7.3.3 Start-Up

Through a lower resistance, the band-gap reference can quickly charge the noise reduction capacitor (C_NR). The TPS7A80 have a quick-start circuit to quickly charge C_NR, if present; see the Functional Block Diagram. At start-up, this quick-start switch is closed, with only 33 kΩ of resistance between the band-gap reference and the NR pin. The quick-start switch opens approximately 2ms after any device enabling event, and the resistance between the band-gap reference and the NR pin becomes higher in value (approximately 250 kΩ) to form a very good low-pass (RC) filter. This low-pass filter achieves very good noise reduction for the reference voltage.

Inrush current can be a problem in many applications. The 33-kΩ resistance during the start-up period is intentionally put there to slow down the reference voltage ramp up, thus reducing the inrush current. For example, the capacitance of connecting the recommended C_NR value of 0.01 μF along with the 33-kΩ resistance causes approximately 1-ms RC delay. Start-up time with the other C_NR values can be calculated as Equation 1:

Equation 1.

Equation 1 is valid up to t_STR = 2 ms or C_NR = 26 nF, whichever is smaller.

Although the noise reduction effect is nearly saturated at 0.01 μF, connecting a C_NR value greater than 0.01 μF can help reduce noise slightly more; however, start-up time will be extremely long because the quick-start switch opens after approximately 2 ms. That is, if C_NR is not fully charged during this 2 ms period, C_NR finishes charging through a higher resistance of 250 kΩ, and takes much longer to fully charge.

A low leakage C_NR should be used; most ceramic capacitors are suitable.