USB Lithium Polymer Battery Charger

The MAX1551 and MAX1555 charge single-cell 3.7V lithium-ion lithium-polymer batteries from USB and AC adapter power. They operate without external FETs or diodes and accept operating input voltages up to 7V. On-chip thermal limiting simplifies printed circuit board layout and allows for optimal charging rates without the thermal limitations imposed by worst-case battery and input voltage.

When the MAX1551 and MAX1555 reach their thermal limits, the charger does not shut off but gradually reduces the charging current. The MAX1551 includes a low-active POK output to indicate the presence of an input power source. If either charging source is active, the low-active POK goes low. The MAX1555 has a low-active CHG output to indicate the charging status. When USB is connected but no DC power is available, the charging current is set to 100mA (maximum). This allows charging from both powered and unpowered USB hubs without port communication. When a DC power source is connected, the charging current is set to 280mA (typical). No input blocking diode is required to prevent battery discharge. The MAX1551 and MAX1555 are packaged in a 5-pin thin SOT23 package and operate over a temperature range of -40°C to +85°C.