN Power-On-Reset (POR) TThe JTMA7322/A keeps monitoring the voltage on VIN and VCC pins to prevent wrong logic operations which may occur when VIN or VCC voltage is not high enough for internal control circuitry to operate. The VCC POR ris- ing threshold is 2.7V (typical) with 0.2V hysteresis and VIN POR rising threshold is 2.3V with 0.05V hysteresis. During startup, the VCC and VIN voltage must exceed the POR threshold. Then the IC starts a starts-up process and ramps up the output voltage to the voltage target. Output Under-Voltage Protection (UVP) In the operational process, if a short-circuit occurs, the output voltage will drop quickly. Before the current-limit circuit responds, the output voltage will fall out of the required regulation range. The under-voltage continually monitors the FB voltage after soft-start is completed. If a load step is strong enough to pull the output voltage lower than the under-voltage threshold, the IC shuts down converter’s output.The under-voltage threshold is 50% of the nominal out put voltage. The under-voltage comparator has a built-in 3µs noise filter to prevent the chips from wrong UVP shut down being caused by noise. The JTMA7322/A will be latched after under-voltage protection. Over-Voltage Protection (OVP) The over-voltage function monitors the output voltage by FB pin. When the FB voltage increases over 125% of the reference voltage due to the high-side MOSFET failure or for other reasons, the over-voltage protection comparator will force the low-side MOSFET gate driver high till the occurence of low-side current limit. This action actively pulls down the output voltage. As soon as the output voltage is less than 0.2V, the JTMA7322/A will be latched off and needs a POR or enable to recover to normal operation. Over-Temperature Protection (OTP) The over-temperature circuit limits the junction tempera ture of the JTMA7322/A. When the junction temperature exceeds TJ = 160oC, a thermal sensor turns off the both power MOSFETs, allowing the devices to cool. The thermal sensor allows the converters to start a start up process and to regulate the output voltage again after the junction temperature cools by 50oC. The OTP is designed with a 50oC hysteresis to lower the average TJ during continuous thermal overload conditions, increasing lifetime of the JTMA7322/A. Current-Limit Protection The JTMA7322/A monitors the output current, flows through the high-side and low-side power MOSFETs, and limits the current peak at current-limit level to prevent the IC from damaging during overload, short-circuit and over- voltage conditions. Typical high side power MOSFET current limit is 3A, and low side MOSFET current limit is 1.9A. Soft-Start The JTMA7322/A has a built-in soft-start to control the rise rate of the output voltage and limit the input current surge during start-up. During soft-start, an internal voltage ramp connected to one of the positive inputs of the error amplifier, rises up from 0V to 0.95V to replace the reference voltage (0.8V) until the voltage ramp reaches the reference voltage. During soft-start without output overvoltage, the JTMA7322/A converter¡¦s sinking capability is disabled until the output voltage reaches the voltage target. Soft-Stop At the moment of shutdown controlled by EN signal, under-voltage event or over-temperature protection, the JTMA7322/A initiates a soft-stop process to discharge the output voltage in the output capacitors. Certainly, the load current also discharges the output voltage. During softstop, the internal voltage ramp (VRAMP) falls down from 0. 95V to 0V to replace the reference voltage. Therefore, the output voltage falls down slowly at the light load. After the soft-stop interval elapses, the soft-stop process ends and the IC turns on the low-side power MOSFET. 八,相关产品
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粤ICP备13004986号-3