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LM2832YSDX Fiches technique(PDF) 11 Page - National Semiconductor (TI) |
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LM2832YSDX Fiches technique(HTML) 11 Page - National Semiconductor (TI) |
11 / 27 page Applications Information THEORY OF OPERATION The LM2832 is a constant frequency PWM buck regulator IC that delivers a 2.0A load current. The regulator has a preset switching frequency of 1.6MHz or 3.0MHz. This high fre- quency allows the LM2832 to operate with small surface mount capacitors and inductors, resulting in a DC/DC con- verter that requires a minimum amount of board space. The LM2832 is internally compensated, so it is simple to use and requires few external components. The LM2832 uses current-mode control to regulate the output voltage. The following operating description of the LM2832 will refer to the Simplified Block Diagram (Figure 1) and to the waveforms in Figure 2. The LM2832 supplies a regulated output voltage by switching the internal PMOS control switch at constant fre- quency and variable duty cycle. A switching cycle begins at the falling edge of the reset pulse generated by the internal oscillator. When this pulse goes low, the output control logic turns on the internal PMOS control switch. During this on- time, the SW pin voltage (V SW) swings up to approximately V IN, and the inductor current (IL) increases with a linear slope. I L is measured by the current sense amplifier, which generates an output proportional to the switch current. The sense signal is summed with the regulator’s corrective ramp and compared to the error amplifier’s output, which is pro- portional to the difference between the feedback voltage and V REF. When the PWM comparator output goes high, the output switch turns off until the next switching cycle begins. During the switch off-time, inductor current discharges through the Schottky catch diode, which forces the SW pin to swing below ground by the forward voltage (V D)ofthe Schottky catch diode. The regulator loop adjusts the duty cycle (D) to maintain a constant output voltage. SOFT-START This function forces V OUT to increase at a controlled rate during start up. During soft-start, the error amplifier’s refer- ence voltage ramps from 0V to its nominal value of 0.6V in approximately 600 µs. This forces the regulator output to ramp up in a controlled fashion, which helps reduce inrush current. OUTPUT OVERVOLTAGE PROTECTION The over-voltage comparator compares the FB pin voltage to a voltage that is 15% higher than the internal reference V REF. Once the FB pin voltage goes 15% above the internal reference, the internal PMOS control switch is turned off, which allows the output voltage to decrease toward regula- tion. UNDERVOLTAGE LOCKOUT Under-voltage lockout (UVLO) prevents the LM2832 from operating until the input voltage exceeds 2.73V (typ). The UVLO threshold has approximately 430 mV of hysteresis, so the part will operate until V IN drops below 2.3V (typ). Hys- teresis prevents the part from turning off during power up if V IN is non-monotonic. CURRENT LIMIT The LM2832 uses cycle-by-cycle current limiting to protect the output switch. During each switching cycle, a current limit comparator detects if the output switch current exceeds 3.25A (typ), and turns off the switch until the next switching cycle begins. THERMAL SHUTDOWN Thermal shutdown limits total power dissipation by turning off the output switch when the IC junction temperature ex- ceeds 165˚C. After thermal shutdown occurs, the output switch doesn’t turn on until the junction temperature drops to approximately 150˚C. 20197566 FIGURE 2. Typical Waveforms www.national.com 11 |
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