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BQ25700A: noise, heating and problem with ADC
Part Number: BQ25700A
I am trying to use a BQ2700A as a boost battery charger.
(Note: in the future I may need higher input voltage, e.g. 12V 3A)
Here's what I have so far:
The problem I'm having is that the charger only seems to operate in buck mode.
Do people have any thoughts??
I got the 'scope out, and I found that the PWM control seems to be really unstable. For 12V 250mA -> 6.1V ?? mA, I get the following waveforms on SW1 (yellow) and SW2 (green). I am using very short probe connections to avoid noise pickup. I would expect SW1 to be nice an periodic, and SW2 to usually sit at 6V with occasional dips to ground to recharge bootstrap capacitor 2.
I checked that the switching frequency is correct and the switches aren't nasty/noisy (scope has 100MHz bandwidth), which is good. However, SW1 and SW2 don't have waveforms like I would expect - I would expect SW2 be high almost all the time.
However, the charger won't boost. Reducing the input to 5.1V, the Vsys sags to (i.e. out of regulation). I'm not really sure what's going on here - SW1 and SW2 seem to be operating at different rates. (Ch4 is the low side gate for SW2 node)
So I did some more investigating and made 2 changes:
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In reply to Jonathan Bradshaw98:
I am reviewing your question and will get back to you soon. I am reviewing the waveforms and test conditions you have captured.
In reply to Joel Hernandez II:
In reply to Steve Preissig:
This sounds reasonable. Once I get a moment free at the office I will see about getting a variable DC load hooked up and try across a wide load range.
Hello again Steve
I finally got some time for some more bench testing. You are right - it's about the transition from pulse / burst mode to fixed frequency mode.
For those who may be interested, I found the following transitions (with 3.3uH inductor, 800 kHz switching freq, 10mR sense resistors and battery charge disabled)
Transition to CCM
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