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LM2831ZSDX Datenblatt(PDF) 11 Page - National Semiconductor (TI) |
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LM2831ZSDX Datenblatt(HTML) 11 Page - National Semiconductor (TI) |
11 / 27 page Applications Information THEORY OF OPERATION The LM2831 is a constant frequency PWM buck regulator IC that delivers a 1.5A load current. The regulator has a preset switching frequency of 550kHz, 1.6MHz, or 3.0MHz. This high frequency allows the LM2831 to operate with small surface mount capacitors and inductors, resulting in a DC/DC converter that requires a minimum amount of board space. The LM2831 is internally compensated, so it is simple to use and requires few external components. The LM2831 uses current-mode control to regulate the output voltage. The following operating description of the LM2831 will refer to the Simplified Block Diagram (Figure 1) and to the wave- forms in Figure 2. The LM2831 supplies a regulated output voltage by switching the internal PMOS control switch at constant frequency 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) in- creases 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 am- plifier’s output, which is proportional to the difference be- tween 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 di- ode, which forces the SW pin to swing below ground by the forward voltage (V D) of the Schottky catch diode. The regu- lator 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 LM2831 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 LM2831 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 2.5A (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. Design Guide INDUCTOR SELECTION The Duty Cycle (D) can be approximated quickly using the ratio of output voltage (V O) to input voltage (VIN): The catch diode (D1) forward voltage drop and the voltage drop across the internal PMOS must be included to calculate a more accurate duty cycle. Calculate D by using the follow- ing formula: V SW can be approximated by: V SW =IOUT xRDSON The diode forward drop (V D) can range from 0.3V to 0.7V depending on the quality of the diode. The lower the V D, the higher the operating efficiency of the converter. The inductor value determines the output ripple current. Lower inductor values decrease the size of the inductor, but increase the output ripple current. An increase in the inductor value will decrease the output ripple current. One must ensure that the minimum current limit (1.8A) is not exceeded, so the peak current in the inductor must be calculated. The peak current (I LPK) in the inductor is calcu- lated by: I LPK =IOUT + ∆i L 20174866 FIGURE 2. Typical Waveforms www.national.com 11 |
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