Connecting Battery for Secondary Power

flux

Great—USB-charged 2‑cell (2S) Li‑Ion with USB as primary is a perfect fit for a buck‑boost charger with NVDC/power‑path, so your system runs from USB when present and falls back to battery seamlessly.
Proven charger ICs (all support 2S, NVDC power path, and buck‑boost from 5 V per datasheets)
  • BQ25713RSNR — 1–4S, NVDC power path, 3.5–24 V input, seamless buck/buck‑boost/boost. Per datasheet: “Charge 1s to 4s… 3.5 V to 24 V… Seamless transition among buck, buck‑boost and boost operations” and “Narrow voltage DC (NVDC) power path management.”
  • BQ25792RQMR — 1–4S NVDC buck‑boost, Vin 3.6–24 V, I2C, adapter detection. Datasheet calls out NVDC power path and wide input (3.6–24 V).
  • MP2762A — dedicated 2S NVDC buck‑boost charger, 4–21 V input. Datasheet: “for 2 cells in series,” NVDC power path, boost charging when Vin 5 V): add a PD sink controller such as STUSB4500QTR or TPS25750DRJKR to request 9–12 V for higher efficiency. Design power stages to support negotiated voltages.
  • Battery side
    • 2S pack -> BAT pins on the charger; connect NTC if the pack provides a thermistor.
    • Use a protected pack, or add a 2S Battery Protection IC. Include polarity protection.
    • Configure charge voltage/current per the IC (resistors or I2C/SMBus, per datasheet).
  • System power
    • Use the charger’s SYS output as your primary system input.
    • Generate rails from SYS with a Buck Regulator (e.g., 5 V and/or 3.3 V).
    • Add input/output caps and inductor exactly per the charger’s layout guidelines.
  • UX/safety (KB best practices)
    • Add charge/power‑good LEDs per the charger’s status pins.
    • Implement an adjustable low‑voltage cutoff (via charger settings or protector) to prevent over‑discharge.
Two quick choices
What I need to finalize parts and values:
  • USB input type: 5 V only, or USB‑C with PD?
  • Your system rails and max current (e.g., 5 V @ 1 A, 3.3 V @ 600 mA)?
  • Do you have a protected 2S pack with NTC, or should I add cell protection on-board?
If you want, I can drop the charger (plus PD controller if needed) and supporting passives into your project and wire up the power-path.
J

johny2000

I want USB-C with PD at 9V. @library suggest a simple PD sink controller + compatible 2S charger.

flux

Got it: USB-C with PD at 9 V, and a 2S Li‑Ion charger. Below are clean, simple pairings that make USB the primary source and seamlessly charge/run the system. Per best practice, size your power stages to the negotiated PD voltage/current you plan to request (e.g., 9 V), and consider headroom if you may later allow 12 V.
USB-C PD sink (standalone, fixed 9 V capable)
  • STUSB4500QTR - Standalone PD sink, widely used for fixed PDOs with NVM profiles.
  • CYPD3177-24LQXQ - Standalone PD sink, very simple bring-up for power-only sinks.
  • AP33771 - Compact PD sink controller option.
2S Li‑Ion charger with NVDC/power-path (buck‑boost, 5–12 V input capable)
  • BQ25713RSNR - 1–4S NVDC buck‑boost charge controller, I2C control.
  • BQ25792RQMR - Compact NVDC charger IC option.
  • MP2762A - 2S NVDC buck‑boost charger with integrated FETs.
Recommended pairs
Next, tell me:
  • Target charge current (e.g., 2 A) and peak system load while on USB.
  • Whether 9 V is fixed forever, or you might later request 12 V.
Pick a pair and I will verify the datasheets for 9 V operation and then add the controller, charger, and minimal support parts to your project.
Pick STUSB4500 + MP2762A
Pick CYPD3177 + BQ25713
Need 2A charge, 4A load
Add parts to project
Compare AP33771 vs STUSB4500
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