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# BQ24105-Q1: Recommended inductor values

Part Number: BQ24105-Q1

Hi TI-team

I saw that "Charging NiMH Battery Using bq24105-Q1 in Automotive Applications”

http://www.ti.com/jp/lit/an/slua843/slua843.pdf

Figure 3. shows the value of Lout is 10μH.

Is this a recommended value ?

Can it be calculated using equation (13) in the data sheet below ?

Please tell me VBAT, VINMAX, Freq, and ΔIL when 10μH is decided.

Best Regards,

Koji Hayashi

• Hi Hayashi-san,

The datasheet equation assumes that the charger is operating in highest efficiency continuous conduction mode (CCM).  Inductance value is determined by the amount of acceptable inductor current ripple in CCM and also the charger's internal feedback loop compensation.   Inductor current ripple is typically selected as 20-40% of average inductor current, in order to have acceptable output voltage ripple.  Datasheet equation 9 gives the LC product equation for stability.

What is your maximum input voltage, maximum VBAT and maximum charge current?  We cannot back calculate these values from inductor size.  Freq is the charger's switching frequency =  1.1 MHz.

Based on other posts, I assume that VINMAX=16V, VBATMAX=3.6V and ICHARGEMAX=50mA?  If so then the Lout equation gives 126uH to operate in CCM instead of DCM but that is impractical.  I suggest using the values in the datasheet L=22uH and Cout=4.7uF or L=33uF and Cout=3.3uF in order to satisfy stability equation 9.  Actually, when charging at high current, the charger must also be able to terminate down to 50mA, it would be acceptable to use Lout=10uF and Cout=10uF but with high inductor ripple current.

If the NiMH charging requirements have changed to slow charge with 50mA, timer termination is acceptable and you do not need BQ24105-Q1 plus host processor for dV/dt or dT/dt termination.  Have you considered a linear charger?  With such a small charge current, a linear charger would not get impractically hot even with high input voltage.

Regards,

Jeff

• In reply to Jeff F:

Hi Jeff-san

Thank you for quick response.

I asked a question to see how 10μH was determined.
There is no need for back calculation.
Are there any TI linear chargers that can be used with NiMH for automotive use ?

I cannot find L = 33uH and Cout = 3.3uF in the data sheet.
Where is it listed ? Or how was it calculated ?

Best Regards,
Koji Hayashi

• In reply to Koji Hayashi92:

Hi Hayashi-san,

We have an automotive qualified linear charger in development but I cannot discuss on this public forum.  Can you friend request me in this forum and then give me your email address so one of our systems engineers can contact you?

Regarding the L and C, I used datasheet equation 9 page 16.

Regards,

Jeff

• In reply to Jeff F:

Hi Hayashi-san,

Is this issue resolved?

Regards,

Jeff

• In reply to Jeff F:

Hi Jeff-san

My customer appears to be reselecting the batteries (reviewing the capacity, etc.).

I will close it once.

Best Regards,

Koji Hayashi