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LM5010MHX Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LM5010MHX Datasheet(HTML) 10 Page - National Semiconductor (TI) |
10 / 19 page Hysteretic Control Circuit Overview (Continued) Typically when the load current increases suddenly, the off- times are temporarily at the minimum of 265 ns. Once regu- lation is established, the off-time resumes its normal value. The output voltage is set by two external resistors (R1, R2). The regulated output voltage is calculated as follows: V OUT = 2.5V x (R1 + R2) / R2 (1) Output voltage regulation is based on ripple voltage at the feedback input, requiring a minimum amount of ESR for the output capacitor C2. The LM5010 requires a minimum of 25 mV of ripple voltage at the FB pin. In cases where the capacitor’s ESR is insufficient additional series resistance may be required (R3 in Figure 1 ). When in regulation, the LM5010 operates in continuous conduction mode at heavy load currents and discontinuous conduction mode at light load currents. In continuous con- duction mode current always flows through the inductor, never reaching zero during the off-time. In this mode the operating frequency remains relatively constant with load and line variations. The minimum load current for continuous conduction mode is one-half the inductor’s ripple current amplitude. The approximate operating frequency is calcu- lated as follows: (2) The buck switch duty cycle is approximately equal to: (3) At low load current, the circuit operates in discontinuous conduction mode, during which the inductor current ramps up from zero to a peak during the on-time, then ramps back to zero before the end of the off-time. The next on-time period starts when the voltage at FB falls below the refer- ence - until then the inductor current remains zero, and the load current is supplied by the output capacitor (C2). In this mode the operating frequency is lower than in continuous conduction mode, and varies with load current. Conversion efficiency is maintained at light loads since the switching losses reduce with the reduction in load and frequency. The approximate discontinuous operating frequency can be cal- culated as follows: (4) where R L = the load resistance. For applications where lower output voltage ripple is re- quired the output can be taken directly from a low ESR output capacitor as shown in Figure 9. However, R3 slightly degrades the load regulation. Start-up Regulator (V CC) The startup regulator is integral to the LM5010. The input pin (V IN) can be connected directly to line voltages up to 75V. The V CC output is regulated at 7.0V, ±6%, and is current limited to 10 mA. Upon power up the regulator sources current into the external capacitor at V CC (C3). With a 0.1 µF capacitor at V CC, approximately 58 µs are required for the V CC voltage to reach the under-voltage lockout threshold (UVLO) of 5.8V (t1 in Figure 8), at which time the buck switch is enabled, and the softstart pin is released to allow the softstart capacitor (C6) to charge up. V OUT then increases to its regulated value as the softstart voltage increases (t2 in Figure 8). The minimum input operating voltage is determined by the regulator’s dropout voltage, the V CC UVLO falling threshold ()5.65V), and the frequency. When V CC falls below the falling threshold the V CC UVLO activates to shut off the buck switch and ground the softstart pin. If V CC is externally loaded, the minimum input voltage increases since the out- put impedance at V CC is )140 Ω at low V IN. See Figures 2 and 3. In applications involving a high value for V IN where power dissipation in the startup regulator is a concern, an auxiliary voltage can be diode connected to the V CC pin (Figure 10). Setting the auxiliary voltage to between 7.5V and 14V shuts off the internal regulator, reducing internal power dissipation. The current required into the V CC pin is shown in Figure 4. Internally a diode connects V CC to VIN. 20119915 FIGURE 9. Low Ripple Output Configuration www.national.com 10 |
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