BQ2000 Frequency Operation Calc

Fernando Santiago

Hi.

I have a problem to found the exactly value of the operation frequency in my application. I didn't find any informations to how can I calculate the operating frequency on my case specifically.

I'm using the following parameters.

NCell = 8 Mi-MH

Vin[MAX] = 36V

Vout= 8 * 1,7 = 13,6

Rsns = 0,056Ohms

Chys = 4,7pF

Ic =0,9A(Charge Current)

So, the ApplicationNote says:

"The six values that determine operating frequency are input voltage VIN, inductance L, sense resistor RSNS, filter resistance RF, filter capacitance CF, and hysteresis capacitor CHYS."

and

"Select the desired maximum operating frequency F.This is the frequency of operation when the output voltage is exactly half the input voltage. At duty cycles of 75% or 25%, the frequency is 75% of this value."

Is this the best method to find the maximum operating frequency? Does exist any other form to calculate the operating frequency of the specific circuit?

over 12 years ago

0 Charles Mauney over 12 years ago

TI__Mastermind 31120 points

There is no calculator since there are some many variables and this is a hysteretic switcher.

The RC on the SNS pin is the primary factor that sets the switching frequency. I typically do this Empirically.

It is important to have the output caps across the battery connection so all the AC current will go through the sense resistor so one gets a good signal to regulate current.

0 Manuel Pfeuffer over 12 years ago in reply to Charles Mauney

Prodigy 30 points

Hi,

I have quite the same problem. I tried to set the switching frequency to about 150kHz but it is always less than 90kHz. I changed the capacitor as well as the resistor, but nothing works.

I want to use the battery controller with 5V USB, therefore the maximum charging current should be about 350mA.

Can anyone help me with my problem?

Attached is my schematic.

7838.BQ2000_test.pdf

Thanks!

0 Charles Mauney over 12 years ago in reply to Manuel Pfeuffer

TI__Mastermind 31120 points

II believe your issue on changing the frequency is that you are using the

ZXCT1009 current sense IC on the high side, which we do not use.

The output impedance of this device is different than the 0.1 ohm sense resistor to ground that is normally used so the RC filter would probably be totally different. I would remove the capacitor and see how the sense waveform changes and then go from there. you may have to use a much lower capacitor.

0 Manuel Pfeuffer over 11 years ago in reply to Charles Mauney

Prodigy 30 points

First I removed the capacitor from the RC filter but the frequency won't change.

Then I removed the 4.7pF hysteresis capacitor and the frequency is also the same.

I can adjust the current by changing the resistor of the RC filter and with this the frequency is also changing, but only a little bit. So with the current circuit I have about 77kHz switching frequency and 320mA current consumption.

Here is my updated schematic:1425.20130912_BQ2000_test_circuit.pdf