Single-cell linear charger for portable builds
The Texas Instruments BQ2057CTSTR is a linear charge-management IC designed for single-cell Lithium Ion or Lithium Polymer battery packs. It terminates at the standard 4.2V full-charge voltage and uses a constant-current, programmable charge profile — you set the charge current with a single external resistor, no serial bus or firmware involved. The input supply can go up to 15V, which gives headroom for unregulated wall adapters or 5V USB inputs with margin.
Setting the charge current — one resistor, no code
The 'Constant - Programmable' current feature means the charge rate is set by a resistor from the PROG pin to ground. No I2C, no firmware, no configuration register — just pick the resistor value for your battery capacity and charge time. This makes the BQ2057CTSTR a straightforward drop-in for a hardware-only charger block. Because it is a linear charger, the pass element dissipates the voltage drop between input and battery as heat. For a 5V input charging a 3.7V nominal cell at 500 mA, that is about 0.65W — the 8-TSSOP package on a reasonable copper area handles it, but keep the thermal path in mind during layout.
Mounting type is Surface Mount, so no through-hole equivalent exists in this series. The base product number is BQ2057, meaning other package variants (e.g., SOIC) share the same die but require a different PCB footprint.