Powerpath instant-on operation
Dead batteries can be especially troublesome in a traditional power architecture where most of the portable product is connected directly to the battery. When the battery voltage is too low for the system to run, the product may appear to be unresponsive even minutes after being connected to a source of input power—possibly generating unnecessary support phone calls. The problem is further compounded when the battery capacity is very large relative to the available charging current (e.g., a USB-powered system with a large capacity battery).
Linear Technology PowerPath products such as the LTC4155 decouple the system power rail from the battery to enable instant-on operation and solve the two most vexing problems caused by deeply discharged batteries.
The first problem is that charge current and system load become indistinguishable when the system power rail is connected directly to the battery. When the battery is deeply discharged, battery manufacturers recommend a greatly reduced initial charge current until the cell voltage reaches a safer level. This trickle charge current must be programmed to a safe level for the battery assuming minimum or no system load current.
Secondly, in a direct-connect battery system, if the system is operational during trickle charging, a significant portion of the charge current intended for the battery is shunted to the system rail. The resulting reduced battery charge current extends recovery time proportionately. A sizable system load can cause the net battery current to reverse, further discharging the battery. For the duration of this low battery condition, the portable system may not be able to respond to the user due to insufficient voltage on the system power supply rail. The duration of unresponsiveness is multiplied by at least a factor of 10 because of the reduced power available to the common-connected battery and system power rail.
The LTC4155 delivers 3.5V to the system rail when the battery is deeply discharged to enable instant start-up. As the battery voltage rises during the precharging phase, the LTC4155 seamlessly and automatically transitions to a higher efficiency mode to speed charging and minimize heat production. Figure 5
shows the voltage available to the system power rail as a function of battery voltage.
Figure 5: VOUT voltage vs battery voltage
The LTC4155 battery charge current is programmed independently from the input current limit to decouple battery charge current constraints from input power constraints. The input current limit can be programmed based only on the limitations of the input supply. Similarly, the battery charge current can be programmed based only on the battery capacity. The LTC4155 always enforces input current limit and prioritizes power to the system load over battery charging if necessary.