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LM5007MM Datasheet(PDF) 6 Page - National Semiconductor (TI) |
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LM5007MM Datasheet(HTML) 6 Page - National Semiconductor (TI) |
6 / 12 page Hysteretic Control Circuit Overview (Continued) The output voltage (Vout) can be programmed by two exter- nal resistors as shown in Figure 1. The regulation point can be calculated as follows: V OUT =2.5x(R1+R2)/R2 The feedback comparator in hysteretic regulators depend upon the output ripple voltage to switch the output transistor on and off at regular intervals. In order for the internal comparator to respond quickly to changes in output voltage, proportional to inductor current, a minimum amount of ca- pacitor Equivalent Series Resistance (ESR) is required. A ripple voltage of 25mV to 50mV is recommended at the feedback pin (FB) for stable operation. In cases where the intrinsic capacitor ESR is too small, additional series resis- tance may be added. For applications where lower output voltage ripple is re- quired the load can be connected directly to the low ESR output capacitor, as shown in Figure 1. The series resistor (R) will degrade the load regulation. Another technique for enhancing the ripple voltage at the FB pin is to place a capacitor in parallel with the feedback divider resistor R1. The addition of the capacitor reduces the attenuation of the ripple voltage from the feedback divider High Voltage Bias Regulator The LM5007 contains an internal high voltage bias regulator. The input pin (Vin) can be connected directly to line voltages from 9 to 75 Volts. To avoid supply voltage transients due to long lead inductances on the input pin (Vin Pin 8), it is always recommended to connect low ESR ceramic chip capacitor () 0.1µF) between "Vin" pin and "RTN" pin (pin 4), located close to LM5007. The regulator is internally current limited to 10mA. Upon power up, the regulator is enabled and sources current into an external capacitor connected to the Vcc pin. When the voltage on the Vcc pin reaches the regulation point of 7V, the controller output is enabled. An external auxiliary supply voltage can be applied to the Vcc pin. If the auxiliary voltage is greater than 7 Volts the internal regulator will essentially shutoff, thus reducing inter- nal power dissipation. 20078305 FIGURE 1. Low Ripple Output Configuration www.national.com 6 |
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