BQ24070_14 Datasheet, PDF (11/33 Pages) Texas Instruments – System Power-Path Management IC

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bq24070, bq24071
SLUS694G – MARCH 2006 – REVISED DECEMBER 2014
8.3 Feature Description
8.3.1 Power-Path Management
The bq2407x devices power 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
IN
(2)
OUT
Q1
40 mΩ
Q2
bq24070/1
BAT
PACK+
+
PACK−
Figure 3. Power-Path Management
System
UDG−04082
8.3.1.1 Case 1: IN Mode (Mode = High)
8.3.1.1.1 System Power
In this case, the system load is powered directly from the AC adapter through the internal transistor Q1 (see
Figure 3). 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 voltage of the battery.
8.3.1.1.2 Charge Control
When in IN mode, the battery is charged through switch Q2 based on the charge rate set on the ISET1 input.
8.3.1.1.3 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.
1. This feature allows the designer to select a lower power wall adapter, if the average system load is moderate
compared to its peak power. For example, if the peak system load is 1.75 A, average system load is 0.5 A
and battery fast-charge current is 1.25 A, the total peak demand could be 3 A. With DPPM, a 2-A adaptor
could be selected instead of a 3.25-A supply. During the system peak load of 1.75 A and charge load of 1.25
A, the smaller adaptor’s voltage drops until the output voltage reaches the DPPM regulation voltage
threshold. The charge current is reduced until there is no further drop on the output voltage. The system gets
its 1.75-A charge and the battery charge current is reduced from 1.25 A to 0.25 A. When the peak system
load drops to 0.5 A, the charge current returns to 1 A and the output voltage returns to its normal value.
2. Using DPPM provides a power savings compared to configurations without DPPM. Without DPPM, if the
system current plus charge current exceed the supply’s current limit, then the output is pulled down to the
battery. Linear chargers dissipate the unused power (V_IN-V_OUT) × I_LOAD. The current remains high (at current
limit) and the voltage drop is large for maximum power dissipation. With DPPM, the voltage drop is less (V_IN-
V_(DPPM-REG)) to the system which means better efficiency. The efficiency for charging the battery is the same
for both cases. The advantages include less power dissipation, lower system temperature, and better overall
efficiency.
3. If possible, the DPPM sustains the system voltage no matter what causes it to drop. The DPPM does this by
reducing the noncritical charging load while maintaining the maximum power output of the adaptor.
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