BQ24075: Short circuit detection at low battery voltage and high current outputs

Anthony,

Thank you for the response. 

In my scenario the system is running entirely from battery power. Section 9.3.4.2 says that short circuit is detected if VBAT - VOUT > 250mV (this is actually between 200mV and 300mV min/max), but considering that section 8.3 specifies IBAT (discharge) as 4.5A it seems strange to me that a short circuit would be detected at less than that. The margin on short circuit detection and valid operation seems to be non-existent. If the dropout voltage can be as high as 100mV with 1A and a short circuit can be detected at as low as 200mV, then even if my battery voltage was good it would still have a chance of cutting out at 2A.

If 4.5A is only capable in supplement mode and battery current is actually limited to 2.9A then the highlighted line is quite misleading.

Are there any pin compatible parts that do not have the short circuit detection, have it set higher, or have lower dropout?

Hi Bryan,

Would you be able to provide a schematic and the conditions of the high current draw when in battery only mode? I see you've mentioned 8.2W but wanted to check if this is 5.5V and 1.5A at the output during these increased spikes? I'd like to also try and recreate the situation to see if there are some adjustments we while looking to recommend a pin to pin part.

The 4.5A is the battery output max, which is not necessarily supplement mode. In a battery only mode, the current from the battery to the output is not regulated and so the limitations of the current draw is whether or not the battery is able to sufficiently provide the power draw. Do you see this issue at higher battery voltages?

Hi Anthony,

Unfortunately, I am not able to share schematics without approval from my client and NDAs being in place. We have a boost regulator that produces 5V downstream of the output. The circuitry can draw up to 1.5A for ~10ms at a time. With small additional loads on 3.3V regulators, battery droop and efficiencies taken into account that is typically about 8.2W but can climb at very low battery voltage to close to 9W.

Yellow = OUT, Pink = IBAT, Blue = BAT

You can't see it in the provided graph but I have verified that the output cuts off 250us after BAT - OUT > 200mV (the measure is ~210mV)

These pulses come in groups with pulses in a group separated by a few hundred milliseconds and groups separated by minutes to hours. We typically see failures after a few successful pulses within a group and only after the battery voltage dips below 3V. The battery is capable of handling the power out and the voltage droops without PCM cutting off the output (this is a custom battery and has been configured to handle these loads). I can use an electronic load to simulate these conditions directly on the battery and have verified it does not cutout.

Hi Bryan,

Appreciate the timely reply. Is it possible to get the the basic configuration of the charger relevant to the battery:

• Battery Capacity
• Bypass connector connected to BAT
• Bypass connector connected to OUT

I understand this is an urgent issue. I've scheduled some time in the lab on Monday to look into the issue so I hope that's ok.

Is it possible to get the same waveforms you've provided with the 0 mA or V reference line on screen as well? Is it also possible to get a waveform with the output current?

Hi Anthony,

• Battery capacity = 1400mAh
• Battery internal impedance = 180mΩ (this was measured with the 1kHz AC method)
• BAT bypass capacitor (I assume you meant capacitor) = 10uF
• OUT bypass capacitor = 10uF (total of 22uF on OUT)

I do not have the OUT current but that should match the BAT current in the previous plot and the below plot as the battery is the only power source.

Yellow = OUT, Pink = IBAT, Blue = BAT

You can see the no load battery voltage is 3.89V. Yes, the battery is drooping quite a bit and we are working with the battery manufacturer to reduce that as much as possible. However, we need the device to operate down to a near depleted battery at ~3.4V so we will be seeing pulses beneath 3V even if we improve the battery.

I'm working on getting approval to share the power path schematic sheet with you, but it will be Monday before I get word back on that.

Thanks,

Bryan

Hi Bryan,

Yes, I did mean capacitor thank you. Would you be able to increase the capacitance on the battery and the output? Currently working on getting a proper setup for this to test this. From what I see, it looks like the load is pulling less than 92mA and then peaking at about 3.3A. I will attempt to recreate the situation and report back to you in the next couple of days. 

Hi Anthony,

I can increase the capacitance, but based on my calculations and simulations it would require something like 1mF to overcome this problem. I can have 0805 spots so I can probably change the 10uF to 22uF.

Thanks,

Bryan

Hi Brian,

Appreciate the information. I'll do what I can with the schematic I've received offline and attempt to recreate the issue. I will update as soon as I have some results.