sensing tiny and huge current without dropping too much power
sensing tiny and huge current without dropping too much power
(OP)
I am having a bit of a conundrum trying to come up with a current detection scheme for a battery and power circuit. There is a boost IC between the LiIon battery and a load. We need to sense when a charging voltage/current has been applied at the output of the boost (battery charge is coming from load device) and then switch off the boost to directly route the charging power to the battery. We were looking at a MAX9928 to do current sensing at the output of the boost, via a ~0.02 Ohm sense resistor, but the issue with that is that at the tail end of the charge cycle, current may be very low, like < 5mA, so with that resistor we are talking about only 0.1mV of voltage across the resistor, which I don't think the IC can detect reliably (based on Figure 2 of datasheet). Trickle charging would be interrupted when current dropped to low to be detected. If I increase the value of the sense resistor to 1 Ohm so that we get several millivolts across Rsense even at 5mA, then we have unacceptable voltage drop during normal charge currents and when the boost circuit is in operation.
[LI-ION] =====> [BOOSTER]======> [RSENSE] =====> [LOAD and CHARGER]
^<-------------------(bypass booster when charging detected)------|
Any ideas for different ways to solve this?
Thanks!
[LI-ION] =====> [BOOSTER]======> [RSENSE] =====> [LOAD and CHARGER]
^<-------------------(bypass booster when charging detected)------|
Any ideas for different ways to solve this?
Thanks!





RE: sensing tiny and huge current without dropping too much power
Put a low offset op-amp across the low ohm resistor in a differential amplifier mode.
Read its output as the value when the output isn't saturated.
Read the resistor directly when the op-amp output is saturated.
RE: sensing tiny and huge current without dropping too much power
Disclaimer - not an expert in this area, so standby for corrections.
RE: sensing tiny and huge current without dropping too much power
RE: sensing tiny and huge current without dropping too much power
RE: sensing tiny and huge current without dropping too much power
RE: sensing tiny and huge current without dropping too much power
Can you post your boost circuit? Methinks it is critical in understanding the solution.
RE: sensing tiny and huge current without dropping too much power
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: sensing tiny and huge current without dropping too much power
They have a large selection of current-sense amps.
Benta.
RE: sensing tiny and huge current without dropping too much power
http://www.ti.com/ods/images/SLVSAQ2B/TPS61230_3.5...
Skogsgurra: I like this idea a lot and took a look at hall effect sensors but they seem quite large. Do you think there is the possibility of being able to detect only milliamps of current with a very limited amount of space? My thought was that it would take many windings to achieve this. It is a 30mm x 5mm board that is designed to go inside a small battery pack.
Benta: The highest gain I see in the TI line is 1000V/V, so if I have a 0.02Ohm sense resistor, that is only 0.0001V of drop at 5mA charge current, which means the charge sense IC output is 0.1V. Did I do that math right for this type of sensor? I could feed that through another 30x gain stage to make it useful for driving other things, but is that too much total gain?
RE: sensing tiny and huge current without dropping too much power
One thing that you remeber for future applications: Hall sensors and GMRs are temperature sensitive. Flux gates are not. Virtually no zero drift at all.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: sensing tiny and huge current without dropping too much power
So you're back to your original idea about bypassing the booster. I think you might need to look a bit closer at the behaviour of the various components. I still don't see what current has to be the trigger. Don't you just want to know when the voltage of your load is higher than the output of your booster? If it's not, then the battery needs to supply the load. If it is, then the battery should be charging. So why not just detect voltage?
RE: sensing tiny and huge current without dropping too much power
Yes I do just need to detect which voltage is greater, but the problem as I understand it is that the point at which I can measure the charger voltage is the same as the output of the booster. So they are one and the same unless I create a little voltage drop between the two using the sense resistor, which is what I am doing now. Maybe I have misunderstood the question?
RE: sensing tiny and huge current without dropping too much power
RE: sensing tiny and huge current without dropping too much power
https://www.youtube.com/watch?v=VkBGfcZjXvM
Z
RE: sensing tiny and huge current without dropping too much power
No, not which is greater (which would require the sense resistor), but whether the charger voltage has exceeded a threshold.
I'm still a bit confused by your circuit, but this is my assumption:
Battery is nominal 3.2V?
Booster outputs 3.5V regulated.
Load runs off 3.5V.
Then the load turns into an energy source and the charger switches on. Voltage rises and you want the booster bypassed.
So just set your bypass circuit to be triggered when the booster output voltage rises above, say, 3.6V. The charger will charge the battery, which will eventually float to 4.2V and enter trickle charge. When the energy source disappears, the load reappears, pulling the battery voltage back down below 3.5V. Set your bypass circuit to switch out when the voltage falls below 3.5V, and you'll be back to the battery feeding the load via the booster.
RE: sensing tiny and huge current without dropping too much power
RE: sensing tiny and huge current without dropping too much power
Yes we considered just monitoring the output voltage, but here was the issue with that: There is overlap because battery voltage could be anywhere between 3.2V and 4.2V at any point in time. Charging voltage could also be anywhere in the same range as well, because most charging algorithms these days start in constant current mode (charge voltage could be anywhere between 3.2V and 4.2V to drive a fixed amount of current into battery) and then switch over to a constant voltage float charge (4.2V) after that cycle is complete.
RE: sensing tiny and huge current without dropping too much power
So I'm afraid I'll have to back out. I can't think of a simple solution given the parameters you've provided. I can only suggest you: 1) look at something that more directly detects the presence of the charger, 2) try to connect the charger to the battery instead of the load, or 3) swap the boost converter for a bi-directional DC-DC converter.
RE: sensing tiny and huge current without dropping too much power
You want to detect as little as 5 mA, so that's a 400:1 range.
Here's a TI circuit (eval board available) that can sense from 10 uA to 100 mA, a 10,000:1 range, far more than you're probably looking for. Yeah, it's only 100 mA and they use a 6.8 ohm sense resistor, so you'll want to scale the sense resistor for the range you need, but it looks to me like it would do what you want.
http://www.ti.com/tool/tipd135?keyMatch=high%20sid...