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Original question:

BQ24074: Input Voltage Regulated Down to 4.3V (potentially Vdppm?)

BQ24074: Understanding BQ24074 ISET limit for Solar Panel Input

Sam G
Prodigy 20 points
Part Number: BQ24074
Other Parts Discussed in Thread: BQ24195, BQ25185

Tool/software:

Hello,

I have a BQ24074 that I use to charge a single cell LiPo from a small solar panel.

My panel can produce a maximum power point of ~0.6W at 5.89v and 0.10mA.

I notice that the charger is letting the panel voltage get down to around 4.3V, this is with a ISET resistor value of 1k which should limit the charge current to ~1.5A.

This seems to be from the vddpm function of the BQ24074, since the panel cannot provide the 1.5A of current, the BQ reduces voltage to until the current recovers.

Since the current will never recover, the panel voltage is set to the lower limit of ~4.3V.

More info here: https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1203115/bq24074-input-voltage-regulated-down-to-4-3v-potentially-vdppm

However, when I put a 9.09k resistor on ISET, which should give a charging current limit of ~98mA, the panel voltage will get up to 5.6V. This behavior is expected.

The interesting part comes when the micro controller is connected, with a power draw of 28mA-130mA, the panel voltage will crash again to 4.3.

Is this the standard behavior, that the vddpm will kick in when the system current draw goes above ISET max charge current value?

If this is the case, there really is no way to hit the maximum power point of my small panel?

If so, is there a recommended charge controller that can set an input voltage lower limit?

I know the BQ24195 has this capability, but I am looking for a chip that does not require communication with a micro controller.

  • +1
    TI__Expert 8235 points

    Hi Sam,

    Thanks for reaching out.

    It sounds like the configured input current limit could be less than the sum of the charge current (programmed by the ISET pin) and the system load current (MCU load). This would cause the output voltage to fall, leading the charger to enter VDPPM regulation.

    Sam G said:
    Is this the standard behavior, that the vddpm will kick in when the system current draw goes above ISET max charge current value?

    The ISET resistor programs the fast charge current, while the EN1 and EN2 pins determine the input current limit configuration. For example, if EN1=LOW and EN2=HIGH, the input current limit is set by the ILIM pin. Could you confirm what the input current limit is set to in your design?

    If the sum of the charge current and the system load current exceeds the programmed input current limit, the output voltage begins to fall as the capacitors on the OUT pin discharge to supply the excess load demand. When the output voltage falls to the VDPPM threshold, the VDPPM regulation loop becomes active and regulates the output voltage (for example, to 4.3V) by dynamically reducing the charge current. If the charge current is reduced to 0A but the system load continues to pull the output voltage below 4.3V, the battery will begin discharging to supplement the input source.

    Many chargers, including the BQ24074, also implement an input voltage dynamic power management (VINDPM) regulation loop, which regulates the input voltage to the VINDPM threshold (stylized as VIN-DPM in the BQ2407x datasheet). VINDPM is active on the BQ24074 when EN1 and EN2 are configured for either USB100 (EN1=0, EN2=0) or USB500 (EN1=1, EN2=0) modes.

    VINDPM is especially useful with solar panel inputs. The charger pulls the charge current and system load current from the input source, so depending on the I-V curve of the solar cell, the current pulled by the charger could cause the solar panel's output voltage to collapse. The VINDPM loop prevents this collapse by regulating the input voltage to the VINDPM threshold while still drawing as much current as the panel can provide at that voltage. The VINDPM threshold is fixed on the BQ24074 but is programmable on other chargers.

    Only one regulation loop can be active at a time. If the input source cannot provide the combined charge and system current, the output voltage falls to the VDPPM threshold and the VDPPM loop becomes active. If instead the output voltage remained above the VDPPM threshold but the input voltage fell to the VINDPM threshold, the charger would enter VINDPM regulation.

    In other words, the output voltage falls if the input current limit or source capability is the bottleneck, and the input voltage falls if the solar panel (or a weak or incorrectly designed USB input source) is the bottleneck due to its I-V curve.

    Sam G said:
    If this is the case, there really is no way to hit the maximum power point of my small panel?

    For a linear charger, the primary goal is often to achieve the programmed fast charge current, not necessarily to operate the solar panel exactly at its MPP, since charging time is most directly determined by charge current. That said, if you are interested in a switching charger, we do have standalone switching chargers with integrated MPPT.

    Sam G said:
    If so, is there a recommended charge controller that can set an input voltage lower limit?

    I recommend the BQ25185. It's our latest standalone linear charger and includes a battery tracking VINDPM feature that makes it great for use with solar panels. Check out this app brief: Using Battery Tracking VINDPM to Make the Most of Solar and High-Impedance Power Sources

    Please feel free to let me know if you have any questions.

    Best regards,

    Alec

  • 0 in reply to Alec Lehman
    Prodigy 20 points

    Alec, thank you very much for your detailed response.

    My current EN config is EN1 = 0 EN2 = 1. With a 9.09k resistor for ISET we get a max current of ~98mA.

    As mentioned, this is OK when the device is sleeping but when the device is on current draw goes above this and the panel voltage falls to 4.3v.

    The way i'm currently set up, it looks like I am relying on DDPM circuit and that lower voltage limit is 4.3v.

    If I switch my EN pins to USB500 mode (EN1=1 EN2=0), I will be relying on the VIN-DPM circuit which has a typical voltage limit of 4.5v.

    This USB500 mode seems preferable for my use case, as 500mA is plenty charge current for any of the typical panels we use, and the higher 4.5v limit will increase my charging power ever so slightly.

    Is it true that if I set up USB500 mode, we will never get into DDPM circuit?

    Thank you for the BQ25185 recommendation, we are evaluating that.

    The power consumption of the BQ25185 and the BQ24074 are similar, it seems the functional difference for my application would be the VINDPM mode.

    The BQ25185 would keep my panel voltage +330mV above battery voltage at all times, whereas the BQ24074 will just keep the panel voltage at 4.3v (provided that current draw > ISET).

    After reading the app brief you referenced, it seems that maximizing the solar panel power is not the best approach for battery charging. This makes my initial concern about the solar panel voltage going to 4.3v not as important as I thought.

    I will reconfigure the BQ24074 to USB500 mode, which sets my panel voltage to 4.5v.

    The next design we may go for the BQ25185 as it has improved charging efficiency when using a solar panel.

    Thanks!

  • 0 in reply to Sam G
    TI__Expert 8235 points

    Hi Sam,

    I'm glad I could help.

    Sam G said:
    Is it true that if I set up USB500 mode, we will never get into DDPM circuit?

    To clarify my previous comments, the VINDPM and DPPM loops can be simultaneously active. In VINDPM, the charger reduces the input current to regulate the input voltage to the VINDPM threshold. If the sum of the charge current and system load current increases beyond the available input current from the input source during VINDPM regulation, the output voltage of the charger falls as the output capacitors discharge. If the output falls to the VDPPM threshold, the charger enters DPPM regulation and reduces the charge current to prioritize the system load and regulate the output voltage to the VDPPM threshold.

    Let me know if you have any questions about the BQ25185 or anything else.

    Best regards,

    Alec