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LM2619ATL bảng dữ liệu(PDF) 11 Page - National Semiconductor (TI) |
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LM2619ATL bảng dữ liệu(HTML) 11 Page - National Semiconductor (TI) |
11 / 15 page Device Information (Continued) less than the 2.8V minimum operating voltage. The LM2619 is designed for compact portable applications, such as mo- bile phones. In such applications, the system controller de- termines power supply sequencing. Although the LM2619 is typically well behaved at low input voltages, this is not guar- anteed. INTERNAL SYNCHRONOUS RECTIFICATION While in PWM mode, the LM2619 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. The internal NFET synchronous rectifier is turned on during the inductor current down slope during the second part of each cycle. The synchronous rectifier is turned off prior to the next cycle, or when the inductor current ramps to zero at light loads. The NFET is designed to conduct through its intrinsic body diode during transient intervals before it turns on, elimi- nating the need for an external diode. CURRENT LIMITING A current limit feature allows the LM2619 to protect itself and external components during overload conditions. In PWM mode cycle-by-cycle current limit is normally used. If an excessive load pulls the voltage at the feedback pin down to approximately 0.7V, then the device switches to a timed current limit mode. In timed current limit mode the internal P-FET switch is turned off after the current comparator trips and the beginning of the next cycle is inhibited for 2.5µs to force the instantaneous inductor current to ramp down to a safe value. Timed current limit mode prevents the loss of current control seen in some products when the voltage at the feedback pin is pulled low in serious overload conditions. DROPOUT CONSIDERATIONS The LM2619 can be used to provide fixed output voltages by using external feedback resistors. The output voltage can be set from 1.5V to 3.6V. The internal reference voltage for the error amplifier is 1.5V. In cases where the output voltage is set higher than 2.5V, the part will go into dropout or 100% duty cycle when the input voltage gets close to the set output voltage. Near dropout the on time of the P-FET may exceed one PWM clock cycle and cause higher ripple on the output for load currents greater than 450mA. This increased ripple will exist for a narrow range of input voltages close to the 100% duty cycle and once the input voltage goes down further the P-FET will be fully on. See Setting the Output Voltage in the Application Information section for further details. In dropout conditions the output voltage is V IN −IOUT (Rdc + R DSON (P)) where Rdc is the series resistance of the inductor and R DSON (P) is the on resistance of the PFET. SOFT-START The LM2619 has soft start to reduce current inrush during power-up and startup. This reduces stress on the LM2619 and external components. It also reduces startup transients on the power source. Soft start is implemented by ramping up the reference input to the error amplifier of the LM2619 to gradually increase the output voltage. THERMAL SHUTDOWN PROTECTION The LM2619 has a thermal shutdown protection function to protect itself from short-term misuse and overload condi- tions. When the junction temperature exceeds 150˚C the device turns off the output stage and when the temperature drops below 130˚C it initiates a soft start cycle. Prolonged operation in thermal shutdown conditions may damage the device and is considered bad practice. Load Transient Response (Circuit in Figure 3) 20057427 FIGURE 8. Line Transient Response (Circuit in Figure 3) 20057428 FIGURE 9. www.national.com 11 |
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