TLV755P: [TLV75518P] [TPS74601] Power up slew rate check for 3.3V input and 1.8V output

Hi Tiger,

Let us know if you want different LDO options with soft start control to better control the turn on ramp rates.

I expect the slew rate will be the same for the 1.8V option.  Based on these waveforms we can estimate the following is happening inside the LDO:

1. There is a voltage reference internal to the LDO which (relatively speaking) turns on nearly instantaneously, like a step function
2. The output of the reference flows into an RC low pass filter
3. During turn on, the R of the low pass filter is disconnected and a fast charge current source charges up C
   1. This gives the linear ramp during turn on
4. After the fast charge is complete (usually around 97% of Vout steady state) the fast charge current source is removed and the R is placed back into the circuit
   1. This gives you the slower roll off to steady state

The feedback resistors set Vout but I do not expect the ramp rates to change.  Thus, for 1.8V output I would expect the same ramp rates as you can see in those waveforms.  The typical methods to slowing down the turn on are using an LDO with an NR/SS pin, SS pin, or placing a Cff across the top setpoint resistor.  None of these options are available to you with these fixed LDO's. 

The only remaining method to slowing down the turn on is increasing Cout to trip current limit during startup.  Keep in mind that as a rule of thumb, current limit protection typically requires 20-50us to engage, so for the initial 20-50us the original ramp rate will still take effect.  Per the datasheets, typical current limit for the TLV755P is 720mA and for the TPS746 it is 1.44A.  Use I = C*dV/dt, with dV = 0.97*1.8V = 1.746V and dt = 300us.  I = the typical current limit values.  Then:

TLV755P: Cout > 124uF

TLV746: Cout > 248uF

Thanks,

Stephen