RT9941

28

DS9941-01 April 2011

www.richtek.com

The RT9941 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 18). The

output SYS is regulated at 5.0 V. 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.

Figure 18. RT9941 Powered by AC Adapter

Figure 17. Power Path Management Flow Chart

LDO

Power Path

Control,

System power

and Current

limit selection

PWR_ID

PWR_IN

Adapter

From adapter

VSYS

ID

V+

GND

V+

GND

Q1

BATT supply

SYS

SYS < BATT

(T8)

AC supply SYS

& BATT

SYS > BATT

(T1,T2,T6,T7)

AC supply SYS &

BATT

Reduce charge

current

SYS = 4.2V > BATT

(T3,T5)

AC & BATT

supply SYS

SYS < BATT

(T4)

NO

NO

NO

NO

NO

SYS Load >

AC Current

Limit

AC Current Limit

ACOK?

AC Current Limit

ACOK?

ACOK?

YES

YES

YES

NO

NO

YES

YES

YES