Other Parts Discussed in Thread: TPS61088
Hi. I'm designing a 5V backup power system which uses the BQ25895 and TPS61088, but I'm facing multiple problems with them.
The BQ25895 is for managing the battery charge stuff and TPS61088 is for boosting voltage to 5V.
My design is almost the same as the reference designs in datasheets, except:
- I don't use the USB handshake & OTG functionality of BQ25895, so I made D+, D-, DSEL, PMID floating.
- I changed some resistors near the FB pin in TPS61088 design for getting my desired voltage (5V).
I connected the SYS pin of BQ25895 directly to the inductor before the SW pin of TPS61088.
And here comes the problems:
1. SYS output of BQ25895 will drop if I connect a power adapter when under load.
I connected a standard 3.7V 3000mAh li-ion battery (charged to about 3.9V) to BAT pin of BQ25895, and then I connected a USB LED lamp to the VOUT of TPS61088. The lamp uses less than 1A of current. At first the lamp worked normally. If I connect power adapter (5V 3A) to BQ25895 (VBUS), the lamp light will go off for about 0.5s, and back on again. I used a multimeter to measure voltage of BQ25895's SYS, which dropped to about 2 V, and TPS61088's VOUT, which dropped to about 0 V, at that time. Then I removed the lamp and then the voltages didn't change when I plugged in the power adapter.
If I use a much higher load, such as two big 5R1J resistors (draws about 2A on 5V), the SYS will go down to about 2 V when I plugged in the power adapter, and never go back. Something on the PCB will start making a humming sound.
Everything worked well with battery only.
2. The system doesn't work without a battery.
Idk if I misinterpreted the sentence "Instant-on Works with No Battery" in first page of BQ25895 datasheet, but if I don't connect a battery, almost any load will make BQ25895's SYS drop to about 2 V and something on the PCB will start making a humming sound as well.
Thank you in advance.
