MAX1555

The MAX1555 is a Lithium Ion Polymer USB Battery Charger IC. It provides a complete electrical interface for charging a single LiPo cell in a safe manner and requires no external components. The IC supports two charging modes, USB and DC. USB is limited to 100mA at USB bus voltage current so as not to overload self-powered USB-hubs while DC can go as high as 280mA and supports voltages up to 7V. When both are plugged in, DC has precedence over USB. The MAX1555 also provides an active-low open-drain CHG pin that indicates when charging current is over 50mA.
 * Manufacturer: Maxim
 * Package: 5 pin SOT-23
 * DataSheet: PDF

When the IC detects overheating, it will gradually reduce the charging current until it reaches a safe level. If the battery is in a critically discharged state (VBatt < 3V), the IC will provide a precharge current of 40 mA so as not to damage it.

The MAX1555 does not require any code to interface with. Its CHG pin can be hooked up to a micro-controller to inform it when battery charging it complete.

How it works
The MAX1555 is a temperature controlled voltage source of 3.7V, the maximum voltage of a LiPo cell. When plugged in USB or DC, it will take over the battery as the power source for the circuit, thereby charging the battery up to its maximum voltage.

Interfacing

 * USB - Power from USB
 * DC - Power from DC
 * CHG* - Active-low open-drain output to indicate charging state
 * GND - Ground
 * BAT - LiPo battery and system load

Since the CHG pin is open-drain, it will require a pull-up resistor to the logic voltage rail.

Design concerns
On battery powered system, the charging rails are never connected directly to the battery and system load side. In other words, the system is always battery powered but when power is plugged in, its the MAX1555 that provides power to the system and the battery. Since the MAX1555 can only provide 100 mA maximum when plugged in, this means your design should consume less than 100mA or the battery will not charge. Current your design consumes is current not charging the battery. Critically discharging a battery should be avoided as it will reduce its life expectancy and will greatly increase its charging. The design should verify battery voltage and indicate the user when it is critically low or outrightly refuse to work.

Soldering
This IC only comes in an SMD package and thus very small and hard to solder. Ideally, it should be affixed to a | breakout board if it is to be used for prototyping purposes.

Availability
Sparkfun

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