LP3945 Datasheet, PDF (12/15 Pages) National Semiconductor (TI) – Battery Charge Management System

Application Notes (Continued)
BIPB PIN
BIPB pin is used to select between charger mode and LDO
mode. It is pulled HIGH internally to the CHG-IN pin, which is
the LDO mode. To select charger mode, this pin must be
connected to ground directly or pulled to ground via the
battery pack ID resistor. In the latter case, BIPB pin pulled
LOW confirms battery connection. Alternatively, this pin can
be pulled to LOW by the system micro-controller for added
flexibility.
LDO MODE
The charger is in the LDO mode when the BIPB pin is left
open or HIGH. This mode of operation is used primarily
during system level testing of the handset to eliminate the
need for battery insertion. CAUTION: battery may be dam-
aged if device is operating in LDO mode with battery con-
nected.
The internal power FET provides up to 1.2 amp of current at
BATT pin in this mode. The LDO output is set to 4.1V. When
operating at higher output currents, care must be taken not
to exceed the package power dissipation rating. See “Ther-
mal Performance of LLP Package” section for more detail.
EN PIN
The Enable pin is used to enable/disable the charger, in both
charger mode and LDO mode, see Figure 5 and Figure 6.
The Enable pin is internally pulled HIGH to the CHG-IN pin.
When the charger is disabled, it draws less than 4 µA of
current.
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FIGURE 5. Power Up Timing Diagram in Charger Mode (BIPB = LOW)
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FIGURE 6. Power Up Timing Diagram in LDO Mode (BIPB = HIGH)
5.6 HR SAFETY TIMER IN CHARGER MODE
Both LP3945 and LP3946 have built-in 5.6 hr back up safety
timer to prevent over-charging a Li Ion battery. The 5.6 hr
timer starts counting when the charger enters constant cur-
rent mode. It will turn the charger off when the 5.6 hr timer is
up while the charger is still in constant current mode. In this
case, both LEDs will turn on, indicating a fault condition.
In order for the 5.6 hr safety timer to function in the LP3945,
pin 13 should be left floating. CAUTION: disabling the back
up safety timer could create unsafe charging conditions. If
disabled, user must provide external protection to prevent
overcharging the battery.
StopModeEN PIN
To provide the flexibility of using an external back up timer,
StopModeEN allows “bypassing” of the 5.6 hr safety timer. It
is achieved by pulling pin 13 on the LP3945 to LOW. As
indicated in the LP3945 Flow Chart, this feature works only
in constant current mode with a Li Ion battery. Therefore, if a
Li Ion battery is in constant current mode and the 5.6 hr timer
times out, instead of the charger being turned off, it proceeds
to maintenance mode.
This is not a recommended mode of operation. Disabling the
5.6 hr timer can potentially expose the battery to prolong
charge cycle and damage the battery. If StopModeEn feature
is used, user must protect the battery from exposure to
prolong charge cycle. For normal operation, pin 13 should be
left floating.
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