RT9945

28

DS9945-01 April 2011

www.richtek.com

The RT9945 powers the system while independently charging the battery. This feature reduces the charge and discharge

cycles on the battery, allows for proper charge termination, and allows the system to run with an absent or defective

battery pack. This feature gives the system priority on input power, allowing the system to power up with a deeply

discharged battery pack. This feature works as follows:

Case 1: AC Mode (PWR_ID = LOW)

In this case, the system load is powered directly from the AC adapter through the internal transistor Q1 (Figure 17). The

output SYS is regulated at 5V. If the system load exceeds the capacity of the supply, the output voltage drops down to

the battery's voltage.

When in AC mode, the battery is charged through the switch Q2 based on the charge rate set on the ISETA input pin. This

feature monitors the output voltage (system voltage) for input power loss due to brown outs, current limiting, or removal

of the input supply. If the voltage on the VSYS pin drops to a preset value (4.2V) due to a limited amount of input current,

then the battery charging current is reduced until the VSYS stops dropping. If the system continues increasing load to

exceed the AC adapter capacity, the battery will start to discharge to VSYS.

Case 2: USB Mode (PWR_ID = High)

In this case, the system load is powered from a USB port through the internal switch Q1 (Figure 18). Note that in this

case, Q1 regulates the total current to the 450mA level as selected on the input. The output, SYS, is regulated to 5V. The

system's power management is responsible for keeping its system load below the USB current. Otherwise, the output

drops (VSYS) to the battery voltage; therefore, the system should have a low-power mode for USB power application.

Figure 17. RT9945 Powered by AC Adapter

LDO

Power Path

Control,

System power

and Current

limit selection

PWR_ID

PWR_IN

Adapter

From adapter

VSYS

ID

V+

GND

V+

GND

Q1

Figure 18. RT9945 Powered by USB port

LDO

Power Path

Control,

System power

and Current

limit selection

PWR_ID

PWR_IN

USB

VSYS

ID

VBUS

GND

VBUS

GND

Q1

D+

D-

USB port

from PC or

Notebook