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七,功能概述 Main Control Loop The JTMA7300 is a constant frequency current mode switching regulator. During normal operation, the inter- nal N-channel power MOSFET is turned on each cycle when the oscillator sets an internal RS latch and would be turned off when an internal current comparator (ICMP) resets the latch. The peak inductor current at which ICMP resets the RS latch is controlled by the voltage on the COMP pin, which is the output of the error amplifier (EAMP). An external resistive divider connected between VOUT and ground allows the EAMP to receive an output feedback voltage VFB at FB pin. When the load current increases, it causes a slight decrease in VFB relative to the 0.925V reference, which in turn causes the COMP voltage to increase until the average inductor current matches the new load current. VIN Power-On-Reset (POR) and EN Under-voltage Lockout The JTMA7300 keep monitoring the voltage on VIN pin to prevent wrong logic operations which may occur when VIN voltage is not high enough for the internal control circuitry to operate. The VIN POR has a rising threshold of 4.05V (typical) with 0.3V of hysteresis.An external under-voltage lockout (UVLO) is sensed at the EN pin. The EN UVLO has a rising threshold of 2.5V with 0.2V of hysteresis. The EN pin should be connected a resistor divider from VIN to EN . After the VIN and EN voltages exceed their respective voltage thresholds, the IC starts a start-up process and then ramps up the output voltage to the setting of output voltage. Over-Temperature Protection (OTP) The over-temperature circuit limits the junction tempera-ture of the JTMA7300When the junction temperature ex- ceeds TJ =+160oC, a thermal sensor turns off the power MOSFET, allowing the device to cool down. The thermal sensor allows the converter to start a start-up process and regulate the output voltage again after the junction temperature cools by 50oC. The OTP designed with a 50 oC hysteresis lowers the average T J during c ontinuous thermal overload conditions, increasing life time of the IC. Enable/Shutdown Driving EN to ground places the JTMA7300 in shutdown.When in shutdown, the internal N-Channel power MOSFET turns off, all internal circuitry shuts down and the quies-cent supply current reduces to 0.3µA. Current-Limit Protection The JTMA7300 monitors the output current, flowing through the N-Channel power MOSFET, and limits the IC from damages during overload, short-circuit and over- voltage conditions. Frequency Foldback The foldback frequency is controlled by the FB voltage.When the FB pin voltage is under 0.6V, the frequency of the oscillator will be reduced to 110kHz. This lower fre-quency allows the inductor current to safely discharge, thereby preventing current runaway. The oscillator’s fre-quency will switch to its designed rate when the feedback voltage on FB rises above the rising frequency foldback threshold (0.6V, typical) again. Over-Voltage Protection The over-voltage function monitors the output voltage by FB pin. When the FB voltage increase over 120% of the reference voltage, the over-voltage protection compara-tor will force the high-and low-side MOSFET gate driver off. As soon as the output voltage is within regulation, the OVP comparator is disengaged. The chip will restore its normal operation. 八,相关产品
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