Improving a simple soft-start circuit

[jmw]

I came across this design for a inrush limiting soft-start circuit:



The initial voltage at vg is set by the capacative voltage divider of C3 and C1 & C2. If C2 << C1, then the series combination of C1 and C2 is approximately just C2, and at the instant of the voltage step the capacitors are much lower impedance than R1, so it too can be ignored, making the startup voltage at vg equal to V1 * C3/(C3 + C2), so C3 is chosen to ensure vg starts high enough that M1 is initially off. The voltage at vg decays until it reaches the turn-on threshold of M1, then negative feedback through C2 will hold vg up to keep the current Iout delivered to the out node approximately constant. If C2 << C1, then the current through C2 is approximately (C2/C1)*Iout, so R1 acts as a current sense resistor while M1 is in saturation. R1 and C2 are chosen so that R1*C2 = Vg/(Iout/C1), where Vg is the voltage that puts the MOSFET in saturation to deliver Iout. In this case, 5V to deliver a desired current of 100 mA yields Vg = 4.1 V, and requires R1*C2 = .041. Eventually v(out) ramps to V1 and the current through the capacitors falls to zero, the MOSFET exits the linear region and the voltage at vg resumes its decay with time constant R1*C3.



It's a simple, cheap circuit that works but has some disadvantages:
1. The ramp up and down of current is slow
2. Choosing C3 with enough margin that Vgs starts below threshold means a long startup delay where the circuit is waiting for the MOSFET to start to turn on and no current is flowing.
3. Once the capacitors are charged, R1 is now problematic since it slows the decay of the gate voltage. The MOSFET is "on", but rDS(on) is probably higher than optimal. Ideally, vg would drop to ground as soon as the inrush is done. Reducing R1 means C2 must increase along with C3, so the time constant is basically fixed by the supply voltage and desired current limit.

Are there easy ways to improve this circuit to address some of its shortcomings?

[SiliconWizard]

https://www.onsemi.com/products/power-management/load-switches/ncp330

 

[jmw]

It seems that part has a fixed 2ms startup. For 5 V to charge 1000 uF in 2ms, an inrush of 2.5 A is required. I want something more adjustable to keep it under 100 mA to meet USB limits when a dedicated charger port isn't detected.

[SiliconWizard]

It seems that part has a fixed 2ms startup. For 5 V to charge 1000 uF in 2ms, an inrush of 2.5 A is required.

Oh, it was meant as a joke. Although you may be able to find a similar integrated solution that fits your requirements.

I want something more adjustable to keep it under 100 mA to meet USB limits when a dedicated charger port isn't detected.

With that application, are you sure it's a soft-start circuit you really need, and not just a current limiter?
A soft-start will just limit the current when switching on, not permanently. You may want to use a switchable current limiter instead? (Which is not exactly the same thing.)

[jmw]

Yep, the input is intended to draw power from a changing port. But an unprotected inrush into a large capacitance will probably glitch or crash a USB data hub if it's mistakenly plugged in to one, and will probably overwhelm the supply even if it's a charger. So I want protection at inrush only, and other parts of the circuit can do charger port detection to see if it's safe to draw more than 100 mA.

[jhpadjustable]

It seems that part has a fixed 2ms startup. For 5 V to charge 1000 uF in 2ms, an inrush of 2.5 A is required. I want something more adjustable to keep it under 100 mA to meet USB limits when a dedicated charger port isn't detected.

Aren't USB device inrush currents limited to 10µF equivalent across VBUS? Is there a reason you need a 1000µF bulk cap at all?

[SiliconWizard]

It seems that part has a fixed 2ms startup. For 5 V to charge 1000 uF in 2ms, an inrush of 2.5 A is required. I want something more adjustable to keep it under 100 mA to meet USB limits when a dedicated charger port isn't detected.

Aren't USB device inrush currents limited to 10µF equivalent across VBUS? Is there a reason you need a 1000µF bulk cap at all?

I agree. Instead of limiting inrush current, I would design the device so that there is a max 10µF equivalent on VBUS by default, and use a power switch, followed by the 1000µF cap (if really needed) and power circuitry, when the additional power is required (and can be safely asked for).

A small series inductor on VBUS can still be used to decrease voltage noise and provide some basic additional inrush current limiting.

[jmw]

I'm using VBUS as the input to a boost converter and a sizable input filter is needed, and even with a power switch a soft-start is needed. Of course, I'm also interested in improving this circuit in the general case for other applications.