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SLUS694B – MARCH 2006 – REVISED AUGUST 2006
FUNCTIONAL DESCRIPTIONS (continued)
Autonomous Power Source Selection, Mode Control Pin
With the MODE input low, the bq24070/1 defaults to USB-mode charging, and the supply current is limited by
the ISET2 pin (100 mA for ISET2 = Low, 500 mA for ISET2 = High). If an input source is not available, then the
battery is selected as the source.
Boot-Up Sequence
In order to facilitate the system start-up and USB enumeration, the bq24070/1 offers a proprietary boot-up
sequence. On the first application of power to the bq24070/1, this feature enables the 100-mA USB charge rate
for a period of approximately 150 ms, (t
(BOOT-UP)
), ignoring the ISET2 and CE inputs setting. At the end of this
period, the bq24070/1 implement CE and ISET2 input settings.
indicates when this feature is enabled.
See
Power-Path Management
The bq24070/1 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:
AC Adapter
VDC
GND
Q1
40
mΩ
BAT
System
PACK+
+
PACK−
bq24070/1
Q2
UDG−04082
IN
(2)
OUT
Figure 4. Power-Path Management
Case 1: AC Mode (Mode = High)
System Power
In this case, the system load is powered directly from the AC adapter through the internal transistor Q1 (see
The output is regulated at 4.4 V (bq24070). If the system load exceeds the capacity of the supply, the
output voltage drops down to the battery's voltage.
Charge Control
When in AC mode the battery is charged through switch Q2 based on the charge rate set on the ISET1 input.
Dynamic Power-Path Management (DPPM)
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 OUT pin drops to a preset value, V
(DPPM)
× SF, due to a
limited amount of input current, then the battery charging current is reduced until the output voltage stops
dropping. The DPPM control tries to reach a steady-state condition where the system gets its needed current
and the battery is charged with the remaining current. No active control limits the current to the system;
therefore, if the system demands more current than the input can provide, the output voltage drops just below
the battery voltage and Q2 turns on which supplements the input current to the system. DPPM has three main
advantages.
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