On Wednesday 13 April 2016 12:23 PM, Mark Brown wrote:
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On Tue, Apr 12, 2016 at 06:59:06PM +0530, Laxman Dewangan wrote:
I have put my understanding based on datasheet and observation but it seems
I am missing some important information which is making difficult to
understand further here.
We are not crossing the maximum limit of the load on the rail per datasheet.
We just changed the output capacitor in the platforms and saw deviation.
Well, we might be hitting an inrush limit as we attempt to ramp the
voltage up.
Yes, and Maxim said that once this is there, there is no way to
calculate the actual ramp time. It just say that ramp will not meet the
advertised value but there is no equation or anything to get the actual
ramp time. :-(
I think I need to go again to Vendor to find out that why changing of
capacitor making the deviation in ramp delay and what is the relation.
Probably, that may help here.
Possibly. It did also occur to me last night that having a Maxim
specific property which lets you specify a raw register value to
configure in cases where the board goes out of spec (as opposed to a
time which could be left specified as the real value) might solve the
problem without being too terrible from an interface point of view,
though something that's directly obvious from the schematic would be
much better.
We had discussion with maxim and he pointed us the datasheet for
descriptions and helped on some query:
/*
Note 20. During a soft-start event or a DVS transition, the regulators
output current will increase by COUT*dV/dt. In the event that the load
current plus the additional current imposed by the soft-start or DVS
transition, reach the regulator’s current limit, the current limit will
be enforced. When the current limit is enforced, the advertised
transition rate (dV/dt) will not occur.
---
13.4 Soft-Start and Dynamic Voltage Scaling
The linear regulators have a programmable soft-start rate. When a linear
regulator is enabled, the output voltage ramps to its final voltage at a
slew rate of either 5mV/µs or 100mV/µs, depending on the state of the
SS_Lx bit.
The 5mV/µs ramp rate limits the input inrush current to around 10mA on a
300mA regulator with a 2.2µF output capacitor and no load. The 100mV/µs
ramp rate results in a 200mA inrush current on a 300mA regulator with a
2.2µF output capacitor and no load, but achieves regulation within 50µs.
The soft-start ramp rate is also the rate of change at the output when
changing dynamically between two output voltages while enabled (dynamic
voltage scaling: DVS).
The LDO soft-start circuitry supports starting into a pre-biased output.
For example, if the output capacitor has an initial voltage of 0.4V when
the regulator is enabled, the regulator gracefully increases the
capacitor voltage to the required target voltage such as 1.2V. This is
unlike other regulators without the start into pre-bias feature where
they may force the output capacitor voltage to 0V before the soft-start
ramp begins.
During a soft-start event or a DVS transition, the regulators output
current will increase by COUT*dV/dt. In the event that the load current
plus the additional current imposed by the soft-start or DVS transition,
reach the regulator’s current limit, the current limit will be enforced.
When the current limit is enforced, the advertised transition rate
(dV/dt) will not occur.
*/
We have more capacitance on the board and this increases the current at
power up. If we are within the specs for the LDO current then you will
notice that Maxim specs are guaranteed. However, Maxim does not
guarantee the timings in case current exceeds the LDO specs. In this
case because of the larger capacitances the currents seen by the LDO is
much more and hence Maxim device enforces current limit. When the
current limit is enforced then the dv/dt will be different than what is set.
Some more query to help SW interface:
Q: Is there any mechanism where we can derive ramp rate based on maximum
load or capacitor or inrush current?
Ans: No. Since the LDO imposes the current limit the standard formula
doesn’t hold correct. That’s the reason Maxim in their datasheet has
mentioned that the advertised rate will not occur.
So based on this, when platform does not have the advertised ramp, we
need to have:
Maxim specific property for register configurations and do not allow run
time changes as this does not hold correctly.
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