SNVSA46B June 2014 – January 2018 LM46001
The voltage regulation loop in the LM46001 regulates output voltage by maintaining the voltage on FB pin (VFB) to be the same as the internal REF voltage (VREF). A resistor divider pair is needed to program the ratio from output voltage VOUT to VFB. The resistor divider is connected from the VOUT of the LM46001 to ground with the mid-point connecting to the FB pin.
The voltage reference system produces a precise voltage reference over temperature. The internal REF voltage is 1.016 V typically. To program the output voltage of the LM46001 to be a certain value VOUT, RFBB can be calculated with a selected RFBT by
The choice of the RFBT depends on the application. RFBT in the range from 10 kΩ to 100 kΩ is recommended for most applications. A lower RFBT value can be used if static loading is desired to reduce VOUT offset in PFM operation. Lower RFBT reduces efficiency at very light load. Less static current goes through a larger RFBT and might be more desirable when light load efficiency is critical. But RFBT larger than 1 MΩ is not recommended because it makes the feedback path more susceptible to noise. Larger RFBT value requires more carefully designed feedback path on the PCB. The tolerance and temperature variation of the resistor dividers affect the output voltage regulation. TI recommends using divider resistors with 1% tolerance or better and temperature coefficient of 100 ppm or lower.
If the resistor divider is not connected properly, output voltage cannot be regulated since the feedback loop is broken. If the FB pin is shorted to ground, the output voltage is driven close to VIN, since the regulator sees under voltage on the FB pin and increases the output voltage. The load connected to the output could be damaged under such a condition. Do not short FB pin to ground when the LM46001 is enabled. It is important to route the feedback trace away from the noisy area of the PCB. For more layout recommendations, please refer to the Layout section.