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LM3674MF-1.8 Scheda tecnica(PDF) 10 Page - National Semiconductor (TI) |
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LM3674MF-1.8 Scheda tecnica(HTML) 10 Page - National Semiconductor (TI) |
10 / 15 page Operation Description DEVICE INFORMATION The LM3674, a high efficiency step down DC-DC switching buck converter, delivers a constant voltage from a single Li-Ion battery and input voltage rails from 2.7V to 5.5V to portable devices such as cell phones and PDAs. Using a voltage mode architecture with synchronous rectification, the LM3674 has the ability to deliver up to 600 mA depending on the input voltage, output voltage, ambient temperature and the inductor chosen. There are two modes of operation depending on the current required - PWM (Pulse Width Modulation), and shutdown. The device operates in PWM throughout the I OUT range. Shutdown mode turns off the device, offering the lowest current consumption (I SHUTDOWN = 0.01 µA typ). Additional features include soft-start, under voltage protec- tion, current overload protection, and thermal overload pro- tection. As shown in Figure 1, only three external power components are required for implementation. The part uses an internal reference voltage of 0.5V. It is recommended to keep the part in shutdown until the input voltage is 2.7V or higher. CIRCUIT OPERATION During the first portion of each switching cycle, the control block in the LM3674 turns on the internal PFET switch. This allows current to flow from the input through the inductor to the output filter capacitor and load. The inductor limits the current to a ramp with a slope of by storing energy in a magnetic field. During the second portion of each cycle, the controller turns the PFET switch off, blocking current flow from the input, and then turns the NFET synchronous rectifier on. The inductor draws current from ground through the NFET to the output filter capacitor and load, which ramps the inductor current down with a slope of The output filter stores charge when the inductor current is high, and releases it when the inductor current is low, smoothing the voltage across the load. The output voltage is regulated by modulating the PFET switch on time to control the average current sent to the load. The effect is identical to sending a duty-cycle modulated rectangular wave formed by the switch and synchronous rectifier at the SW pin to a low-pass filter formed by the inductor and output filter capacitor. The output voltage is equal to the average voltage at the SW pin. PWM OPERATION During PWM ( Pulse Width Modulation) operation the con- verter operates as a voltage-mode controller with input volt- age feed forward. This allows the converter to achieve ex- cellent load and line regulation. The DC gain of the power stage is proportional to the input voltage. To eliminate this dependence, feed forward inversely proportional to the input voltage is introduced. While in PWM mode, the output voltage is regulated by switching at a constant frequency and then modulating the energy per cycle to control power to the load. At the begin- ning of each clock cycle the PFET switch is turned on and the inductor current ramps up until the comparator trips and the control logic turns off the switch. The current limit comparator can also turn off the switch in case the current limit of the PFET is exceeded. Then the NFET switch is turned on and the inductor current ramps down. The next cycle is initiated by the clock turning off the NFET and turning on the PFET. 20167275 Internal Synchronous Rectification While in PWM mode, the LM3674 uses an internal NFET as a synchronous rectifier to reduce rectifier forward voltage drop and associated power loss. Synchronous rectification provides a significant improvement in efficiency whenever the output voltage is relatively low compared to the voltage drop across an ordinary rectifier diode. Current Limiting A current limit feature allows the LM3674 to protect itself and external components during overload conditions. PWM mode implements current limiting using an internal compara- tor that trips at 1020 mA (typ). If the output is shorted to ground the device enters a timed current limit mode where the NFET is turned on for a longer duration until the inductor current falls below a low threshold, ensuring inductor current has more time to decay, thereby preventing runaway. SOFT-START The LM3674 has a soft-start circuit that limits in-rush current during start-up. During start-up the switch current limit is increased in steps. Soft start is activated only if EN goes from logic low to logic high after Vin reaches 2.7V. Soft start is implemented by increasing switch current limit in steps of 70mA, 140mA, 280mA, and 1020mA (typ. switch current limit). The start-up time thereby depends on the output ca- pacitor and load current demanded at start-up. Typical start-up times with 10µF output capacitor and 300mA load current is 350µs and with 10mA load current is 240µs. LDO - LOW DROP OUT OPERATION The LM3674-ADJ can operate at 100% duty cycle (no switching, PMOS switch completely on) for low drop out support of the output voltage. In this way the output voltage www.national.com 10 |
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