BQ24295 output voltage in buck and charging mode

Mrudang Shelat

Other Parts Discussed in Thread: BQ24296, BQ24295

Hi,

We are using the BQ24295, but we would like to know that while in buck and charging mode, how much output voltage we get on PMID and SYS pin while providing 5V input supply.

Regards,

Mrudang

over 9 years ago

0 Jeff F over 9 years ago

TI__Guru*** 148875 points

In buck mode, PMID is connected to VBUS through the Q1 FET, so V(PMID) = V(VBUS) - I(VBUS) *RDSON(Q1).

0 Mrudang Shelat over 9 years ago in reply to Jeff F

Prodigy 190 points

Hi Jeff,

Thanks for the solving the query.

On top of that I would like to know that, during boost mode operation (Vbus is not available as a input supply) at that time what will be the status of Q1?
If Q1 gets on then boost supply is also available on Vbus.
Can I use PMID pin for output power for both buck and boost mode?
And what If i don't use USB D+/D-? I have just USB 5V and GND. Can you please provide me it's reference design with D+ and D- divider circuit?

Regards,

Mrudang

0 Jeff F over 9 years ago in reply to Mrudang Shelat

TI__Guru*** 148875 points

Regarding 1 and 2, Q1 is turned off.

Regarding 3, yes but keep in mind that the input current limit feature is controlled through Q1 so any current pulled through PMID will not be available to the system or to charge the battery.

Regarding 4, bq24296 does not have D+/D- but its boost output is from VBUS not PMID. If you don't want to use D+/D-, shorting D+=D- gives ILIM at startup = 1.5A while putting 15kohm to ground on each gives ILIM at startup=0.5A. Once the host is up and running, you can change ILIM.

0 Mrudang Shelat over 9 years ago in reply to Jeff F

Prodigy 190 points

Hi Jeff,

Thanks for support.

Please see my requirement as below.

I am working on Li-Ion battery battery charger.

My Input is 5V@500mA (USB Power). My system Load is 5V @ 200mA max. And I have 10AHr or 20AHr battery for charging. And I want both Buck and Boost output from single pin out (PMID).

I know charging process will be slow.

Could you please tell me Whether I can use BQ24295 for my require circuit?
With How much charging current of battery along with System load (5V @ 200mA max)?
If Vbus input is available then on PMID pin how much voltage and power will i get?
If Vbus input is available then on PMID do we get Vbus or boost output?
What is Q1 mosfet's current specification?

Please provide your valuable feedback.

Regards,

Mrudang

0 Jeff F over 9 years ago in reply to Mrudang Shelat

TI__Guru*** 148875 points

Regarding 2, with only 5V@500mA input power available, using datasheet figure 3 to estimate efficiency at 93%, the maximum output current available for system and charging (assuming 3.6V battery) is 5V*0.5A*0.93/3.6V = 0.645A. So with 200mA load, you will have only 0.445A available for charging. A 20AHr battery will need slightly longer than 20AHr/0.445A = 45 hours to charge. This is longer than the bq24295's max safety timer of 20Hrs. So, you would need to add additional circuitry to reset the safety timer in order for the battery to complete charge.

Regarding 3, V(PMID) = V(VBUS) - I(VBUS)*RDSON(RBFETQ1) where RDSON(RBFETQ1) is 41mohm max. Any power taken from PMID reduces the power available to system and charging.

Regarding 4, no. Boosted 5V output on PMID is provided from the battery and is only available when no adapter is present.

Regarding 5, the RDSON for Q1 is 41mOhm Max. It will handle up to the 3A continuous per recommended operation table, with 8A for 20ns per absolute max table.

Regarding 1, bq24295 will charge your battery if you can increase your input power.

0 Mrudang Shelat over 9 years ago in reply to Jeff F

Prodigy 190 points

Hi Jeff,

Thanks for the support,

As I have told you. I have 5V@500mA USB power but i don't have any D-/D+ pin for input current limit.

Please find my charging circuit attached along with query.

How can I make charger circuit with External Adpter or USB power(without D-/D+ detection)?
What will be the value of Rilim = R3 = ? for my ckt?

Kindly provide your valuable feedback.

Please let me know if any other changes required for both Buck and Boost output from single pinout (PMID)..

0 Jeff F over 9 years ago in reply to Mrudang Shelat

TI__Guru*** 148875 points

As a reminder, in buck mode, the output from PMID is NOT from the buck converter but is simply the input power at VBUS after passing through a switch (Q1). In buck mode, you do not have access the battery from PMID.

Your table is a bit misleading. The ILIM resistor does not directly set charging current but sets the current available from PMID. The efficiency of the buck converter is needed to predict the amount of charging current.

Are you not using the SYS output from the buck to power your system? Even if not used, you must have the recommended output capacitance on SYS.

Regarding question 1, if you only have one input port (i.e. J1), then I suggest you do the following:
1. Connect D+/D- from J1 to Dp/Dm of U1 - most adapters with usb micro connector have D+/D- shorted. This will result in ILIM being set to ILIM. Therefore, you need RILIM set to the adapter's maximum output current.
2. Place 15kohm pull down resistors on DpDm - this will force the charger's Dp/Dm detection into 500mA (unless an adapter is connected as explained above)