Other Parts Discussed in Thread: OPA1632,
Hi there,
we use TPS7A4701 and TPS7A3301 to supply +/- 5V for very low noise analog application. below is sch. Some load IC is OP amplifier (OPA1632), we usually need the +/- 5V power to apply to amplifer nearly simultaneously. Which means the soft-start ramping time of 4701 and 3301 should be about the same. This limit the choice of Noise reduction cap.
For 3301, we would like to refer to datasheet section 8.1.4 "TI highly recommends using a 10-nF feed-forward capacitor and a 1-μF noise-reduction capacitor to minimize noise and maximize AC performance. ". but C_NR = 1uF will lead to 1.2s start time. this is to slow to be accepted. So we use 0.1nf as section 8.1.7 writes "To maximize noise and PSRR performance, TI recommends including 47-μF or higher input and output
capacitors, 100-nF noise-reduction capacitors, and 10-nF feed-forward capacitors, as shown in Figure 32 ". Thus, t_ss = 123ms, taking into the CFF effect on soft-start.
As a result, For 4701, we need a 4.7uF NR cap to achieve about 116ms t_ss to be close to 123ms of 3301. This 4.7uf is much larger than 1uF mentioned in the datsheet section 8.2.2.1.2 "Larger capacitances, up to 1
µF, affect noise reduction at lower frequencies while also tending to further reduce noise at higher frequencies ". Not sure NR_cap = 4.7uF will be ok for TPS4701.
Hope some TI expert could give some suggetions on my concerns. Thanks.
