BQ24630: Charge current (ISET1) set doe s not match the actual current

Hello,

For 1A charge current, you can still use a 10mOhm sense resistor and change the value of VISET1 around to reach 1A charge current.

The charge current regulation accuracy varies based on the voltage across the sense resistor. Please refer to the datasheet EC table for charge current regulation accuracy.

You can use the calculation tool for bq246xx below to find the optimal inductor and capacitor value. Make sure the current ripple does not exceed the cycle by cycle current limit.

www.ti.com/.../getliterature.tsp

The sense resistor value can be changed up to 40mohm. 100mOhm is not going to work..

"The sense resistor value can be changed up to 40mohm. 100mOhm is not going to work.."
Why isn't it going to work?
Where is this magic number coming from? Please explain.

And what does not work? The max current of 1A is tested and if I set ISET1 (try and error) to 1.7V I get 750 mA charge current.

Let me explain why I what to increase Rsr...

For safety:
On a single fault the charge current must still be within battery specification (IEC62368). By increasing Rsrs to 100mOhm the max current is set to 1A (tested and working). If the pull-own resistor on ISET1 is not connected or the pull-up is shorted, there will be 3V3 on ISET1 and the max charge current is set. With 10m Ohm this is 10A, way to high for my battery and this is not allowed by the safety standard (IEC62368).

With ISET1 the current is set to 750mA, that is the charge current I need. I Have a 1500mAh 2S1P battery (LiFePo), charge current is 0.5C, max 1C.
But... the charge current is only 580mA, in stead of 750mA. If I use a 10m Ohm and I set ISET1 again the charge current is 750mA on the target and on the evalutation board.

The datasheet tells that it is possible to increase Rsr, but it does not tell me anything except a higher value is better for the accuracy.
The datasheet assumes Rsr = 10mOhm and for that a L and Co is given. Can I use these values with Rsr = 100m Ohm?

The calculation sheet does not calculate the L or Co, only the LC output resonant frequency and Iripple is calculated. There is also nothing in the datasheet about, except typical recommended value using a 10m Ohm.

Voltage between SRP and SRN is max 100mV. I'm allowed to charge with 1C, this is 1.5A for a 1500mAh battery, is I can decrease the Rrs to 68m Ohm. I noticed that when Rsr is 100m Ohm that the regulation became kind off unstable with a charge current of 750mA, The U rimple waveform is different.

We did some calculation using general buck formula: Rsr = 100m Ohm, L=47u, Co = 3u3. I will test it tomorrow.

Hello,

The charge current is going to be limited by the input current as well. This means if your input current limit is set such that the battery charging current can only reach 1A, the charge current can not reach 10A even if you want it to...Thus for safety, I would highly suggest to use the input current regulation function as second level protection.

Whether or not the charge current can reach 10A is also depending on the adapter's current capability, but that is not the point of this discussion.

As of why 100mohm will not work, the device has an internal cycle by cycle over-current protection at 160% of DC charge current. This means the IRIPPLE/2 cannot exceed 0.6*ICHG_DC. --> IRIPPLE < 1.2*ICHG_DC

ICHG_DC = (VSRP-VSRN)/RSR

IRIPPLE <1.2 * (VSRP-VSRN)/RSR

If you switch 10mohm to 100mohm, it is 10 times larger, which means the IRIPPLE needs to be 10 times smaller in order not to trigger the cycle by cycle current protection.

Fore example, VSRP -VSRN range is 0 to 100mV. With 10mohm sense resistor, the charge current range is such that the IRIPPLE does not reach 12A.

With 100mohm, the IRIPPLE range will be not exceeding 1.2A. Depending on the chose of LC, this might be achievable, but need to keep in mind that might be an issue if something does not work as expected.

Hi Jing,

I can't limit the adapter current, my system needs more that 1A

You are assuming L = 8u2, but the inductor current can be decreased by increasing the inductor, so a larger Rsr can be used. As voltage over Rsr does not exceed 100mV the value of Rsr is not important.

I tested my calculated values (L = 47u, C = 3u3) and this works.
* Icharge = 750mA
* Imax is 1.1A when ISET1 = 3.3V,
* Uripple = 35mV).

My calculation:

Icharge = 750mA
ISET1 = 1.5V
ISET2 = 0.75V

D = Uout/Uin = 6.5/12 = 0.54

Iripple = 20 .. 40% Icharge → 150 .. 300mA

L = (Uin * D(1-D)) / (fs * Iripple) → L = 66 .. 33uH

47uH → Iripple = (12 * 0.54(1-0.54)) / (300k * 47u) = 211mA → K = Icharge/Iripple = 28%

Isat > Icharge + (Iripple/2) = 750m + (211m / 2) = 0.86A → SRN8040-470m = 1.4A

Fo = 10k..15kHz → Fo = 1/(2PI * SQRT(LC) → C = (1/(2PI * Fo * SQRT(L))^2 → 5.39u .. 2.39uF → 3,3uF → 12.8kHz

Regards,
Rody

Hello Jing,

What do you mean? Load transients are not expected IMHO. The charger only charges the battery while the system runs on the adapter. The battery charge current is a constant current. When running on the battery it is not charged and the buck is switched off.

Regards,
Rody.