BQ24251 Output Voltage(SYS) dropping at input voltage below 4.7V

Harvinder Sidana

Other Parts Discussed in Thread: BQ24251

Without Battery connected, Output voltage of BQ24251 drops when the input voltage is approx. 4.66V. Load current is approx. 800mA.

Datasheet specifies min. operating voltage as 4.35V. I would like to know why the PWM On period is reducing at 4.66V and why the output voltage is dropping.

Also would like to know what happens when the input voltage is less than 4.35V.

Above plot shows voltage at SW pin (Top) and SYS pin (Bottom) When VIN=4.78V

Below plot shows voltage at SW pin (Top) and SYS pin (Bottom) When VIN=4.66V

over 8 years ago

0 Jeff F over 8 years ago

TI__Guru*** 143970 points

The charger has VINDPM feature which monitors the input voltage and reduces input current to prevent the IC from crashing the power source. With VDPM pin shorted to ground, your VIN_DPM threshold is 4.68V per d/s table 2 page 22.

0 Harvinder Sidana over 8 years ago in reply to Jeff F

Prodigy 60 points

We were referring to 2014 BQ24251 data sheet where in VIN_DPM threshold is specified as 4.35V.

We have set charging current as approx. 1A and current limit as 1.8A. Our load current is approx. 0.5A and we have not connected battery. So there is no charging current. In that case, ideally the output voltage should not drop.

0 Jeff F over 8 years ago in reply to Harvinder Sidana

TI__Guru*** 143970 points

The latest datasheet (dated August 2015) has the correct VINDPM value of 4.68V when VDPM pin is shorted. To confirm that VINDPM is reducing input current to prevent crashing the input voltage rail, can you apply a higher voltage than 4.7V to the input and see the output voltage no longer droops?

0 Harvinder Sidana over 8 years ago in reply to Jeff F

Prodigy 60 points

As we mentioned earlier, our load current is much below the set current limit of 1.8A. By how much percentage is the input current reduced ? Does this mean, that independent of load current, output voltage would drop for input below 4.7 V ?

0 Jeff F over 8 years ago in reply to Harvinder Sidana

TI__Guru*** 143970 points

The VINDPM control loop consists of an error amplifier that reduces the input current, as controlled through the FET between the IN and PMID, to 0A if necessary in order to keep V(IN) from drooping below 4.68V. Therefore, I can't give you a percentage. As a quick test, you can short IN to PMID.